Pierre Cotillon
Stratigraphy With 115 Figures
Springer-Verlag Berlin Heidelberg New York London Paris Tokyo Hong Kong Barcelona Budapest
Professor Dr. Pierre Cotillon Departement des Sciences de la Terre Universite Claude-Bernard Lyon I 27/43 Boulevard du 11 Novembre F-69622 Villeurbanne Cedex France Translated by Professor James P.A. Noble Department of Geology University of New Brunswick P.O. Box 4400 Fredericton, N.B. Canada E3B 5A3
Title of the original French edition: Pierre Cotillon, Stratigraphic
© Bordas, Paris, 1988
ISBN-13:978-3-540-54675-7 e-ISBN-13 :978-3-642-77025-8 DOl: 1O.l007/978-3-642-77025-8 Library of Congress Cataloging-in-Publication Data Cotillon, Pierre. [Stratigraphie. English] Stratigraphy/Pierre Cotillon; [translated by James P.A. Noble]. p. cm. Includes bibliographical references and index. ISBN-13:978-3-540-54675-7 1. Geology, Stratigraphic. 1. Title. QE651.C7313 1992 551.7-dc20 92-19762 CIP This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitations, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution under the German Copyright Law.
© Springer-Verlag Berlin Heidelberg 1992 The use of general descriptive names, registered names, trademarks, etc. In this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Typesetting: Best-set Typesetter Ltd., Hong Kong 32/3145 - 5 4 3 2 1 0 - Printed on acid-free paper
Foreword
"The poor world is almost six thousand years old." Shakespeare, As you like it
Stratigraphy, the study of stratified rocks is with sedimentology, the science of sedimentary rocks, which recently has became independent from it. Its two principal objectives, to evaluate the course of time (geochronology) and to reconstruct past geographies (paleogeography), have, however, remained uniquely stratigraphic questions, unchanged by the progress associated with other sciences and techniques. Fossils may have attracted the attention of man since time immemorial, but the consequences of their study, such as the measure of time and the determination of ancient shorelines, were barely understood before the eighteenth century, when the Neptunists promulgated their extremist views that the entire crust of the Earth was precipitated from the oceans. It was only in the nineteenth century that stratigraphy in the proper sense established itself as an autonomous science. However, it could only solve problems of relative time, allowing the older to be distinguished from the younger, without being able to give a real age. The Earth was old, older than Shakespeare believed, but how old? Towards the middle of the twentieth century, radioactive isotopes began to provide answers to this question, giving stratigraphy its unit of time, millions of years. From that point on, the stratigraphic calendar was supplied with a reference system defined in relation to measurable units of time with names borrowed from geography. This first revolution was followed by another, resulting from the determination of former magnetic fields (paleomagnetism), which means that every point on the Earth could be tracked in its successive positions during time, giving a scientific foundation to the old concept of the mobility of continents, proposed earlier with such foresight by A. Wegener. From then on it was possible to reconstruct the sequence of past geographies as they unfolded in time, i.e. paleogeography. Many other techniques have been developed in recent years to make stratigraphy a new science. Pierre Cotillon, by his work on the'sediments of yesterday's
VI
Foreword
seas in the Alps and on the oceans of today is ideally suited to outline in this short volume the new approach to the history of the Earth, which is like an opera, with stratigraphy being the score. Jean Aubouin
Preface
The major purpose of this work is to outline the successive achievements of one of the oldest geological disciplines, whose basis and major principles date from the nineteenth century. The methods of stratigraphy have been improved, to the same extent as the other Earth sciences, not only by contributions from biology, paleontology, sedimentology, geochemistry, geophysics, and global tectonics, but also by the requirements of petroleum exploration and the large international programs of ocean drilling with respect to age dating. Stratigraphy enables the construction of paleogeographic syntheses which are the basis of all historical reconstructions. The histories of three very unequal segments of time, the Precambrian, the Paleozoic, and the Mesozoic-Cenozoic, are analyzed in the last chapter. For each of these periods, plate tectonics, variations of sea level, climatic trends, and marine and continental sedimentation are discussed successively. Only a few brief lines are devoted to biological phenomena, in spite of their close connections with geological aspects, but they have been treated fully in two books of this same series. A major effort has been to show the interdependence of all the events which constitute the history of the Earth and which have a principal driving force in common residing in the deeper layers of the Earth. Only the most relevant works and specialized articles are mentioned in the bibliography. I am very grateful to Prof. Jean Aubouin, Member of the Institute, who entrusted to me the writing of this book and who willingly criticized and corrected the first manuscript. I thank also, for their advice, my colleagues Raymond Enay, Jean Chaline, and Herve Charnley. Finally, I have benefited from the efficient assistance of Helene Trunde with regard to the text, and of Andre Duivon for the illustrations; I thank them warmly. Villeurbanne, July 1992
Pierre Cotillon
Contents
Chapter 1 Fundamentals of Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1 2 3
1 1 3
Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chronology of Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Principles of Correlation. . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 2 Elaboration of the Fundamentals of Stratigraphy . . . . . . . . . . .
1 2 2.1 2.2 3 3.1 3.2 3.3 4
Lithostratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biostratigraphy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Evolution, the Reference System for Age Dating. . . . . . The Zone Concept of Oppel ....................... Chronostratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. The Concept of the Stage. . . . . . . ... . . . . . . . . . . . . . . . .. Event Stratigraphy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. The General Chronostratigraphic Scale. . . . . . . . . . . . .. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
7 8 9 10 12 12 15 17 17
Chapter 3 Modern Stratigraphy
19
1 1.1 1.2 1.3
19 20 24
2 2.1 2.2 2.3 2.4 2.5
Refinement of Concepts and Time Scales. . . . . . . . . . .. Evaluation of Geologic Time Intervals and Rates ..... New Biostratigraphic Approaches. . . . . . . . . . . . . . . . . .. Search for a Rigorous and Universal Chronostratigraphy .................. New Methods of Correlation . . . . . . . . . . . . . . . . . . . . . .. Correlation by Sedimentary Rhythms. . . . . . . . . . . . . . .. Correlation by Mineralogic and Geochemical Markers. Correlation by Paleomagnetism. . . . . . . . . . . . . . . . . . . .. Extraterrestrial Correlations ....................... Conclusions......................................
33 37 37 47 56 62 62
x
Contents
Chapter 4 From Stratigraphy to Paleogeography ., . . . . . . . . . . . . . . . . ..
65
1 1.1 1.2 1.3 2 2.1 2.2 2.3 2.4 2.5 2.6
65 65 67 68 75 75 77 77 78 81
Principles and Methods of Paleogeography . . . . . . . . . .. Facies........................................... Paleobiogeography............................... Cartographic Syntheses. . . . . . . . . . . . . . . . . . . . . . . . . . .. Factors of Paleogeographic Evolution ............... Deformation of the Lithosphere ............ . . . . . . .. Volcanic Eruptions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Interplay of Erosion and Sedimentation. . . . . . . . . . . . .. Eustasy......................................... Polar Wandering ................................. Conclusions: the Earth in Relation to Other Planets of the Solar System .......... . . . . . . . . . . . . . . . . . . . ..
82
Chapter 5 The Major Stages of Earth History . . . . . . . . . . . . . . . . . . . . . ..
83
1 1.1 1.2 1.3 1.4
83 83 85 86
1.5 2 2.1 2.2 3 3.1 3.2 3.3
The Precambrian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Boundaries and Subdivisions . . . . . . . . . . . . . . . . . . . . . .. Methods of Study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. The Geography of the Precambrian ................. Early Segregation and Establishment of Fundamental Processes ......................... Conclusions on the Precambrian . . . . . . . . . . . . . . . . . . .. The Paleozoic: the Formation of Pangea ............. Lower Paleozoic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Upper Paleozoic .................................. The Mesozoic and Cenozoic: Breakup of Pangea. . . . .. The Mesozoic .................................... The Cenozoic .................................... Conclusions on the Mesozoic and Cenozoic ..........
87 100 100 101 114 132 133 155 171
General Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 173
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 177 Subject Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 185
Chapter 1
The Fundamentals of Stratigraphy
1 Definitions The aim of stratigraphy, or the science of geologic strata, is to study the distribution in space and time of these strata and the events which formed them, i.e. to reconstruct the organization and history of the outer crust of the Earth on the basis of the lithologic documentation obtainable from these superficial layers. The rocks record in their facies the signature of all or part of the dynamic events constituting this history, biological, physical, and chemical. In normal usage, the term stratigraphy is reserved for sedimentary rocks which occur as bedded successions; however, some stratigraphic methods are also applicable to crystalline rocks.
2 Chronology of Events Any history presupposes a succession of events of variable duration within a certain time framework; it is this succession of events, arranged against an appropriate time scale, which represents history in the most natural way. Just as the falling sand of an hourglass gives a notion of time, so does a sedimentary layer formed during a particular time interval also represent that interval, albeit fossilized. Prior to all historical reconstruction, therefore, a stratigrapher must establish the order of deposition of all beds under study, assuming, for normally stratified beds, that the lower bed of any superposed pair is the older (principle of superposition). However, a few exceptions to this principle are illustrated by alluvial terraces, sedimentary veins, cave deposits, etc. (Fig. 1). The order of deposition of different sedimentary beds defines ipso facto the relative chronology of the events which they represent. A succession of sedimentary beds provides a local or regional history, though generally incomplete, by virtue of the record of events it contains. These include metamorphic and plutonic rocks, volcanic flows, veins which cut one another on a regional or thin-section scale, continental erosional and depositional structures, tectonic deformations, and inclusions (Fig. 2). All
Fundamentals of Stratigraphy
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Fig. 2. Local history. A Photographed on Mars by Viking. 1 Old impact crater partially filled with lava; 2 volcanic cone later than 1; 3 impact crater later than 2. B Regional observations: the granitic batholith 2 is later than formation 1 and its deformation. The erosional surface 3 is later than 2 but earlier than the discordant rocks 4. C Observations under the microscope: the foraminifer 1 included in the fragment 2 is older than it. The fragments 2, forming part of the rock, were deposited at the same time. The vein 3, cutting the shell fragment 2 is later than the formation of the rock but earlier than the vein 4 which cuts and offsets it
geological disciplines, therefore, must use stratigraphic principles whenever they wish to refer to the geologic time scale. A local history cannot be used directly to help reconstruct the general history of the globe. The duration and extent of any gaps that the succession contains are unknown. Thus, in a stratified succession the total active periods (i.e. of sedimentation) may be only a fraction of the "dead time" (represented by planes of nondeposition and diastems) during which no new geological documentation is added and some part of the old may be destroyed.
Principles of Correlation
3
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1 - Granite of the basement; 2 - Permian sandstone Infilling depressions of basement reliefs; 3 - The "Conglom6rat principal" forming the first cuestas of the Paris basin and overlying the Vosges sandstones; 4 - Voltzia sandstones and Wellenkalk; 5 - anhydritgruppe; 6 - Upper Muschelkalk (second cuesta); 7 - Keuper; 8 - Rhetian carbonate sandstones with Avicula contorta; 9 - Levallols marls (Upper Rhetlan); 10 - Hettangian sandstones (basal Jurassic and third cuesta) (After Pommerol1975)
Fig. 3. Trias section from the Vosges to Lorraine (NE France): 2-3 sandstones; 4-6 dominantly carbonates; 7 evaporites
3 Principles of Correlation In order to contribute to general Earth history, the local histories must be related to one another by correlation, i.e. compared with respect to their characteristics and chronology. For example, in eastern France and the Germanic Basin the oldest rocks, constituting the basement, are covered with red beds, which pass upwards into a dominantly carbonate assemblage and then into varicolored evaporitic beds. This sequence of three sedimentary events, grossly simplified, constitutes the Trias (Fig. 3)1. But it has been demonstrated that the three lithological groups are not synchronous across the area in question. Furthermore, the Permo-Triassic red beds, or just the Trias, are often discordant on a basement of older, deformed, metamorphic rocks. This discordance can be considered as an important break in the continuity of a geologic history (see below). The correlations of local histories can have two results: 1. An inventory of events and determination of their lateral extent (paleogeographic stage). 2. Documentation of major events of widespread importance, useful for the erection of a global framework subdivided into distinct periods (geochronologic stage). Correlations are effected in two ways: 1. By attempting to follow beds or bed boundaries (litho horizons) from one region to another, the principle o~ lateral continuity is applied. This 1
Of which the lower sandy part is a continuation of the underlying Permian red beds.
4
Fundamentals of Stratigraphy
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method can only be applied to limited areas because overburden or erosion generally interrupt the outcrop continuity. The beds so followed are only isochronous if they formed by strictly vertical sedimentary accretion. In contrast, they are diachronous where sedimentary accretion, partly or totally lateral, is controlled by currents (delta front for example; Fig. 4). 2. By seeking comparable sequences in different places (sequence stratigraphy). Stratigraphic correlations are effected taking the following principles into account (Fig. 5):
Principles of Correlation
5
a) The duration of an event, as well as its beginning and end, can vary from place to place. For example, a faunal migration will result in such a variation. Therefore, a stratigraphic correlation is not necessarily a time correlation. b) New events can appear between two areas (C, G), and others can disappear (E). c) One event can be laterally replaced by another (lateral facies variation for example; A, A'). d) Gaps in events (lacunae), due to nondeposition or erosion, can exist in any lithologic sequence without necessarily being recognizable (B, E). e) Evidence for events may also be altered by diagenesis or metamorphism. Events of limited lateral extent are of little use in correlation. On the other hand, they can be useful in characterizing the environment. In contrast, major widespread events are very much sought, for they permit longdistance correlations, many of which are regarded as time correlative. The ideal would be a series of events of worldwide extent that are easily recognizable. The search for such a series is one of the major tasks of stratigraphy, as the history of this science demonstrates. To this end, tectonic, biological, climatic, eustatic, chemical, and paleomagnetic events have all been sought; so far, a truly universal stratigraphy has not been possible. However, the search for worldwide correlations today has the advantage of plate-tectonic theory, which does consider geologic phenomena and their causes on a global scale. This theory, if used cautiously, can enrich stratigraphy by providing new means of correlation. The value of an event in geochronologic correlation depends also on its duration. The shorter it is, the less diachronous its beginning and termination are likely to be. The disappearance of many groups of organisms at the Cretaceous-Tertiary boundary is not as abrupt as one might imagine from its supposed link with some cosmic cataclysm. This extinction is, in fact, gradual over a period of several hundreds of thousands of years. And no proof exists of the perfect synchronism of this event throughout the globe. The history of those outer layers of the Earth, capable of being described today, can thus be deduced from a juxtaposition of local, more or less well-correlated histories, allowing the recognition of the most important events. The latter are fundamental for long distance correlation and for the construction of a stratigraphic framework necessary for the division of geologic time. The recognition of these events is a precondition to all paleogeographic reconstructions. In other words, the task of stratigraphy is to solve a gigantic three-dimensional jigsaw puzzle. The pieces of the same age must first be assembled before it is possible to reconstruct the successive pictures of the Earth's history.
Chapter 2
Elaboration of the Fundamentals of Stratigraphy
1 Lithostratigraphy The first European stratigraphers set out initially to describe local histories illustrated by vertical lithologic sequences. Among them, William Smith (1769-1839) is generally considered the founder of stratigraphy, including biostratigraphy. He saw in the succession of sedimentary deposits a sort of representation of the passage of time. He recognized their continuity in space and was able to use fossils to distinguish lithologically similar beds. Inspired by this, Quenstedt and Leopold de Buch subdivided the rocks of the Swabian Jura into three parts: (1) a lower group or "Black Jura" (Lias), formed of marls and dark shaly limestones; (2) a middle group or "Brown Jura" (Dogger), consisting of ferruginous layers; and (3) an upper group or "White Jura" (MaIm), composed of light-colored limestones. In addition, three superposed sequences of sands were soon distinguished in the Paris area: lower, middle, and upper sands, separated by shaly or calcareous formations. This objective lithologic stratigraphy, or lithostratigraphy, is still the basis of descriptive sedimentary geology. It is the basis of the measured section in the field and its representation as a stratigraphic column. It is also the starting point for sequential analysis. Finally, the cartographer is above all a lithostratigrapher who attempts to follow previously defined sedimentary units around the land surface. The first European geologic maps, like those of Guettard (18th century) and those of Dumont (19th century) were strictly lithologic, without any chronologic significance. The basic lithostratigraphic unit is the Formation, whose genetic basis implies deposition under uniform conditions. Its limits are placed where the lithology changes or where there are significant breaks in the continuity of the sedimentation. Formations are subdivided into Members and associated into Groups. They were originally named in various ways, by figures, numbers, lithologic character, and names of the places where the units were particularly well exposed (stratotypes). The present nomenclature is in many cases inherited from those original names, in spite of the stratigraphic codes that have since appeared l . Figure 6 shows, for example, the stratigraphic 1 Suggesting the use of lithological characteristics and stratotype locality. Example: Comblanchian limestone (Bathonian, C6te-d'Or).
8
Elaboration of the Fundamentals of Stratigraphy
PseudolHhographic Corallian or
limestone.
(RICHE 1898)
lubcorallian faclel
Spherll. layera
(ENAY 1966)
Dogger
Fig. 6. Lithostratigraphic relations of the Oxfordian sequences in the southern Jura. The author and date when each lithologic unit was defined are shown in parentheses (After Enay 1966, with names of authors added)
nomenclature of the Oxfordian in the southern Jura, according to Enay (1966). The formations are seen to have only limited distribution and their limits are not necessarily isochronous. They are named after place-names, lithologic or paleontologic characteristics, and even a particular position in the succession (passage beds, boundary beds). In many countries outside Europe, especially in the United States, lithostratigraphy remains the fundamental tool of the sedimentary geologist, a tool evidently used with objectivity in the descriptions and correlations of natural successions, until the local lithostratigraphic scale and the general chronostratigraphic scale (see below) can be tied together. However, this methodology unfortunately does produce a multiplicity of unit names; in 1938 the stratigraphic lexicon of North America counted more than 13000!
2 Biostratigraphy The history of the Earth must be reconstructed in its continuity, but the successive sedimentary events, arranged in time sequences using lithostratigraphic methods, cannot always be correlated with one another. The
Lithostratigraphy
9 Time
4
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® Fig. 7. Discontinuity of sedimentary events. A Sedimentary events recorded in a stratigraphic section: 1 and 2, continuous deposition; 2,3 and 4 are beds separated by diastems. B The same events in a time framework; 3 slow deposition; 4 rapid deposition; 1 and 2 continuous deposition; 2,3,4 discontinuous deposition; hachures denote lacunae corresponding to diastems in stratigraphic section
simple phenomenon of stratification implies, in effect, a discontinuity in deposition, with a resulting hiatus of unknown importance (Fig. 7). Also, many sedimentary events are diachronous and likely to be repeated during the course of geological history. It quickly became apparent, therefore, that there was a need for a reference to phenomena independent of sedimentation and of a continuous nature more clearly representative of the flow of time.
2.1 Evolution, the Reference System for Age Dating Most important in this regard is biological evolution, manifest in the emergence of species, a phenomenon continuous, nonrepetitive and irreversible, but having the disadvantages of being more or less dependent on the environment and proceeding at variable rates. As early as 1831, Deshayes established in the Paris Basin that the fauna changed from one formation to another, leading to the concept of successive disappearances and creations as time markers. Thus, Alcide d'Orbigny (1850-1852) said "the first thing to obtain from a paleontologic study is the age;" and Albert Gaudry in 1896 added "of two different outcrops, I affirm that in one the animals will indicate a state of evolution less advanced than in the other. I conclude from this that the first is from an older epoch." Biostratigraphy was thus born. By consideration of fossil remains, their positions in the strata, and their place in the evolution of animals and plants,
10
Elaboration of the Fundamentals of Stratigraphy
it attempts to characterize the different segments of geologic history by a particular fossil or by an assemblage of fossils. The correlations between fossiliferous beds therefore, represent time correlations, and two beds possessing the same fossiliferous content are said to have the same age, i.e. within the limits of resolution they were formed at the same time. This method is obviously only valid for that epoch of Earth's history called the Phanerozoic, characterized by determinable and useful fossils, and it cannot be applied to rocks too severely affected by metamorphism. The principles of biostratigraphy were applied early. As long ago as 1829, Morton and Vanuxem proposed a correlation between the chalk of the Upper Cretaceous of Europe and certain formations of the east coast of the United States on the basis of their similar ammonite faunas. The same procedure was adopted for the limestones of Savoie and the chalk of Rouen by Cuvier and Brongniart (1822), who advocated the use of fossils rather than lithology to correlate different areas.
2.2 The Zone Concept of Oppel With Oppel (1856), all reference to lithology disappears. Faunas alone are considered stratigraphically useful, being considered, justifiably, as more stable than lithologic facies over long distances. Adopting the subdivisions of Quenstedt and choosing the fossil group showing the most rapid vertical changes, he proposed 33 ammonite zones for the Jurassic of Wurtemberg and showed, by 1856, that this zonation is repeated in northern Germany, England and France. Oppel's biozones can be defined as the volumes of rock corresponding to the vertical and horizontal ranges of two or more taxa, each not necessarily occupying the same space. These units are named from the most typical, frequent or characteristic fossil (index fossil), which may, however, be locally missing. The best zones are those with the shortest vertical ranges (high rates of evolution) and the widest horizontal ranges. Certain Oppel zones have been recognized as far away as Madagascar and South America. It was already apparent by the middle of the last century that certain fossil groups differed markedly in their rates of evolution. Some evolved rapidly (tachytely), for example the ammonites, especially in the Late Triassic and Jurassic, and the graptolites, whose taxa tend to be spread widely and rapidly independently of the nature of the sediments. This wide distribution is due to a biological cycle which includes a planktonic larval stage (planktotrophic larvae), and for the ammonites, extensive postmortem dispersal of their adult shells by virtue of their buoyancy. For this reason, correlations using ammonites are considered practically synchronous. Moreover, these zones are almost worldwide in the Lias since they are recognizable in Europe, North America and the Andes. They subsequently become more restricted during the course of the Mesozoic
11
Lithostratigraphy
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due to increasing climatic zonation and consequent increased faunal provincialism. Other groups evolved slowly (bradytely), are geographically restricted, and appear to be confined to certain types of sediment. They are, therefore, dependent on sedimentary facies, a concept also introduced in the last century by the Swiss geologist Gressly (1838). Finally, the fossils can be classified according to their usefulness in biostratigraphy. The data of Fig. 8, however, are only generalizations. Among the groups considered there can be exceptions. For instance, the hippuritids of the Upper Cretaceous, although dependent on a reef environment, evolved rapidly and are very useful stratigraphically. Since Oppel, and following his lead, many biostratigraphic studies have been produced. Ammonite zones have been refined and increased in number, especially in the Jurassic where that group's evolution was particularly rapid. The practice of biostratigraphy in a growing number of countries, however, has revealed that the horizontal extensions of biozones are limited. With the exception of the Liassic zones mentioned above, some are restricted to Europe, while the majority do not extend beyond certain faunal provinces (Fig. 9). Interprovincial extensions of a zone are only possible when certain taxa overlap in their geographic ranges. And these extensions can be complicated by the phenomenon of migration. Certain genera or species, responding most often to variations of the environment, modify their distributions through time, expanding or contracting them, or even displacing entire distributions (Fig. 10). In an invaded region, the vertical range of a taxon will be shorter than in its province of origin, and its
12
Elaboration of the Fundamentals of Stratigraphy
Fig. 9. The Tethyan domain and its faunal provinces (ammonites) from the Upper Bajocian to the Middle Bajocian (after Cariou et al. 1985). The Phylloceratina dominate the Mediterranean province of deeper-water environments. Faunas very diversified in the European Submediterranean province, less diversified in the Ethiopian. J Probable land; 2 epicontinental seas; 3 zones of ocean floor; heavy lines denote boundaries of provinces
sudden appearance will seem to be a new taxon. These phenomena show also that a biostratigraphic correlation is not necessarily a time correlation.
3 Chronostratigraphy 3.1 The Concept of the Stage The biostratigraphic zones are related to the historically older chronostratigraphic stages. The stage was originally defined as a group of beds deposited during a specific interval of geologic time, this interval being a geochronologic unit. It can be divided into subunits and is, by definition, universal. The first subdivisions of European sedimentary series into stages by d'Orbigny, Brongniart, Marcou, Oppel, etc. were based on both their lithologic character (facies) and some of their faunal elements. The facies refers to the nature of the beds in the region where the stage was defined.
Chronostratigraphy Time
13
®
~
Fig. 10. Change of areas of faunal distribution with time. A Restriction; B spreading; C displacement
The faunas provide a time reference as well as a means of recognizing the stage anywhere, even if the lithology has changed. Recognition may be direct if the faunas are the same, and indirect, through intermediate steps, if they change. Theoretically, the stage can be identified throughout the world even in the absence of the faunal suite characteristic of its type region. The faunas are also used to subdivide the stage into finer units. The stages of d'Orbigny (ten in the Jurassic, seven in the Cretaceous) have been defined by reference to sections or stratotypes which are adequately fossiliferous, well exposed and well defined at their upper and lower limits. Such sections are common in epicontinental successions which are more easily subdividable into distinct lithologic units. For example, in the Paris Basin, the Cretaceous consists of green sands recognizable in the Aube department (Albian stage) while the Jurassic of Semur occurs as beautiful exposures of limestones with Gryphea and Arietites (Sinemurian stage). Figure 11 shows a list of Jurassic stages together with their stratotypes. However, in an epicontinental series, like that of the Paris Basin, the clear lithologic distinctions used to define the stages are often rendered more distinct by the sedimentary discontinuities which separate them. These include the depositional gaps (nondeposition and/or erosion) which are evidence of the numerous oscillations of sea level marking the basin's history. These hiati accentuate also the contrasts between the paleontologic content of successive stages, giving the impression that each corresponds to a renewal of the fauna. The higher-order subdivisions are based on the same principle. Thus, Deshayes in 1831 introduced a major break between the top of the Chalk and the base of the Tertiary in the Paris Basin on the basis of a comparitive study of their respective faunas. The upper limits of the Devonian, the Permian and the Trias also coincide with massive disappearances of taxa, giving rise to the terms still in use today: trilobite era (Paleozoic); ammonite and reptile era (Mesozoic); and mammal era (Cenozoic). For d'Orbigny, imbued with the creationist ideas inherited from Cuvier, the history of the Earth consisted of 27 stages, each possessing its own fauna and each separated from the next by catastrophic events of tectonic origin (global revolutions). At each revolution, faunas and floras
"
G,)'(i;
-.
~:;
" €'~
::S~
~.~
-.
OJ ~ -l'" ::s
Rhetian
Hettangian
Sinemurian
Pliensbachian
(= Charmouthien)
Toarcian
Blijocian Aalenian
Neocomian pro-parte Portlandian p.p.
Kimmeridgian stricto sensu. Sequanian s.s Sequanian Rauracian
Lotharingian Sinemurian S.s.
Carixian
Domerian
Angoumian Oxfordian s.s
Callovian Bathonian
Oxfordian
Kimmeridgian
Portlandian
Pubeckian facies
Stages and sub-stages
W. Gumbel, 1861 and E.Renevier, 1864
A.d'Orbigny, 1849 G. Bonarelli, 1894 A. Oppel, 1858 J. Lang, 1913 E. Haug, 1910 A. d'Orbigny, 1849 E. Renevier, 1864
A.d'Orbigny, 1852 1. Omalius d'Halloy, 1831, and A.d'Orbigny, 1852 A. d'Orbignyi, 1852 C.Mayer-Eymar, 1864
J. Marcou, 1848 A. Brongniart, 1829
A. Brongniart, 1829 J. Thurmann, 1832 J. Marcou, 1848 1. P. Greppin, 1870
A. Brongniart, 1829
Authors of terms
Shales and limestones from Rhetic Alps (Engadine, Switzerland)
I
Marly limestone from Thouars (Deux Sevres, France) Marls and limestones from Monte Domaro(LombardY,Italy) Marls from Pliensbach (Wurtemberg, Germany) Marls from Carixia (= Charmouth, England) Limestones and marls from Lorraine (France) Limestone from Semur-en-Auxois (Cote d'Or, France) Sandstone from Hettange (Moselle, France)
Oolithic limestone from Bayeux (Calvados, France) Black marls from Aalen (Wurtemberg, Germany)
Sandy limestone from Kelloways (England) Oolithic limestone from Bath (England)
Limestones and sandstones from Portland (Dorset) Black marls from Kimmeridge (Dorset) Limestone from Franche-Comte, France (= Sequania) Corallian limestones from Rauraces country (dealing with a tribe from Jura bemois) Grey marls from Argovie (Swiss Jura) Black marls from Oxford (England)
Lacustrine deposits from Purbeck (Dorset, England)
Origin of terms
Fig. 11. Origin of stage and substage names of the Jurassic. (Note that the substage names Argovian, Sequanian, and Rauracian are no longer used)
-180M.A. Triassic
~
Cl
0
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OJ
e
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Subsystem boundaries
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15
Chronostratigraphy Eras
Systems Quaternary
Holocene Pleistocene
-------------------- -- ------Neogene
Pliocene
Miocene
Cenozoic Paleogene or Nummulitic
1======-= ==
Oligocene
Maim Jurassic
Pasadesian Rhodanian Attican Savian Helvetan Pyrenean
Paleocene
===== ===== :: ~
Cretaceous
Mesozoic
-s - 23 - 34 - 65
-
I--
- 130
Cimmerian
-
~
Palatinian
- --
Saalian Asturian Sudetian Bretonian
- f-- - 21)0
Austrian Neocimmerian Andinianor Nevadian
======:-
Permian
Carboniferous Devonian
Caledonian Silurian Ordovician
Taconian Salai"r
Cambrian
1::-====== ======: ======: t= Precambrian
----
- 1,6
---
Laramidian
Triassic
Paleozoic
- --
DOJlSer Lias
------------- -------
Time in Ma
Orogenic stages
Assyntican
- f-- f-= --
- 204 - 245
- 360 -400
~
- 425
~
- 495 - 530
Fig. 12. Principal subdivisions of the general chronostratigraphic scale. Orogenic phases from Stille. Radiometric ages after Odin et al. (1982b)
were destroyed to be renewed in the succeeding strata. The principle of a stratigraphic paleontology capable of dating rocks far apart, without having to consider their lithologic similarities or differences was thus established.
3.2 Event Stratigraphy D'Orbigny thus divided time according to a stratigraphy of events both biological and tectonic, the former being secondary to the latter. Others placed the major breaks exclusively at times of major lithospheric movements: Suess (1880) tied the rhythmic history of the Earth to marine oscil-
16
Elaboration of the Fundamentals of Stratigraphy
lations related to positive and negative epeirogenic movements. This was also the theme of the German school under the influence of Stille during the years 1920-1930. Thirty orogenic phases were distinguished during the entire geologic time scale, each corresponding to a peak of activity. Their effects are identifiable not only in the orogen but also at some distance from it as emergence, synsedimentary deformation, instability of areas of deposition (slumps, turbidites), discordance, deposition of coarse detrital sediments (fault breccias, olistoliths, flysch, molasse), or by certain minerals (predominance of chlorite and illite f. It is generally supposed that the lateral extent of a tectonic event depends on its magnitude, and in principle there is no reason why a really major orogenic phase should not be felt throughout the world, even if only as a sea-level variation or as a slight epeirogenic movement. This is assumed implicitly in many geologic time scales, for example when the Late Jurassic and Late Cretaceous regressions are related to the Neocimerian and Laramide orogenic phases, which are hardly of worldwide extent (Fig. 12). In practice, such correlations are difficult to establish for several reasons, including the following: 1. Of variable intensity and often barely discernable, the manifestations of a tectonic event may be difficult to follow over large distances. 2. Tectonic events are often not synchronous from place to place, in so far as they may have variable durations in different locations and may migrate laterally. For example, the discordances which bound many chronostratigraphic units are often diachronous, and have only a regional significance. This diachronism becomes less clear in oldest successions, while the stratigraphic resolution is lessened.
The concept of tectonic phases is, therefore, out of date today, especially since the fundamental mechanisms for orogeny became known. The movement of lithospheric plates gives rise, in fact, to slow, gradual, continuous, and shifting deformations. It follows that an orogen is active throughout the course of its history, but especially in its beginning, i.e. in the basin where synsedimentary tectonics are now commonly recognized. In this continuum some periods can, nevertheless, be discerned in the intensification or generalization of movements, sometimes accompanied by important regressions, detrital pulses, etc. Only such major events can be used in long-distance correlation, but they do not permit a fine division of time. At the best they can be used to indicate certain major cycles like orogenic cycles or sedimentary cycles. For example, the PaleozoiclMesozoic boundary has been related to a global geodynamic event, the beginning of fragmentation of Pangea, also rep-
2 However, characteristics of the clay sediments have as much climatic as tectonic significance.
Conclusions
17
resented by a general transgression. This boundary coincides also with the beginning of the Alpine Orogenic Cycle. In general, the sharper and more important the boundaries as judged by the number and extent of coincident phenomena, the larger are the associated stratigraphic gaps. Between a folded and metamorphosed basement and the beds which lie discordantly above it, a very large part of the global history may be missing.
3.3 The General Chronostratigraphic Scale This has been developed progressively since the eighteenth century, when geology began. It bears the stamp of the diverse schools which have inspired it and the regions where it was developed. The principal units of this scale, recognized worldwide, are shown in Fig. 12 with their duration in millions of years. Excluded are the lower order units (stages), still used only locally or regionally, and which are mentioned in several figures in Chapter 5 (Figs. 74, 82, 90, 107). Systems are groupings of stages; their boundaries coincide with major discontinuities resulting from emergence and deformation3 • Systems have various origins noted in Chapter 5 (see the boundaries and subdivisions of the Lower and Upper Paleozoic, the Mesozoic, the Cenozoic). The boundaries of eras correspond not only to important geodynamic events but also to significant faunal renewals: the beginning of the Paleozoic, which is also that of the Phanerozoic (= appearance of life), is distinguished, among other characteristics, by the appearance of the first abundant faunas, well preserved and widespread. At the end of the Paleozoic, trilobites and fusulinids disappear; at the end of the Mesozoic, the ammonites, rudistids and large reptiles disappear, while the nummulitids appear in the Cenozoic.
4 Conclusions Such were the first steps of a stratigraphy aimed at a useful subdivision of time (stratigraphic scale) based on the lithologic, fossil, and deformation information observable in sedimentary sequences. The litho- and biostratigraphic scales are the most objective and their scope is regional or provincial. Lithostratigraphy can be applied to all rocks, sedimentary, volcanic or metamorphic, but each lithostratigraphic unit, a result of specific physical phenomena, can be repeated several times in the course of time. Biostra-
3This was not always the case. In 1830, Lyell subdivided the Cenozoic into three systems (Eocene, Miocene, Pliocene) characterized by an increasing percentage of modem species.
18
Elaboration of the Fundamentals of Stratigraphy
tigraphy can only be applied to fossiliferous rocks, but biostratigraphic units, based essentially on the unique events of biological evolution, cannot be repeated during the course of time; thus their great utility. Theoretically, some of these units have a chronostratigraphic value, to the extent that appearances and disappearances of fossil species can be assumed to be synchronous everywhere. However, we will see in the following chapter how the influence of the environment often renders this assumption invalid. Biostratigraphy, therefore, has two components: one, irreversible, based on evolution, the other, reversible and related to factors of the environment. Chronostratigraphy, the normal end product of a regional study, follows from the other two aspects; it depends on a division of strata according to geologic time and therefore has a universal value. Future studies will perfect the methods of basic stratigraphy, fill the gaps in our historical documentation, forge new tools of correlation and synthesize absolute data on the duration and speed of events.
Chapter 3
Modern Stratigraphy
Stratigraphy has contributed, like other subdisciplines, to the spectacular progress in Earth science made during the second half of the twentieth century, mainly due to three important factors: 1. A deeper knowledge of evolutionary phenomena; 2. The increase of petroleum exploration with its constant demand for greater precision in the recognition of specific stratigraphic units; 3. The development of plate tectonic theory and consequent programs of deep ocean drilling.
In the latter two cases, the drilling techniques have spawned new stratigraphic tools, seismic and downhole logging methods in lithostratigraphy, micro- and nannofossil time scales in biostratigraphy. The stratigraphy of ocean sediments is constructed on oceanographic ships by international teams, an important factor in the establishment of a chronostratigraphic framework with worldwide validity, in the rigorous redefinition of old established units and in the sharpening of concepts used in stratigraphy. This need for consensus in the stratigraphic codes used has generated numerous discussions and yielded numerous resolutions at international meetings. A fortunate consequence of plate tectonics has been to elevate scientific debate to a planetary level, and stratigraphy, drawn into this movement, has felt compelled to find new methods of correlation dependent on events and phenomena of global significance: eustasy, oceanic geochemistry, climate and paleomagnetism. Similarly, striving to escape slowly from the relative nature of its chronology, stratigraphy is more and more supported by precise measures of absolute age. Finally, one can observe a will to adapt stratigraphy to the progress realized in paleontology (evolution, species concept), in paleoecology (influence of the environment) and in sedimentology (interpretation of sedimentary discontinuities, reworked beds and variable rates of sedimentation).
1 Refinement of Concepts and Time Scales Since the 1950s, stratigraphy has had to respond to an increasing demand for more precise dating as science and subsurface exploration progressed. Since
20
Modem Stratigraphy
100m more or less of drilling can have a considerable effect on the budget of an exploration company, there is a necessity for a finer subdivision of the stratigraphic scale in order to increase the resolving power of stratigraphy. This need has also led to the establishment of other scales (micro- and nannofossil) utilizable in drilling and related where possible to a radiochronologic scale. Stratigraphy has also had to redefine its concepts more rigorously in light of new methods of correlation and dating and contributions from other disciplines.
1.1 Evaluation of Geologic Time Intervals and Rates The estimation or calculation of geologic time intervals is necessary: 1. For the comprehension of past phenomena and their comparison with present-day or recent phenomena whose durations are known by direct reference to human history. 2. For the construction of a consistent geochronologic scale with correct relative placement of different divisions of geologic time. Two courses are possible: 1. Reference to sedimentary rhythms of known and constant time durations corresponding to seasonal or annual cycles, such as varves which are formed as alternations of light and dark millimetre-thick laminations, each couplet representing deposition during 1 year. Varves can be perfectly preserved over great thicknesses, indicating a lack of deforming compaction and an absence of bioturbation typical of anoxic lacustrine or marine environments. The calculation of duration to within a few years is, therefore, possible. 2. Reference to a physical transformation, which is unidirectional and irreversible and is a known function of time, for example, the transformation of a radioactive element into a stable one.
1.1.1 Radiochronology A radioactive element A, contained in a mineral at its crystallization, will disintegrate progressively and be transformed into a daughter element B, said to be radiogenic. The ratio of concentration AlB will depend on the time duration of the disintegration and on the half-life T of the element (time required for the disintegration of half of the element) or on the decay constant (coefficient of decrease of the element as a function of time). t = 11l0g(1 + N'IN), where N' is the number of atoms of the radiogenic element B (daughter element) and N is the number of atoms of the element A (parent element) after time t.
21
Refinement of Concepts and Time Scales
Nand N' are measured in a mass spectrometer to yield a value for t which is the isotopic age (radiometric age) of the mineral containing the element. The measured age is from when the system closed, which generally causes the cessation of exchange of fluids between mineral and pore waters. Given certain conditions, it may then be possible to determine the age of the crystalline rocks or sedimentary formation in which the mineral occurs. The method is only valid if the decay constant is well defined and the mineral containing the radioactive element was a closed system throughout the decay time. Other methods also used include: 1. Measurement of the concentration of a radioactive element. The carbon atoms of CO2 derived from the atmosphere to form living matter or biochemical carbonates have a 14C content 4 C/ 12C = 1.2 x 1012 ) which remains constant over time. After the system closes (death of organisms), the 14C content decreases with time 4C ~ 14N) so that its measurement yields the amount of time since the system closed (i.e. since death). 2. Measurement of the ratio between two stable elements A' and B' derived from the decay of two radioactive elements A and B (e.g. 238U and 235U) of different half-lives. Since A'/B' varies as a function of time, its calculation yields the amount of time since the system closed.
e
e
Principal transformations used:
Half-life in years 87 Rb~ 232 Th ~ 40K ~ 238 U ~ 235 U ~ 234 U ~ 230 Th ~
14C 3T
~ ~
87 Sr 208 Pb 40Ar 206Pb 207 Pb 230Th 226 Ra
14N
2H
5 or 4.7 x 1010 13.9 X 109 11.9 X 109 4.6 X 109 7.0 X 108 250000 75220 5568 12.26
The results of these age dates must be used with great caution, since the resolving power of the method decreases with increasing age so that the error can be 50-l00m.y. for the Precambrian. Also, the radiometric age may correspond to a first event (e.g. formation of a rock) when the chemical system closed and the radiometric clock was set, or to the latest of its transformations (metamorphic or deformational) which resets the clock at zero by reopening the chemical system. Age dates of plutonic and volcanic rocks are the least problematic. Many silicates are suitable for radiochronology because their crystallization usually corresponds to the rock formation. In metamorphic rocks, the date
Modern Stratigraphy
22
+ '-0:::-----_--_-------+ + -f-=-+ + + + + + + + + + +
Ma .40(
+
+
+ + (j)+
HARLAND et aI.
+
+
LAMBERT
(1971)
+
+
o
(1982)
(1980)
(1964) Devonian
I
..... Silurian ? .-
-500 ~?
......
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Cambrian Cambrian
~
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Ordovician
Ordovician
Ordovician
Ordovician
.....~Silurian ~
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Silurian '"
Ordovician
:-600
GALE
ARMSTRONG GALE (1978) et al.
'. Devonian., ._ Devonian
-
Fig. 13. Dating of an azoic sedimentary formation by plutonic rocks. The age of the formation (2) is later than that of the granite (1) but earlier than that of the laccolith (3)
.-'
.........
-
ii
. .
-
Fig. 14. Different geochronologic scales for the Lower Paleozoic, after Gale (1982). Note the indication of limits of error related to geochemical and analytical techniques for the latest scale
generally corresponds to the later recrystallization phase and not to the original formation of the rock; in sedimentary rocks, the minerals used are usually detrital and, therefore, older than the age of the rock. Glauconite is exceptional since this authigenic mineral is practically synsedimentary. It can be dated by K/Ar and Rb/Sr but it is sensitive to later diagenetic changes which can reset the radiogenic clock. This mineral is also often reworked and present in condensed sequences. Ages obtained by this method range from Cambrian to Pliocene. Sedimentary formations which are azoic, nonglauconitic and older than 40000 years are only datable relative to crystalline rocks which cut them or predate them (Fig. 13). For beds younger than 40000 years, 14C dates are the best, though subject to errors
23
Refinement of Concepts and Time Scales
60 Ma
Va Hinle 1976
7o -
M
Ca
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90
~
~
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~
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liauterlrilll
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140
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Fig. 15. Different radiometric scales for the Cretaceous
M.crichlilo M
Ap ~
n.. Ba 12U f - - Ha
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T
Ce 100
OdIn eill. 1982
Benlullll
Fi 7. 91
97. 5
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119 124 131 138 144
150
related to variations in the magnetic field!, to climatic variations or to isotopic fractionations due to "vital effects" in biological systems. In contrast, the complex stratigraphy of Precambrian rocks can only be ascertained with the aid of isotopic dates which extend back to 3800m.y., the age of the Earth's formation being 4600m.y.
1.1.2 Radiometric Calibration of Stratigraphic Scales. Durations of Stratigraphic Units The first geologic time scale combining the principal chronostratigraphic boundaries and radiometric age dates was published by Holmes (1932). Since then it has been continually improved (Fig. 14), so that it is now possible to assign durations for all stages. At first, it was possible only to estimate average stage durations by dividing each system duration by the number of its stages. Subsequently, ages have been determined for stage boundaries, allowing separate and different durations to be calculated. Results vary according to different authors (van Hinte 1976; Kennedy and Odin 1982; Kent and Gradstein 1985; Fig. 15). Westerman (1984) and Kent 1A reduction of the latter during the last 7000 years has resulted in an increase in the production of atmospheric 14C from the action of cosmic rays on nitrogen.
24
Modern Stratigraphy
and Gradstein (1985) adopt the following principle for the Jurassic. Based on a system duration of 64-74 m.y. (according to different authors), they calculate a duration for each stage proportional to the number of ammonite subzones it contains. The reliability of this method assumes that all biostratigraphic units are defined homogeneously and that biological evolution proceeds at a constant rate. Knowledge of stage time durations then allows recognition of even shorter time spans. In the Toarcian, for example, a stage duration of 5 m.y. and the recognition of 27 ammonite zones suggests that an average ammonite zone lasted 185000 years. Many pelagic deposits are formed of a series of alternating limestone and shale couplets defining global cycles (see Chap. 3). From the number of cycles in different stages ranging from Jurassic to the Quaternary it has been possible to calculate the average time duration, varying from a few thousand years to a few tens of thousands for each cycle.
1.1.3 Rates Once time durations are known, it is possible to calculate rates, especially rates of sedimentation. These are generally average rates, but in the case of continuously deposited pelagic sediments they are also close to instantaneous rates2 • Rates of erosion, uplift, subsidence, and lithification are also determinable.
1.2 New Biostratigraphic Approaches 1.2.1 Factors in Biostratigraphic Development 1.2.1.1 The Modern Species Taxonomic concepts of modern biology have had an important influence on paleontology and stratigraphy. Typological concepts based on the representation of a species by an arbitrarily chosen individual have rightly given way to the dynamic concept whereby the population and as many factors as possible are taken into account in the definition of the species. The paleontologist-stratigrapher then has the problem of distinguishing between species and the problem of their transitions. Distinction. Only biometrics and statistics allow a proper study of variation in a population and enable conclusions to be drawn on its monospecific or polyspecific character.
2 These are calculable in near-surface sediments by the decrease in the 230 Th/232 Th ratio compared to this ratio at the surface.
Refinement of Concepts and Time Scales
25
/Sub-
~-+--f--:i~H"f-~ speCieS} e
SpeclesE
.~+-;/,~H'-T--'¥--t Subspecies
_____ 'L _____ _ SpeclesB
A
-
abc d e Morphotypes
dm A
f
dm
B
Fig. 16. A Cladogenetic evolution and B anagenetic (after Tintant 1972). dm Morphologic variation; T time; f frequency; A,B,C,D,E distinct species; B,B' ,b,c,d,e subspecies
Transitions. The change from one species to another in a lineage can occur by continuous transformation (anagenesis; Fig. 16), but the problem is how this can be used to establish discrete biostratigraphic units, or in the words of H. Tintant (1972) "how does the continuous flux of the evolution of life flow within the discontinuous framework of stratigraphic division?" If the morphologic stages of a lineage represent a succession of barely perceptible changes, it is only by using numerical indices that biostratigraphic subdivisions can be constructed (autochronology). However, a species may also originate abruptly by the branching of a lineage (cladogenesis; Fig. 16), and this sudden event is frequently used to define higher-order stratigraphic units. It should be noted, however, that biostratigraphic units are not all based on single lineages but may also be based on the succession of species belonging to different lineages (allochronology). 1.2.1.2 Relationships Between Paleontology and Other DiscipHnes
The multidisciplinary approach using paleontology, paleoecology, sedimentology, and paleogeography to solve problems is now a common practice, to the benefit of biostratigraphy. For example, knowledge of a paleoenvironment (bathymetry, degree of energy, temperature, salinity, etc.) from a study of the sediments is indispensible for understanding the true significance of faunal changes over time, because it can allow the environmental component of the change to be separated. Faunal changes due to the environment are reversible because they are generally not yet fixed in the genotype; they are repetitive and can be diachronous within a single basin. Changes due to evolution have the opposite characteristics. Thus, in the Late Pleistocene, the direction of coiling of the two species of Globigerina, Gl. pachyderma and Gl. hirsuta is temperature controlled; in the cool waters of glacial periods, the coiling is dominantly sinistral, while in the warmer waters of the interglacial periods it tends to be dextral. It is only because we know a great deal about the biology of these globigerines that we can avoid confusing these intraspecific variations with mutations in the
26
Modem Stratigraphy J J I
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Ammonite Zones
Fig. 17. Comparison between the distribution of the principal foraminifers of the Ardeche border (thick lines) and of the Vocontian Basin (thin lines) during the Valanginian (Moullade 1979). 1 Otopeta; 2 Pertransiens; 3 Campylotoxum; 4 Verrucosum; 5 Trinodosum; 6 Callidiscus; 7 Radiatus (After Darmedru 1984)
direction of other species. This allowance for the effect of the environment on fossil species is necessary before any biostratigraphic subdivision is attempted; and those index forms too dependent on facies should be eliminated. The benthonic representatives are especially affected, for example the nummulites, which favor shallow calcareous facies, and numerous small foraminifers of the platform and the basin. In the Valanginian of southeastern France, for example, several calcareous (Lenticulina) and agglutinated (Dorothia) species have vertical distributions which differ in the southern sub-alpine chains (Drome, Hautes-Alpes, Alpes de HauteProvence) from those in Ardeche (Fig. 17). These areas correspond respectively to a basin and its border and therefore represent very different
Refinement of Concepts and Time Scales
27
environments, and especially depths. Therefore, when biozonations over large distances are attempted, the influence of the environment on the temporal distribution of species is apparent even for planktonic forms; the less obvious this environmental dependence is, the more rapid is the evolution of the taxon (e.g. ammonites). The rate of evolution is, in turn, also influenced by climatic and eustatic variations3 • A close scrutiny of any sedimentary structures or directional indicators should be included in any study of a fossiliferous unit if it is required to reconstruct its formation and to classify it as one of the two categories: life assemblage or death assemblage. This study may also confirm the existence or not of a reworked fauna, i.e. the presence of older fossils eroded elsewhere and transported into the indigenous community. However, low rates of sedimentation can lead to condensed faunas which may appear like reworked faunas, especially if the beds have been bioturbated and bedding destroyed. Understanding of some of the diagenetic history should allow the discrimination between the effects of mechanical (deformed or crushed fossils) or chemical (dissolution) processes on the fossils at the time of burial. Finally, biostratigraphy should consider the role of certain paleogeographic factors in the space-time distribution of taxa. Climate is undoubtedly the most important, as illustrated in the following two examples: 1. In the Paleogene, nummulites are abundant in the Paris Basin, but never
extend further north than a line approximately between London and Brabant and, therefore, never invaded the North Sea Basin. 2. In the Callovo-Oxfordian, boreal ammonites never extended beyond the North Tethys margins. Many other groups, especially among the nannoplankton, have a distribution very dependent on climate, but natural barriers also play an important role. For example, in the Plio-Pleistocene, the Atlantic and Pacific were separated by the formation of the Isthmus of Panama with a consequent divergence of faunas in the two oceans, but in a unique climatic setting. The opposite effects occurred in the terrestrial faunas of the two Americas, because their faunas were able to mingle.
1.2.2 Diversification and Perfectioning of Biostratigraphic Scales Certain fossil groups were selected in the establishment of the first biostratigraphic zonation schemes: trilobites, graptolites and vascular cryptogams for the Paleozoic; ammonites for the Mesozoic; and pelecypods, echinoids, large benthonic foraminifera and mammals for the Cenozoic. Subsequently, other groups have been sought with the characteristics of 3Transgressions, by favoring speciation, increase the rate of evolution.
~9~~ETe:j;3
s:
= o·
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t=; ('1) o.el
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('1) ::s ('n1) o ...... 1\)
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9
::s (=i'::S 0 ('1) ~"'O 1\)"11\)='<('1)'" ::to ::to n ~ 0. o o~ 1\)::S('1) ::s § ::s .... ::s .... ~
~
--
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3
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~
6
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9
10
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g
11
Gracilis Macrocephalus
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17 16
15 14
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19 18
ZONES
AMMONITE
''Cenocel'llS'' Untlfltus et heudllfll!lidt!l IIGceki
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-----
---- -----
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Miihlbersi, Latoyalis
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___ !!~!.~f- _.
it:
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t .~
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ECHINOIDS
FORAMINIFERS
1
--
-
spectrum
Lenticulina d'Orbignyi
-
Pillniinvolutil ctJrilltltll
Collyrltes IlCUtll Flexuosa --------Dorsoplicata _ J>!lIi&ru:.ana___ CoUyritu elliptiCII Aromuiensis nn . "I. Almerasi ... ~enuiformis paffiiCa - - - Dorothill owwlenm Divionensis "Tithooill" Boueti yIIIvulllltlluscll blondeti Globata CircumdL - -LentkuUnll -------What1eyensis ntkuilltll OyfJftl, __ _ L_btl.!!D!ie_",!! ___ BivaData i 2,Q ~~~.,.cr emil Buseysiaca Trlpllllill btlrt_teini 1-"/001 Dumortieri ------ Mevendoifillltl btlthonicil Voultensis r----B -----Pturlcidlll'u Ferryi
_
ZONES
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l'Ieudocenocenu ctllloviense
CymlltonllU tilus julii
lltkus -- --tIftlII ------
heudDllflnides
NAUTILOIDS
BRACHIOPOD
Fig. 18. Comparison of different biostratigraphic successions of the Middle Jurassic useful in southeastern France (After Elmi 1984)
-
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Refinement of Concepts and Time Scales
29
Numerous indicators of evolution (appearance, disappearance of species) are actually diachronous on a global scale because of very different reactions by different groups to environmental changes (temperature, salinity etc.) in space and time. This results in mismatches, sometimes significant, between different biostratigraphic zonations, and makes it necessary to calibrate them with the radiochronologic or magnetostratigraphic scales (see below), independently of biological processes. Despite this problem, the use of multiple zonation schemes has certain advantages, such as the following:
1. Possibility of dating where index fossils are absent or rare (dilution of fossils due to high sedimentation rate or sampling methods etc.). The rocks supposedly "azoic" are in fact rarely without such readily disseminated organic remains as coccoliths or pollen4. 2. Possibility of dating with very small volumes of material, using microand nannofossils. These fossils were used increasingly during the 1970s as a consequence of the early oceanic drillings. 3. Improvement of precision in stratigraphic subdivisions. Fifteen graptolites zones have recently been created in the Ordovician of West Texas and 25 zones of calcareous nannofossils in the Paleogene of northwestern Europe (Aubry 1983). This is as much progress as had previously been accomplished using trilobites in the first case and nummulites, vertebrates, and planktonic foraminifera in the second. 4. Better correlations between biozonations of faunal provinces, basins, or different environments. The 25 zones previously mentioned allow better correlations between the principal basins of Western Europe. In the Cretaceous, good correlations are possible between the rudistid zones (specific to the platform limestones) and the zones of the planktonic foraminifera (for the pelagic deposits), using the orbitolinids. Biozonations of marine and continental sequences are difficult to correlate in the absence of an intermediate sequence where the two interdigitate, but the palyniferous zones are useful for this since pollen can be dispersed on land as well as in the oceans.
1.2.3 The Different Concepts ofthe Zone Zones have always been characterized in a way that was as simple and convenient as possible, but several methods are possible, depending on the faunal elements available and on their evolutionary character (Fig. 19). Assemblage zones or zones of association designate a collection of beds characterized by a natural association of fossils. In most cases only a few
4The latter are very resistant except to metamorphism and prolonged oxidation (red beds).
=n= :c x
Modern Stratigraphy
30
Inlerzone
I
_J._ I I
Subzone b Subzone a
3
rI 4
Ir
6
YI A
Upper boundary of taxons Lower boundary of taxons
Fig. 19. Different types of biozonation. 1 Assemblage zones; 2 range zones; 3 overlapping range zones; 4 oppel zones; 5 abundance zones; 6 interval zones (After Hedberg 1979)
forms, often only two, are considered out of the total present; for instance, the Rotalipora montsalvensis-Rotalipora cushmani zone, which is the third subdivision of the Cenomanian according to Porthault (1974), and is designated Cn3. Range zones correspond to the vertical and horizontal ranges of a given taxon (species, genus or family), for example, the Acanthodiscus radiatus (ammonite) zone of the basal Hauterivian, or more simply the Radiatus zone. This zonal concept goes back to that of d'Orbigny. If an homophyletic biostratigraphy is practiced, i.e. one based on the evolution of a single phylum, as advocated by certain authors, then species in a continuous evolutionary lineage or clade have to be separated. This is not easy and often leads to more or less arbitrary zonal boundaries (phylozones). Moreover, the use of range zones assumes that the chosen species had short time ranges and that their distributions within a formation are perfectly known. Overlapping range zones are defined by the overlapping parts of ranges of several taxa. Oppel's zones are of this type. This method uses taxa of restricted vertical range within the zone. It follows, as we have already emphasized, that all taxa used in this way are not necessarily present in all locations and their coexistence may correspond to only the middle part of a zone. Abundance zones or acme zones are based on the abundance or maximum development (acme or hemera) of certain forms independent of their time range. Clearly, this is somewhat subjective and dependent on the original environment as well as on the hazards of collecting. Also, the actual moment of this maximum is often difficult to fix because it does not necessarily correspond to the beds enclosing the most abundant fossils. Finally, some species are mono, poly, or ahemeric.
Refinement of Concepts and Time Scales
31
Interval zones represent the stratigraphic interval between two biohorizons, i.e. between two surfaces possessing distinctive biostratigraphic characteristics (see below). Each of these different types of zones is useful according to the circumstances: faunal or floral abundance (often linked to the rate of sedimentation), rate of evolution and diversification, environment of deposition. The object is to gradually improve the resolution, the universality and the facility of use of biostratigraphy. These attributes are necessary in ocean drilling for precise and rapid correlation and have been made possible by the use of widely applicable time scales. From this point of view, refinements of the calcareous nannofossils have proven to be particularly useful. Nevertheless, the limits of ranges of taxa are not precisely known and not perfectly isochronous due to the hazards of sampling, especially in those beds in which first appearances or disappearances are supposed to take place. Biozonation can also be augmented by the use of biohorizons, which are surfaces or very thin beds corresponding to particular biological phenomena, such as first appearance, disappearance, evolutionary change etc. These are also called horizons, reference levels or marker beds. Invaluable for correlation, they are equally valid at the boundaries of biozones as they are within them. Examples include the extinction of Pseudoemiliana lacunosa and the first appearance of Emiliana huxleyi (coccoliths), synchronous with certain chemostratigraphic reference levels (stages 12 and 18 of the 8180 curve, see below). Both define horizons of worldwide validity. In practice, extinctions (which are first appearances during drilling) are preferable to first appearances because the latter are often gradual (phyletic gradualism) and difficult to detect during drilling because of contamination. Moreover, the emergence of new species by geographic isolation is a diachronous phenomenon by definition.
1.2.4 A Biostratigraphy Based on Degree of Evolution Another biochronologic method, used currently by mammologists, is based on the evolutionary level within anagenetic lineages (Gourinard 1984), rather than on vertical ranges of taxa. Population studies enable numerical indices to be defined whose average values are supposedly identical at the same time over very wide areas. The evolution of this mean index is calibrated in millions of years, so that it is possible to assign a numerical age to this or that population (Fig. 20).
1.2.5 Quantitative Stratigraphy For about the last dozen years, researchers have tried to eliminate unmeasurable factors and all subjectivity from biostratigraphy. These are related to
Modern Stratigraphy
32 _______ Datlngs Locations -..............
K/A r
IIquitaine (France) Provence and Corsica Italy Sardinia Algeria Morocco
•
15
*
•
••
Datum P1anea
.. o
6 V
<>
5 10
o
15
20
20
25
25
30~--7~0--~6~r~~50~~4~0~3~0--~2±0--~1±0~~030 -400 -300 -200 -100 0 .. 100
Fig. 20. Curves expressing rates of evolution from biometric indices. A represents the calibration in millions of years of the Scott index, ratio of height to width of the principal opening of Globigerinoides of the primordius-trilobus line; the index multiplied by 100 in the abscissa varies from 0-70. B represents the calibration of the gamma index of Drooger for the Miogypsinidae of the evolutionary assemblage made up of the Miogypsinoides complanata and Miogypsina gunteri-intermedia groups. The index varies from -400 to + 100. Points shown by solid symbols correspond to potassium-argon ages; points with open symbols correspond to datum planes. Different symbols indicate different geographic locations of samples. The geographic region defined is the minimum area of validity of the curves (After Gourinard 1984)
the chance elements of samplingS, to the differential solution of calcareous microfossils, to the choice of biological events for the establishing of a time scale, and to the environmental influence. The observed data are therefore computerized and analyzed using appropriate programs (Gradstein et al. 1985). Data consist of events of low or high frequency or simple presence or absence. According to Davaud (1982) the most reliable type of zonation is that similar to Oppel's. It is defined by the "unitary association", which include the largest group of compatible species, i.e. those which have lived together for a certain time. The computer procedure consists of: 1. Eliminating all species not compatible with this unitary association (reworked fossils, as with many nannofossils, those due to sampling error, etc.). 2. Erecting a composite biostratigraphic scale, with relative positions of all events superimposed, based on as many sections as possible because a biostratigraphic sequence based on only one section reflects only the 5 In
the same section, the base of the NP25 zone, defined by the appearance of nannofossils, has a position which can vary up to 45 m according to different authors.
Refinement of Concepts and Time Scales
33
order in which that locality was colonized, and not necessarily the real chronology of biological events. 3. Selecting the most significant subdivisions of the scale by multivariate analysis. For example, from 100 species of Jurassic radiolaria determined in 210 samples coming from 43 localities, Baumgartner (1984) has defined 14 unitary associations distributed among 7 biozones. 4. Establishing correlations with certain confidence limits.
1.3 Search for a Rigorous and Universal Chronostratigraphy 1.3.1 Weakness of the Initial Concept of the Stage. Revision of Stratotypes It has been seen (Chap. 2) that chronostratigraphic units were initially created from sedimentary sequences with evident discontinuities in lithology and faunal assemblages. The pioneers of stratigraphy considered these breaks to be simply episodes of short or no time duration. It is now known that these intervals may be very long, even longer than the periods of sedimentation separating them. This is shown by the presence of nonclassifiable faunas in the d'Orbigny stages. Occurring in the transition beds between units, these faunas prove that the stratotypic successions are generally incomplete. For instance, at Biarritz, the Lutetian-Stampian interval is represented by 1500 m of limestones, marls and marly limestones deposited in the relatively deep environment of the Aturian Gulf. It is a continuous sequence, apparently complete and difficult to subdivide stratigraphically. In the Paris Basin, the same interval is about one-tenth the thickness and consists of 18 distinctive lithologic formations evidently resulting from alternating marine and nonmarine conditions, and yielding, therefore, a sequence riddled with gaps and a very imperfect picture of the passage of time. However, the Paris Basin has been selected for the Eocene and Oligocene stratotypes. A revision of the European stratotypes was therefore undertaken during the 1960s, with a view to improving the definitions and zonal subdivisions. Two views have arisen: 1. Inappropriate stratotypes should be abandoned in favor of sections with continuous pelagic deposits. 2. Stratotypes should be preserved with these defects but should be restudied so as to evaluate the magnitude of their hiati.
The choice between these two solutions has depended on the actual case. At Valangin and Hauterive (Switzerland), urban development has resulted in the disappearance of most of the sections on which the Valanginian and Hauterivian stratotypes were based. Additional type sections (hypostrato-
34
Modern Stratigraphy
types) have therefore been selected in the Subalpine Basin of southeastern France, where sediments are thick and sedimentation was continuous. Ammonite biozonation has been refined and parallel time scales based on varied fossil groups have also been erected. However, for stratotypes reasonably well preserved, a thorough revision was undertaken as follows: 1. A precise relative dating of any sedimentary discontinuities, especially at the base or top of a type sequence. 2. A systematic inventory, both qualitative and quantitative, of the biologic content. 3. A description and relative dating of the bio- and lithohorizons. This type of study made it possible to verify whether the original definition of the stage in terms of physical and biological characteristics and subdivisions (substages, zones) corresponds to the objectively redescribed reality. If it does not, it had to be corrected. One of the consequences of all this revision work is that other countries have been able to propose new stratotypes which are better than the originals for any given time interval. Another consequence is the emphasis put on the heterogeneous character of stratotypes in general, distributed among two groups of faunal associations: those of the epicontinental regions with hiati now defined with respect to localization and significance; and those of the deep pelagic regions. From all this it is seen that stages, stratigraphically superposed, often represent a succession of ages separated by gaps but also with overlaps; thus the necessity for a redefinition of their boundaries.
1.3.2 Redefinition of Boundaries To be certain of the perfectly sequential nature of stages, it was decided that only their lower boundaries would be defined and this boundary would be taken as the upper limit of the underlying stage. Because of this practice, all subsequently discovered beds which fall between two stages are included in the upper part of the older stage. Therefore, interfaces between successive units, of theoretically zero duration, are substituted for the previously assumed breaks with their implied time discontinuities. The simplest scheme, following Oppel, would be to make the lower boundaries of stages coincide with biozone boundaries, which would make the biozone a part of the stage. It is known, however, that zone boundaries are not necessarily isochronous and therefore would be difficult to apply to chronostratigraphic boundaries. Today the method is more arbitrary: whether in a stratotype or in another sequence or representative part of a sequence called the boundary stratotype, a decision is made to define the base of a certain stage at a particular bed, coinciding perhaps with a biozone boundary at this point, but perhaps elsewhere with other events such as the appearance or disappearance of taxa, the beginning of a zone of mineralization, volcanic
35
Refinement of Concepts and Time Scales 55
60
Fig. 21. Position of some stratotypes of the Paleeocene and Lower Eocene in the chronostratigraphic scale of the Anglo-French-Belgium basin. Demonstration of the different time ranges for different stratotypes (After Berggren et al. 1985)
Late Paleocene
Early Eocene
(Selandlan)
(Ypresian)
Million years
_.
Selandlan Thanetlan
t--
--
Lutetian
Ypreslan
Sp~~_
-- -
Stratotype positions
ash beds, magnetic reversals, etc. In short, either a biomarker or a lithomarker can be used as a reference. Also, it is necessary that this boundary be related as precisely as possible to several biostratigraphic scales so that it is as widely applicable as possible. The stages redefined in this way between precisely defined boundaries related to a clear reference system then correspond to the totality of time, and in so doing clearly demonstrate the incomplete character of the majority of stratotypes (Fig. 21). Is it therefore necessary to determine precisely the lithologic and faunal content of a stage at one locality, knowing that this can change rapidly in a lateral sense? In other words, do the stratotypes, or even the stratotypic regions advocated by some, have any use? The higher-order boundaries have also been redetermined more precisely, with international working groups seeking the best boundary stratotypes. For example, for the Cretaceous-Tertiary boundary, two sections are in competition: that of Gubbio in Italy and that of The Kef in Tunisia. In both cases the boundary has been suggested where the greatest number of important events occur, such as climatic variation, faunal renewals, magnetic reversals, geochemical anomalies etc. Thus modern research on global phenomena, while helping to define major chronostratigraphic boundaries, rejects Lyell's principle of uniformitarianism (uniform distribution through time of all geodynamic processes). Instead, it reintroduces in modernized form the "revolutions" of d'Orbigny.
1.3.3 Refinement of the Chronostratigraphic Scale A time correlation of two geographically distant events implies that they could not be separated in time because of the inadequate resolving power of the stratigraphic tool employed. Since precision in correlation is necessary for true paleogeographic reconstructions, it is reasonable to try to improve this precision by using smaller units than the stages, which have average durations of 10, 5.7 and 4m.y. for the Paleozoic, Mesozoic and Cenozoic,
36
Modem Stratigraphy
respectively. One such smaller unit would be the zone, even if it is a biostratigraphic unit. However, for the system to be consistant, the zone used in a chronostratigraphic sense must become a chronozone. The duration of a chronozone is clearly inversely related to the rate of evolution of the paleontologic group used. The average duration of Ordovician graptolite chronozones is about 5 m. y., while the Devonian, Permian and Mesozoic ammonoid chronozones average about 1 m. y. The 185000 years average duration for the Toarcian (Sect. 1.1.2, this Chap.) chronozones in its type area almost certainly represents a record for precision. All requirements for greater precision, and all refinements of stratigraphic tools clearly result in new constraints, new revisions of boundaries and an increased need for rigor in definitions. As in all scientific disciplines, stratigraphy will, therefore, require continual adjustments.
1.3.4 Modern Trends: Biostratigraphy Slowly Replaces Chronostratigraphy Much more work is necessary before the chronostratigraphic scale becomes precise, reliable and universal. Because of this, its subdivisions remain somewhat abstract. In comparison, biostratigraphic units are more concrete and easier to use, especially when they are based on microorganisms generally well represented in the sediments. Founded on the irreversible evolution of living organisms, these units inevitably acquire a chronostratigraphic significance, even if they do not always lead to time correlations. The natural heirs of catastrophism, d'Orbigny's stages owed their existence to the unconformities bounding them. In the thick continuous sequences where stratigraphers later labored, such stages are difficult to recognize without recourse to purely paleontologic arguments such as the appearance or disappearance of a fauna, genus or species. This is more valid the smaller the stratigraphic unit sought. The zone, therefore, tends more and more to replace the stage as a material representation of a segment of Earth's history. This is certainly a deviation from the original concept of the zone, but it is justified for practical reasons. As Pomerol (in Pomerol et al. 1980) wrote: "One can observe a replacement of chronostratigraphy by biostratigraphy, and stratigraphy gains in effectiveness what it loses in rigor." This loss of rigor derives mainly, as we have seen, from the diachronism of certain biozones, especially at times of strong climatic gradients. Example: the Ericsonia subdisticha (coccolith) zone which cuts diachronously across the Eocene-Oligocene boundary from low to high latitudes in the northern hemisphere (Fig. 22). For this reason and for others which similarly prevent the synchronism of the appearance and disappearance of species (facies variations, fossil preservation, fossil abundance, faunal migrations), the establishing of biozonations of universal validity appears at the moment to be beyond realization.
37
New Methods of Correlation
w
Z
w
Zonation by planktonic Foraminifers (W.H.BLOW)
Germany 52° lat. N
Italy 45° lat. N
Florida 30° lat. N
g P19 (!)
~
P18
P 17 w ~ P16
u
ow
P 15 P14
2"
1
Fig. 22. Ericsonia subdisticha zone in the North Hemisphere, defined by two diachronous markers. 1 Disappearance of Discoaster barbadiensis and/or of D. saiponensis. 2 Disappearance of Cyclococcolithus formosus (After Cavelier 1979)
2 New Methods of Correlation After basing itself essentially on lithology and paleontology, stratigraphy turned during the 1950s towards physical, chemical and mineralogical methods for new forms of correlation or even new time scales. To a large extent this tendency stemmed from a desire to relate the stratigraphic scales to physical and chemical phenomena possessing certain properties such as: 1. A regular periodicity, giving them the character of clocks rhythmically ticking the march of time; 2. An instantaneous character on a geological scale, i.e. a duration not exceeding a few thousand years; 3. A very wide occurrence, in some cases worldwide. These new methods have been developed largely within the context of the remarkable technical progress in areas like geophysical exploration and improvement of analytical methods over the last few decades.
2.1 Correlation by Sedimentary Rhythms A rhythm is, by definition, a regular repetition of a certain feature or interval. The arrangement of sediments into beds is itself a rhythm. In order that correlations can be made by this method, the sedimentary rhythms should be related to periodic phenomena with a synchronism recognized over very wide areas. This type of correlation is now in general use due to
38
Modern Stratigraphy
1---1
Clay
E:] ....
Sands
E:i9
Platform carbonate
~
deep limestone
~
Conlinental red beds
~
Platform limestone and marl
c:
~
~
Fig. 23. Representation of lithologic units of a basin on a space-time diagram on the basis of seismic and drilling data, offshore West Africa (After Vail 1977)
the large amount of subsurface data available from petroleum exploration. The principal geophysical methods directly applicable to stratigraphy will, therefore, first be reviewed.
2.1.1 Subsurface Methods of Investigation 2.1.1.1 Seismic Methods Seismic waves generated at the ground surface are reflected by lithologic surfaces of discontinuity separating lithologies of different elastic and/or density properties. The recording of these reflections, therefore, permits the localization of the principal breaks in a sedimentary sequence (bed surface, surface of erosion or nondeposition, fractures, etc.) with a resolution of a few tens of metres. Sedimentary units and their relationships as well as the geological structure can then be reconstructed. According to Vail et al. (1977), bed surfaces and discontinuities revealed by seismic are useful in correlation but also have a chronostratigraphic significance (see below). Their characteristics enable the relative importance of stratigraphic gaps to be gauged and the lithology or environment of deposition to be known in some detail (Fig. 23). The seismic method, using data from ocean drilling as a reference, is also the basis of the geologic map of sediments overlying the ocean floor (Fig. 24) and thus provides a test of the reality of ocean-floor spreading at the mid-ocean ridges.
39
New Methods of Correlation
D
Pleistocene
i. . . ; "I
Jurassic
1=-: Ilower Cretaceous ~ Upper Cretaceous [](D
Basalt + Pliocene
Fig. 24. Geological map of sediments in contact with the Atlantic Ocean floor (from the Geological Atlas of the World of Freeman, Lynde and Tharp, in Daly 1984) . Isochronous bands symmetrical about the mid-oceanic ridge offset by transform faults
2.1.1.2 Well Logging Methods (Diagraphic)
These are measurements of different physical and chemical properties of rocks (lithology, mineralogy, nature and importance of fluids , texture , bedding, grain size distributions and dips) encountered during the drilling of a well. These measurements (natural radioactivity, spontaneous potential,
40
Modern Stratigraphy
.,:::>
o
'"o
~ U
Sandstone Dolomitic limestone Dolomite and anhydrite
:ii '
'is c
Limestones and marly intercalations
«I
t: o
0..
800
o
.,
Marls and marly limestones
"r;;
f!
Limestones
3
Marls and marly limestones
...,:::>
700
c «I "a' "C .~
E E
c «I
"c
:.::
~~.....I----H~+---4l-L,:+--+I-L~----.i+!::=t--+
800
Limestone and marly limestone
Sublithographic limestone Valiant-St.Georg.. 1 Grandvilia 101 (/) Gelannes1 Nozay 1 Mailly 102 ~
g
Mentioned heights (in meters) are calculated from soil surface and refer conventionally to the top of the Sequanian
Romi Ily-sur-Sei
Fig. 25. Example of electric-log correlations between holes in the Paris basin (After Perrodon 1968) abc
abc
a
a
b
I
T7 c
1
c
b
b
a
a
2
Fig. 26. 1 Cyclic and 2 rhythmic sequences a,b,c Lithologic units
41
New Methods of Correlation WEST
CONDAT
EAST
Sequences
---
Rhythm C
100 RhythmB2
80
--
--
60
Rhythm B1
"'!I-f--......~~-+--
__
---
Rhythm A
20
o Fig. 27. Sequence correlation (here the rhythms A, B1, B2, C) in the Middle Jurassic of Quercy (after Delfaud 1972). Ideal sequence: 1 lignitic marl; 2 micrite with gypsum pseudomorphs; 4 azoic micrite; 5 micrite with algal balls; 6 sparite with ooliths
density, porosity, resistivity and sonic) define for each bed what Serra (1972a,b 1986) has called an "electrofacies", expressed by the character of the recorded curves, and may be used in correlation. Especially the electrofacies makes it possible to determine the lithology and the internal structure of the rocks (beds, rhythms, discontinuities). For example, shales, porous and full of retained water rich in ions, are much less resistant to electric currents than a compact limestone. The two are, therefore, easily separable by a resistivity log. Reference markers such as unconformable surfaces (enriched in phosphates), cinerites and derived claystones (tonsteins) are recognizable by radioactive logs. The degree of resolution in the analysis of strata obviously depends on the resolving power of the various tools used: lOcm for the more classic methods, 1 cm for dipmeter logs. A "composite log" combining all types of measurement is a convenient method of defining a lithologic sequence. These can then be used by comparing the shape of the curves and their cyclic character to establish correlations between wells within a basin (Fig. 25). The degree and reliability of the correlations may be quantified using appropriate coefficients. In sum-
42
Modem Stratigraphy
mary, the logging technique is essentially lithostratigraphic. It can, however, provide results of chronostratigraphic significance in two situations: 1. In reference to pyroclastic horizons, which are more or less radioactive
and distributed independently of facies; 2. Where wells have a number of similar log characteristics, as shown by maximum coefficients of correlation over an interval limited in space and time. According to Serra (1972a), "the probability that over a certain time duration a non-synchronous cycle of sedimentation can be perfectly duplicated laterally in all its characteristics is so remote that one can say on the one hand that the reliability of the correlation is maximal and on the other that it must be synchronous".
2.1.2 Sequence Stratigraphy A stratigraphic sequence of sediments is a consequence of an evolution in sedimentation controlled by changing external factors. If these factors change cyclically and with more or less equal periods, they give rise to a rhythmic sequence of repeated lithologies. These rhythms can consist of continuous variations (= cycles in the strict sense of abcba type) or they may be syncopated, i.e. punctuated by sharp reversals (saw-tooth cycles of abcabc type) (Fig. 26). These reversals are generally marked by sedimentary discontinuities through erosion or nondeposition. In any sequence, rhythms of first, second, third or even higher orders may be defined (Delfaud 1972). Rhythm stratigraphy entails the definition, in a given location, of a particular vertical rhythm and its correlation with the same rhythm found elsewhere. To facilitate comparisons it is usual to express the rhythms as lithologic curves. Delfaud (1972) distinguishes three degrees of correlation (Fig. 27): 1. Between sequences of analogous facies. 2. Between sequences of different facies but within the same sedimentary domain. 3. Between sequences of different facies and different sedimentary domains (e.g. between a platform and a basin).
In all cases, the correlation can be lithologic and nonchronologic. In fact, the persistance of the lithologic characteristics of a formation over a wide area6 demands a contemporaneous uniformity of environment rarely observed, especially in the platform domain. Diachronism is, therefore, probably the rule. Among numerous examples known is that of the "lithoclinal sequences" making up the Dogger of the Paris Basin (Purser 1972). In this example, facies boundaries cut across time lines (Fig. 28). Diachronism 6 Sequence
1981).
correlation is possible from England to Germany for the Jurassic (Hallam
43
New Methods of Correlation Lorraine
Callovian IIthoclin. Upper
Bathonian
--v-v-
Discontinuities
• • • • • • • Isochronous lines
~ Etrochey limestone ~ "Daile nacr6e-
1_ _ I
Marls
Fig. 28. Relations between time units and lithologic units in the Callovian lithocline along the eastern border of the Paris basin (after Purser 1972). These relations imply a displacement of the zone of deposition with time
Isochronous line
Fig. 29. Israelski Principle. Sedimentary cycle determined by a transgression followed by a regression. The dashed line joining the points of furthest advance of each facies is an isochronous line representing a section of an isochronous surface (see also Kauffman and Hazel 1977, p. 228)
can result from progradation, as commonly seen in the deposition of sedimentary prisms at the edges of basins. Variation in sea level appears to be one of the major causes of this progradation, with diachronism being more pronounced, the slower the changes in sea level. When the eustatic movement is reversed, the direction of sedimentary progradation must also be reversed, and the positions of the points of reversal theoretically define an isochronous surface, in the absence of tectonic movements and given uniform subsidence (Israelski principle; Fig. 29). A generalization of the relation between sequences and sea-level fluctuations has been proposed by Vail et al. (1977). According to Vail, the sequences of strata defined on the continental margin by seismic reflection are objective stratigraphic units bounded by discontinuities. The majority of the major sequences are deposited under the double control of rate of sedimentation and the cyclic variations in relative sea level. The latter generally involve a rapid rise, a period of stability and then a rapid drop, with a total average estimated duration, according to Vail, of about 1 million years. The discontinuities which bound the sequences may be due to nondeposition during periods of stability, or both erosion and nondeposition
44
Modem Stratigraphy
during the fall of sea level. They are considered as practically isochronous, regionally, to within a few hundred thousand years. It has been possible to use such markers for the correlation of biostratigraphic scales established in different faunal provinces, for example in the Cenomanian between the Tethys and the temperate zones. Struck by the similarity in the character of continental margin submergence when compared by sequential analysis, Vail has proposed a curve of relative sea-level fluctuations of global scale (Fig. 30) 7 . Superimposed on this curve is a true stratigraphic scale of global eustatic cycles related to the chronostratigraphic scale. Seen from the eustatic point of view, therefore, the sedimentary sequences may be regarded as tools of correlation, as well as being useful in dating, but their resolving power remains rather low. Also, the chronostratigraphic significance of the Vail curves is refuted by some scientists. Although traditionally used to solve lithostratigraphic problems, the sequences are also very useful in the search for economically useful resources such as petroleum and water, and in paleogeographic studies (see below).
2.1.3 Use of Cycles of Punctuated Aggradation (PAC) According to Goodwin and Anderson (1985), basin sedimentation is controlled by periodic elevation of the depositional base level. These elevations, resulting generally from rapid transgressions, are marked by surfaces of nondeposition synchronous within a basin. These surfaces bound minor cycles of deposition (PAC), totally independent of formations, and these cycles are considered fundamental chronostratigraphic units deposited in a few thousand years, and therefore capable of very precise correlation.
2.1.4 Use of Binary Rhythms This method is applicable to low-energy environments where sedimentation is essentially of suspended fine-grained terrigenous matter and planktonic skeletons, deposited alternately in cyclic successions of the two components. 2.1.4.1 Pelagic Limestone-Marl Alternations Deep-water sediments often form monotonous sequences consIstmg of limestone beds and marl interbeds in intergrading units of decimetre thickness (Fig. 31). Many authors believe that these alternations represent cyclic global climate changes affecting both the oceanic environment, particularly temperature, dynamics, and productivity, and the continental environment, as source of clays. This relationship is clear for recent sediments where the 7 This
curve has been criticized because it is based mainly on data derived from the Atlantic margins, which have been appreciably affected by vertical movements.
45
New Methods of Correlation 2ND ORDER CYCLES PERIODS
RELATIVE VARIATIONS OF SEA LEVEL
EPOCHS
Plio-Pleistocene TERTIARY
...
P_ Rising_
1
...r:==~M~lo~ce~n~eL:==~_
......-..,p.?~a;r.IO;~~'!lcne~ene.....;..~_- -
s
CRETACEOUS
-
_
-~
M
TRIASSIC
......---:-I-..:..:..:c-S ---;
-fb-= _-_-_-_-_ :-~.:-
_~_~ ______
......- - - - - - I - - - I - - - - - - f S
PERMIAN
Lowering ~ _ = _ ~;Td.-O
Tc
_
I-------+....-_-_-_-_-_-_-_-_-~·:s'_·-_-_:r- - - - JURASSIC
~
NOTATIONS
- -
,- - - - -
p:s:(J-1 ~_~~~
Ka
UJ
J
~
sea l e v e l .
-----r ..----
1!;
~
FP
1-:300
\.
M
t---------ir--r--:I-===:;::~ CAMBRIAN
_200 ~
P
o-S ORDOVICIAN
~
TR -
"
PENNSYLVANIAN
-100
Kb
M
~
-
--------
PRECAMBRIAN
Fig. 30. Major cycles of global sea-level variations (After Vail et al. 1977)
Fig. 31. Example of limestone-marl alternations in the Lower Cretaceous of the Vocontian basin, southeastern France. The transition between the two lithologies is gradual
8 5 i=
. 1-500
COo
46
Modern Stratigraphy
Fig. 32. Correlation of a bundle of beds and interbeds in the Valanginian of the Vocontian basin, southeastern France. The sections occur in the deep zone indicated on the map (After Cotillon et al. 1980)
Site 534
t.. Core numbers
Vergons
Fig. 33. Example of correlation by cyclograms in the Campylotoxum zone of the Lower Valanginian. Vergons is a locality of the Alpes de Haute-Provence (France). 534 is the site of a drill hole in the Atlantic off Florida. The horizontal bars represent the relative thicknesses of the cycles defined by alternations of limestones and marls (After Cotillon and Rio 1984)
alternation of calcareous and terrigenous muds can be directly correlated with glacial and interglacial stages during the last 700000 years. Already by the end of the last century, Gilbert (1894), struck by the regularity of the Upper Cretaceous cycles of Colorado, interpreted them as due to climatic oscillations induced by the 21000-years cycles of the precession of the equinoxes. By counting the couplets in a formation, he was thus able to measure its duration. Today it is believed that cyclic variations of three orbital parameters, precession of the equinoxes with a period of 21000 years, obliquity of the Earth's rotational axis on the plane of the ecliptic with a period of 41000 years, and the eccentricity of the Earth's orbit with periods of 106000 and 410000 years, interfere to produce a complex fluctuation of solar heat at the Earth's surface, which in turn controls the
New Methods of Correlation
47
sediment rhythms. On this basis, it is believed that limestone-marl couplets can be correlated over very large distances with sometimes a resolving power at the level of the individual bed. These correlations are made directly bed by bed (e.g. Lower Cretaceous of the Vocontian Basin, Fig. 32), or by graphical representations of thicknesses of cycles plotted against time (cyclogram of Fig. 33). 2.1.4.2 Varved Microalternations Varves have been already mentioned as a means of calculating time durations. These indicators of anoxic lacustrine and marine environments can also be used for very precise correlation within basins if certain cycles or successions of cycles can be readily recognized.
2.2 Correlation by Mineralogic and Geochemical Markers Mineral or organic constituents, elements, even some isotopic ratios characterizing deposits of a certain age or region, can play an important role in correlation. The precision and geographic significance of this will depend on the spatiotemporal distribution of the markers. Many of them are related to cyclic phenomena or gradational processes, while others represent geologically brief events not necessarily repeated in the same place.
2.2.1 Clay Minerals Widespread because of their small particle size, clays are useful for precise correlation to the extent that they are inherited from the continents. According to Chamley et al. (1978), the clays of the North Atlantic are distributed through time as a function of major geodynamic events such as climate, plate mobility and orogeny. Smectite appears in appreciable amounts from the Upper Jurassic, but especially in the Cretaceous, and is indicative of the erosion of tropical soils. From the Eocene, the increase in content of the primary clays illite, chlorite, mixed-layer illite-smectite and chlorite-smectite, reflects the intensification of the Alpine Orogeny, and the global cooling evident from the Eocene-Oligocene boundary but especially in the Upper Miocene (first arctic ice) and in the Plio-Quaternary (growth of the Greenland and Alpine glaciers). There are also other indicators more time-specific; attapulgite in the Albian, attapulgite and sepiolite in the Paleocene and Lower Eocene. These also are minerals characteristic of a certain cratonic margin provenance. It appears, too, that newly formed minerals are also useful in correlation. Frohlich (1982) has observed that the vertical change in mineralogic composition of the azoic red muds of the Indian Ocean (composed of clays, clinoptilolite, phillipsite and amorphous silicates), represents a veritable stratigraphic sequence of extremely wide significance.
48
Modem Stratigraphy
2.2.2 Heavy Minerals These have the advantage of being very resistant, easily identifiable, and provenance-specific in their petrographic characteristics. Their associations are sometimes useful for characterizing lithologic formations. The Lower Trias Buntsandstein sands of the Vosges and the Rhenan region, for example, can be stratigraphically subdivided on the basis of the type of tourmaline (Henrich 1961) as follows: 1. Lower Vosgian sands: tourmaline rounded and angular. 2. Upper Vosgian sands: tourmaline rounded only. 3. Principal Conglomerate and Purple Boundary Beds: dominantly angular.
tourmaline
Useful for correlation, the heavy minerals can also help to trace the tectonic evolution of a region by their distribution in time. In the perialpine molasse, for example, glaucophane is a valuable marker indicating the first erosion of the Schistes Lustres of the Piemontaise zone. Figure 34 summarizes the distribution of the principal minerals of the alpine association during the Tertiary in the molasse of the region from Switzerland to Isere (Latreille 1969).
2.2.3 Volcanic Ash This is the basis of tephrostratigraphy. Explosive volcanic eruptions can eject pyroclastic material into the upper atmosphere where prevailing winds carry it over very great distances. In principle, the finer the ash, the more widely distributed will it be. For example, 300000 km 2 were covered by 8 billion tons of ash from the Quizapu volcano in the Chilian Andes in 1932; and 3500 years ago, 20 times that amount was ejected from the Santorin volcano. The ash falling into quiet environments forms beds of cine rite which tend to be altered later by diagenesis. Deposited with detrital sediments, they are little altered, although when included in beds of plant material (future coal for example) their feldspars are hydrolized in the presence of humic acids to form kaolinite. Every cinerite bed has its own characteristics identifiable in thin section, even when derived from the same volcano. Bouroz (1972), for instance, distinguishes five types of cinerites (called gores or tonsteins) in coal successions, according to their mineralogy and microstructure. This petrographic signature is often significant when tonsteins of different basins are compared. For example, in northern France the coal sequences of SaareLorraine and North Pas de Calais are correlatable by means of about ten beds of tonstein. Eight tonstein beds have allowed the sequences of the Cevennes Basin and the Jura Basin to be correlated much more precisely than by plant biostratigraphy (Bouroz 1972). At the boundary between the Lower and Middle Burdigalian, tuffites and cherts have been described from
49
New Methods of Correlation N Switzerland
Haute-Savoie (France) North Usses
Savoie (France)
South Usses
HELVETIAN
BURDIGALIAN
AQUITANIAN
OLIGOCENE
EOCENE
Fig. 34. Principal mineralogic groups in the detrital subalpine and peri alpine Tertiary (after Latreille 1969). Gr Gamet; Ep epidote; Gl glaucophane >5%
around the entire periphery of the western Mediterranean (Lorenz 1984). These are the products of acid volcanism from a source not yet located. Because they are products of very brief events, cine rites are perfect isochronous markers, chronohorizons in the sense of Hedberg (1979), allowing correlation between continental and marine sequences. Their only limitation is their moderate geographic distribution, rarely more than 1000km for the distinct beds, slightly more for those beds somewhat diluted but still detectable by their radioactivity.
2.2.4 Chemical Elements and Isotopes One of the implications of the principle of uniformitarianism is the constant chemistry of ocean water and, therefore, of the carbonates, sulphates and halides precipitated from it. However, this assumption has never been proven, even for deep oceanic sediments which have never been affected by meteoric diagenesis or other continental influences. Also, the names of periods such as Carboniferous and Cretaceous are an implicit recognition of global geochemical variations during geological history. In 1952, Arrhenius observed that two recent sediment cores taken from the East Pacific Rise showed synchronous fluctuations in their CaC0 3 content. Many other observations have since shown that in the Quaternary and the Neogene, such variations dated by microfauna and carbon 14 are correlatable throughout the Pacific, Indian and Atlantic oceans. The car-
Modern Stratigraphy
50
.."
-" "
0
"'u :2.,
90 100
~a;
0
110
~~ ~u
120
Fig. 35. Change in strontium content of pelagic carbonates since the Upper Jurassic (After Renard 1985; see also Renard in Pomerol et al. 1987)
bonate content of pelagic deposits must, therefore, depend on general factors such as primary production or rate of dissolution of carbonate sediment during or after deposition. In both cases, global changes in water chemistry are implied, perhaps related to the marked eustatic and climatic fluctuations of the recent periods. Whatever the causes, these variations are synchronous8 and are, therefore, very useful for correlation. They are the basis for chemostratigraphy. 8Given the short mixing-time constant of the oceans (less than 1000 years) all chemical changes of seawater are essentially synchronous in the world's ocean.
New Methods of Correlation
51
2.2.4.1 Trace Elements A number of elements occurring in carbonates are now being used for correlation. Strontium in pelagic sediments generally decreases from the Cambrian to the Jurassic and then increases to the Present while showing marked variations in certain periods (Fig. 35). Its maximum concentration occurs in the Miocene. According to Renard (1985), the strontium curve may be related to changes in submarine hydrothermal activity and its calcium supply at the mid-oceanic ridges. The variations in concentration of strontium may also be related to variations in the CCD (Carbonate Compensation Depth), which controls the dissolution of calcite and, therefore, the release of strontium for incorporation into the residual sediment. However, the CCD curve is related to the sea level curve, therefore the observed coincidence between the strontium curve and transgressiveregressive cycles, as pointed out by Renard, supports this hypothesis. Iron and manganese vary through time very much like strontium. Their concentrations in calcareous pelagic deposits probably depend on the amounts produced at the mid-oceanic ridges and, therefore, on the latter's activity. Other attempts at correlation have been based on Na, K, Ca/Mg. Finally, some elements appear in exceptional concentrations at certain brief moments of geological time, during what may be called geochemical events. This is the case for Iridium, which is concentrated at the CretaceousTertiary boundary at about 50 oceanic and continental sites distributed worldwide. The same concentration is found at the Eocene-Oligocene boundary. The significance of these concentrations is still controversial. Two causes have been suggested: one extraterrestrial (fall of giant meteorites), the other volcanic (periods of intense magmatic activity). 2.2.4.2 Stable Isotopes In 1955, Emiliani demonstrated variations in the () 18 09 of the calcareous shells of planktonic foraminifers from the Quaternary of the North Atlantic and the Caribbean. These variations may be used to define isotopic stages (Fig. 36) which correspond rather closely to the glacial and interglacial stages. This correspondance derives from the fact that, at thermodynamic equilibrium, the 180/160 ratios of a carbonate and the water from which it precipitated (chemically or biochemically) are different, and that this difference increases as the temperature of precipitation decreases. Many subsequent studies have shown the synchronism of these variations in the marine environment (Fig. 37) and, therefore, their utility as a correlation tool. The generalization of this method to pre-Quaternary formations poses numerous problems related to diagenesis, such as recrystallization and temperature effects which change the original isotopic composition of oxygen. Nevertheless, data as far back as the Paleocene show the existence 9
52
Modern Stratigraphy
IX!
o
a.. o
~
o CD -co
NORMAL BRUHNES
o
2
3
4
REVERSE MATUYAMA
5
6
7
8
9
10
11
12
13
14
Depth (m)
Fig. 36. Curve of /) IH O of planktonic foraminifera correlated with the magnetostratigraphic scale in a core from the Pacific (after Shackleton and Opdyke 1976). The numbers up to 17 refer to Emiliani's (1955) isotopic stages
-2
Fig. 37. Variation in composition of oxygen isotopes from tests of foraminifera in four cores from 1 the Caribbean sea; 2 the Indian Ocean; 3 the Mediterranean; and 4 the Pacific. Time scale in millions of years. On the ordinate, variations of /)180 in parts per mil relative to PDB (Peedee Belemnite Standard) (after studies by Emiliani 1966; Shackleton and Opdyke 1973; Be and Duplessy 1976; Cita et al. 1977)
of variations related to fluctuations of arctic and antarctic ice and, therefore, to oceanic temperatures (Fig. 38). In contrast, the isotopic ratio of carbon (13 CP2C), expressed as 013C, is practically unaffected by burial diagenesis. Moreover, it appears to vary, synchronously, in the world's ocean, showing sharp changes principally at the Cretaceous-Tertiary boundary (65m.y.), at the Paleocene-Eocene boundary (53 m. y.), and in the Upper Miocene (about 6.1 m.y.; Fig. 39). This synchronism derives from the fact that the 013C of pelagic carbonates seems to be an indicator of the paleodepth,
53
New Methods of Correlation
Ma
PPliol
Miocene
lp
2,0
10 I igocene I
-2
l
j
1
0
0
o 2
p 10
o
0
JPalaeocene
of
+-0
~
I0"),+
00
6.0
~o
o/~'w
: +
0
~O+•• O
1
Eocene
40
30
C\)
t
0
0
o
0
0
00 0
0
t
o h;\/o 1}ty"'~+"++.o .. ....++ .. +'!.o >,\-o 0
~40 ,\.t • I +
101801
Fig. 38. Temporal variation of the oxygen isotope composition of total carbonate (0) at two sites in the northeastern Atlantic and planktonic foraminifera ( +) at three sites in the South Pacific (after Vergnaud-Grazzini 1979). Arrows indicate events
Ages ~ ot
g
Paleo cene
Miocene
Late
Late Cretaceous
Jurassic
+6:
u
M
+
('t)
'Ma
b
,
10
,
20
jo
, , 50
40
6'0
,
70
I
80
I
90
i i i
100
110
120
1!m
,
140
Fig. 39. Change in carbon isotope ratios of pelagic carbonates (total carbonate) since the Upper Jurassic (After Renard 1985)
and the depth of the euphotic zone. It increases during transgressions and decreases during regressions. Modifications of the continental and marine biomass (fixing 12C preferentially) lead to modifications of the 013C of sediment. For example, sediments enriched in organic matter on the shelves during transgressions, or significant deposits of coal, increase 013C. In summary, the isotopic variations of oxygen and carbon in marine carbonates reflect changes in the temperature, geochemistry, and other parameters of the environment of formation related more or less directly to global changes in climate and sea level. These variations are sharp at certain times and may then be used as stratigraphic markers. Other isotopic ratios under study may also become tools of correlation. The 034S curve, expressing changes in the 34SP2S ratio, defines a megacycle
54
Modern Stratigraphy Total volume of evaporite. (t O· km')
0,25
0,75
Neogene Paleogen
Cretaceous
Jurassic
,-, , I
Triassic Permian
Carbonlfero~'s .. ,
,
\
\ ,- .-
Devonian I
Silurian
---- --
,,-- --
Fig. 40. Temporal variation of sUlf.hur isotope ratios 4 S/ 3 S) in sulphates (solid line) and of the volume of evaporites (dashed line). Inspired by Odin et al. (1982a) and Tardy (1986)
... - -"
"
e
, f
Ordovician ,
10
20
--'
rn a:: c(
w
>z
0
:J ...J
~
~
w
~ ~
20 ",' 60 ./ ...... J 100 I 140 I 180 ....... 220 ~ 260 \ '\ 300 ........... ",' 340 380 I .~ 420 460 500 540 580 0
Tertiary Cretaceous
\
Jurassic
.
Triassic
---
Permian Carboniferous
'.
<::"">........
...o
...
o· 87 Sr/86 Sr ratio
Devonian Silurian Ordovician Cambrian Precambrian
o
01
o
~.
Fig. 41. Change of the strontium isotopic ratio (After Faure 1982)
New Methods of Correlation
55
(Fig. 40), with a minimum value in the Permo-Trias. In general, but not always, times when evaporites (rich in 34S) were important were also times when marine waters were enriched in 32S and, therefore, had lower 034S values. The 87Sr/86Sr ratio reflects the contributions of submarine volcanism, related to ocean spreading, and continental erosion. When plotted against time, this ratio varies inversely with the activities of the mid-oceanic ridges, the lows in the curve corresponding to periods of major oceanic spreading or, as in the Permian, to periods of major fragmentation (Fig. 41). 2.2.4.3 Organic Matter Geological history has been punctuated by periods characterized by sediments abnormally rich in organic matter. This imparts a dark color to the sediments ("black-shales") and results in a high content (greater than 1 or 2%) of organic carbon. The classic black shales include the graptolitic shales of the Paleozoic, the bituminous shales of the Upper Lias, the CallovoOxfordian black marls of the Tethys and those of the Cretaceous known from the North and South. Atlantic, from the United States, from Western Europe and from North Africa, deposited during phases of maximum extension between the Upper Aptian and the Coniacian. The most remarkable black shales, by virtue of wide geographic distribution and short stratigraphic duration, were formed at the Cenomaniah-Turonian boundary (Fig. 42). This was truly a black shale event. Similarly, the Upper Lias bituminous shales were deposited over a large part of western Europe. They are known in France as "carton shales". In the Quaternary of the eastern Mediterranean, beds of black sapropels have been encountered in drilling. The apparent causes of such events are varied, but all appear to be related to global phenomena such as climate, eustasy, and distribution of continental masses, guaranteeing their synchronism over very large distances. The composition of organic molecules can also be a basis for stratigraphic correlation. Recent studies (Brassell et al. 1986) have shown that Quaternary marine sediments of the last million years contain unsaturated organic components (alkenones) derived from certain coccolithophorids. The index of unsaturation of these products is a function of the surface water temperature at the time of their synthesis. The variation of this index through a sequence parallels the 0180 curve, which is also (partly) related to temperature. The index of unsaturation curves may, therefore, be used in correlation in the same way as the 0180 curves. They are especially useful for low-carbonate sediments deposited below the CCD. Alkenones have been used as far back as the Cretaceous, but the importance of further diagenetic change in older formations has yet to be evaluated. Of all the mineralogic and geochemical markers, it is the latter which promise to be of greatest use. The majority of them express the chemistry of former seas, with temporal variations in this chemistry being immediately valid for all the world's ocean; thus the great significance of chemostratigraphy for correlation. The use of this method, however, is limited by
Modem Stratigraphy
56
Fig. 42. Anoxic deep water (dotted areas) at the Cenomanian-Turonian boundary (After Graciansky et al. 1986)
diagenesis, the influence of which increases with the age of the sediments, decreasing the precision of measurements and, therefore, of correlation.
2.3 Correlation by Paleomagnetism 2.3.1 The Principle The ferromagnetic minerals (magnetite, iron sesquioxide, ilmenomagnetite) possess the property of taking on a magnetism when they are placed in a magnetic field. Included in substances undergoing cooling (lavas), these minerals preserve a thermoremanent magnetism acquired during their cooling at temperatures below their Curie point (578°C for magnetite, 600°C on average for most minerals). These minerals acquire the characteristics of the Earth's magnetic field at that time (intensity, declination lO , inclination), but they are wiped out if the ambient temperature subsequently exceeds the Curie point. On the basis of measurements taken from carefully oriented and dated material of the last 2000 years, it has been possible to show changes of inclination and declination with time. These parameters have varied even during historical time as secular variations of 20° to 30° lOVery close to the direction of the geographic meridian.
57
New Methods of Correlation Age (Mal
Fig. 43. Magnetic reversal scale based on recent volcanic rocks
about an average direction. Having established these temporal changes, it is possible to date archeological material or recent lavas, for example, by measuring their remanent magnetism.
2.3.2 Magnetic Polarity It has been observed that throughout the Quaternary and Tertiary, geological materials such as volcanic rocks have preserved magnetic fields whose directions and inclinations have about the same degree of scatter about the modern values as those values for the last 2000 years. In contrast, the actual direction of magnetization can be either similar to the present (normal magnetization) or opposite (reversed magnetization). These reversals, practically instantaneous on a geological scale, with a duration of only a few thousand years, have been recognized as worldwide synchronous events. Following advances in dating of relatively young rocks by the K-Ar method, it has been possible since 1963 to construct a detailed stratigraphy of magnetic reversals. Four major periods of about 1 million years average duration have been defined. They are, from the youngest, the Brunhes period (normal), the Matuyama (reversed), the Gauss (normal) and the Gilbert (reversed; Fig. 43). Within each major period, shorter periods of reversals less than 10000 years long have also been recognized and given names. These are called "excursions" by anglo-saxon authors and cor-
58
Modem Stratigraphy
respond to shifts of the magnetic pole of more than 45° of latitude in relation to their normal position. Paleomagnetism is also measureable in sediments and sedimentary rocks, where the preserved permanent magnetism can be attributed to three principal phenomena: 1. Orientation of magnetic particles at their deposition or shortly after (detrital remanent magnetism); 2. Magnetization of crystals at their formation during diagenesis or alteration in the Earth's magnetic field (iron oxides and sulphides for example) creating a secondary magnetization (chemical or crystalline remanent magnetism) ; 3. Parasitic magnetization (viscous, anhysteretic, etc.). Only remanent magnetism of the first type (primary signals) can be used as a stratigraphic tool, and using this a succession of magnetic reversals has been constructed similar to that derived from volcanic rocks. The other types of magnetization can be removed in the laboratory by various processes, such as by heating or by applying alternating magnetic fields.
2.3.3 Magnetostratigraphy The utilization of paleomagnetism in stratigraphy is difficult. The Earth's history shows that only two types of magnetic polarity are possible, as against an infinity of other types of events, especially those nonrepetitive events related to biological evolution. A magnetic reversal, therefore, has in itself little chronologic significance, other than allowing conclusions on the different ages of rocks in different locations by virtue of their different polarities or similar polarities but different magnetic declinations. However, stratigraphic information can be greatly improved by reference to a standard paleomagnetic time scale. Figure 44 illustrates such a time scale with the superimposed oceanic magnetic anomalies numbered 1 to M29 and several polarity periods or magnetozones (5 to 22) following the four previously designated (Bruhnes to Gilbert). Since anomalies and magnetic zones are correlated with the radiometric time scale, their relative durations are known and they consequently are valid geochronologic units. In addition, some higher order groupings are apparent, specifically "disturbed" periods with numerous reversals of polarity and "quiet" periods, with mainly normal or mainly reversed polarities. How has this time scale been constructed? The first succession of magnetic reversals was constructed by Cox et al. (1963a,b) for the last 7 million years and based on exposed lavas. This time scale was continued back into the Cenozoic and Mesozoic using positive and negative magnetic anomalies measured on the seafloor. It is known that the latter is formed from a continuous supply of basaltic rocks with ferromagnetic minerals which fossilize the Earth's field as they cool. This takes the paleomagnetic time scale back to the Upper Cretaceous, on the assumption
New Methods of Correlation
59
." ;;)
0 III U ~
~
'"'
III
~
'"
u
>-
• ...l ~
III
•
NORMAL POLARITY
D
REVERSE POLARITY DISTURBED ZONE FROM THE SINEMURIAN AT ~EAST
Fig. 44. Magnetic polarity scale for the Mesozoic and Cenozoic (After Channell 1982; and, for the Jurassic pre-Kimmeridgian, after Kent and Gradstein 1985)
60
Modern Stratigraphy 200m
250
300
-
II
.....0 - - - - -
2
-
-
3 34
...
_
II
K'T
6 2 5 k m - - -...... ~
II
.... 0-----
350 I
I
I
I
1_
I
I
9 5 5 k m - - -.........
III.,. 33
5 1 5 k m - - -.........
4
Fig. 45. Comparison of different polarity sequences established for the Upper Cretaceous. 1 Gubbio limestones (Italy); 2 North Pacific (40 N); 3 North Indian Ocean (88 E); 4 South Atlantic (38 S); (After Channell 1982) 0
34
,-3_3",-_
0
0
of a constant rate (1.9 cm/year) of ocean floor spreading for the South Atlantic l l . This method has two disadvantages: 1. It is difficult to date the ocean basalt anomalies, the potassium-argon method being inaccurate beyond 5m.y. This often makes it impossible to separate radiometrically two adjacent magnetozones. 2. The paleomagnetic signals weaken considerably with the age of the oceanic crust because of the alteration of its upper part. The sequence of paleomagnetic polarities has been subsequently compared with events used in biostratigraphy, such as first and last appearance of taxa, using sections on land and wells drilled in the ocean. These have made it possible to extend backwards the magnetostratigraphic time scale. The Cretaceous has been most studied on land, in central Italy, where it has been possible to correlate the biostratigraphy with the succession of polarities. This vertical sequence is then comparable directly with the horizontal sequence of oceanic magnetic anomalies, to provide them with the same calibration (Fig. 45). The anomalies should also be datable by microfossils in the sediments immediately overlying the basalts, but this is somewhat inaccurate because of the discontinuities which can exist between the basalt and this sediment. The data for the Jurassic come from boreholes in the ocean floor and from sections in Italy and northern Spain. The oldest oceanic data are from anomaly 29 of Oxfordian age. Before that was a long period of stability or weak magnetic field, as in the Middle and Upper Cretaceous, which lasted until the Callovian. This followed a disturbed
11 It has since been shown that this approximation is correct for many sectors of the world's ocean.
New Methods of Correlation
61
Fig. 46. Example of zonation by calcareous nannofossils in the Paleogene, and relation to the chronostratigraphic and magnetostratigraphic scales (After Aubry 1983)
period recognized in recent studies from the Bathonian to the Sinemurian (in Hallam 1984). In summary, magnetostratigraphy appears to be the best method of calibrating biostratigraphic zonation against absolute age derived from the radiometric time scale (Fig. 46). Widely used to date ocean spreading, it can aid considerably in the correlation of offshore wells and sequences on land. Magnetochronology is now being combined with biochronology and radiochronology to establish a unified geologic time scale.
62
Ma
Modem Stratigraphy
Moon stratigraphy
0
1000
COPERNICIAN
1800
ERATOSTHENIAN
3000' IMBRIAN
~OOO
4?1V1
NECTARIAN PRENECTARIAN
Fig. 47. Geologic time scale for the Moon (Van Eysinga 1985)
2.4 Extraterrestrial Correlations The interpretation of images of celestial bodies provided by space probes is based partly on relative dating and correlation of rocks according to their surface morphology. The planetary reliefs all show meteoritic impacts, particularly numerous about 4.5 billion years ago, i.e. very shortly after the creation of the solar system. Since that time, both the frequency of impacts and the size of meteorites have decreased. A stratigraphy based on surface formations is therefore possible for those planets, such as Mars, the Moon, and Mercury, which have experienced little crustal evolution. Two outcrops, for example of volcanic flows, having essentially the same density of large craters per unit surface area are correlatable in the sense that they began being bombarded by meteorites at the same time. Their formation must, therefore, have been also about the same time. For the Moon these observations, together with dating of samples brought back to Earth, have led to the construction of a rudimentary chronostratigraphic scale (Fig. 47).
2.5 Conclusions Geophysics, geochemistry, mineralogy, sedimentary cycles, and geomorphology all now contribute to stratigraphic methodology. This is not to say that they replace or even compete with classical lithostratigraphy and biostratigraphy, which remain the basic tools, but they are an indispensible complement, allowing, for example, the correlation of different paleon-
New Methods of Correlation
63
to logic zonation schemes and the testing of the synchronism of the appearance and disappearance of taxa at different latitudes. For instance, the comparison of various biological events with the paleomagnetic scale shows that many of them are synchronous to about O.1-0.4m.y. According to Johnson and Nigrini (1985), the disappearances of species are much more synchronous than their appearances, based on Cenozoic radiolaria of the Indo-Pacific region. In this way, a more useful biostratigraphy and lithostratigraphy not influenced by facies is slowly being established. Conversely, biostratigraphy can be used to test the value of a physical or chemical marker. This reciprocal control by different methods is one of the more significant factors contributing to the progress in stratigraphic methodology during the last two decades.
Chapter 4
From Stratigraphy to Paleogeography
The principal task of stratigraphy is dating. This allows the correlation of contemporary events, a necessary prerequisite to the reconstruction, chapter by chapter, of the Earth's history. This paleogeographic stage is dominantly spatial rather than temporal.
1 Principles and Methods of Paleogeography Paleogeography aims to paint the successive pictures of the Earth's surface from the beginning of its history. This synthetic discipline is therefore fundamental to geology. In a narrow and classical sense it attempts to trace the former boundaries between land and oceans and to reconstruct the vanished continental surfaces with regard to their topography, climates, life, and geodynamics. Paleogeography in the wider sense is also concerned with the oceans and their sedimentation, currents, depths, chemistry and living components. To achieve its synthetic purpose, paleogeography must extract information from the lithological and paleontological documents at its disposal in order to interpret particular environments of deposition.
1.1 Facies This term, introduced by Gressly in 1838, refers to all the physical, chemical and biological characteristics of a sedimentary rock reflecting its depositional environment. Thus, a facies (or isopic) map is implicitly a paleoenvironmental map. Lithofacies and biofacies represent often the two major components of a facies, the one physical and chemical, the other biological (fossils and/or traces). For example: 1. The Triassic red sandstones of the Vosges have characteristics which suggest a slightly inclined alluvial plain, with meandering rivers and sparse vegetation (Voitzia, Equisetum), and a climate like modern Sudan with contrasting wet (when silicate iron dissolves out) and dry (when ferric iron precipitates on sand grains) seasons.
From Stratigraphy to Paleogeography
66
EC9 Tithonian facies
Transltlonallacles
I;:;·.~.:J
'Calcaires blancs de Provence'
o
Gravelly limestone Pelagic Neocomlan
o
II7a1 High energy biohermalfacles ~
B
White limestones with green clay interbeds
Neritic Neocomian
Fig, 48. 1 Geometric relations of different facies near the Jurassic-Cretaceous boundary (dashed line) in Haute-Provence (Castellane region). 2 Restored section for the beginning of the Cretaceous, based on facies interpretation. Note the platform carbonate to basin transition (After Cot ilion 1975)
2. The Quaternary sapropels of the eastern Mediterranean (Chap. 3, Sect. 2.2.4.3) consist of dark shaley beds, decimetric, laminated, with organic material and planktonic remains, alternating with calcareous muds. Detailed analyses, including geochemical, of their lithofacies and biofacies allow precise conclusions to be drawn with respect to their environment of deposition: marine waters, stratified due to lack of vertical mixing caused by a hyposaline surface layerl. The deep water becomes therefore anoxic, so preserving the organic matter. The names given to facies show clearly their close relationships with the environment. They may refer to a place characterized by a particular type of deposition (Germanic facies of the Trias, Dauphinois facies of the Jurassic) or they may refer to a particular environment (reef facies, pelagic facies). Thus the first step in paleogeography is the analysis of rocks as indicators of the environment. The second step is the recording of all contemporary facies in a region and an examination of their mutual relations, the purpose being to reconstruct a terrestrial or submarine landscape; for example, the transition from the "Calcaires Blancs" of Provence to the Tithonian limestones of Haute-Provence in the Portlandian-Berriasian (Fig. 48). The geometric rela1 This freshening comes from continental waters brought by the Nile during climatically wet periods.
Principles and Methods of Paleogeography
67
tions between these two facies show that the transition from one to the other is both gradational and by interdigitations. The change of facies along a N-S cross-section suggests the transition from a platform to a basin with significant bathymetric variations. In addition, the subfacies within the "Calcaires Blancs" enable variations in the environment of the platform to be mapped.
1.2 Paleobiogeography Contributing to the knowledge of the environment (water depth, oxygenation, temperature, climate etc.), the biofacies is a fundamental element in paleogeographic syntheses. It provides information especially on the distribution of biological populations, the basis of paleobiogeography, whose aim is the study of the relations between the evolution of life and the evolution of the Earth. The first paleobiogeographic synthesis (Neumayr 1872) involved Jurassic rocks and introduced the idea of faunal provinces, generally very large regions often containing smaller areas defined by the occurrence of different taxa. Uhlig (1911), for example, distinguishes in the northern hemisphere in the Mesozoic: 1. A temperate zone in the north, or Boreal province. 2. A warm zone in the south, or Tethyan province, subdivided into several subzones (Mediterranean, Caucasian, sub-Mediterranean, Himalayan, Japanese, Pacific)2. None of these regions can be defined precisely because of faunal exchange across their boundaries. Western Europe was often the locus of such exchanges, espcially in the Toarcian, Callovian and Neocomian. These events complicate, as we have seen, the problems of time correlation but they do allow correlation between adjacent provinces with different biostratigraphic zonations. They also help to differentiate certain paleogeographic domains: for example in the Callovian-Oxfordian, the faunal expansions of boreal origin never crossed the north Tethyan boundary in Europe (Cariou et al. 1985) which is, therefore, identifiable by its immigrant faunas. For much older epochs, the faunal distributions may not always be related to climatic gradients; this could be the case, for example, for the two major trilobite provinces of the Lower Cambrian (Fig. 49), the Olenellus province of North America, Scotland, and Scandanavia and the Redlichia province of Asia and Australia.
The paleobiogeography helps to verify the paleogeographic reconstructions deduced from continental movements, which must be compatible with the known migration paths of this or that species, suggesting, for example, is now common to assume for the Jurassic a Boreal domain, a Tethyan domain (with Mediterranean, Indo-Southwest Pacific and East Pacific provinces), and from the Tithonian an Austral domain (Enay 1980). 2 It
68
From Stratigraphy to Paleogeography
Fig. 49. Faunal provinces in the Lower Cambrian, from trilobite distributions. The arrows indicate the possible exchange of faunas between provinces. Land indicated by cross-hatching (After Cowie 1971 in Pomerol and Babin 1977)
barriers or freeways. Several periods in the Albian - Turonian have been proposed for the opening of the communication between the North and South Atlantic oceans. Moullade and Guerin (1982) prove, using benthonic and planktonic foraminifera, that the South Atlantic was clearly open to the Central Atlantic from the Middle Albian (Fig. 50). The warm water Tethyan species are present at this time in the northern part of the South Atlantic but disappear to the south, due without doubt to the cooler waters. This shows that the only possible migration route was north-south from the Central Atlantic and not from the south via an eastern route around Africa. Moreover, the presence of certain planktonic forms shows that the arm of the sea separating Africa and South America was at least lS0-200m deep in the narrowest part. We should note, however, that the propagation of even planktonic species does not always require open ocean. According to Enay (1980), an opening can be preceded by faunal exchanges across shallow epicontinental waters. Thus, from the Lower Jurassic "pre-oceanic freeways" above a sialic basement may have linked the Tethyan and East Pacific oceans.
1.3 Cartographic Syntheses Just as geography cannot be conceived without cartographic illustrations, so paleogeography is usually illustrated by synthetic maps of variable scales, including, for different epochs, the maximum of information on continental
69
Principles and Methods of Paleogeography
\(
~
30
30
•• 60
Fig. 50. Migration of Central Atlantic foraminifera to the South Atlantic in the Albian. Points represent studied wells (After Moullade and Guerin 1982); arrow denotes direction of migration
and marine distributions and their boundaries. Paleogeographic maps have been improved over time, but whatever their degree of perfection, their image of the globe is never instantaneous but is always the average of a series of superimposed images representing a certain interval of time. The more recent the time and the larger the scale, the smaller need be the interval of time illustrated.
1.3.1 Facies Maps They are the basis of paleogeographic maps. They represent the lateral distributions of the different facies (therefore different environments) of chrono- or biostratigraphic units (stages, zones). Marine and continental deposits are clearly differentiated and their boundaries sometimes delineated (Fig. 51), thus representing a transition towards a true paleogeographic map. The facies boundaries do not necessarily correspond to depositional boundaries, but often to erosional boundaries as seen in the Upper Jurassic formations which surround the French Massif Central today, but probably covered it entirely at the time of deposition. Isopachous contours indicating the thickness of various formations may be superimposed on the facies maps. For regions which have been strongly deformed tectonically, it is often useful to restore the facies to their pre tectonic
70
From Stratigraphy to Paleogeography
Fig. 51. Facies map of the Upper Cretaceous showing the distribution of continental basins (vertical hachuring) and the maximum extension of marine sediments (dashed line and dotted area) in the Rhodanian basin (After Debrand-Passard et al. 1984)
positions. The maps resulting from this unfolding are palinspastic maps (Fig. 52).
1.3.2 Paleogeographic Maps These are interpretive maps showing basically the boundaries between oceans and continents, but may include continental relief, ocean depths, shoreline movements, faunal migrations, tectonic movements, provenance of sediment, or sediment transport directions. On the continents, the zones of deposition with their facies and the zones of erosion with their lithology and age may also be indicated. For example, in southeastern France, basement movements of Middle Cretaceous age have caused the erosion of Permian rhyolites and the deposition of bipyramidal quartz derived from the rhyolites. Formerly, because of insufficient data to determine the positions of the major cratons, the paleogeographic maps were based on the present distribution of land and sea (Fig. 53). The interpreted shorelines were shown, therefore, as landward encroachments more or less beyond the present coastline or conversely exposing parts of the continental platform. Such simple distinctions obviously could not represent all the observed sedimen-
71
Principles and Methods of Paleogeography
Fig. 52. Palinspastic map of the western Alps in the Senonian (After Ricou 1984)
ft~t~!&J
Continental areas
o
Dinosaurian fields
Marine area
Fig. 53. Paleogeographic map of the Upper Jurassic showing the uncertainty regarding the distribution of oceans of that time (After Furon 1972)
tary and tectonic phenomena. For example, when some detrital sediments implied the existence of some offshore continental source, subsequently submerged beneath the ocean, this "source land" could be located and its extent determined only with great difficulty. Moreover, the great distances
72
From Stratigraphy to Paleogeography
between continents bearing identical biological communities posed the problem of migration across vast oceans by swimming, or on rafts of vegetation, or via intercontinental bridges, somewhat as a present-day isthmus allows the exchange of faunas between emerged lands. In a general way it was difficult to imagine, assuming the fixed nature of continents, the paleogeography of areas corresponding to the large oceans of today (Fig. 53). Since the 1960s, the petroleum exploration of the continental margins, the theory of plate tectonics and deep ocean drilling have revolutionized the way of paleogeographical expression which is now presented as global maps on which the ocean floors (continental platforms, slopes and abyssal plains) are given as much significance as the continents. The boundaries between land and sea are placed in their most probable positions for each epoch of geologic history (Fig. 54). From the present day back to the Middle Jurassic, the successive geographies have been deduced from the following: 1. Paleomagnetic data, collected on land as well as from submarine oceanic crust. The magnetic declinations measured on different continents allow the reconstruction of their relative positions at different epochs, and the magnetic inclinations their latitudinal positions. 2. Unfolding of tectonically deformed continental crust. For older periods only continental data can be used, because the corresponding oceanic crust has disappeared by subduction or has been transported as blocks on to the edge of cratons (obduction). According to whether an ocean is now bounded by a stable or an unstable margin, estimates of its original position at different epochs can vary between 100 to 1000 km (Dercourt 1984). These estimations also depend on the particular sequence of paleogeographic evolution chosen from all the possible sequences: once constructed, the global paleogeographic maps will show gaps which correspond generally to vanished oceanic segments, like the Tethys, the only evidence for which are the ophiolitic sutures (Fig. 111). Other information helpful in the localization of oceans and continents are the following: 1. The paleo depths of oceanic domains are calculated on the basis of the subsidence curve for the ocean floor basalts as a function of time and thickness of overlying sediments. 2. The rate of detrital deposition within or marginal to the craton yields information on the extent of continental relief. 3. The tracing of former shorelines proceeds from two approaches: a) From the character of the most landward deposited marine facies, it is possible to determine whether the shoreline was close to or well beyond the present outcrop limits of this facies (Fig. 55). b) By examination of the eustatic curves (Vail et al. 1977; Hallam 1978) some corrections of estimates based on the first approach are possible (Fig. 56).
73
Principles and Methods of Paleogeography
Emerged lands
ti"jlTj.YJ
Oceanic crust
Thick continental crust Platform limestone
:: >..':' Sand lone
AI: Albonn
=ffi..
Thinned oceanic CruSI
Pelagic limes lone
B : B~onnai.
MAa : Middle A.I_.pine
SI: Sila
t:X·.:. f
AClive Ridge
ST) : Stilo
K : Klbylil
-
Fault
.=-.=- = L:
~ Ophiolites obdUction
Clay
Lom_
101M : Morroclll 101....
TA: Tatra.
~!aJeolatilude
TR : Triclentin
LA.: Lower AUlbo.aJpine OM: OrIll... M.....
UAa :. Upper Au._a1pin.
VL: Valoi.
Fig. 54. Example of a global paleogeographic map at the Jurassic-Cretaceous boundary (130m.y.; Dercourt et al. 1985). A relatively narrow Tethys separated Africa and Europe, partly covered by an epicontinental sea. To the south of Spain, the ocean was reduced to a narrow trough whose dynamics reflected the sinistral movement of Africa
4. The percentage of land covered by sea can lead theoretically to a calculation of the altitude of sea level in relation to the present day (Fig. 57). In fact, knowledge of the exact part played by eustasy in determining shoreline positions is necessary, since the latter is also controlled by subsidence, sediment compaction, and tectonic movements. 5. The oceanic and atmospheric paleocirculations depend on two factors: a) The general global climate: the distribution of facies on a global scale gives a good illustration of this because it indicates the significance of latitudinal temperature gradients, which control the major circulations of air and water. b) The distributions of land and sea on the Earth's surface. Taking into account these distributions, several models have attempted to recreate
From Stratigraphy to Paleogeography
74
•
..
.
• 0'
:::::', ','. : .. :.. " . ~.
, ..... :.
... . . . :,': : : : .. . '. ,. · :., :'.. :: .:: ....... 0°,·,' •
'.'
','
,"'
· ·.··.·.:·:.:·.,',0',.,' ,' ....... ' ... ' ..........
....... .. '
•
'
.'
: ,·0
.
•
Fig. 55. Paleogeographic map of the Oxfordian constructed from outcrops on the continents (after Hallam 1975). Interpreted area of continental inundation is probably maximal, especially in Europe
Sea level
•
Tithonian Kimmeridgian Oxfordian Callovian Bathonian
?
Bajocian Aalenian Toarcian Pliensbachian Sin em urian Hettangian
Fig. 56. Variation of average sea level on the continents in the Jurassic (After Hallam 1978)
75
Factors of Paleogeographic Evolution
E 8000 .~ 6000 "C
.a ~
Fig. 57. Hypsometric curve representing the percentage area of land above a given sea level. A Present area; B area in the Upper Cretaceous
E .S
g.
.t::.
Present marine 0
2000 4000 6000 8000
0
0 10
30
50
70
the circulations on the basis of locations of supposed high- and lowpressure atmospheric zones (Fig. 58). It appears that in the Cretaceous, the traditionally assumed relation between low thermal gradients and slowness of current flow is doubtful, the atmospheric circulation appearing not to be too low at this time (Parrish et al. 1982).
1.3.3 Mapping of Volcanics One particular case of cartographic synthesis concerns rapidly evolving volcanic zones. A computer-aided evolutive cartography at a scale of 1 :25000 has recently been produced by the BRGM (Bureau de recherches geologiques et minieres) for Piton de la Fournaise (Reunion island). These data will be used for a survey of modern volcanic activity.
2 Factors of Paleogeographic Evolution The face of the Earth, represented cartographically, is continually modified through time. This evolution results from the interaction between two dynamics: one, internal and manifested by a continual deformation of the lithosphere, the other external, shaping the surface of the Earth through its enveloping fluids and biosphere.
2.1 Deformation of the Lithosphere 2.1.1 Plate Tectonics This is the essential agent of paleogeographic evolution. It is the basis of orography, whose main characteristics, continental and oceanic rifts, transcurrent faults, accretionary prisms, subduction trenches and mountain chains are associated with plate boundaries. In addition, the global tectonics
From Stratigraphy to Paleogeography
76
Conttnents and epicontinental seas Upwelling areas
H: High pressures
lID
Maln'eontlnental rellets
L' Low pressures
_
W ni d trend
Fig. 58. Model of winter atmospheric circulation in the North Hemisphere in the Tithonian (After Parrish and Curtis 1982)
control the distributions of continents around the Earth, which in turn control the oceanic circulation and the climate. Geologic history seems to indicate periods of dispersion of continental crust and periods of assembly, ending in the formation of supercontinents of the "Pangea" type (Wilson cycles). Two cycles have been recognized within the last 900 million years; their cause probably has to be found in the asthenosphere or deeper within the Earth (variations of heat flow). The consequences of such cycles are fundamental for the history of the Earth, because the creation of Pangea was generally associated with a lower sea level and a deteriorating climate, marked by the appearance of ice sheets and profound changes in the course of biological evolution .
2.1.2 Epeirogenic Movements These are vertical movements of the lithosphere, with no apparent relation to orogeny but without doubt also linked to variations in heat flow. When the latter increases, the lithosphere dilates, becomes less dense and rises (thermal updoming). The opposite effects occur when the heat flow decreases (thermal subsidence). The structures seen as broad cratonic domes and basins could be of this origin. For example, in central Siberia, a dome brings up the basement to an elevation of 1700m, while the Amazonian Shield is depressed into as immense synclinal basin. Thermal subsidence also plays a role in epeirogenesis, as defined above. It effects not only all parts of the progressively cooling lithosphere, especially the oceanic crust just after
Factors of Paleogeographic Evolution
77
its formation, but also the continental margins and cratonic regions. It also influences the loci of sedimentary basins and their evolution. Finally, epeirogenesis appears also to effect continents as a group. According to Worsley et al. (1984), the thermal tumescence of continental masses, which are weak conductors of heat, is proportional to their surface areas and inversely proportional to their rate of displacement above the asthenosphere. Under these conditions, the supercontinents of the Pangea type would not be invaded by seas because of their relatively high mean elevations. Epeirogenic movements result also from isostatic adjustments. The lithosphere, overloaded by an ice sheet, sediments or lavas, sinks into the asthenosphere, indicating that a part of the subsidence of basins is due to the weight of sediments. Periodic volcanic eruptions (the Hawaiian islands, for example) similarly create a sagging of the lithosphere, thereby reducing a part of the former relief. Conversely, a lessening of the load on the lithosphere leads to its uplift. When the Wiirm ice sheet covering northern Europe began to melt, Scandinavia rose. This movement began 12000 years ago and continues today. The maximum uplift, centred on the Gulf of Bothnia, reached about 400m. Unloading of the lithosphere can also result from erosion of its upper part. Since the intensity of this process is generally a function of its elevation, it follows that an elevation of the lithosphere by thermal tumescence will tend to increase the effect of erosion.
2.2 Volcanic Eruptions Continental volcanism is the cause of some of the highest mountains of the Earth (Kilimandjaro in Africa of nearly 6OO0m; Aconcagua in the Andes, 7026m). Enormous flows have built great basaltic piles (200000km 2 wide and 2000 m thick) in the Upper Cretaceous Dekkan plateau of India and massive trachyandesites in Chile and Patagonia (700 000 km2 wide and several thousands of meters thick). Oceanic volcanism is active in the zones of plate divergence, building the great mid-oceanic ridges, a major morphologic feature of the oceans. Their length totals about 60000km, with a width of l000-3000km and an average height of 2000 m. These ridges are cut by transform faults. Apart from ridges and volcanic arcs associated with subduction, oceanic magmatism is rare. The alignment of volcanic belts parallel to the direction of movement of the plate carrying them suggests the presence of a hot spot beneath the plate. This is the explanation for the Hawaian islands.
2.3 Interplay of Erosion and Sedimentation All topographic relief is inexorably doomed to destruction by erosion. The products of erosion are then carried by several agents, the main one being gravity, towards the low areas of the Earth's crust, i.e. the oceans, where
78
From Stratigraphy to Paleogeography
they are deposited. This continental transfer of material from the continents to the oceans should eventually flatten the continents. It has been calculated that in only 50 million years the continents would disappear in spite of isostatic readjustments due to this unloading. Such a scenario appears to be reflected in the peneplains which are developed at the end of orogenic cycles, before the transgression which initiates the following cycle advances over the erosional surface. However, as long as the internal motor of the Earth remains active and therefore capable of deforming its crust, the destruction of all continental relief can only be transitory for any given area and cannot be a condition of the whole globe at anyone time. Submarine relief also seems to be largely due to mechanical and chemical erosion, as direct observations from submersibles have recently revealed. Sedimentation acts in a sense opposite to that of erosion. It fills basins and can in some situations lead to emergence. In the marine environment, this may occur only in the relatively shallow epicontinental regions. This is illustrated in nearshore areas by the progradation of deltas and coastal spits and the construction of barrier reefs. Beyond the continental platform, sedimentation plays an important morphologic role at the base of the continental slope, where submarine deltas are built in front of large rivers.
2.4 Eustasy 2.4.1 Paleogeographic Effects Whatever their causes, variations of sea level have profound repercussions, especially on the continental margins, although the shifts of paleoshorelines are generally due more to local or regional deformation of the cratonic margin than to eustatic fluctuations. Drowning terrestrial topography, and especially the low-lying valleys, a marine transgression creates an incised coastline with isolated islands. Regressions are characterized by smoother coastlines. The degree of immersion of the continent controls the base level of the fluvial system, and, therefore, the interplay of erosion and sedimentation, both continental and marine. A high sea level decreases erosion in the low fluvial valleys and increases the sedimentation on the continental shelf; a low sea level causes erosion of the shelf and the terrigenous products are transferred directly to the base of the slope, where they accumulate as submarine deltas or may reach as far as the abyssal plain (Fig. 59). The variations in sea level also affect oceanic circulation. A eustatic rise creates new communications by drowning sills and highs. It therefore facilitates interprovincial exchanges. A eustatic drop tends on the contrary to isolate basins from each other and may cause the development of restricted, brackish or hypersaline environments. Paleobiogeographic evolution is directly affected by these variations.
79
Factors of Paleogeographic Evolution
Low sedimentation rate
Fig. 59. Influence of eustacy on oceanic detrital sedimentation. 1 High sea level; 2 low sea level
Finally, eustasy controls to some extent the global climate. Transgressions increase the ocean's surface and decrease the average albedo of the Earth, thereby creating a warmer and more humid climate (if other factors remain the same). Regressions have the opposite effects. The Jurassic and Cretaceous, characterized by two major transgressions, were generally warm and humid periods. On the other hand, the end of the Cretaceous, marked by a major regression, was typically cold and dry. The end of the Jurassic, which also coincided with a regression, but of less significance, was also a time of relatively dry conditions, according to Hallam (1984).
2.4.2 Causes of Eustasy The curve according to Vail et aI. (1977), which attempts to represent eustatic variations (see above, Fig. 30), can be useful in distinguishing the principal factors involved. This curve consists of cycles of several orders:
1. First order cycles of very long periods (200-400m.y.), of which there are only two from the end of the Precambrian to the Present. They are parallel to the Wilson cycles and consequently follow the rhythm of the contraction and dispersion of continental crust. 2. Second order asymmetric cycles of 10-100m.y., taken generally from the major subdivisions of geologic time. Their limits coincide, therefore, with several of the discontinuities separating the systems and subsystems of the stratigraphic scale. 3. Third and fourth orders cycles of 1-10 and of 10000-lm.y. The larger
80
From Stratigraphy to Paleogeography
cycles are asymmetric and may correspond to stages, especially in the Jurassic and Tertiary. The smaller cycles can be seen from the end of the Miocene (Messinian) to the Present. This curve has been revised by Haq et al. (1987). Three major types of causes of eustasy are generally accepted: 1. Geotectonic causes. All are effective in modifying the volume of oceans. They can, according to Vail, be applied to the first-, second- or thirdorder cycles, at least in part. For example, the orogenies resulting from plate convergence induce a shortening of the continental crust with a corresponding increase of ocean widths. If the ocean volume remains constant, this must result in slightly lowered sea level. This effect is obviously greater when the continents are assembled into a supercontinent, when the thermal updoming is also at a maximum (see above). Consequently, the formation of a Pangea is always accompanied by a very low sea level. Conversely, cratonic tensional regimes, while creating sedimentary basins and rifts, reduce the areas of the oceans and therefore raise sea level. According to Bureau (1985), these tensional effects alone would have caused a eustatic rise of about 80 m in the Mesozoic. Variations in mid-oceanic ridge activity also modify the ocean floor morphology and, therefore, the space available for the ocean (Pitman 1978). The marked expansion of the basaltic crust results in a growth of ridge volume and a rise in sea level 3 . This was the situation in the Upper Jurassic and in the middle of the Upper Cretaceous. Some authors do not accept the global character of Vail's secondorder cycles, suggesting that they result from an interaction between firstorder cycles and local deformation, thermal subsidence of continental margins, flexure of the lithosphere etc. 2. Climatic causes vary the volume of ocean waters much more abruptly than magmatism and are at least partly responsible for the third- and fourth-order cycles. During glacial periods, water is bound as ice at the poles, lowering sea level. The alternation of glacial and interglacial periods, well known since the Miocene, thus induces eustatic cycles. Recent work has concluded that such cycles have existed since the Trias, when the North Pole was located to the northeast of Siberia. It should be added that the ocean volume depends also on temperature, a drop of 1°C resulting theoretically in a 1 m drop in sea level. 3. Causes related to deformation of the geoid. Of variable origins (coremantle relations, distribution of oceanic masses, variations of gravity and of global rotation, etc.), these deformations can cause local eustatic variations so that a transgression in one part of the globe could correspond to a regression in another part (Marner 1981). 3 According
to Pitman, a variation of 3 cm/year at a rate of expansion along a ridge 40 000 km long can raise sea level by 1 cm/lOOO years.
81
Factors of Paleogeographic Evolution 90W
Fig. 60. Apparent displacement of the North Pole relative to Europe (solid squares), and North America (solid circles) from the Cambrian to the Present (after McElhinny 1973). Cb Cambrian; S Silurian; D Devonian; Cl Lower Carboniferous; Cu Upper Carboniferous; P, Permian; Tr Triassic; Trl Lower Triassic; Tru Upper Triassic; J Jurassic; K Cretaceous
In summary, the paleogeographic evolution produced by transgressions and regressions represents a complex interplay between eustasy and vertical movements of the continental crust (subsidence, isostasy, epeirogenesis, tectonism), and even of the sediments (compaction).
2.5 Polar Wandering We have seen above that the directions and inclinations of paleomagnetic fields vary within certain constant limits about a mean value, the mean declination giving the direction of the geographic poles. It is therefore assumed that the magnetic and geographic poles have always coincided throughout geologic time. The numerous paleomagnetic measurements made all over the world since the 1950s have shown that, after making allowance for continental movements, the poles must have drifted slowly at an average rate of 4 cm per year since the Precambrian. The polar wandering curves are peculiar to each continent because of the latter's relative movement (Fig. 60). They record the major geodynamic phenomena. Thus, the northern polar wandering curve, seen from North America, changed its direction between 120 and 50m.y. ago and also around 200m.y. ago. The Laramide and Nevadan orogenic phases may be related respectively to these changes. The mobility of the poles, in addition to that of the continents, has played a very important role in regional climatic change because it has resulted in the displacement of the latitudinal climatic zones, especially the
82
From Stratigraphy to Paleogeography
evaporite zone indicative of aridity and high temperatures. It is known that this zone moved across much of Europe during the Mesozoic (see below). Pole migration is, in fact, only apparent in so far as the Earth's mean rotational axis is considered stable with respect to the Earth as a whole. It is therefore necessary to assume that the totality of the lithosphere is mobile and can move with respect to the deeper zones of the Earth.
2.6 Conclusions: the Earth in Relation to Other Planets of the Solar System The paleogeographic changes evident at the Earth's surface are characteristic of a planet often called "living", not by reference to its biosphere, but rather to the dynamic nature of each of its envelopes. This leads to continual modification of its surface and its climate. The energy needed for this is of thermal origin, coming mostly from internal sources and partly from the sun (atmospheric and hydrospheric movements). Terrestrial relief, therefore, does not depend on external agents such as meteorites, which cannot leave any durable imprint on the surface. Other planets of the solar system, especially the inner "telluric" planets, owe their surface morphology primarily to very intense crater-forming impacts at approximately 4600m.y., i.e. at the end of the period of planetary accretion. Mercury is most typical; being very close to the sun, it lost its fluid envelopes very soon and preserved its primitive surface, pockmarked by large craters. On the Moon, also lacking water and an atmosphere, volcanic outpourings of lava testifying to an internal activity have locally masked its primitive surface. Mars is much more like the Earth, but tectonism there has remained somewhat rudimentary, partly due to a very thick lithosphere (200km). However, erosion and deposition due to wind and runoff, as well as to volcanic flows, have destroyed the initial relief of its surface. The Earth long ago lost all trace of its primitive morphology. It is also the only planet (together with Venus?) manifesting the processes of plate tectonics.
Chapter 5
The Major Stages of Earth History
1 Precambrian Time We will define the Precambrian as the period of time from the formation of the Earth to the lower boundary of the Cambrian, although a narrower definition starts the Precambrian from the first dated rocks (3800m.y.). This traditional term is not the best because it does not fit with the names of later eras, Paleozoic, Mesozoic, Cenozoic. Thus, some people prefer the term Archeozoic. The Precambrian is five times as long as all the other eras combined, which are commonly grouped into a single chronostratigraphic higher-order unit (eon) named the Phanerozoic. The Precambrian should, therefore, be regarded as the first, or even first two eons of geologic history.
1.1 Boundaries and Subdivisions The Precambrian can be subdivided according to two concepts (Fig. 61): 1. Classic concept. According to this concept, the Precambrian covers two eons: the Archean, including in North America very metamorphosed rocks, bounded at the top by a major break at 2S00-2600m.y. marking the end of strong tectonic and magnetic activity recognizable on all the shields; then the Proterozoic (from the Greek Proteros, "first"), discordant on the Archean and much less metamorphosed. The Proterozoic itself includes several units corresponding to orogenic cycles. These units are often named according to local terms and therefore are equivalent to formations. At the top of the Proterozoic is a unit, the Eocambrian, which is equivalent to a part of the French Infracambrian. Its lower part (600 m. y.) coincides with glacial formations and its upper part is overlain by the first Cambrian beds with trilobites. The Eocambrian contains the first remains of metazoa with mineralized skeletons, shells, spicules, etc. 2. The concept of Salop (1979). In this concept, the term Archean disappears because it is too poorly defined at the base. It is integrated partially into a very large unit, the Protozoic, or rocks of primitive life,
84
The Major Stages of Earth History
o
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USA Mexico
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Late Proterozoic
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Daslandian
France
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Pharusian
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Ven· dian
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Late Middle Early Proterozoic :t.rchean~rcheanl
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Svecocarelian
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Neoprotozoic
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Fig. 61. Some stratigraphic scales of the Precambrian now in use
which is subdivided into Paleo-, Meso-, Neo-, and Epiprotozoic, defined by orogenic megacycles. Units older than 3500m.y., which seems to be the time of a major break, are placed in a new eon, the Eozoic (dawn of life), the beginning of which is difficult to define precisely but whose termination is defined by the era called Katarchean (lit. "below the Archean"). Finally, the Eocambrian can be placed in the Phanerozoic (organized life) since it already contains the remains of complex organisms lacking a skeleton or shell. Moreover, this unit coincides with the beginning of a major transgression which culminates in the Cambrian. These two contrasting stratigraphic concepts are mentioned in order to show the complexity in the study of this initial period of Earth history, which is not only the longest but also the poorest in decipherable records.
85
Precambrian Time
1.2 Methods of Study These illustrate the special situation of the Precambrian among geological formations. The problems inherent to the study of these rocks derive from their composition, their correlation, and the difficulty in applying the principle of uniformitarianism to them. The oldest deposits of the Earth's crust were intensely altered during several orogenies. They include crystalline schists, more or less metamorphosed, migmatites, and granites, all strongly deformed and recrystallized. In these conditions the criteria for determination of upper and lower parts of a layer do not exist; therefore, the fundamental stratigraphic principle of superposition is often not applicable. However, even though badly altered, many Precambrian rocks still preserve important primary structures such as grading, erosion marks, current ripples, oblique stratification, cross-bedding, and even certain sedimentary cycles. Correlation of such sequences is always difficult because they are generally azoic (originally, or due to metamorphism) or they lack organisms having any stratigraphic value. The only fossils usable are stromatolites and certain unicellular organisms (see below). Moreover, many Precambrian cratons formed continental blocks which were subsequently fragmented during the course of several orogenies. The historical reconstructions of the separate parts of these gigantic puzzles are not easily correlated with one another. Also, generally speaking, the only practical correlations are lithostratigraphic, although it has been necessary to adapt the method to metamorphic rocks by assuming that the alterations were isochemical, i.e. they preserve the geochemical identity of successive beds which can therefore be distinguished by their compositions. In the old shields, the most obvious stratigraphic breaks generally mark the end of orogenic cycles and separate units of different metamorphic grade. They have allowed the recognition so far of 16 orogenic cycles in the entire Precambrian. The basic data of relative age associated with these breaks must first be established in the field, especially discontinuity surfaces and plutonic intrusions. In thin section, the successive tectonic phases are interpreted from successive recrystallizations, which also help to establish the timing of plutonic emplacements. These different methods lead to a regional lithostratigraphy which can be applied in anyone region as far as tectonic structures can be continuously traced. If, however, it is necessary to correlate separate and structurally different domains, then radiochronology must be used. Three methods are commonly used for the Precambrian: Rubidium-Strontinum (87Rb/86Sr) on whole rock or micas; Potassium-Argon (40K/40Ar) on whole rock or on minerals rich in potassium; and Uranium-Lead or Thorium-Lead 38UI 206Pb, 235Upo7Pb, 232ThPo8Pb) on zircons (ZrS04) and apatites (Ca5(P04hOH, F, CI). The use of these three methods on the same sample makes it possible to date with an error of 2-3% the formation of the
e
The Major Stages of Earth History
86
o
Phanerozoic
_
Proterozoic Archean
fZ:iJ
Fig. 62. Distribution of Precambrian cratons of Pangea in the Permian (After Windley 1984)
igneous rock and sometimes the phases of deformation and metamorphism. In practice, only coarsely crystalline and unaltered igneous rocks will give reliable results . As information on the Precambrian accumulates, the difficulty of a strict application of the principle of uniformitarianism for a period of time so distant becomes increasingly apparent. The laws of physics have remained the same since the formation of the Earth, but the conditions of their application have changed considerably . Thus the atmosphere, the hydrosphere, erosion, weathering and the entire dynamics of the lithosphere were very different in the Precambrian from what they are today. Their evolution to the present situation occurred either gradually or sporadically, with certain stages marked by metal concentrations, first red beds (iron oxide), first ophiolites, first organized metazoa, weathering surfaces, igneous eruptions of particular compositions, glacial formations, etc.
1.3 The Geography of the Precambrian The Precambrian terranes constitute the backbones of the eXlstmg continents. Their outcrops define the shields which are covered peripherally by weakly deformed Phanerozoic rocks. These peripheral zones, where the Precambrian basement is accessible only by drilling, correspond to the platforms. In addition, some more or less important elements of the basement are reactivated in various orogenies postdating the Precambrian. Examples are known from Scotland, Bohemia, Spain, and France, where they occur in the
Precambrian Time
87
Fig. 63. Section in the Grand Canyon of Colorado (after Pomerol and Babin 1977) . I Basalts and diabases; 2 Hotanta conglomerate; 3 Bass limestone; 4 Hakatai variegated shale; 5 Shinumo quartzite; 6 Dox stromatolitic sandstone; 7 Chuar shale; 8 Tapeats sandstone; 9 Bright Angel trilobite shale; 10 Muav limestone; I I Temple Butte lenticular limestone; 12 Red Wall Productus limestone; 13 Supai sandstone; 14 Hermit shale; 15 Coconino sandstone; 16 Kaikab fusulinid limestone
majority of the old massifs. The map in Fig. 62 shows the distribution of shields and platforms in the Permian Pangea. A famous section is that of the Grand Canyon of Colorado (Fig. 63) where two major unconformities are visible. One between the Archean and the Proterozoic, the other between the Proterozoic and the Cambrian (Huronian unconformity).
1.4 Early Segregation and Establishment of Fundamental Processes 1.4.1 The First Crust Once the original material of the Earth had accreted and become heated internally by the energy derived from gravitation, radioactivity and meteorite impacts, the principal elements and minerals were segregated by density to form the different layers of the Earth. Water and gases were expelled to the surface, where they formed the primitive hydrosphere and atmosphere, the latter replacing the essentially hydrogen-rich initial atmosphere. In contrast, the heaviest elements sank to the centre of the Earth where they formed the dense nickel-iron core. Between the atmosphere and the core, a mantle of iron and magnesium alumino-silicates was formed, with an upper silica-rich part which became differentiated by partial fusion and cooling into a crust
88
The Major Stages of Earth History
composed largely of the lighter elements such as Si, K, Na and Ca. The subsequent history is still being debated. One possible scenario, according to Kroner (1984) is as follows. From the stage when a crust was established above a mantle, an embryonic plate tectonics began to operate. The mantle, undoubtedly hotter than at present, would be stirred by vigorous convection currents. At the surface where these currents emerged, the crust would be thinned and broken into rigid fragments, much like one sees today in lava lakes. These plates would be rapidly recycled in the underlying mantle, into which they would sink by virtue of their high density, or under the impact of meteorites whose maximum effect was between 4500 and 4000m.y. From 4000m.y. the heat flux and temperature of the mantle diminished, the primitive crust therefore persisted longer, and the volcanism resulting from partial melting of the upper mantle was able to thicken certain plates and thus make them more buoyant. The first silica crust, composed essentially of granitoid plutons, then developed progressively at the expense of subcrustal magmatic differentiations. The end result was a relatively light crust composed of two constituents: at the base, high-grade gneiss complexes and at the top, a mixture of volcanics and granitoid intrusions. When this crust emerged above sea level, erosion produced the first sediments, some of which are believed to be still extant: for example, the gneisses of the Limpopo belt in South Africa, dated at 3800m.y. (Fig. 64). The evolution of the continental nuclei into blocks too light to be assimilated into the mantle occurred slowly by vertical accretion, leading to thickening of their roots. This scenario therefore suggests vertical accretion as the mechanism for the formation of primitive continental crust, which was perhaps 25 - 30 km thick by 3500 m. y. It also rejects the uniformitarian explanation of marginal accretion during collision, although it seems to be valid for Phanerozoic times.
1.4.2 The Primitive Hydrosphere and Atmosphere Study of the gaseous envelopes of cosmic bodies less evolved that the Earth, or differently evolved, can shed much light on the Earth's primitive atmosphere; all planets of the solar system should have started out with similar atmospheres. Figure 65 summarizes the principal atmospheric components of these planets today. The outer planets, of greater mass and strongly shielded from solar radiation, have certainly retained most elements of their primitive atmosphere, which should, therefore, resemble that of the Earth very early in its history. To be noted are the abundance of the light elements and the reducing character of this atmosphere with CH 4 , NH 3 , N2 , H 2 , H 2 0, H 2 S. However, the atmospheres of the telluric planets, which include the Earth, were rapidly enriched in CO2 and H 2 0 coming from the degassing of the crust and mantle, and, therefore, became very dense (pressures of 70 bar at the Earth's surface, according to some estimates). The greenhouse effect resulting from this huge quantity of CO2 could have led to very high
Precambrian Time
89
Fig. 64. Simplified mantle plume model for the origin and growth of primitive continental crust. Arrows denote movements of convection (Kroner 1984, after Condie 1980)
Outer or gaseous planets (and satellites)
Inner or telluric planets Mercury and Moon
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
N2 O2 CO 2
CO H 2O H2 He Ne
A NH3
----
CH4
Fig. 65. Principal atmospheric constituents of the planets in the solar system
temperatures at the surface (70-100°C in the oceans). The Earth is at the moment the only inner planet to still have small amounts of the lighter elements (hydrogen, helium). It has also been able to retain its nitrogen derived from the dissociation of ammonia under the influence of solar
90
The Major Stages of Earth History
radiation. The presence of free oxygen in large quantities, a condition specific to the Earth, is due principally to the photosynthetic activity of plants (see below), and slightly to the photo-dissociation of CO 2 and H 20 by UV radiation. In the high atmosphere, an ozone (03 ) layer makes a protective screen against UV radiation. Whatever the hypothesis for the origin of dissolved salts in seawater, and their proportions, no real idea of the composition of primitive seawater yet exists. It was probably very acid because of the very large quantities of CO2 dissolved in it and the presence of strong acids. It was in this perhaps ubiquitous oceanic environment, already containing most of the salts we find today (except sulphates) and in the presence of a reducing atmosphere, that the first organic compounds were synthesized. Later, by about 3500 m. y. or earlier, these would lead to the emergence of life. Soils may also have played a role in this emergence. This first period corresponds to the Katarchean of Salop (1979, 1983). It terminates, according to this author, with the Saamian orogeny (37503500m.y.) characterized by deformations, plutonic and hypabyssal intrusions and metamorphism.
1.4.3 From the Archean to the Eocambrian: the Establishment of a Dynamic System 1.4.3.1 Internal Dynamics In the Archean, the first evidence of lithospheric activity is seen in the greenstone belts i.e. the zones of sediments and volcanics, folded and metamorphosed (chlorite, epidote, serpentine) with gneisses and granitic plutons. The greenstones represent the fillings of old basins, and some authors interpret them as the product of intraoceanic or marginal subductions analogous to those of the Phanerozoic. Others, like Kroner (1983a, 1984), propose an ensialic origin linked to continental rifting (Fig. 66); in fact, the existence since 3600m.y. of fragments of continental crust up to several hundred kilometers across is generally acknowledged. At point or linear hot spots the still unstable crust broke up, thinned, and then sank along elongate grabens widened by the classic mechanisms of tilted blocks. Continental basins were so created and became filled with sediments and essentially basaltic basic igneous rocks! from partial melting of the upper mantle. Crustal melting can also lead to granitoid intrusions. If the crust breaks completely, small oceanic basins appear. Their closure, after disappearance of the point or linear hot spots, results in deformation of the basin filling as recumbent folds and thrust faults. In total, the formation of greenstone belts results in an enlargement of continental crust as well as its thickening by magmatic differentiation and 1 In the Archean, many lavas were peridotitic komatiites derived from the deep mantle. These types disappear in the Proterozoic.
91
Precambrian Time
c
B
o
\(
T .ripnou. ledimentl,
biotenic c.bonate.
1l1li.................
E
Malic: 10 u1tr.101ic RICO or oc_ floor llllnity + •
c........ or .......... of... ...., or_.de""",
..
Fig. 66. Dynamics of the Archean lithosphere. A Evolution after small-scale convection in the upper mantle. B-E Different stages of formation of a green stone belt (After Kroner 1983a)
cooling. This latter process is estimated to have formed 85% of the crustal section between 2800 and 2500m.y. In the Lower and Middle Proterozoic (2500-900m.y.), the strongly cratonized continental masses were relatively stable, with large basins which became filled with volcano-sedimentary sequences, especially in West Africa, Brazil and Canada. These basins sometimes developed along rifts or aUlacogens. These fold belts always arise, according to Kroner (1983b), from an intracrustal or ensialic tectonism, implying weak plate movements
92
The Major Stages of Earth History
(verified by paleomagnetism), delamination of the mantle lithosphere and crustal imbrication (Fig. 67). The delamination is itself followed by metamorphism and intense granitization. Relatively deep basins were created by these movements and received the first flysch sediments. Once formed, the belts remain zones of weakness for a long time, later becoming loci for large continental faults, for example, the Grenville belt (Fig. 68), which is often interpreted in modern plate tectonics terms as the opening of a Proto atlantic around 13oom.y., oceanic closure and collision around llOOm.y. Also, according to Kroner (1984), the subduction at that time did not affect a hydrated lithosphere as it did in the Phanerozoic, thus explaining the absence of island-arc calc-alkaline volcanism. Several phases of intensified plutonism resulted in intrusions of gabbros, ultrabasites and granitoids, as well as migmatizations, granitizations, and mineralizations of Cr, Ni, Pt and Cu. At the beginning of the Upper Proterozoic, around 900m.y., plate tectonics of the Wilson cycle type 2 was initiated and resulted in continental accretions by collision. This is demonstrated by evidence of active margins and island arcs, by crustal shortening suggesting collisions, by the appearance of ophiolites, and by high pressure-low temperature minerals. This new dynamics, evidence perhaps for an acceleration of plate movement, must have been established gradually between 900 and 650m.y., as shown, for example, by the occurrence of the Pan-African orogeny (see below). But Kroner's (1984) interpretations are rejected by many authors, who believe they see evidence for a modern type of plate tectonism from the Lower Proterozoic. For instance, they interpret the greenstone belts as back-arc basins. The absence of HP-LT minerals and ophiolites before the Upper Proterozoic is explained as being due to a hotter oceanic crust than today and narrower oceans. Whichever is true, it seems certain that by plate movements at least four amalgamations of continental crustal blocks into a supercontinent of the Pangea type have taken place during the course of the Precambrian (Worsley et al. 1984), with each time a renewal of volcanic activity and a major emergence of the continental platform. It was not until after 1oo0m.y., when the oceans increased significantly in size, that the first paleogeographic characteristics were outlined. The African and South American shields were formed, solidly attached to one another, while two immense continental assemblages, one Northern, the other Gondwanian, also gradually took shape. The movements of these two megablocks were to influence practically all tectonic history of the globe, outside the Pacific, from the Precambrian to the Present. For example, at the end of the Precambrian the destruction by subduction of the Celtic ocean, which was a precursor of the North Atlantic, was the principal motor of the Cadomian orogeny (Fig. 69). 2 See
Chapter 4, Section 2.1.1.
93
Precambrian Time
A
+ -
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-----~C .. ,
I
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Subcrustallithosphere
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Fig. 67. Formation of orogenic belts folded by intracrustal (or ensialic) tectonism. A Rifting; 8 stretching and delamination of the lithosphere; C continental subduction (or ensialic) resulting in shortening and orogeny (After Kroner 1983b)
c
-"
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-----:~- - p
+
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Fig. 68. Grenville belt. Outcrops of formations affected by the Grenville orogeny shown in black; their probable extension shown hachured (After Windley 1984)
The principal effects of the dynamic evolution of the Earth during the Precambrian were as follows: 1. The thermal flux decreased gradually from the Archean when it was 2.5 times as high as at present. 2. The crust increased its heterogeneity as well as its thickness and rigidity, mainly in the Archean with the formation of granulites. Related to this, the cratons grew in size and stabilized to their final dimensions by the
94
The Major Stages of Earth History
C A
o o M I
A N
o R o
G E N
Fig. 69. Main elements of the Cadomian orogeny (after Cogne and Wright 1980). 1 Subduction zone; 2 island arc on the Pentevrian microplate; 3 marginal extension basin (Upper Brioverian)
Lower Proterozoic. The tectonic processes became intensified with time, giving rise in particular to increasingly evolved structures such as the aUlacogens. Also, the calc-alkaline granitic plutonism and acid volcanism characteristic of stable cratons became dominant in the Proterozoic, rather than the basic and ultrabasic outpourings so abundant in the Archean, the time of higher upper mantle temperatures. In fact, the Archean-Proterozoic transition appears to correspond to a significant and rapid lowering of the Earth's thermal regime. Volcanism and plutonism have often been associated with the precipitation of heavy metals (Cu, Fe, V, Au, Zn), especially in association with the greenstone belts. The presence of these elements, of considerable economic value, is proof of the strong activity of the Earth's internal layers during the Precambrian. Finally, it seems that until the Middle Proterozoic, the internal dynamics of the Earth were also activated by the impacts of large meteorites on a still barely consolidated crust. Some of the gneissic domes which are
95
Precambrian Time
Fig. 70. The Vredefort dome (Transvaal), dated as 1970 ± loom.y., and probably caused by meteorite impact (After Daly 1947)
1: Upper Paleozoic - 2-5: Proterozoic 6: Archean granite - 7: Metamorphic aureole
scattered across the shields and which denote local high thermal fluxes could be due to such impacts (Fig. 70). The Precambrian ended with a series of more or less contemporaneous orogenies: Assyntic, Cadomian, Baikalian and Pan-African, during which the continents were displaced significantly before coming back together (Fig. 71). These movements affected Australia, China and North America, initially joined (Eisbacher 1985), then separated during a phase of rifting between 800 and 700m.y. 1.4.3.2 Atmosphere, Hydrosphere and Climate
The reducing and acid combination of the primitive atmosphere was slowly modified during the course of the Precambrian, profoundly affecting the climate, sea water composition and sedimentation. At the beginning of the Archean, the atmospheric pressure was probably 10-20 bar because of the abundance of water and CO2 maintaining a high surface temperature (greenhouse effect) and reducing the penetration of solar radiation. The sea was warm (60-90°C), somewhat acid, chlorine and carbonate-rich3 . Wider than in the Phanerozoic, it was also shallower on average. The climate was very uniform and scarcely affected by latitudinal gradients. These initial conditions were gradually modified, as a number of sedimentary indicators show. 3 According
to another hypothesis (Kempe and Degens 1985), the oceans were sodic, therefore very alkaline, with low concentrations of Ca and Mg. Only after l000m.y. did they become chlorine-rich from the leaching of oceanic crust.
96
-
The Major Stages of Earth History
~
I II III IV
V VI VII VIII IX X
Pan-African range Pan-African Pangea boundary West Africa craton Congo craton Kalahari craton Guyana craton Sao Francisco craton North America craton Antarctic craton Australia craton China craton Siberia craton
Fig. 71. The Pan-African chain at about 6OOm .y. (After Black 1978)
The first red beds, with iron oxide, appeared about 2000m.y. (±200m.y.) in the Lower Proterozoic, together with some free oxygen in the atmosphere. They subsequently became more abundant, with hematite (Fe203) and goethite (FeO-OH) gradually replacing siderite (FeC0 3) in the sediments but especially in the paleosoils. Also, certain gold and uranium minerals requiring a reducing atmosphere for their bacterially controlled formation disappeared after the Lower Proterozoic. It is possible that at about 1S00m.y. (beginning of the Upper Proterozoic) the atmospheric oxygen content reached the Pasteur level (1/ 100 of today's concentration) and by the end of the Cambrian, one-third of the present level. The first abundant evaporite deposits indicate that the seas were becoming more chlorine and sulphate-rich (towards the end of the Proterozoic). Carbonate deposits never formed in the Archean because of the acidity of the oceans, but they accumulated in the Proterozoic, implying a reduction of atmospheric CO 2, The relative abundance of dolomites is explained by a higher Mg/Ca ratio and a CO 2 concentration in sea water higher than today, allowing a direct precipitation of dolomite. Oxygen isotope ratios of siliceous rocks indicate a lowering of sea-water temperature to about 30-40°C at the end of the Eocambrian. This gradually produced a climatic zonation. Tillites, however, show that marked climatic deterioration occurred eight times, leading to glaciations, some of which were of near-global extent (Fig. 72), with their glacial deposits useful as stratigraphic markers. The duration of these different cooling periods seems to be of the order of a few
Precambrian Time
97
Fig. 72. Precambrian glaciations. Solid circles, glaciation of the Lower Congo (950m.y.) and Sturtian glaciation (750m.y .). Open circles, Varangian glaciation (650m.y.). After Seyfert and Sirkin 1973
EOCAMBRIAN
LATE PROTEROZOIC
± 30
-600
u
6777
± 40
•
750 0 758 eB
50 •
775
0 820 850 900
.2300
02000 2200 n 2450 u
ARCHEAN
~
Soha,lan Inlandsls. South pole close to NW AI,lca coosts
6950 EARLY PROTEROZOIC
n
6816
580 Sinian glaciation 610 ~ 610 . 650 } Var.ngi.n 660 · 720 glaciation 790 \ Sturtian 800 glaciation
I
II!! 820 Lower Congo iii 870 Glaciation
} Generalized marks
on continents
4 glaciations North America
2750 South America. Transvaal
Fig. 73. Precambrian glaciations. Dates with triangles from Steiner and GriIImair (1973), with solid squares from Crowley (1983), with open rectangles from Salop (1979), with crosses in squares from Hambrey and Harland (1985)
tens of millions of years (Fig. 73) . These events are very distinctive and are unlike Quaternary glaciations in the following respects: 1. Their universal character; 2. Their higher ambiant temperatures, more or less equally distributed across the Earth's surface. Their origins must, therefore, be sought outside the Earth, perhaps in a temporary reduction of solar heat due either to variation in solar
98
The Major Stages of Earth History
radiation or to the presence of dust or gaseous screens between the sun and the Earth. A reduction of CO 2 in the atmosphere has also been invoked, especially for the Eocambrian and Archean glaciations. It should be noted, however, that the two glaciations of the Upper Proterozoic and that of the Eocambrian coincided with the existence of a Pangea formed as a result of continental collisions which formed mountainous relief and, therefore, some cooling. Also, the existence of these large continental masses increased the chances that one, at least, of the poles would coincide with emerged land, a situation favorable for long-lasting glaciation. According to Eisbacher (1985), the latest Precambrian glaciations were associated with rifting deformation. These glacial episodes, apart from the more recent Eocambrian, are not unanimously accepted, because of the interpretive nature of much of the tillite evidence and because it is difficult to prove their synchronism on a global scale. Also, large displacements of the poles and their ice sheets during the Upper Precambrian, as suggested by some, would mean diachronous glaciations. The original hypothesis of Williams (1975) is revealing in this respect, suggesting that the inclination of the Earth's axis of rotation in the plane of the ecliptic was much greater than at present, leading to a climatic uniformity at the Earth's surface as well as a marked seasonal contrast. Thus warm water (summer) deposits would occur at all latitudes. However, the existence of an important Eocambrian ice sheet extending from the Sahara to the Congo is generally accepted. 1.4.3.3 Sedimentation and Environments
During the Precambrian, different and contrasting environments of deposition became established. Initially, a single ocean, probably shallow, covered all the Earth. Then the continents emerged, at first of limited dimensions and unstable, later thicker, more rigid and therefore more durable. Soon after emergence, the land provided the first detrital deposits of Archean age, very immature (e.g. greywackes), from which the gneisses of the Limpopo belt in South Africa could have been derived. The gradual enlargement and thickening of the continents, together with the appearance of active margins in the Proterozoic, was to gradually accentuate the morphological differences between continents and oceans, the former becoming higher and the latter deeper. A consequence of this was a growing and more diversified deposition of clastics in the marine environment, often dominating over carbonate and siliceous sedimentation. Detrital sediments are abundant, especially near the coastlines and on the continents. Molasse first appears in the Archean in association with orogeny, psammites, sandstones with cross-stratifications, current ripples, desiccation cracks, tidal and deltaic sediments, greywackes and arkoses, etc. The erosion of the greenstone belts produced deltaic conglomerates locally rich in uranium and gold (Lower Proterozoic of Transvaal, Canada, Brazil) and placer gold and diamonds (Middle Proterozoic of India, Brazil, Siberia,
Precambrian Time
99
Africa). Flysch sediments appear episodically only in the Proterozoic, evidence of deepening environments. The abundance of sediments at the continental margins slowly increased the continental areas as large prograding shelves covered by epicontinental seas, especially in Africa and North America. Extensive already from the Middle Proterozoic, these shelves accumulated mainly detrital sediments, but also carbonates, sometimes mineralized (Pb, Cu, Zn, Mn in Siberia and Anti-Atlas), phosphorites, sedimentary iron minerals (siderite, hematite), glauconite, and evaporites. The latter, known from 3500m.y., are indicative of marginal-littoral environments of some aridity. Some aluminous deposits are also known and these are indicative of extreme continental weathering. Restricted environments are indicated by shungite (algal carbon from the Middle Proterozoic of Finland), black slates and graphitic schists (Lower Proterozoic of Karelia and Transvaal). The most notable chemical and biochemical deposits include stromatolitic dolomites and the banded iron minerals or jaspers. The former appear in the Archean and become increasingly important. The latter, particularly abundant in the Lower Proterozoic, consist of alternating beds of silica and iron-rich minerals (hematite, magnetite, siderite). They appear to be the products of chemical precipitation controlled by bacteria or algae, but opinion is divided on the environment of deposition. Narrow marine basins, subsequently closed tectonically, as well as other types of basins, shelves, and even lakes have all been suggested. In the Archean, iron and silica were clearly of igneous origin, but later, weathering of large areas of emerged land was an additional source. With the increasing abundance of oxygen in the atmosphere and hydrosphere during the Precambrian came a decrease in the leaching of reduced iron from soils, especially since the latter were devoid of plant organic matter. This may explain the absence of jasper in the Upper Proterozoic. Whatever the reason, these deposits constitute the major occurrences of iron of old basements dated from the Lower Proterozoic (Lake Superior in the United States, northern Europe, and Gabon). The growing importance of the continents during the Precambrian is also shown by the increased deposition of continental sediments; red beds indicative of dry climates, alumina-rich sediments from deeply weathered profiles (metabauxites of the Steap Rock Group, Archean of South Africa, Eocambrian bauxites of Siberia). In general, there was a thickening of the sedimentary cover accumulating within basins of increasing diversity, intracratonic at first, then marginal and oceanic. This epicrustal accretion contributed by metamorphism to lithospheric thickening, and thus to better thermal insulation of the mantle. This may have lead, according to some authors, to increased tectonism with time. 1.4.3.4 Development of the Biosphere The earliest Archean (3800-3700m.y.) already contains the traces of anaerobic microorganisms, undoubtedly prokaryotes, and evidence of their metabolic activity (stromatolites, oncolites) is signaled by 3400m.y.
100
The Major Stages of Earth History
In the Middle Proterozoic, oxygen from photosynthesis slowly built up in the atmosphere and hydrosphere, allowing the development of respiratory metabolism, eukaryotes (1400-1500m.y.), and cyanobacteria. Metazoa appeared around l000m.y. In the Eocambrian, a primitive but diverse soft-bodied fauna appeared at about 590-570m.y. (Ediacara fauna), and biocalcification appeared at about 570 m. y., probably in response to increasing levels of calcium in the oceans. At the end of the Precambrian, the first occurrence of hydrocarbons (in Siberia) and significant deposits of phosphates attest to the importance of the biosphere.
1.5 Conclusions on the Precambrian The first chapter of the Earth's history is very distinctive. It is very long and the rocks which represent it are badly altered and mostly azoic, making its history difficult to unravel. Appropriate stratigraphic methods are necessary and precision is low. Thus reconstructions of this earliest stage in the Earth's history are still conjectural, and based mainly on the recognition of certain events indicative of some internal geodynamic event, some compositional change in the Earth's fluid envelopes, or some biological evolution. One of these events was the rapid decrease in heat flow at the end of the Archean, a period characterized by intense basaltic magmatic activity and the rise of heavy metals to the Earth's surface. Another event was the change from a reducing to an oxidizing atmosphere at about 2000m.y., significantly affecting the mobilization of iron and the course of evolution. Finally, it appears that plate tectonics of the Phanerozoic type only started when the crustal blocks had become sufficiently large and thick in comparison with the intensity of heat flow. This dramatically significant moment (about 900 m. y.) separates a primitive unstable world from one organized and predictable, where life could establish itself and contribute significantly to the geochemical evolution of the atmosphere and hydrosphere.
2 The Paleozoic: the Formation of Pangea This era, lasting 285-340m.y. according to different time scales, corresponds to a major part of the penultimate Wilson cycle (see above), i.e. to a gradual assembling of previously dispersed continents into a supercontinent. This accretion is marked by two major collisions, the Caledonian and Hercynian orogenies, which have profoundly affected the paleogeography, eustasy, sedimentation, climate and biological evolution of the Earth.
101
The Paleozoic: the Formation of Pangea
•
STAGES
lab Systems
Subsystems
400 39'"
Europe S A L
Late SILURIAN
A
423 Early
425 435
Cayugan
WENLOCKIAN
Late
445 450
""'"Ardenian
Niagarian
LLANDOVERIAN
~ Ashgillian
430
Orogenic stages
LUDLOVIAN
0 p I
N
North America
Medinlan
Taconian I
Cincinnattian.
CARADOCIAN LLANDEILIAN
455
Champlain ian LLANVIRN IAN ORDOVICIAN Early
-----ARENIGIAN
SKIDDAVIAN
480
Canadian
TREMADOCIAN
495 1,-
Sardinian SHIDERTINIAN Late
51S
TUORIAN
Crolxian
MAYIAN CAMBRIAN
Middle
Albertian
LEN IAN
54G Early
530 570
AMGIAN
ALDAN IAN
Waucobian
Cadomian
-!...-
Fig. 74. Subdivisions of the Lower Paleozoic. Two radiometric age scales: a according to Odin et al. (1982b); b according to Van Eysinga (1985)
2.1 Lower Paleozoic This interval, of 170-175m.y. corresponds regionally to the Caledonian cycle. It is represented by rocks initially studied in Great Britain (Caledonia is the Latin name for Scotland). Caledonian time marks a transition between an old and a new world, beginning when the Earth had already cooled
102
The Major Stages of Earth History
considerably and the continental crust had stabilized. However, the principal focus of activity took place at the boundaries between the continents and the oceans. Sediments, augmented by significant contributions of biogenic origin, spread across the broadening shelves and became much more diverse, although detrital sediments remained dominant because continents still lacked vegetation. Biological evolution, stimulated by the growing amounts of oxygen in the atmosphere, initiated its major future patterns, while living organisms utilized all marine ecologic resources as well as setting foot, for the first time, on the continents.
2.1.1 Boundaries and Subdivisions Applied to Paleozoic rocks, biostratigraphy is much more useful because it could be based on organisms such as trilobites and graptolites which were both complex and evolved rapidly. Stratigraphic units and boundaries are, therefore, more reliable and of wider application than in the Precambrian. Nevertheless, there is still no agreement on a universal stratigraphic scale, because biozonations were developed mainly in Great Britain and affected by undeniable provincialism. Consensus exists only for the major systems; Cambrian at the base (Cambria is the Latin name for Wales), Ordovician (of the Ordovices, a Welsh tribe), and Silurian (of the Silures, another Welsh tribe). The classic lower boundary, coinciding with the Assyntic unconformity separating the Eocambrian beds from the transgressive beds with Olenellus (trilobites), has no general validity because of the magnitude of the hiatus between these beds (see above). Also, the Olenellus beds are sometimes underlain by archeocyathids, such as in Australia, Siberia, and Morocco. The base of the Silurian was fixed in 1984 at the base of the Parakidograptus acuminatus zone, near Moffat (England), while the base of the Devonian (i.e. the top of the Silurian) was fixed by international agreement in 1972 at the base of the Monograptus uniformis zone (see below). The subdivisions into series and stages are still the same as those developed in the major areas where the initial basic studies were carried out. Figure 74 illustrates two stratigraphic scales, one European, the other American. It should also be added that the stratigraphic use of conodonts will probably result in a stratigraphy of more global utility.
2.1.2 Plate Mobility During the Lower Paleozoic The lithospheric movements before the formation of Pangea at the end of the Paleozoic were for a long time conjectural. From a wealth of data, they are now known as well as those of the Mesozoic and Cenozoic. 2.1.2.1 Available Data Paleomagnetic measurements prove the existence of five continental masses at the beginning of the Paleozoic (Fig. 75); Proto gondwana (comprising
103
The Paleozoic: the Formation of Pangea
2 Fig. 75. Distribution of the continental masses in the Lower Paleozoic. 1 Cambrian; 2 Silurian (After Seyfert and Sirkin 1979)
South America, Africa, India, Australia, Antarctica, and Madagascar), North America, Europe, Siberia and China. This distribution suggests that after being joined at the end of the Precambrian, the continents were again separated during the Cadomian and Pan-African orogenies by ocean spreading, often initiated at old sutures formed during previous collisions. Such was the case with North America and Europe separated by the ProtoatIantic, or Iapetus, ocean, and a Nordic group and Protogondwana separated by Paleotethys. The western part of the latter was a mid-European sea separating southern Europe, which was part of Gondwana (or a block separated from Gondwana by an arm of the Paleotethys (Fig. 76), from Fenno-Sarmatia, comprising Scandinavia and the Russian platform. During the Lower Paleozoic the continents assembled once more, but the details of timing and processes are well known only for the Caledonian orogeny, a consequence of the closing of the ProtoatIantic (Fig. 76). The paleomagnetic data also trace the polar wandering curves for the Lower Paleozoic (Fig. 88). Starting from offshore Morocco in the Cambrian, the South Pole traces two possible paths, one ending in central Africa, the other in Argentine at the end of the Silurian. The structural, petrographic, and sedimentary data, derived mainly from the continental margins, have enabled the reconstruction of the evolution of those margins in relation to the dynamics of the opening and closing of the oceans by processes comparable with those known for the
104
The Major Stages of Earth History
Fig. 76. Paleogeography of West Europe in the Lower Ordovician (After Babin et al. 1980)
Mesozoic and Cenozoic. Thus, the Caledonian and Appalachian orogens, although considerably deformed and altered, are sources of data for the reconstruction of passive and active margins. Passive margins are represented by the following deposits: 1. Platform deposits of relatively shallow environments and diverse lithologies (limestones, shales and sandstones, etc.). When they are thick and localized at the continental margin they define miogeoclines4 or marginal cratonic basins. 2. Slope deposits, generally detrital and often turbidites; 3. Deep-water deposits, at the foot of the slope; they are generally thick and characterize marginal ocean basins. Sometimes metamorphosed, they consist of detrital sediments and volcanics. Active margins are characterized by obducted ophiolites, "melanges" or olistostromes formed near subduction zones, glaucophane and lawsonite (high pressure-low temperature minerals) schists, island arcs, acid volcanism and plutonism at the continental margins. Important structures, related to continental collision, include the large thrusts described from the Caledonian and Appalachian orogenies and implying significant crustal shortening. Paleontological Data. The movements of continents by plate tectonic mechanisms can lead to the gradual removal of faunal barriers and the mixing of
4 Miogeocline
is used increasingly to replace the older term "miogeosyncline".
105
The Paleozoic: the Formation of Pangea
1....-_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
1::.\: ~·:':.:·I
•
Emerged lands Pacific trilobites
1a: Scotland 2b: E Newfoundland
~ o
1b: England 3: Nova Scotia
Epicontinental seas Atlantic trilobites 2a: NW Newfoundland 4: New Britain
Fig. 77. Pacific and Atlantic faunal provinces on both sides of the Protoatlantic, or Iapetus, at the Cambrian-Ordovician boundary (After Windley 1984)
provincial faunas. When two provinces defined by their particular benthic associations thus lose their identities, the region is assumed to have become shallow and to have lost ocean crust by continental collision. These paleobiogeographic criteria have been used to track several phases of collision in the Caledonian and Appalachian orogens and also in the Buller chain (New Zealand). 2.1.2.2 Example: Closure of the Protoatlantic and the Caledonian Orogeny The Proto atlantic was initiated by a separation between North America on the one hand and Europe and Africa on the other, at a time difficult to specify, but either the Upper Proterozoic (630, 675 or 81Om.y., according to different authors) or the basal Cambrian. The Upper Proterozoic hypothesis is supported by the presence, within the Proterozoic section, of a thick detrital unit (Torridonian in Great Britain) intruded by basic plutonic and volcanic rocks which could represent the initial crustal thinning and rifting prior to plate divergence. However, this evidence does not fit with the hypothesis of a latest Precambrian Pangea. In either case, from the Lower Cambrian two trilobite provinces can be distinguished in western Europe: (1) the Atlantic province (or Acado-baltic); and (2) the Pacific province (or American), thereby demonstrating the reality of the Protoatlantic (Fig. 77).
106
The Major Stages of Earth History
Dalradian basin
-==- --
··:·.·.:.:.-::.:-.-.~~.r
Uplands
Midlands
...
Iapetus
--
1 Grampian
Iapetus
Skiddaw
Lake District
Irish sea horst
f
~~~M 2
Grampian
1: Middle Cambrian 4: End Silurian
Iapetus
2: Early Ordovician
Snowdon
3: Late Ordovician
Fig. 78. Evolution of the British Caledonides (After Watson and Dunning 1979)
Its maximum width in the Lower Ordovician is still debated, but estimates vary between 1000 and 3500 km. This ocean spread transgressively on to the continental margins as a shallow sea depositing thick sequences of sands and tidal and subtidal carbonates up to 3000 m thick in the Appalachians. An immense carbonate platform, which Rodgers (1970) has compared to that of the Bahamas, extended from Texas to Newfoundland in the Appalachians, then into northwest Europe and eastern Greenland. On the continental slope and base of slope, thick detrital sediments (up to 15000 m) accumulated, including some flysch and volcanics. On the margin of the American continent, there were marginal seas isolated by microcontinents, the Piedmont and Avalon terranes in the Appalachians, and the Southern Upland terrane in the Caledonides. Certain margins rapidly became active, for example, in northern Great Britain and at the Atlantic border of the United States (Figs. 78, 79). Evidence for a subduction zone in the MidCambrian of the Midlands of Scotland can be seen in the blue schists and
107
The Paleozoic: the Formation of Pangea Early Cambrian - Early Ordovician
GrenVille orogeny (900 Ma)
1
o/t.N.A~rica;0WJ Alrl~
2
~,A;;lca
3
Late Precamblan EBR-P CB,(:SB ~;'rtamDNLL
5
Late Precamblan
-
Ordovician - Silurian
-
Late Precambrian - Early Cambrian
4
lJJTI7J2b",
~
,~5>
....,.7,~rm""~~_7%lJ'-"",?~_ _-.zlJC"'lrlca
a
zzz=-C;
~
6
@l)22* ~=~~77. Devonian
QAirica
7
()
4». ---..:
~
CB
CS8
lfI,77J%'17.7#J13~~~~~~z=i~~C8 Carboniferous - Permian MolasseV&R
8
BZ
1Zti+*¥5i'I~~c.
l'Ig. 1':1. bVOlUtJOn 01 me eastern margm of North America between the Grenville (Upper Precambrian, 9OOm.y.) orogeny and the formation of the Appalachians (PermoCarboniferous). EBR Eastern Blue Ridge; P Piedmont; CB Charlotte Belt; CSB Carolina Slate Belt; BR Blue Ridge; V and R Valley and Ridge; BZ Brevard zone (after Hatcher and Odom 1980). This interpretation does not take into account the Pangea stage at 6OOm.y. subsequent to the Pan-African and Cadomian orogenies. 2 to 4 Opening of the Protoatlantic; 5 to 8 closing of the Protoatlantic, then collision of Africa and America. Continental crust hachured, oceanic crust in black
mineralized (Pb, Zn, Au, Ag) ophiolites forming the Ballantrae Complex. Instability of the margin is documented by megabreccias derived from the platform and resedimented in the marginal ocean basin. In the Ordovician, the marginal seas closed with collision of the American continent and the microcontinents . At the beginning of this period, the ophiolites at Ballantrae were obducted onto the continent and a volcanic arc was initiated. A strong deformation (Grampian phase) affected the basin to the north. At the same time, a volcanic arc appearing in the southern Appalachians and the "Dunnage Melange" of Newfoundland, consisting of an olistostrome with sedimentary and volcanic components resting on ophiolites, was emplaced thus signifying the emplacement of a northwest-dipping subduction zone. It is associated with copper and iron mineralization. All the early deformations, dated as Lower and Middle Ordovician, define a first Taconic phase. In the Lower Ordovician, a second subduction zone symmetrical to the first occurred along the southeastern margin of the Protoatlantic in the Caledonian domain, resulting in the closure of this ocean. In the Upper Ordovician, the faunal provinces began their first exchanges. A consequence of this subduction was the appearance of an andesitic volcanic arc in North Wales and mineralization of gold and copper. The Ballantrae region was uplifted several thousand metres at this time and partly eroded, yielding a thick flysch sequence. In the Appalachians, the collision of the American continent with the Piedmont microcontinent defines the second Taconic phase (Middle and Upper Ordovician), characterized by metamorphism, northwest-thrusting, including slices of crystalline Precambrian basement,
108
The Major Stages of Earth History
and emergence of the western part of the Appalachian orogen. In the Silurian, the trilobite faunal provinces of the Caledonian region became indistinct, indicating total closure of the ocean. This was due to the collision of the Baltic and Canadian shields starting in the northeast and continuing progressively toward the southwest, defining in the Upper Silurian the Ardenne phase of the Caledonian orogeny. Like the early phases, this was characterized by significant thrusting to the northwest (in the Irish Sea and the Lake District, but especially in North Scotland with the Moine Thrust, whose displacement may have been about 100 km) and to the southeast (Narvik Thrust in Norway), as well as by the transcurrent Great Glen fault (Scotland, North Sea) with a displacement of 120 km. These movements continued until the Mid-Devonian. Some andesites and granitic plutons, indicating continued subduction, were formed in Scotland at about 380-400m.y., and the final collision did not affect eastern Canada until Mid-Devonian. From the Mid-Silurian, when the oceanic crust of the Proto atlantic disappeared, the ocean slowly filled with a thick flysch until the Mid-Devonian, when North America and Europe were finally connected by land. In the Appalachians, the erosion of the Taconic mountains produced great clastic wedges of coarse detritus on the continent. Echoing the Ardenne phase, a certain sequence of events occurred: folding, metamorphism and plutonic intrusions in Quebec and Newfoundland (Silinic disturbance), and granitizations in the northern Appalachians. The closure of the Pro atlantic by collision of the American continent and Africa did not take place until the Upper Paleozoic, but even from the Upper Silurian, detrital sediments from the African continent can be recognized in the marginal ocean basins of the American continent. 2.1.2.3 Other World Events With the exception of the Tethys margin, the margins of the great continents of the Lower Paleozoic often became active when they coincided with subduction zones. As in the Caledonian and Appalachian domains, this activity was characterized by deformation, plutonism and the volcanism of island arcs. This was also the case in the Cordilleran and Andean domains at the western margins of the American continents, in the Uralian domain between Europe and Siberia, at the southern margin of Gondwana, and around the Siberian and Chinese continents. The Cambrian, a rdatively quiet period, was nevertheless affected by two tectonic phases. The first, in the Lower Cambrian, was localized in the Salai"r region of Siberia, the second (Sardinian phase) at the end of the Cambrian, was more widespread, in Sardinia, central Europe, the Iberian meseta, the Montagne Noire, the Himalayas, and Tasmania. In addition, southern Europe was affected by intense acid volcanism in the Upper Cambrian, probably associated with a phase of extension, and the Canadian Cordillera was fragmented into large blocks also by extension.
The Paleozoic: the Formation of Pangea
109
The Upper Ordovician was marked by a major orogenic phase, the Taconic orogeny, throughout the Protoatlantic and beyond. In the Urals, folding was accompanied by granitization, ultrabasic intrusions, and chromium, asbestos and platinum mineralizations, as Europe and Siberia collided. In Brittany, there is a major unconformity, while deformation and uplifts occurred in Greenland and the Andes and generally acidic volcanism in the Montagne Noire, Bohemia, southern Europe, Spain, northern Africa, Kazakhstan, and eastern Australia (where an island arc developed). The Upper Silurian shows the same phenomena as a result of the Ardennian tectonic phase. The huge Eurasian continent began to be assembled in the east with the approach and collision of the Siberian and Chinese plates. The Urals record several phases of folding contemporaneous with an island arc developed in Angara in southern Siberia. The effects of the Caledonian orogeny were varied and widespread, from the Ukraine, Greenland, Ardennes, circum-arctic region, northern Africa, Sahara, China, Antarctic (uplift of the transantarctic chain). Some volcanism has been recorded in Brittany, central Europe, the Urals, Siberia, the Lower Himalayas, and in Australia, sometimes followed by granitization and metal-rich concentrations (Urals, Aral Sea).
2.1.3 The Marine Environment Compared with the previous epochs, the marine environment was more widespread. Covering extensive platforms during transgressions, it resulted in a greater diversity of sediments and a vastly increased capacity for the development of organisms. 2.1.3.1 Shoreline Fluctuations These were important during the Lower Paleozoic and appear to have been dependent on climatic variations. For instance, the major Cambrian transgression, continuing from the Eocambrian, is eustatic and a consequence of the melting of the glaciers at the end of the Precambrian, which coincided with an acceleration of ocean expansion. This transgression continued in many places through the Ordovician, especially in the Sahara. But in other places (certain European countries, central United States) there was a regression. This became general in the Upper Ordovician as a result of the Taconic movements and glaciation affecting the South Polar regions of northwestern Africa at that time. The drop in sea level at that time is estimated to be not more than about 20 m. The melting of this ice cap must have been at least partly responsible for a new major transgression beginning in the Lower Silurian and culminating in the Middle Silurian. This advanced further in some places (especially in Africa) than it had in the Ordovician. Finally, a regression marked the end of the Silurian.
The Major Stages of Earth History
110
~ ~
CJ
Reefal belts enclosing evaporite basins Normal saline water supplied to the basins
Fig. 80. Paleogeography of the Upper Silurian in northeastern United States (After Alling and Briggs 1961)
2.1.3.2 Sedimentation Depositional environments of the platform contrasted with deep-water zones corresponding to the continental slope and base of slope environments (marginal ocean basins). The truly oceanic pelagic environments have left few traces, having mostly disappeared by subduction: but some obducted formations, always metamorphosed (schists, greywackes, volcanics) and deformed, do exist, for example in the interior zones of the Caledonides. The platforms were covered by shallow seas which often advanced far onto the land during transgressions, as on the Russian, American and Gondwana platforms. Several transgressive sequences are known from the Lower Paleozoic. The most widespread, in the Cambro-Ordovician, consists of basal sandstones (often glauconitic), shales, and limestones and dolomites at the top attesting to very shallow environments (shelly limestones, oolites, stromatolites, desiccation cracks). Frequently found on or at the margins of continents, this sequence may sometimes be replaced by detrital sediments (Andes, Cap region), or by carbonates (Canadian Cordillera). On the shelves, areas of active subsidence sometimes accumulated carbonates up to thousands of metres thick in marginal cratonic basins (Appalachian,
The Paleozoic: the Formation of Pangea
111
Caledonian regions). In the Ordovician, these basins also received coarse detritus from newly formed island arcs. Another transgressive facies consists, in the Ordovician, of Armorican-type sandstones, as seen in Brittany, Normandy, southern England, Spain, Portugal, and Morocco. An adjacent facies is represented by the Tigillite sandstones (bioturbated by tubular annelids). On the platforms, reefs gradually became more common. They were rare at the beginning of the Paleozoic, although these constructive associations were richer than in the Precambrian when stromatolites were the only reef-forming organisms. In the Cambrian, these consisted of sponges, archeocyathids, bryozoa, corals, crinoids, and stromatoporoids. In the Ordovician, reefs and other limestones spread to a tropical zone covering arctic America, Scandinavia, Siberia, and eastern Australia. The Silurian saw the maximum reef development, which was widespread in North America, Europe, the Arctic, Japan, and the Tethys. Algal activity also contributed considerably to reef development. A famous example is the barrier reef of the Michigan Basin in the Upper Silurian (Fig. 80). Shales are also common on the platforms, especially in the Ordovician with its graptolitic black shales, an indicator of anoxic conditions at the seafloor, and calymenid shales. Less common facies include: (1) phosphates found between 45° north and south and indicative of upwelling of deep waters; (2) oolitic iron minerals (hematite), mined in the Armorican Massif, Morocco, Bohemia, and Sweden; and (3) evaporites deposited mainly around the margins of Gondwana during the Precambrian-Cambrian transition in association with the initial rifting phase of the Early Paleozoic continental dispersion. The deep-water zones, corresponding (generally) to the marginal ocean basins, were strongly deformed during the orogenic phases of the Lower Paleozoic. Apart from typical radiolarites, these deposits show much evidence of continental margin activity, including coarse detrital sediments (sands, conglomerates, shales, and greywackes), volcano-sedimentary sequences, ophiolites, and olistostromes (see above). 2.1.3.3 Biological Phenomena In the Lower Paleozoic, patterns of evolutionary development were conditioned by the important events taking place at the Precambrian- Cambrian transition. Thus, more than 900 species of invertebrates among 9 phyla rapidly appeared during the Lower Cambrian. The majority lacked mineralized structures of support or protection. With biological competition, the stromatolites were the first victims. In terms of numbers and diversity of faunas and variety of marine environments colonized5 , the Ordovician appears as one of the peaks in the development of life. In the Silurian, the continents were gradually colonized, especially by plants and fishes.
5
Related to the importance of the epicontinental seas.
112
The Major Stages of Earth History
2.1.4 Climatic Evolution 2.1.4.1 The Available Evidence From the Lower Paleozoic, climatic reconstructions can be based not only on tillites, but also on the normal criteria used for more recent periods of the Phanerozoic. Reefs in the Cambrian, and especially in the Ordovician and Silurian, consisted of diverse and complex calcareous organisms requiring warm waters. Evaporites indicate arid conditions typical of latitudes between 5 and 35° Sand 15 and 40° N. This aridity was accentuated on the continents by the lack of vegetation. Red beds, often associated with evaporites, attest to a tropical climate and relatively high temperatures such as those seen today between the 30° latitudes; an example is the continental Silurian in the central United States. 2.1.4.2 The Major Climatic Factors The apparent displacement of the poles during the Lower Paleozoic (from offshore Morocco to central Africa or to Argentina for the South Pole), added to that of the continents, had significant consequences for the climatic evolution of different continents. The warm latitudinal belt straddling the equator persisted in America and in northern Europe, but also swept across other large continental areas such as Siberia, China (from north to south), Antarctica, India, and Australia (from south to north). The strongest evidence of this zonation, almost the reverse of today's, includes: 1. Salt deposits (Lower Cambrian of Siberia, India, Silurian of North America, especially the Michigan Basin); 2. Evaporites, dolomites, and limestones (Ordovician and Silurian of the Canadian Arctic islands and northeast Greenland); 3. Reefs, very developed in the Silurian in North America and western Europe.
Another basic climatic indicator is the presence of an ice cap in the Upper Ordovician, covering the Sahara, part of Brazil and perhaps Arabia, an area of about 6 x 106 km2 around the South Pole of that time. Other indicators of glaciation described from southwest Africa, New Zealand, Spain, France, and Scotland could correspond to material transported by icebergs. 2.1.4.3 Global Climatic Evolution In the Cambrian, the wide distribution of algal and archeocyathid reefs in a broad equatorial belt (to 75° S in Morocco) suggests a warm climate even in high and middle latitudes. Evaporites and red beds were deposited in a zone between 15 and 30° Nand S. In the Upper Cambrian, however, a slight cooling, possibly related to the Sardian phase, caused a retreat of reefs and limestones. In the Lower and Middle Ordovician, the polar regions again became free of ice and the climate generally warmer.
The Paleozoic: the Formation of Pangea
113
In the Upper Ordovician, continental glaciers formed at high latitudes and lowered average temperatures appreciably, so creating a varied climate resembling that of the Pleistocene. The Gondwana glaciation lasted 10 to 15 m. y., from the Caradocian to the Lower Llandoverian, with significant fluctuations giving rise to four interglacial stages. In the Silurian, a general warming and more widespread aridity resulted in the melting of Gondwanian ice and an expansion of reefs, evaporites and red beds to a belt between 40° Nand 40° S. The end of this period was marked everywhere by renewed cooling, fewer limestones and the presence of mountain ice sheets near the South Pole (Brazil, South Africa, Falklands). This change was enhanced, if not initiated, by the Caledonian orogeny.
2.1.5 Conclusions on the Lower Paleozoic In the Cambrian, five great cratons were scattered about the globe. In this period of extension, sedimentary basins grew larger and filled up, especially near the continental margins. The Cambrian was also marked by a major transgression. During the Ordovician and Silurian, the gradual convergence of the continental masses gave rise to a succession of orogenic phases: 1. The Sardinian phase at the Cambrian-Ordovician boundary was perhaps
a consequence of the initial approach of Africa and Europe. 2. The Taconic Phase (Upper Ordovician), with worldwide effects, coincided with the beginning of closure of the Protoatlantic and with the initial collision between Europe and Siberia and the formation of the Urals. 3. The Ardennian phase, during which all components of the great Eurasian continent came into contact (China approached Siberia and the Baltic Shield collided with the Canadian Shield). The final closure of the Protoatlantic in western Europe resulted in the uplift of the Caledonian Mountains at the end of the Silurian. This orogeny marks the official end of the Caledonian cycle which, however, continued elsewhere because the collision between Europe and America did not reach eastern Canada until the Middle Devonian and the compression and subduction continued until this time in the Caledonides. Conversely, the Hercynian cycle was initiated in the Silurian when Africa began to approach Europe and North America. The two orogenic cycles of the Paleozoic are, therefore, largely overlapping. Cooling and regressions also took place during the Taconic and Ardennian phases. However, the major climatic event was the glaciation of part of Gondwana at the end of the Ordovician, a phenomenon partly controlled by the apparent displacement of the South Pole across Africa. The significant advances in biological evolution during the Paleozoic were undoubtedly dependent on the major transgressions and favorable environmental conditions (temperature, atmospheric oxygen). In this respect, the
114
The Major Stages of Earth History
conquest of the terrestrial environment may have coincided with the formation of an ozone layer in the high atmosphere, sufficiently effective to protect the Earth's surface from UV radiation damaging to life.
2.2 Upper Paleozoic This period corresponds classically to the Hercynian or Variscan cycle, terms used in Europe for events of this period in the geographically separate Hercynian and Caledonian orogens respectively, although, as we have seen, a certain degree of overlapping in time between the Hercynian and Caledonian orogenies existed. In other areas, and especially in the Appalachians, the distinction between Caledonian and Hercynian deformation is much more arbitrary, each representing a continuum of deformation in a unique paleogeographic context. Considerable progress has been made in the last 10 years or so in the analysis, dating, and interpretation of Upper Paleozoic rocks. Also, the increasing abundance of paleomagnetic data has ensured greater knowledge of the movements of continental masses during that period. A significant fact derived from this knowledge is that the last stages of the convergence of the continents leading to the Pangea stage characterize the transition from the Paleozoic to the Mesozoic. The consequences of this final convergence were important for orogenic, sedimentary, climatic, and biological evolution. The continental environment, totally colonized by living organisms, became for the first time as important for biological evolution as the marine realm.
2.2.1 Boundaries and Subdivisions The numerous tectonic phases of the Upper Paleozoic have complicated the stratigraphy, which has had to be constructed from many areas with different sequences, both in the marine and continental realms, and with tricky problems of correlation between different stratigraphic scales. The lowest boundary of the Upper Paleozoic has been defined as the base of the continental "Old Red Sandstones" which rest unconformably on marine Silurian. In the Ardennes, on the other hand, the base of the transgressive marine Gedinnian is taken as the lower boundary, also with a hiatus between the Lower and Upper Paleozoic. This unsatisfactory situation finally caused the boundary to be redefined in a marine and continuous sequence. By international decision (International Congress of 1972) the base of the Devonian (also the base of the Upper Paleozoic) was fixed as bed number 20 in the Klonk section (Czechoslovakia) at a level corresponding to the maximum number of significant biological events (including the sudden appearance of Monograptus uniformis). Three major systems have been distinguished:
115
The Paleozoic: the Fonnation of Pangea
Leon-Asturias
Ruhr (Germany)
Fig. 81. It is not possible to construct a unified biostratigraphic scale based on fossil plants for separate coal basins fonned at different altitudes (After Bouroz, in Pomerol and Babin 1977)
1. The Devonian, originally defined in Devonshire6 , is sometimes considered as the transition between the Caledonian and Hercynian cycles, sometimes as the dawn of the modern age, because of the beginning of terrestrial colonization by living organisms. 2. The Carboniferous was named in England for the coal-rich rocks. The best definition of its base in western Europe occurs in the Visean Basin, where there is a continuous marine sequence permitting the use of foraminifers and conodonts. The coal-bearing continental series is more difficult to subdivide stratigraphically because the plant materials used are very sensitive to environmental conditions, particularly altitude (Fig. 81). Most useful are the paralic sequences, deposited near sea level and likely to contain marine intercalations which can be dated from cephalopods, foraminifera, and conodonts. American geologists subdivide the Carboniferous into two parts only: the Mississippian below and the Pennsylvanian above. 3. The Permian. This name evokes the Finno-Hungarian kingdom of Permya. Its boundaries are still unsettled, but its lower boundary may be fixed in the Donetz basin (USSR), where sedimentation was continuous, at the stratigraphic level where the foraminifera Schwagerina appears. Figure 82 summarizes the principal subdivisions of the Upper Paleozoic.
2.2.2 Formation of Pangea: Orogenic and Climatic Consequences Already well under way in the Lower Paleozoic, the assembling of the continents into a supercontinent was completed during the Hercynian cycle as shown by the polar wandering curves. Eurasia to the east became a single continental mass through the total closure of the channel separating Europe and Siberia, resulting in the uplift of the Urals, and also by the final joining of China and Siberia (Fig. 83). Gondwana became embedded in the North America-Eurasia block, closing what remained of the Protoatlantic to the
6The actual type region is not Devon, where the rocks are very defonned and metamorphosed, but the Ardennes.
The Major Stages of Earth History
116
•
No
245
b
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STAGES
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Late
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----.....!SJ!!tg~
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Artinskian
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:zao Late
300
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320
355 360
315
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Middle 370
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Namurian
~co
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~ Pennsylvanian
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is 345
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,-
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335
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Fig. 82. Subdivisions of the Upper Paleozoic. Two radiometric scales; a according to Odin et al. (1982b); b according to Van Eysinga (1985)
east of America as well as the oceanic areas of middle Europe (Fig. 84). These latter aspects will be considered in more detail. 2.2.2.1 Collision Between West Africa and North America: the Appalachian Chain Along the eastern border of North America, the situation inherited from the Silurian was an active margin with a subsiding platform to the west and a closing sea to the east (Fig. 79).
Lower to Middle Devonian: Acadian Orogeny. The border of the platform was deformed and uplifted into a mountain chain stretching from Newfoundland to Pennsylvania, by the accretion of a microcontinent called "Avalon-Florida". The erosion resulting from this uplift spread sediment over a large alluvial plain including the red molasse of the Catskill Delta.
117
The Paleozoic: the Formation of Pangea
Fig. 83. Paleogeography of the Upper Carboniferous about 300 m. y. ago, showing the formation of Pangea (Daly 1984, after Irving 1977). Note the difference between this and Matte's reconstruction (Fig. 84). Dashed lines indicate boundaries between oceans and epicontinental seas
A
B
Fig. 84. The Hercynian orogenic cycle and its principal motor, the approach of Gondwana and the North American-Eurasian (or Laurussian) block. A The situation in the Silurian before the beginning of the cycle; B in the Permian at the end of the cycle. 1 Caledonian orogen; 2 Appalachian orogen; 3 Hercynian orogen; 4 Mauritanian orogen; 5 Ural; a Iapetus; b Paleotethys (After Matte 1986)
This molasse filled the basin by progradation, slowly pushing the sea westwards.
Carboniferous and Lower Permian . This detrital regime continued, interrupted locally by episodes of limestone and coal deposition suggesting
118
The Major Stages of Earth History
lacustrine and swamp environments, especially in the Pennsylvanian. The sea withdrew from the platform at the beginning of the Permian.
Middle Permian: Alleghanian Orogeny. This affects the whole of the Appalachian margin. This final phase of deformation, characterized by large thrust faults to the west (up to 50 km of transport in the southern Appalachians) was the result of collision between Africa and North America (Figs. 79 and 84). The Acadian and Alleghanian orogenies also affected the marginal ocean basins, filled with detrital and volcanic sediments, which were folded, metamorphosed, and intruded. The collision between Africa and America also deformed the African border, forming a complex mountain chain (Mauritanides) with large thrust faults to the east, implying a significant crustal shortening in the Permo-Carboniferous. 2.2.2.2 Collision of Europe and Africa: the Hercynian or Variscan Chain The Hercynian orogen extends approximately 5000 km from southern Spain, where it forms an arc (Iberian-Armorican arc) to the Caucasus. In Europe, it has been subdivided lengthwise into several regions differing in paleogeography and types and ages of deformation (Fig. 85). The latter provide indisputable evidence of the collision of two large continental masses (Gondwana in the south, Laurasia in the north), but by mechanisms which still need to be deduced. In particular, was there closure of one or two oceans, one of which could have corresponded to a Paleotethys? Was the shortening taken up partly in large transcurrent faults?
Some Paleogeographic Facts. The oldest (Proterozoic to Lower Paleozoic), most metamorphosed, and most granitoid-rich rocks crop out in a central belt called the Saxo-Thuringian zone. The Lower and Middle Carboniferous are occasionally represented by thick limnic deposits and some basins with syntectonic sediments (flysch of "culm" facies and molasse). The Upper Carboniferous sedimentation was often paralic. In the north, the Rheno-Hercynian zone was transgressed from the south by a Devonian sea deposition, first coarse detrital sediments, then limestones, often as reef deposits, and finally marls and shales. In the second transgression, during the Lower Carboniferous, a carbonate platform developed, passing eastwards into a relatively deep "culm" facies. In the Upper Carboniferous, the continental to subcontinental lagoons and marshy coastal plains provided the necessary environments for the development of the coaly deposits which are so common in Europe. The Rheno-Hercynian zone overlaps to the north a foreland, covered in the Devonian by continental sands characteristic of the Old Red Sandstone continent, and derived from the erosion of the nearby Caledonian mountains. The Carboniferous transgression deposited platform limestones on this area, then, in paralic environments, molasse and coal deposits.
119
The Paleozoic: the Formation of Pangea
----
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.
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3
5
¥ ~
6
7
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/
"
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A: Rhenohercynian zone B: Saxolh urlnglan zone C: Moldanubian zone
Fig. 85. Structural elements of the Variscan chain of Europe (after Matte 1986), with paleogeographic zones. 1 Principal thrust faults; 2 internal crystalline nappes and ophiolitic sutures; 3 domains of flow cleavage or foliation ; 4external basins of DevonianCarboniferous age; 5 platforms or blocks with little or no Variscan deformation; 6 direction of transport of nappes and verging directions of major recumbent folds; 7 major ductile transcurrent faults
To the south, in the Moldanubian zone, the Devonian is detrital, with black shales and even culm facies in the Upper Devonian. In contrast, in Bohemia it consists of bioclastic limestones and nodular limestones with planktonic and nektonic faunas. The Lower Carboniferous is a facies of goniatitic shales and flysch (culm facies) or limestones (Armorican massif). In the Upper Carboniferous, intermontane limnic coal basins appeared which continued until the Permian in the French Central Massif. Further south (Montagne Noire, Pyrenees, northern Spain, and Sardinia) the dominant
The Major Stages of Earth History
120
. . .}::\. . . . : :. J~.• .·.• :.{.~L. . . L.
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190
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2
Fig. 86. Evolution of the Variscan chain in Europe as seen in an Ardennes-Massif Central transverse section (after Matte 1986). 1 Silurian, initial stage, closure of the Rheic ocean (to the north) and Galicia-Massif Central (to the south) by subduction of oceanic crust. 2 Lower Carboniferous, final stage, hypercoliision with intracrustal thrusting
sediments of the Devonian are limestones such as the Griotte marble. This facies continued until the Lower Carboniferous in Asturia, then early paralic sedimentation, still influenced by the marine environments of Asturia, was established. In the Pyrenees, the Upper Carboniferous tends to be detrital and of flysch facies below the Permian beds. Tectonic Evolution. Two periods can be distinguished. 1. An Alpine period, in the Devonian, marked by one or two ocean closures followed by subductions, obductions, and collisions between Africa and Laurussia. The scenario proposed by Matte (1986) is: opening of two oceans in the Cambro-Ordovician, the Rheic in the north, the Galicia-Central Massif in the south 7 ; their closure by subduction of the oceanic crust beginning in the Silurian (Fig. 86) with completion in the Devonian as shown by subductions and collisions associated with ophiolitic sutures in a northern belt, Cape Lizard-North Bohemia, and a southern belt, northwest Spain, Groix island, Vendee, Central Massif, Bohemia (Fig. 85). These belts are marked by high-pressure metamorphism (eclogites, granulites, blueschists), volcanism, and intrusions. A "barrovian" metamorphism is associated with the first stage of the emplacement of the major crystalline nappes in Spain, Vendee, Central Massif, and southeast Bohemia. The Upper Devonian movements define the "Breton" tectonic phase. 7 Other authors assume only a single middle European ocean between a peri-Gondwanian platform and Laurussia.
The Paleozoic: the Formation of Pangea
121
2. A "Hercynian" period, with hypercollision or ensialic orogeny of the "continental subduction" type (Mattauer 1983, 1985) caused by a decollement of the crust and mantle. The result was a stacking, during the Carboniferous, of large thrust units bounded by flat intracrustal decollements. This appears to be confirmed by seismic research of the ECORS8 program. The crustal thickening induces an intense metamorphism, the formation of granulites, partial fusion of the crust, and the emplacement of anatexic granites. Also, ductile transcurrent faults, both dextral and sinistral, were produced parallel to the chain. They appear similar to structures described from the Himalayas. Extension subsequent to the major collision phase was responsible for varied manifestations of volcanism, tholeitic eruptions in the Lower and Middle Carboniferous of central and western Europe, and potassic ignimbrites in the Moldanubian zone in the Upper Carboniferous. Three phases of deformation are classically distinguished: 1. A Sudetic phase (Lower Carboniferous) during which central Europe emerged with paralic coal basins. From this period on, the erosion of the rising mountain chain filled first the syntectonic basins and then the paralic environments with detrital sediments. 2. An Asturian phase (Upper Carboniferous) affecting mainly the RhenoHercynian zone and its foreland and especially the coal-measure sediments. The result of this tectonic phase was a fan-shaped orogen, in cross-section, i.e. an orogen with a double vergence in which metamorphism and deformation became younger from the interal zones to the external zones in the north and south (Fig. 86). 3. A Saalian phase between the Autunian and Saxonian, consisting of compressive and extensional readjustments. Conclusions. The collisions resulting from the Lower Paleozoic convergence of North America, Europe, and Africa, during the reassembling of Pangea, created two major orogenic belts: (1) the Appalachian-Caledonide belt; and (2) the Hercynides-Mauritanide belt. The mechanics of the collision between Africa and Europe are still unknown in many respects. Some, like Matte, believe that a major dextral transcurrent fault between Africa and Europe played a major role in separating, temporally, two orogenies; on the one hand, the Variscan chains and the northern Appalachians (compression SE-NW between Africa and Europe-America); and on the other hand, the Mauritanides and the southern Appalachians (compression WSW-ENE 8A
study (ECORS = Etude des Continents et des Oceans par Reflexion Sismique) of the continents and oceans by reflection and refraction seismic, undertaken by several French research organizations, has demonstrated the presence of the Dinant Nappe, transported more than 100 km and limited to the north by the Midi Fault, which is also found in North America as the Frontal Thrust of the Appalachians.
122
The Major Stages of Earth History
between Africa and North America). The rotation of Africa towards the west may have resulted in the opening of a Paleotethys, until then nonexistent to the south of Europe. According to another model, the West African craton, acting as a ram against the North American craton, may have caused the European craton to slide sinistrally to the northeast along the Great Glen fault (Sect. 2.1.2.2, this Chap.). Recent paleomagnetic measurements allowing more precise fixing of relative movements of the continents may enable this question to be resolved. Finally, it is worth noting that the Variscan chain, in contrast to the Caledonian chain, is particularly rich in mineralizations: Pb, Zn in the platform limestones; Cu, Pb in the rifts (Silesia, Rio-Tinto); Sn, W, V in the Carboniferous granitic plutons of Cornwall, Brittany, Central Massif, Germany, and Siberia. 2.2.2.3 Other Tectonic Manifestations in the World The formation of the Asian continent as a single unit began in the Lower Paleozoic with the approach of the Siberian, Chinese, and European plates, and the first deformations of the Angara and Ural belts, continuing until the Upper Paleozoic. Consequently, the ocean separating Europe and Siberia disappeared in the Upper Carboniferous and the Uralides arose as a consequence of deformations which continued into the Upper Permian. Similarly, China and Siberia were joined in the Upper Carboniferous. In Afghanistan, the Asian margin was the locus of successive orogenic phases induced by ocean consumption and collisions with continental blocks and island arcs. Elsewhere in the world, the Variscan orogeny had numerous repercussions: 1. At the active peripacific and periarctic margins. These include the western American (Cordilleran area with the Antler orogeny, the Andean area, and the Franklin belt of northern Canada), the Tasmanian, New Zealand, Antarctic, and Japanese margins. In most cases deformation occurred from the Middle to Upper Devonian, climaxing in the Carboniferous (northern Canada, Tasmania, Japan) often with mountain building. The last traces of activity are recorded in the Upper Permian. Apart from deformation, granitization, metamorphism, and volcanism have been observed in most ofthe mentioned orogens. 2. On the cratons. It appears that the principal period of tectonic activity occurred in the Lower Carboniferous, especially between the Upper Visean and the Namurian, when it was almost worldwide in its effects, including North America, the Russian platform, Siberia, Altai, southern China, and the Sahara. An important regression was associated with it. In North America, the uplift of the Marathon Mountains in southern Texas occurred at this time together with thick local accumulations of detrital sediment, as shown by the 4000 m of red sandstones, conglomerates, and evaporites in the Paradox Basin of Utah. Mineralization
123
The Paleozoic: the Formation of Pangea
*
Paleopoles
~ Very humid
E=-:3
Humid
1;:;'-:1 Subhumid
[.:.::;.;] Arid
Fig. 87. Distribution of climatic zones on Pangea at the end of the Paleozoic as proposed by Hay et at. (1981)
is another indication of tectonic activity, affecting, for example, the red sandstones of the Colorado Plateau. The fact that copper is associated with lagoonal sediments and uranium with fluviatile sediments indicates an environmental control. 2.2.2.4 Climatic Effects The formation of such a vast continental mass as Pangea and its succession of orogenies strongly affected the climate during the Upper Paleozoic. The increase in continentality and an average elevation twice as high as that of today's continents increased the climatic contrast (Fig. 87) and the aridity of the land. The common occurrence of evaporites in the Permo-Trias is evidence for this. Their effects, however, were modified by the development of vegetation on the land. In addition, from the Devonian to the Permian the South Pole was located in Gondwana, a situation which must have favored the important Permo-Carboniferous glaciation (Fig. 88). In contrast, the North Pole was located in an ocean.
124
The Major Stages of Earth History
Fig. 88. Distribution of ice in the southern hemisphere in the Pennsylvanian and Permian (after Eicher and McAlester 1980). Arrows indicate flow directions of the ice. Apparent South Pole polar wandering curve in the Paleozoic according to Eicher and McAlester (1980), dashed line, and according to Caputo and Crowell (1985), solid line
Lower and Middle Devonian. The climate was warm and rather dry. Reefs flourished in the oceans up to 40° latitude; the continents were somewhat dry with little vegetation covering the high relief of the Caledonian mountains. The continent of the Old Red Sandstone, therefore, had a tropical semiarid climate of the Chad type. In fact, evaporite basins formed at this time in North America. Upper Devonian. A relative deterioration of the climate occurred, related to the first phases (Breton and Acadian) of the Hercynian orogeny, which closed the ocean and its east-west circulation between Gondwana' and Laurasia. Humidity increased and the oceans cooled. Minor glaciations took place in the eastern United States (New York State and Pennsylvania) and north of the 40° S latitude in Brazil and perhaps Niger and Ghana. Lower Carboniferous. The same climatic trend continued, as shown by the rarity of reefs in the oceans, partly maintained by the newly formed continental relief and in spite of being moderated by the great Dinantian transgression. Thus, in spite of a warm humid climate without seasons in the equatorial zone, mountain glaciers developed in Argentina and South Africa. According to Chumakov (1985), however, there was no glaciation at all in the Lower Carboniferous. Upper Carboniferous. The humidity, together with the other factors mentioned above, caused the development of mountain glaciation in Europe and Australia as well as the expansion of forests. The equator was at this time located in the southern United States and in southern Europe, where the
The Paleozoic: the Formation of Pangea
125
Hercynian relief had just been created, with evaporite subtropical zones at the present high latitudes. The Northern Hemisphere was slightly warmer than the southern one, as shown by minor reefs in Japan at a paleolatitude of 650 N. Stephanian. This was the epoch of the major Gondwanian glaciation (Fig. 88), traces of which have been found north of the 65 0 S paleolatitude in Brazil, Argentine, Uraguay, Falklands, South Africa, Oman, Dekkan, Malaysia, Pakistan, and Australia. The structure of wood, from that time provided with annular rings, demonstrates the appearance of distinct seasons. This climate contrast may be related to the general lowering of sea level caused by the glaciation. Permian. Glaciation continued into the Lower Permian, especially in Australia and Antarctica, while Africa and South America slowly became free of ice. The climate became generally colder, drier, and seasonal, although the Tethys remained mild and deposited fusulinid limestones, but no genuine reefs. The aridity is indicated by red beds (New Red Sandstones derived by erosion of the Hercynian mountain chain) and evaporites. Desert dunes formed in northern Great Britain have enabled the determination of the direction of the trade winds of the time. In the Middle and Upper Permian a warming trend melted the Gondwana ice cap, leaving glaciers only in the mountains. The European landmass became increasingly arid, as clearly shown by the sequences of continental sedimentation, from the coals of the limnic basins, through coals with halophytic floras, to the desert deposits of the peneplained Variscan chain.
2.2.3 The Marine Environment 2.2.3.1 The Major Sea-Level Fluctuations At the end of the Lower Paleozoic and with the formation of the Caledonian chain, regression was dominant, as is normal during an orogeny, thus the Old Red Sandstone continent of North America and Europe was largely emergent. However, the transgressive seas of the Upper Paleozoic covered much of this continent and also much of Gondwana. Devonian. This period began with a widespread transgression of probable eustatic origin because Europe, North America, and Africa were all affected by it. In the Middle Devonian, faunal exchanges between distant provinces such as the Sahara and North America were facilitated by the widespread nature of the seas. In the Upper Devonian, a general regression was related to a cooling trend and to the Breton and Acadian orogenies. Carboniferous. A new eustatic rise in sea level took place in the Dinantian, leading to one of the most widespread transgressions in geologic history,
The Major Stages of Earth History
126
§j
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Fig. 89. Europe in the Permian (After L ..... _. _.. ___ ..... _. ___ ~I
with its maximum in the Visean. In Europe, the Old Red Sandstone continent was everywhere covered except in Ireland and Scotland. However, between the Lower and Middle Carboniferous the transgression was halted by the Sudetic orogenic phase, causing central Europe to emerge and the Tethys to withdraw southwards. Faunas became reduced in the Namurian because of the shrinking seas which continued into the Middle and Upper Carboniferous with the uplift of the Variscan mountains. The Asturian orogenic phase was manifested in the emergence of southern Europe, North Africa, Himalayas, eastern Siberia, and China. It should also be remembered that the Permo-Carboniferous glaciation, superimposed on the orogenic effects, resulted in a net eustatic character of sea-level changes. The retreat of ice also caused a slight rise in sea level beginning in the Autunian. Permian. The seas withdrew again from the continents. Was this the effect of an emergence of Pangea due to a heat flow not easily dissipated, as suggested by Worsley et al. (1984)? In any case, the epicontinental seas were reduced at this time, and in the arid environments many dried up
The Paleozoic: the Formation of Pangea
127
completely. A last transgression, limited in its effects, occurred in the Upper Permian and the Zechstein sea reoccupied part of the European region (Fig. 89). Then general emergence took place again, more clearly evident at the end of the Permian when the seas were restricted to the Asian part of the Paleotethys, in Armenia, Iran, Salt Range, and southern China. 2.2.3.2 Sedimentation The importance of detrital sedimentation in the Upper Paleozoic should first of all be noted. There are three possible causes:
1. The absence of vegetation on the continents during the Devonian Old Red Sandstone; 2. A marked aridity in the Permian, reducing the vegetation cover; 3. The frequency and importance of orogenic movements: these were associated, as in previous periods, with highly significant sediments such as molasse, widely deposited in the foreland basins of the mountain belts, or flysch, more proximal and earlier, as seen, for example, in the culm facies with graded sequences. Most of the time, this material was derived from island arcs or the edges of uplifting platforms, and was widely distributed. But chemical and biochemical sediments are also well represented, especially in the Devonian, a period of warm seas, and especially in the Tethys where temperatures remained mild in spite of the climatic vicissitudes. There were several periods of limestone and dolomite deposition, sometimes reef deposits, especially in the Lower Devonian, Lower Carboniferous, and Upper Permian. Of the chemical deposits, evaporites were important from the Upper Devonian to the Permian, but especially in the Permian during the final stages of formation of Pangea. The largest known salt deposits date from this time: 1500 m thick in southeast New Mexico and 1500 m thick near Stassfurt, Germany. The sedimentary environments were distinct, and quite comparable to those of modern times. The cratonic areas, often largely covered by shallow seas, mainly deposited chemical sediments but also reworked detrital sediments. Sometimes these sediments are immature, indicating local tectonic movements at the time of their deposition. The continental margins had platform environments that were generally limited oceanwards by barrier reefs, and deeper environments in the marginal ocean basins, where the sediments were detrital and included some volcanics. For example, in the Lower Carboniferous, the margin of the European continent was a carbonate platform bordered by a reef rimming a deep basin of culm facies. Marginal ocean basins were present offshore of most of the principal continents, North America, Eurasia, and Gondwana, and they were generally filled with more or less coarse detrital sediments. Occasionally, graptolitic shales (western American Cordilleran belt) or even cephalopod limestones (Permian of Timor) were deposited.
128
The Major Stages of Earth History
On the adjacent platforms, certain epochs of the Upper Paleozoic were characterized by a particular type of deposit, very widely distributed: Lower Devonian. At the edges and on the platforms of North America and Eurasia, carbonates and shales were dominant, with occasional terrigenous debris eroded from the Old Red Sandstone continents, as in western Europe. From Poland and Scotland, where they formed the Rhenish facies, these sediments passed southwards and oceanwards into the more carbonaterich sometimes reef Hercynian facies. Around Gondwana the platform sediments were dominantly sandy. Middle Devonian. Carbonate and reef sediments were prevalent worldwide except for the peri-Gondwana belts, which were characterized more by sands and shales. Local detrital sediments can be related to the Acadian orogeny, as in the Catskill Delta of the Appalachians. Carboniferous. After an initial period of carbonate sedimentation (interior USA, Franklin belt of northern Canada, Europe, Siberia, and Himalayas) the Carboniferous became universally detrital and often coarse (culm facies) due to the orogenic activity of this period. Marine facies were often lacking in the Gondwana regions as a consequence of the extension of the southern hemisphere ice sheet. On the continents, chemical sediments accumulated wherever terrigenous sediments were absent. Thus, 1330 m of evaporites in the Paradox basin of Utah, in the United States, and similar sediments in the arctic archipelago were deposited. Permian. The typical Permian sequence is: detrital quartz and clay rich sediments; followed by limestones and dolomites; and finally evaporites, seen especially in the Urals and Europe. Very often incomplete, this sequence represents a progressive reduction in tectonic activity, a growing aridity of climate and perhaps also a general uplift of Pangea (see above). Arms of seas extending onto the land tended, therefore, to be cut off and become supersaline. The Zechstein sea is a very good example of this (Fig. 89). The Castile Sea in the southwest United States shows a similar evolution.
2.2.3.3 Marine Life The Lower Devonian transgression revitalized the biological activity in the oceans by providing new biotopes. The tidal and subtidal zones, and especially the reefs, were habitats teeming with life. Similar conditions returned in the Lower Carboniferous when warm seas favored the development of brachiopods and corals. However, apart from these two epochs, the formation of Pangea and the related orogenies combined to restrict epicontinental seas, and limited the growth and expansion of varied faunas by virtue of the increased biological competition, at the same time reducing
The Paleozoic: the Formation of Pangea
129
provincialism. These trends were accentuated at the end of the Permian, a period when numerous groups disappeared, the first distinct reversal of the unlimited expansion of the early faunas. There has been much speculation about the reasons for these extinctions. Certainly the orogenies, the deterioration of climate, and especially the Permian restriction of epicontinental seas and the abnormal salinities were among the contributing factors. However, there may have been other causes. Intense volcanic activity as shown for example by the basaltic flows of Siberia, (Officer and Drake 1986), and subsequent high atmospheric dust, could have decreased solar radiation and induced a general cooling.
2.2.4 The Continental Environment This increased sharply in extent in the Upper Paleozoic, at the time of a remarkable combination of circumstances; the formation of Pangea and the existence of biological systems sufficiently developed to be able to conquer it. For the first time, the data are numerous enough to allow reconstruction of the different components of the terrestrial environment: the landscape, the sedimentation, tectonism and its life. The formation of a supercontinent in the Upper Carboniferous has been proven in several ways. 1. Paleomagnetic data prove it, and are compatible with several hypotheses for the particular mode of assembling of the continents. 2. The glaciation in Gondwana. The evidence for this is now dispersed on continents which are very far apart. The reconstruction of an ice sheet of reasonable dimensions, therefore, implies that all these continents were joined in the Upper Carboniferous and Permian (Fig. 88). A particular flora, the Glossopteris flora, was associated with this glaciation and, though more widespread than the glacial sediments, its distribution leads to the same conclusion. 3. Distribution of terrestrial faunas. A Pangea implies the existence of homogenous terrestrial faunas cosmopolitan over very wide areas. For example, the Permian reptile Mosasaurus, adapted to fresh water and a poor swimmer, is found in Brazil and South Africa. These areas, therefore, must have been joined in the Permian, for it would have been impossible for the mosasaurs to have swum 5000 km across the South Atlantic as it is today. 4. Structural continuity. The juxtaposition of the now fragmented orogenic belts also leads to the same conclusion that there was a single large continent. In this way, it is possible to reconstruct an Upper Paleozoic belt stretching from South Africa to Australia across Antarctica. Similarly, the joining of northwestern Europe and northern Africa shows the alignment of the Grenville-Gothide (Precambrian), the CaledonianTaconic, and the Acadian-Hercynian orogenic belts.
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The Major Stages of Earth History
2.2.4.1 Sedimentation Devonian. The Old Red Sandstone continent was a remarkable paleogeographic entity, covering Spitzbergen, northwestern Russia, eastern Greenland, western Norway, and Scotland. The products of erosion from the Caledonian chain (arkosic sands, silts, shales, conglomerates) were spread over extensive alluvial plains with deltaic deposits where crossstratification, current ripples, and mud cracks have been recognized. Lacustrine deposits were also formed, in Scotland for example. In places these sediments were very thick in intermontane and extramontane basins. Their bright colors (red and white sandstones and varicolored shales) attest to the predominantly arid climate, as do facetted pebbles, frosted sand grains, aeolian dunes, and desert crusts. It should be noted, however, that the first coals were formed in northern Russia. The Old Red Sandstone facies is also known from the Siberian platform (Angara), from Kamtchatka and from China. The deposition of the "Lower Continental beds" in the Sahara, consisting of detritus distributed across a vast slope, drew to a close in the Lower Devonian, having started in the Cambrian. Carboniferous. A striking contrast was established between the northern and southern hemispheres in the Upper Carboniferous. In Gondwana, sedimentation was essentially glacial, with tillites containing striated pebbles, varves, eskers and floors channeled by ice flow. The measurements of flow directions from these traces suggest that there were two centres of glaciation, one in southwestern Africa, the other in eastern Antarctica (Fig. 88). In North America and Eurasia, on the other hand, coal-rich sediments were deposited in a warm climate over most of the emerged land, forming the coal-measure strata of North America and western Europe, in the latter case occurring in a great elongate coal basin which can be followed from England to Silesia. The coal-measure sedimentation was influenced by the tectonic evolution during the Carboniferous. In the Middle Carboniferous, while the land was still without much relief, paralic basins with coastal swamps were characteristic. The combination of sea-level fluctuations, subsidence, prograding sheets of continental detritus from meandering rivers, and a luxurious vegetation, lead to cyclic deposition, often very thick (5000m in the Saar and Lorraine basin with 560 coal beds). Stratigraphic correlations between basins depend on fossil plants and on tonstein horizons (see methods of correlation above). In the Upper Carboniferous, the basins became limnic and localized in the depressions of the Variscan chain as it was uplifted (e.g. French Massif Central). Permian. Marine regression and a climate once again arid created conditions similar to those of the Devonian. In Western Europe, erosion of the Variscan chain leads to the deposition, below 40° latitude, of red molasse (New Red Sandstone), often with evaporites, in intermontane basins. These were relatively restricted in size, rapidly subsiding, and often localized along
The Paleozoic: the Formation of Pangea
131
transcurrent faults. Carbonaceous limnic basins were developed above 40° latitude (e.g., in Siberia), with a warm climate flora. In Gondwana, land emerging from the retreating glaciers was covered again with vast alluvial plains, especially in South Africa and Madagascar (Karroo Formation). Carbonaceous sediments were able to form in temporary lakes and swamps situated at the foot of wooded slopes. The climate tended to be cold, as shown by coals with Glossopteris deposited at only 5° from the Soutp Pole. From the Carboniferous to the Permian, therefore, a migration of coal deposition took place from the equatorial zone to higher latitudes. 2.2.4.2 Life on the Continents Plants were the first to conquer the land in the Silurian. In the Upper Paleozoic, three conditions converged to permit living organisms, especially the more evolved woody plants, fishes, tetrapods, and arthropods, to consolidate this conquest and to carry it further: 1. The existence of very large continental masses, allowing faunal exchange and reducing provincialism. 2. The appearance of a "breathable" atmosphere 9 permitting lung and tracheal breathing. 3. Thickening of the ozone layer in the high atmosphere, causing the elimination of most UV radiation. The coal measures represent the most important evidence of the success of the terrestrial conquest.
2.2.5 Conclusions on the Upper Paleozoic Initiated during the Caledonian cycle, the assembling of the continents to form Pangea was completed in the Hercynian cycle. The collisions consequent to this resulted in numerous mountain chains almost throughout the globe. The orogenies and marked continentalism had the following important consequences: 1. Epicontinental seas were of minor significance on the continents. Only two major transgressions occurred, one in the Lower Devonian, the other in the Lower Carboniferous. A last one in the Upper Permian was relatively minor. 2. Abundant detrital sediments, both continental and marine, were eroded from the newly created relief. 3. The climate was sometimes extreme, with glaciation in the southern hemisphere, warm and humid conditions in the northern hemisphere in the Upper Carboniferous, and arid conditions with red beds in the Devonian and Permian. 9With an oxygen content of about 80% of the present value.
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The Major Stages of Earth History
4. There was a systematic colonization of the continents by plants and animals, which slowly moved out of the aquatic realm. Fish, amphibians and reptiles appeared successively, the acquisition of lungs apparently requiring an oxygen content in the atmosphere of about 80% of the present level. Also, the vascular cryptograms, the seed ferns and the conifers spread throughout immense forests which were the precursors of the widespread coal measures. After this tremendous leap forward, the pace of biological evolution suffered a serious setback at the end of the Permian with the disappearance and reduction of many animal groups.
3 The Mesozoic and the Cenozoic: Breakup of Pangea Starting about 230-245 m.y. ago, Pangea once more fragmented into several blocks, broken off during rifting phases and separating from one another during ocean spreading. The variable times of opening of the oceans and the finite surface of the Earth meant that the opening of one ocean had to be counteracted by the closing of another ocean. The fact that certain openings that began in the Jurassic are still continuing today means that we have at our disposal oceanic sediments which have never been emergent, and which reflect, in the most direct way, the entire history of the Mesozoic and Cenozoic oceans (see for example Chap. 3, Fig. 24). This is not possible for older sediments because they have all been incorporated into emerged continents. The Alpine cycle is closely related to the evolution of the Tethys which consisted of an initial opening phase, then a phase of closure due to the juxtaposition of the Eurasian landmass and several Gondwana elements. The end result was a succession of mountain chains, collectively called the Alpine system, which stretch from Spain to the Himalayas and which are aligned to the east with the Indonesian and peri-Australian belts. This cycle extended over two eras as follows: 1. Mesozoic, corresponding to the initial dispersal of continents, to the opening of the Atlantic, and to the first movements of the Alpine orogeny. 2. Cenozoic, characterized by the uplift of the Alpine chain and a slow deterioration of climate leading to the late Tertiary and Quaternary glaciations.
This last page in the Earth's history seems at first glance to be the richest in variety of events, as indicated for example by its finer subdivisions of geologic time. However, in reality this merely reflects the greater and more detailed knowledge that we have of post-Permian time, and which progressively increases towards the Quaternary.
133
The Mesozoic and Cenozoic: Breakup of Pangea No b 65
72
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Systems
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Barremian Hauterivian
cornian
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Early
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Fig. 90. Subdivisions of the Mesozoic. Two radiometric age scales: a according to Odin et al. (1982b); and b according to Van Eysinga (1985)
3.1 The Mesozoic 3.1.1 Boundaries and Subdivisions The duration of the Mesozoic was 180 million years. Its lower boundary is difficult to determine, generally occurring in the middle of continental facies, the Permo-Trias of French geologists. Only the Palatine phase, corresponding to the uplift of the Urals, introduced a break (discordance) in the East European and Russian sections. In contrast, the upper boundary is sharp, at least in a biological sense, because it coincides with a significant faunal renewal, the real cause of which is still far from being understood. In North America, the Laramide phase also emphasizes the importance of this boundary.
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The Major Stages of Earth History
Fig. 91. Paleogeography of 1 the Triassic and 2 the Upper Jurassic about 200 and 150m.y. ago . Fragmentation, then dispersion of Pangea (Daly 1984, modified from Irving 1977). Dashed line indicates boundary between oceans and epicontinental seas
The Mesozoic consists of three major sedimentary cycles which have been used to subdivide it into Trias, Jurassic, and Cretaceous lO , each in turn subdivided into stages (Fig. 90) . The top of the Trias coincides with the top of the Rhetian, and the initiation of the major Jurassic transgression, which was effective until the Hettangian . In Southeast Asia at this level, a discordance has been recognized indicating the Cimerian orogenic phase. In many areas, the Trias was a period of shifting paleogeography and ephemeral seas. For this reason, two stratigraphic scales have been constructed in Europe, one for the Germanic Trias of continental and lagoonal facies, the other for the Alpine Trias, of entirely marine facies. The JurassicCretaceous boundary coincides generally with the end of a major regression in the Upper Jurassic, seen in numerous areas around the world. It coincides also with a phase of orogenic activity, the Nevadan phase in North America and the Neocimerian phase in Asia . IOThe Trias refers to the three major lithological assemblages which constitute it in Europe, the Jurassic to the region where this system is particularly well represented , and the Cretaceous to the chalk facies so common in its upper part.
The Mesozoic and Cenozoic: Breakup of Pangea
135
Fig. 92. Closure of the Paleotethys and opening of the Neotethys during the Trias and Lias (inspired by Sengor 1979) Arrows denote trends of oceanic opening
3.1.2 The Major Geodynamic Events 3.1.2.1 Trias-Jurassic: Fragmentation of Pangea (Fig. 91)
The first indications of a breakup of Pangea are already discernible at the end of the Permian, with the appearance of megafractures in Scotland, Norway (Oslo graben), North Sea, France (Vosges, Esterel), and Gondwana (first break between Africa, Madagascar, and India, opening of the Mozambique Channel), and the eruption of calc-alkaline volcanics in Europe, Asia, and Gondwana. The first fractures in Australia even date from the Devonian.
From the Paleotethys to the Neotethys. The Tethys of the Paleozoic continued into the Trias as a gulf between Gondwana and Eurasia opening to the Pacific in the east and closed to the west somewhere in Asia Minor. This was the permanent Tethys (Aubouin et al. 1980) or Paleotethys. To the south of this region, the Neotethyan ocean gradually opened from the SE to the NW beginning in the Trias (Fig. 92) with a classical rifting stage (subsidence of continental crust) and then spreading (with formation of oceanic crust). The Cimerian continent (Tibetan), extending westwards as Iran and Afghanistan, broke away from Gondwana during this opening, drifted north, and so first reduced the Paleotethys then closed it by collision with Asia. This closure, complete by Dogger time l l , is recorded in the Indosinian suture running from northern Iran to China. These events, together with major volcanic outpourings on the Siberian platform, can be 11 In another scenario two openings are envisaged; the Mesotethys and the Neotethys, producing two continents (North and South Tibet), and leading to two collisions with Asia, the later one occurring between the Upper Jurassic and the Neocomian.
136
The Major Stages of Earth History
Fig. 93. Tethys in the Upper Jurassic (after Bernouilli and Lemoine, 1980). 1 Caribbean Tethys; 2 Central Atlantic; 3 Liguro-Piemontain ocean; 4 open Tethys due to drift of the Cimmerian oceans; 5 margin of northwestern Australia. A Apulian block; EM East Mediterranean; P possible remnant of Paleotethys
related to the Cimerian tectonic phase culminating at the end of the Trias and known from Iran, the Balkans, and southeast Asia. In the Upper Trias, the Neotethyan rifting spreads to the submerged East Mediterranean platform, fragmenting the thick limestones which had been deposited on it into large blocks. Further west, i.e. from the western Mediterranean, the rifting began also in the Upper Trias but was conditioned by the earlier formation of SW-NE Variscan faults. The rifting here also affected the emerged land bordering the future Central Atlantic, including Morocco, northwestern Africa (Liberia , Sierra Leone), and the eastern border of North America. These grabens, generally accompanied by basic intrusions, functioned as continental basins filled with sediment which formed the base of most Mesozoic sequences. A tensional phase of tectonism also occurred in northern Africa, Spain, southeastern France (Cevenole margin), and in the Pyrenees region (outpourings of ophites) . The Jurassic Tethys. After the Trias, rifting continued but generally affected structures different from the preceding ones. Evidence for this is seen in the tilted blocks, which affect the Trias but also the earliest Jurassic platform deposits of northern Africa, southeastern France, the Subbetic domain, and the Apennines, for example. Following this rifting, there was plate movement and the formation of oceanic crust. To the east of the Adriatic promontory (Apulian block), rifting began early during the Lias, while to the west (Fig. 93) it was not until the end of the Dogger that two Neotethyan segments, separated by the Maghrebian-Ligurian transform domain, (Bernouilli and Lemoine 1980) began to open as the Liguro-Piemontais
137
The Mesozoic and Cenozoic: Breakup of Pangea Middle Jurassic 3
Oxfordian to Kimmeridgian 2
~r Fig. 94. Evolution of the western margin of North America in the Jurassic (After Roure and Sosson 1986)
1: America-Mexico craton 2: FranCiscan Basin 3: North American craton
Ocean and the Central Atlantic Ocean. As a result, the American block was displaced to the west relative to northern Eurasia and there were two further consequences: 1. The opening, at the beginning of the Upper Jurassic, of the westernmost Tethyan segment, comprising the Gulf of Mexico to the north and the Caribbean to the south, separated by a transform zone between the two Americas. 2. An activation of subduction in the East Pacific region at the western margin of the American Plate. According to Aubouin et al. (1986) this subduction was to the west during the Jurassic and resulted in collision between the West-American margin, passive since the Paleozoic, and a volcanic island arc (American-Mexican block corresponding to the Sierra Nevada, Klamath Mountains, and Blue Mountains). The American margin was overridden by this block in the Upper Jurassic (Nevadan phase), while the Franciscan intra-arc basin was formed (Roure and Blanchet 1983; Roure et al. 1986; Fig. 94). Further north, on the other hand, it appears that the Pacific was subducted under Alaska. Finally, ophiolites were obducted on to the Canadian margin in the Lower and Middle Jurassic.
Other World Events. From the Upper Trias to the Dogger, fractures and associated basaltic extrusives were generated between the Africa-South America and Madagascar-India-Antarctica-Australia blocks and in South Africa (Triassic basalts of Drakensberg). These groups became separated at the end of the Jurassic, giving rise to the Mozambique Basin and to the Indian Ocean. In the Upper Jurassic, deformation was widespread. Apart from the already mentioned Nevadan phase in the North American Cordillera, an Andean phase in the Kimmeridgian affected the western margin of South American. Finally, at the end of the Jurassic, the Tethys was subjected to compression, especially at its two extremities, probably a consequence of the initial opening of the South Atlantic (see below). To
138
The Major Stages of Earth History
Fig. 95. Paleogeography of I the Lower Cretaceous and 2 the Upper Jurassic at about 125 and 75 m.y. ago (Daly 1984, modified from Irving 1977). Dashed line indicates boundary between oceans and epicontinental seas
the west occurred thrusting, folding and metamorphism in the Caribbean (Venezuela, Colombia); to the east (inner Hellenides, Dinarides, Turkey, Iran, Arabia, Caucasus, Crimea and the Far East) deformation, subduction with granodiorite plutonism and andesitic volcanism, metamorphism and obduction of ophiolites. In the same epoch, the Rangitata orogenic phase occurred in New Caledonia and New Zealand at a time when the latter began to separate from Australia. A mountain chain formed in Japan at the end of the Jurassic and in the earliest Cretaceous time due to a collision between Eurasia and the microcontinents. These latest Jurassic events define the Neocimerian phase, one consequence of which may be the rather general regression which characterized the end of the Jurassic. 3.1.2.2 Cretaceous: Opening of the Atlantic, Initial Closing of the Tethys
Formation of the Great Southern Ocean (Fig. 95). The Atlantic Ocean opened gradually from south to north during the Cretaceous and part of the Tertiary. In fact, the separation of South America and Africa had already begun in the Upper Jurassic following the phase of rifting. The basaltic volcanism associated with this is seen up to the Upper Valanginian at the continental margins of Brazil and Namibia. The sea invaded this rupture, in the Oxfordian to the south, in the Aptian to the north. In the Middle Albian, marine pelagic faunas were able to pass from the Central to the South Atlantic, a movement facilitated by the higher sea level characteristic of this epoch (Fig. 50). At the end of the Cretaceous, the South Atlantic was 3000 km wide. The opening of the Atlantic, from south to north, reached the Tethys near the Central Atlantic and enlarged the latter from the Middle Albian. Beyond this area, i.e. north of the Maghrebian-Ligurian transform
139
The Mesozoic and Cenozoic: Breakup of Pangea Tithonian to Neocomian Post-tectonic plutonism
Albian to Campanian Franciscan collision
Late Cretaceous
Continental subduction
Plutonism
Fig. 96. Evolution of the western margin of North American in the Cretaceous (After Roure and Sosson 1986)
domain, the Atlantic opened between Eurasia and North America, where already since the Upper Trias an arm of a northern epicontinental sea had spread into the rift valleys. It is probable that opening took place near offshore Spain from the Upper Aptian, following rifting initiated at the beginning of the Cretaceous. That would explain the cutting off of western Europe from the Tethys and its final dependence on the North Sea and the Atlantic, beginning in the Albian. At the beginning of the Upper Cretaceous, a rupture appeared between Greenland and Europe with only minor subsequent separation until the Paleocene, when the opening of the North Atlantic to the Arctic ocean was completed. However, also in the Upper Cretaceous, another passage was opened between the Baffin and Labrador seas.
Consequences of the Opening of the Atlantic. The acceleration of the movement of the two Americas to the west intensified and multiplied the tectonic phases of the Pacific Cordillera bordering these continents, and increased magmatic activity. It also led to the collision of Asia and Alaska in the Cenomanian. The western margin of North America overrode the Franciscan ocean basin, which became closed (Fig. 96).
140
The Major Stages of Earth History
o
~ 190
131)
Fig. 97. Relative movement of Africa (shown as the coast of Libya) in relation to Europe from the Pliensbachian (Middle Lias) to the Present. The line represents the path of a point in central Libya during that time. The figures are millions of years (After the paleogeographic maps of Dercourt et al. 1985)
From the Albian to the Turonian, a new arc-continent collision took place (Franciscan collision, contemporaneous with the Oregonian phase). This built the Coast Range Mountains and was characterized by blueschist metamorphism, plutonism, and volcanism and also, it seems, by a "collage" of foreign blocks called "suspect terranes" transported often from far away by displacement along transcurrent faults, sometimes interpreted as oblique subduction (Aubouin et al. 1986). The West-American chain thus acquired a doubly vergent structure: to the west in the Franciscan domain of the Coast Ranges, to the east in the Cordilleran domain extending beyond the Sierra Nevada. This latter domain is progressively tectonized from west to east as far as the Rocky Mountains, which were formed at the end of the Cretaceous during the Laramide phase. The great thrust nappes resulting from this tectonism are interpreted by Roure et al. (1986) as evidence of continental subduction. A doubly vergent chain was also formed along the western margin of Mexico. The western margin of South America was just as active, with a subduction zone, marginal basins, and island arcs distinct since the Jurassic. From the Albian, the active shift of South America to the west affected the development of this marginal complex either by closing, as was the case with the oceanic marginal basin of Magellan in the southern Andes, thereby creating a collision mountain belt, or by failing to develop at all (as was the case with the Andean continental margin basin with a sialic basement, and with the Cretaceous Subandean basin of Peru, which remained a continental graben filled by 6000m of sediment). The underthrusting of a system of arcs
The Mesozoic and Cenozoic: Breakup of Pangea
141
below the American continent, giving rise to vertical movements so characteristic of the Andes, may also have taken place. Several phases of deformation, folding, and thrusting, followed one another, with centres of activity moving successively further to the east, and accompanied by intense volcanism (up to 15 km of volcanics in the southern Andes in Upper Cretaceous). In the central Andes, it appears that the Upper Cretaceous marked the end of a phase of extension, dominant from the Trias, and the beginning of a regime of compression continuing until present day. The complex history of the Caribbean was conditioned by the movements of sinistral transcurrent faults between the two Americas. A CircumCaribbean orogenic phase in the Lower Cretaceous and the opening of the Caribbean Sea in the Upper Cretaceous, when North America began to drift to the northwest, then to the north, are both attributable to these fault movements. Volcanic arcs were formed around the Caribbean Sea near the subduction zones. Finally, the difference in rate of opening between the North Atlantic and the Central Atlantic decreased appreciably in the Campanian (80 m. y.). As a result, the sinistral faulting between Africa and Eurasia, already oblique since the beginning of the Cretaceous, gave way to a distinct N-S convergence between the two continents (Fig. 97). Initial Closure of the Tethys. This also was probably a consequence of the opening of the South Atlantic, which pushed Africa and Arabia against Eurasia. This compression began in the latest Jurassic (see above), and in the Cretaceous it led to oceans closing and sometimes to the obduction of ophiolites. Two paroxysmal periods have been noted, the Albian and the Maestrichtian. However, no major volcanism occurred outside the Carpathians and the Balkan-Caspian magmatic arc. The closure of the Tethys was apparently complex and variable, but it did not prevent a reopening in the Valaisan-Carpathian basin and in the East Mediterranean basin in the Lower Cretaceous, for instance. Major compression began in the Aptian, when the North Atlantic opened and when the eastern part of the Arabian-African block exerted greater pressure against Eurasia (Fig. 97). Principal consequences were as follows:
1. Resorption of oceanic crust by subduction and overthrusting by obduction. These are preserved today in the strongly tectonized blueschist ophiolitic belts. An example can be seen in the belt stretching from the Alpine arc (earliest metamorphism of high pressure-low temperature type, dated at 80m.y.) to Iran and northern Pakistan 12 • 2. Calc-alkaline volcanism at active margins, as seen in the Balkans and southern Carpathians. 12The mechanics of this obduction in the Oman region, where it is of Upper Cretaceous age, have been discussed in the classic works of Boudier and Michard (1981).
142
110
The Major Stages of Earth History
~----- ...
... ....
Fig. 98. Relative movement of Spain in relation to Europe from the Aptian (110m.y.) to the Cretaceous-Tertiary boundary (65m.y.). Arrows indicate sense of transcurrent movement of the North Pyrenees (After the paleogeographic maps of Dercourt et al. 1985)
3. Collisions, exemplified by the Apulian block which, detached from Africa by opening of the East Mediterranean basin, came into contact with the Brian~onnais margin in the Campanian, after closing of the Liguro-Piemontais Ocean. These developments were followed by the Austro-Alpine overthrusting. 4. Rotation of the microplates. From the Upper Aptian to the Senonian, the Ibero-Corso-Sardinian block rotated about 35 0 in an anticlockwise direction (opening of the Gulf of Gascogne) while sliding eastwards along the European margin (Fig. 98)13. The Pyreneo-Provencale compressions beginning in the Albian (Austrian phase) and continuing in the Upper Cretaceous (E-W folding in the Lower Turonian and Upper Coniacian in the Subalpine chain) were a consequence of this rotation. The Apulian block also rotated 20-30 0 in an anticlockwise direction in relation to Africa during the Middle and Upper Cretaceous. 5. Metamorphism, beginning in the Aptian. Other major events during the Cretaceous include the following: 1. Continuation of the fragmentation of Southeastern Gondwana. While the latter shifted towards the South Pole, the Mozambique channel widened until the Turonian, and the Tasmanian Sea between Australia and New Zealand opened almost until the Campanian. Rifting developed between 13 This sinistral movement along the North Pyrenean Fault created pull-apart basins in the Albian, and associated crustal thinning as indicated by high temperature metamorphism and the emplacement of massive peridotites (lherzolites).
The Mesozoic and Cenozoic: Breakup of Pangea
143
Australia and Antarctica. Freed from the Antarctic-Australia-Madagascar block, India shifted northwards, beginning at some time still debated but either Lower Cretaceous or Upper Cretaceous (Enay 1980). The subduction of the Indian plate under Eurasia (southern Tibet) was initiated in the Aptian. In South Africa, numerous veins of diamond-rich kimberlite were emplaced in the Upper Cretaceous. 2. Activity around the Pacific fringe. Volcanic arcs formed in Indonesia in the Middle Cretaceous. In the Upper Cretaceous, marginal seas and island arcs began to appear in the northwest Pacific with the opening of the China, Coral, and Tasmanian seas. In Japan, a number of tectonic phases, serving as a prelude to similar events, appeared only in the Paleogene. Finally, the Brooks orogeny occurred in Alaska and the Arctic Ocean opened in the Lower Cretaceous. At the Cretaceous-Tertiary boundary, a large number of varied events made this transition one of the most distinctive in geologic history. 1. Tectonic. The Laramide phase of North America (uplifts, deformations, batholithic intrusions, and temporary unification of the two Americas), the Arctic archipelago, Central America, and the North and South Caribbean (folding, overthrusting, metamorphism, and plutonism) had repercussions over much wider areas. In the Tethys region, for example, a pulse of compression took place in the Maestrichtian. It was felt throughout the Alpine area (Arvinche phase), and the Pyrenees, and was expressed in the English Channel and North Sea areas by uplift and gentle folding. The effects of this phase were also felt in central Iran. In addition, the Indonesian belt was affected by extension, while Timor and much of the Asiatic continent suffered compression due to the approach of Australia. 2. Eustasy. The great worldwide regression of the Late Cretaceous was undoubtedly a consequence of the Laramide orogenic phase. 3. Volcanism. At the Cretaceous-Tertiary boundary, huge basalt flows such as the Dekkan traps, 106 km 3 in volume and 500000 km 2 in area, generated by a hot spot, were widespread in India during a very brief period of about 500000 years (Courtillot et al. 1986). 4. Climate. The general cooling at the end of the Cretaceous was the end result of a change (see below) owing its origin to a fundamental modification of the system of ocean currents. The growth of continental masses and of the Earth's albedo had a similar effect. Another possible cause of the cooling was the enormous amount of volcanic dust associated with all the volcanic activity in India, producing a circum-terrestrial screen of aerosol which also would have increased the Earth's albedo. This activity could also explain the increase in iridium observed at the CretaceousTertiary boundary, which some attribute to extraterrestrial causes (meteorites, passage of a comet, or an interstellar cloud).
The Major Stages of Earth History
144
Fig. 99. The Sundance Sea in the western United States in the Callovian. The Morrison formation (Upper Jurassic) shown as dotted area; hachured area shows the Sundance Sea
All these events appear to have affected the course of biological evolution, as we will see below. However, according to Plaziat and Ellenberger (1982), no exact synchronism between biological breaks, regression, and tectonic movements can be demonstrated at the end of the Cretaceous.
3.1.3 Environments and Sedimentation Apart from the geodynamic activity, the major changes in sea level and the climate affected the nature, the rhythm, and the intensity of sedimentation. The important Jurassic and Cretaceous transgressions increased significantly the areas of epicontinental seas, although major advances of the sea were interrupted by phases of retreat (Fig. 56). The best examples include: 1. The Sundance Sea in the western United States during Dogger times (Fig. 99) and the Saharan Gulf of the same period. 2. In the Cretaceous, the Neocomian transgression, followed by the great Cenomanian transgression and beginning actually in the Aptian, continued in fluctuating mode until the Senonian. This was a result of the accelerated production of ocean crust as manifested, for example, in the opening of the North Atlantic. The effects were worldwide. The transgressions made Europe the crossroads of the Atlantic, the North Sea and the Tethys with epicontinental connections across France, Russia, and Iberia, but without covering most of the old massifs. The regressions, also significant, were characterized by particular facies: (1) Purbeckian lagoonal-lacustrine at the Jurassic-Cretaceous boundary; (2) Wealdian fluviatile in the Lower Cretaceous; (3) bauxitic and lacustrine in Provence in the Middle Cretaceous and at the end of the Cretaceous.
The Mesozoic and Cenozoic: Breakup of Pangea
145
The Mesozoic climate was warmer on average than that of the Paleozoic, a possible consequence of the absence of major orogenies and the noncoincidence of poles and emerged land. The evolving climate on the continents was controlled partly by the apparent shift of the poles, which from the Trias was in a meridian containing the Earth's present axis of rotation. The widespread occurrence of Triassic red beds reflects tropical climates and contrasting seasons. The Upper Trias was somewhat arid, especially in Europe, as is shown by a band of evaporites stretching into North Africa between 10° and 40° of latitude. In the Jurassic, the breaking up of Pangea and the eustatic rise in sea level created a more humid climate in general, with subtropical conditions existing between the 60th parallels north and south. The warm seas favored the formation of carbonates and reefs 14 . However, from the Pliensbachian, a Boreal province and a Tethyan province can be distinguished on the basis of ammonites. Well established in the Jurassic, they persisted into the Cretaceous. The Boreal province was dominated by siliciclastic sedimentation, the Tethyan province by carbonates. On the continents bauxites were widespread, while coal-forming forests flourished in the Boreal province (e.g., Siberia, Greenland, and Spitzberg). Some aridity is recorded in Upper Jurassic sediments of southern Eurasia (Hallam 1984), and the American continents. In the Cretaceous, the equator was 20° from its present position and the climate was always warm with less contrasting seasons than today because of a weaker latitudinal thermal gradient 15 • However, it appears that the climate in general had become a little cooler (shown by a decline in the madrepores) and a little drier; although, according to Hallam, the reverse tendency occurred in the Atlantic and Tethyan regions where bauxites formed. In the north hemisphere, the Tethys, open at both extremities, played an important role. A warm surface current flowed westwards, promoting the formation of platform carbonates. The closure of this sea in the Upper Cretaceous, and the access of cooler water from the South Pacific to the Central Atlantic when the South Atlantic opened, resulted in a narrowing of climatic zones and an increase in humidity. Europe, Russia, and North America thus became more temperate. The northern hemisphere probably cooled gradually during the Mesozoic, but especially at the end of the Cretaceous, in contrast to the southern hemisphere which very quickly rid itself of its Paleozoic glaciers. Some authors do not share this point of view, believing that the highest temperatures were concentrated in the Cretaceous (Crowley 1983), with a high in the Albian to Turonian period. An appreciable cooling, however, is generally accepted from the Campanian.
14 A reef complex 300 km long stretched between the Ardennes and Morvan in the Paris Basin in the Upper Oxfordian. 15 10 ± 3°C average annual temperature at 85° N latitude in the Albo-Cenomanian according to recent studies, but an equatorial zone warmer than it is today.
146
The Major Stages of Earth History FRENCH WESTERN ALPS Rhodanian-Dauphinois zone
~
Basement
I
Brianconnais
11km
I
Piemontais zone
BATHONIAN
L - -_ _....J
lookm
Fig. 100. Graben subsidence of the European margin after the Dogger in the Alps (After Argyriadis et al. 1980)
3.1.3.1 Trias-Jurassic
Tethyan Domain. Outside the permanent Tethys, where facies remained pelagic, calcareous, or shaly, the Trias-Jurassic sediments reflect the various stages of opening of this ocean. Red beds, fluviatile, deltaic, and lagoonal, initiate the succession and are often very thick (up to 8 km of Upper Trias in the eastern United States) due to deposition in subsiding grabens. These sediments were transgressed by the "re-invading Tethys" (Aubouin et al. 1980) from east to west between the Mediterranean region (flooded in the Trias) and the Caribbean region covered partly by an arm of an epicontinental sea in the Lower Lias (Thierry 1982). After confining itself initially to the early rift grabens, the sea spread onto the adjacent platforms, became hypersaline and precipitated evaporites. Off the coast of northwest Africa these have been dated as Upper Trias in the north and Lower Jurassic in the south (Guieu and Roussel 1984). A volume of 10 million km 3 has been estimated for the salts deposited in the rifts associated with the breakup of Pangea. This suggests a decrease in the salinity of the oceans during the Mesozoic, and the figure of 20% has been put forward for the Cretaceous. The continuity of the Tethys from the Mediterranean region to the Central Atlantic was established only in the Domerian-Toarcian, which was a period of deepening of the Liassic seas. In rocks of the same age, the first mixtures of Tethyan and sub-boreal faunas can be seen in Portugal. Platform carbonates overlying the evaporites indicate the reestablishment of marine conditions. These deposits, beginning and terminating at different times in different places, are of Middle Trias (eastern Tethys) to Upper Jurassic (Caribbean) age. They were affected by tensional phenomena, rifting, stretching, and separation, and by thermal subsidence, which gradually restricted the platforms to horsts and the top of tilted blocks
The Mesozoic and Cenozoic: Breakup of Pangea
147
Fig. 101. Paleogeography of Europe at the maximum Triassic transgression (Keuper), (after Pomerol 1975). 1 Continents without sediments; 2 continental redbeds, generally lagoonal in the Keuper; 3 Germanic Trias (continental, then marine sands of the Buntsandstein, marls, carbonates and evaporites of the Muschelkalk, marls and evaporites of the Keuper); 4 Alpine Trias (sands, then thick carbonates, entirely marine); 5 limits of occurrence of red facies; 6 limits of the Keuper sea
and finally caused these to founder. In the Mediterranean region, this evolution took place from the Upper Trias to the Dogger in the east and from the Upper Lias to the Upper Jurassic in the west. Deep-water deposits (shales and radiolarites), therefore, succeeded the carbonates and became dominant in the basins durin.g the MaIm (radiolarian crisis of the Oxfordian in the Mediterranean region), while encrinites and ammonitico-rosso facies, often with cryptalgal structures, covered the sunken margins and the horsts. The Brian<;onnais zone of the Alps, which is a beautiful example of a foundered margin (Fig. 1(0), shows the following sequence: platform carbonates (Trias to Dogger), then deep-water red nodular limestones and micritic limestones of Callovo-Oxfordian age. This transition to deep-water deposits was accelerated by the Callovo-Oxfordian transgression which was widespread and accompanied by a rise of the CCD (Carbonate Compensation Depth) and the deposition of black shales in restricted environments such as the Dauphinois basin in southeastern France and in the Central Atlantic. Other parts of the platform remained stable until the end of the Mesozoic, as in the Dinarides, external Hellenides and the South Appenines. The top of the Jurassic sequence consists of pelagic limestones (Upper Kimmeridgian to Tithonian), generally attributed to a renewal of planktonic activity. The mobility of certain regions (Carpathians, Hellenides, Dinarides,
The Major Stages of Earth History
148 SE
NW Marginal oceanic basin (Tethyan Ocean)
Continental margin
Marginal cratonic basin
Stable platform (Paris basin. England. Europe)
Plemontais lacies
BrianQOnnais lacles
Dauphinols lacits
Epicontinental facies
Calc-shisls with radiolarites and ophiolites
Condensed deposlte with •Ammonitico-rosso' lacies
Palagic successions with alternallng mari and limestone tayers (several thousand meters thick)
Maris and limestones Reefal lacies - Lagoonal lacies (Purbecklan) at the top of the Jurassic
Fig. 102. Facies variations in the Jurassic from the Tethys (Piemontain Ocean) to the western European platform
and Maghrebids for example), during the Neocimmerian orogenic phase, is emphasized by the presence of flysch and submarine breccias. Cratonic Domain. In western Europe, transgressions of the Tethys spread across a flattened continent covered with detrital sediments. Until the Keuper, the marine invasion was spasmodic and the east-west differentiation of facies particularly strong (Fig. 101). Subsequently, a more complete transgression led to normally saline conditions and a connection between the Tethyan and Nordic regions and in the Upper Jurassic with the Atlantic domain. Figure 102 illustrates the facies variations across the epicontinental seas. 1. North America and Gondwana. On these great cratons, marine influences were relatively modest, leaving immense tracts available for continental sedimentation, derived mainly from reworked Paleozoic material. In North America, continental red beds more than 10 km thick accumulated during the Trias an~ Lower Jurassic, with basalts and andesites intercalated during the Trias. The Sundance sea (Fig. 99) invaded the western regions from north to south during the Dogger, but withdrew in the Upper Jurassic at the time of the Nevadan orogeny, when continental sands with reptile remains (Morrison Formation16) were deposited. Extensive deposits of evaporites were formed around the Gulf of Mexico during the Middle and Upper Jurassic. They mark the establishment of a connection between the Tethys and Pacific oceans from the Middle Jurassic. In Gondwana, Triassic sands with reptile and fish remains were spread widely across East and South Africa, South America, Madagascar, India, and Antarctica. Coal beds of the same age are known from Australia and Antarctica. Evaporitic environments extended to southern Tunisia and the Sahara in the Upper Trias and in the Lias. In the Jurassic, the detrital sheets of the Nubian sands covered the Sahara and 16 Suggesting
a landscape of savannas, lakes, and swamps with a warm climate.
The Mesozoic and Cenozoic: Breakup of Pangea
149
spread across the South Tethyan margin as the shale-sand sequences of the Monts des Ksour in Algeria. Similarly, the platform deposits of the east part of the Arabian peninsula were covered by detritus from the emerged lands to the west. 2. Circum-Pacific Belt. With the emplacement of subduction zones around this belt, thick detrital sequences with radiolarites and volcanics accumulated in marginal ocean basins. In addition, basins around the border of the craton were filled with detrital and carbonate sediments, including the Tithonian to Neocomian molasse coming from the destruction of the Nevadan mountains. 3.1.3.2 Cretaceous The importance of tectonism in the Cretaceous is such that sedimentary domains are best classified according to their stability.
Cratonic Domain. The immense shields resulting from the formation of Pangea were covered by seas only during the major transgressions, and then only partially. This situation allowed their colonization by an abundant and diverse reptile fauna. They were also a source of large quantities of detrital sediment eroded from their highlands. The centre of the North American continent was covered by two transgressions, one from the Gulf of Mexico, the other from Alaska. Before these seas joined at the end of the Albian, they spread across Utah and Nevada over thick detrital sheets (Dakota Group) with reptile remains. The provenance of this material was the Cordilleran Mountains to the west, formed during the tectonic phase of the Middle Cretaceous. The Laramide orogeny resulted in the emergence of all the region that was covered at that time by Lake Laramie. The Lance Formation, derived from it, is very thick (more than 100m) with coal intercalations and remains of the last dinosaurs. South America's history was rather similar. A major transgression spread seas across Peru from the Albian to the Coniacian and created an inland sea in the Maestrichtian in Argentina and Bolivia. Brackish and lagoonal detrital sediments formed in the Amazon basin, while 5000 m of red beds with dinosaur remains, mixed with volcaniclastic deposits, accumulated in the South Andes following a tectonic phase in the Albian. Coal-rich beds and bauxites indicate a warm and humid climate. However, in Argentina in the Middle Cretaceous, evaporites were deposited. In Africa, continental deposits are represented mainly by the "continental intercalations" of Upper Jurassic to Lower Cretaceous age. These are red sandstones and shales found in the Sahara, Egypt, Mauritania, Sudan, and Nigeria. From the Albian to the Lower Turonian one of the most widespread transgressions of the African continent occurred, dividing the region by an arm of a sea stretching from the Tethys to the Gulf of Guinea (Fig. 103). Silty shales with ammonites, intercalated with gypsum in the Sahara, were deposited in this sea. From the Upper Turonian, however,
The Major Stages of Earth History
150
Fig. 103. Upper Cretaceous transgression in West Africa (After Kogbe 1976)
,---,
, ,"
\
I
,
'.. __
'---- .. ,
, Fig. 104. The West European archipelago in the Santonian. Horizontal lines, land; dotted line, deep-water subalpine zone and its Valaisan continuation; dash-dotted line, plate boundaries
The Mesozoic and Cenozoic: Breakup of Pangea
151
this connection was lost and the Sahara was covered by Cenomanian evaporites over an area of 340000 km2 • A second transgression from the Maestrichtian to the Paleocene followed the same route and spread onto the Arabian platform to the east. In central China, a lacustrine environment was established in the Jurassic and Cretaceous. Cratonic Margins. The sediments here are totally or partially marine and many have been tectonically deformed, as in the following:
1. Western Europe: marine conditions were permanent from the Jurassic to the Cretaceous in the relatively deep Subalpine domain (Vocontian basin, Dauphinois zone, Helvetic zone), with broad connections to the Tethys ocean (Fig. 104). Sediments are thick, up to 2000m, pelagic to hemipelagic and predominantly marly and marly-calcareous. The Barremo-Bedoulian, consistently more massive and calcareous, passes into the Urgonian reef complex at the margins of the Vocontian basin. An unconformity within the Aptian separates it from the Gargaso-Albian "blue marls". The latter resemble somewhat, in their facies, the "black shales" of the Atlantic Mid-Cretaceous, which indicate an anoxic event 17 at the same time as a renewal of continental erosion marking the beginning of a "Middle Cretaceous crisis" contemporaneous with the opening of the North Atlantic. The Upper Cretaceous is represented by a shallowing sequence contemporaneous with a tectonism of the basin: marly-calcareous at the base (Cenomanian) and limestones at the top, terminated by an emergence beginning at different times, depending on the location, between the Cenomanian and Maestrichtian. Extending to the west and southwest from this Subalpine sea, the Cretaceous transgression spread during latest Jurassic and earliest Cretaceous time to the peripheral lagoonal environments (Purbeckian facies in the Jura and in Provence), then to the fluviatile and deltaic environments (Wealdian facies of the Paris basin and southern England, and also of the eastern United States). The centre of the Paris basin was reached in the Aptian, joining a north-south arm of the Boreal sea. In the Albian, the transgression reached England. The sediments are predominantly carbonates and bioclastic with minor shaly and sandy beds (Albian). The transgression continued in the Upper Cretaceous, transforming Europe into an archipelago and making connection with the Atlantic. From the Cenomanian to the Maestrichtian, chalk sedimentation dominated all other facies in the Paris basin, England and Northern Europe, while 17With little exchange between deep and surface waters, sediments were enriched in organic matter. Four anoxic crises have been recognized in the Atlantic: CailovoOxfordian, Gargaso-Albian, Cenomanian-Turonian boundary, and Coniacian-Santonian. The three older ones also affected the Alpine Tethys. Bituminous shales in the Lower Toarcian (Western Europe) and Upper Devonian (Europe, North America) indicate older anoxic events.
152
The Major Stages of Earth History Coast
+
+ +
i'tTi' + T + T T
+ + i'+
+
o
Progradation prism
1:--:1 Open marine deposits It::-" 1fluviatile deposits
~
deep saa fans
~
Nearshore and shore deposits
D
lacustrine and fluviatile delta deposits
o
Palustrine then lacustrine deposits
Piemont deposits (alluvial fans)
-
Discontinuities
1
0 0 0 001'
IIi1 Salt
Fig. lOS. Sediments of the West African continental margin (Gulf of Guinea). I Upper Jurassic to Neocomian; Il Aptian to Oligocene; III Neogene and Quaternary (After Delteil et al. 1975, see also Moullade and Nairn 1978, p. 393)
carbonate platforms were developed further south (Aquitaine, Provence, Iberia). The latest Cretaceous regression, somewhat earlier or later according to location, did not affect Holland and Denmark. It was followed by red-bed deposition (southwestern France, Provence), and by erosion (Paris basin). 2. North Africa: the situation here was appreciably different due to strong tectoriic activity and to siliciclastic sedimentation coming from the African Shield. On the platforms extending to the south of the troughs of Rif, Tell and North Tunisia, carbonates were common and evaporites occurred here and there. Emergence was frequent and sandy detrital beds more or less developed (up to 400m of sandstone in the Saharan Atlas), but the Upper Cretaceous transgression was dominant. In Morocco the transgressions generally came from the Atlantic. Continental Margin Domains. Passive margins. The history of the Atlantic margins, including the ocean spreading and the principal events unfolding in the regions behind the margins, has been recorded in the diverse marginal basins, and revealed by seismic methods. On the eastern margin of the South Atlantic, from Angola to Cameroon, the Cretaceous can be divided into two assemblages (Fig. 105): (1) A Lower Continental Unit I (Upper Jurassic to Neocomian) deposited during the initial rifting between Africa and South America. It includes detrital sediments of the piedmont, followed by lacustrine and fluviatile sediments. (2) An Intermediate Unit II (Aptian to Oligocene) recording the gradual establishment of marine conditions during the ocean opening. Significant deposits of salt in the Aptian indicate the beginning of a transgression in a narrow and more or less confined basin.
The Mesozoic and Cenozoic: Breakup of Pangea
153
2 ----,-.-:~
'-' '. ......
Oceanic crust
.....
L......
.......
-
;;...0"
5
" 'III II
II ~ == \I =
If.;::; __
4
.~ -
~
;
q ~""II "'" It /I
-~~v '~""!- ...."-==
--- ,,,
3
___
_ --::i!,::3:==~~::::::::~ . . :.
-"""'..,..,
~
1: subduction trough; 2: Volcanic arc; 3: Deformed sedimentary series of Franciscan basin 4: Great Valley Series; 5: Granodlorltlc plutons of Serra Nevada Klamath axis
Fig. 106. The West American margin (California) in the Middle Cretaceous (After Roure 1981)
The truly marine beds, sands and shales, although intercalated with continental sands, begin in the Cenomanian. Active margins. In the Tethys domain, the often very intense deformation of most of the continental margin has made it difficult, if not impossible, to reconstruct the Cretaceous paleogeography. However, facies sequences restored to their approximate original positions permit the following zones to be distinguished. 1. Highs, often called ridges, with carbonate deposits, either neritic or reefs (South Alps, Carpathians, Hellenides, Dinarides, Appeninnes), or relatively deep slope deposits (for example the condensed and incomplete deposits of the Brianc;onnais). 2. Deep zones where pelagic sequences, submarine breccias, and flysch sediments were deposited. The latter are of varied ages like the deformations which gave rise to them. In the Lower Cretaceous, they can be followed from Gibralter to the Balkans via the Maghrebides, the Appennines, the Alps, and the Carpathians, marking a narrow structural zone of nappes and the boundary between the external and internal zones of the mountain chain. These deep zones became widespread in the Middle Cretaceous (Maghrebides, Pyrenees, Ligurian domain), and were still common in the Upper Cretaceous (Dinarides, Hellenides, external Carpathians, Alps with Helminthoides flysch 18).
In the Circum-Pacific domain, the Cretaceous history of the active margins of the western Americas has been reconstucted in some detail. For example, from the formations of the Coast Ranges in California it is possible to recreate, for the Middle Cretaceous, the following succession from west to east (Fig. 106; Roure 1981): (1) a subduction trench marked by thick detrital deposits, turbidites, and conglomerates; (2) a volcanic arc; and (3) the 118 Which
follows schistes lustres deposited from the Kimmeridgian to the Upper Cretaceous.
154
The Major Stages of Earth History
Franciscan basin undergoing closure, with an accretionary prism of folded and thrusted Mesozoic sediments penetrated by slabs of oceanic crust with radiolarites, pillow basalts, metagabbros, serpentines, eclogites, blue schists, flysch and greywackes together making up the Franciscan complex. The eastern margin of the basin is occupied by the Grand Valley sequence consisting, from west to east, of deep water facies (thick turbidites) overlying ophiolites, then platform deposits. Finally, there was a volcanic arc to the east whose basement now forms the batholith of the Sierra Nevadas and the Klamath mountains to the northwest. Other subduction zones have been interpreted on the basis of deepwater facies overlying ophiolites, for instance in Japan, Sumatra, and New Zealand.
3.1.4 Biological Events The Mesozoic represents a distinctive period in the history of life on Earth because of the exclusive presence of certain fossil groups. It is also a period of transition from the archaism of the Paleozoic faunas to the modernism of the Cenozoic faunas. This transition took place between two mass extinctions marking the boundaries of the Paleozoic and Mesozoic and Mesozoic and Tertiary, boundaries which themselves correspond to particular paleogeographic events. Biological events took on different aspects during the Mesozoic: (1) whole faunal renewals, as seen in the hexacorals replacing the tabulates and tetracorals in the Trias; (2) increased abundance, as seen in the protists, green algae, terrestrial plants, pelecypods, echinoids, fish and birds; (3) total dominance, as in the ammonoids and reptiles; (4) discreet presence, mammals; and (5) extinctions, depending partly on paleogeographic and climatic changes. The regression at the Jurassic-Cretaceous boundary, for example, was damaging to the molluscs and to the algae. But the main disappearances were at the end of the Cretaceous, as much in the oceans (foraminifers, algae, molluscs) as on the continents (reptiles and plants), affecting especially the most specialized and most rapidly evolving species. The massive extinctions of the plankton at the Cretaceous-Tertiary boundary are recorded in sediments by a decrease in CaC03 and 13e. The possible causes of extinctions are numerous and have probably all played a role. Among the more important are the following: 1. Biological competition, relentless on land where the birds and mammals were evolving, and in the epicontinental seas which shrank during the regression at the end of the Cretaceous. 2. Cooling, as shown by the change in terrestrial vegetation. Assuming that this is due to a diminution in solar radiation, related perhaps to an atmospheric screen of aerosol (see above), the reduction in abundance of photosynthesizing plankton can also be explained, as well
The Mesozoic and Cenozoic: Breakup of Pangea
155
as the destabilization of the food chains whose bases are formed by these plankton. 3. Continental fragmentation and dispersion, which reduced the possibilites for migration of various organisms, terrestrial and marine and, therefore, intensified biological competition. 4. Intense volcanicity in India, contributing to acid rain, has also been suggested as a cause.
3.2 The Cenozoic With a duration of 65m.y., this is the shortest of the geologic eras. 3.2.1 Boundaries and Subdivisions (Fig. 107) The lower boundary corresponds to the major break at the top of the Cretaceous. Its wide distribution and significance restrict the number of locations where a continuous transition from the Cretaceous to the Tertiary can be observed. One such site is located south of Copenhagen (Stevns Klint cliffs, at the coast). The Maestrichtian is overlain by bryozoan limestones in which no Cretaceous fauna is discernible and which contain a typical Tertiary fauna of brachiopods, echinoderms, planktonic foraminifera (Globorotalia) and nannoplankton. These beds, overlain without discordance or unconformity by Tertiary marls, constitute the stratotype of the Danian stage, at the base of the Cenozoic. The upper boundary corresponds to the Recent. The stratigraphic subdivision of the Cenozoic is based classically on the evolution of the .;chinoderms, lamellibranchs, gastropods, and large benthic foraminifera. More recently, planktonic foraminifera (globigerinids, globorotalids), nannoplankton organites, pollen, hystrichospheres, and the remains of microforaminifera have been used. The major subdivisions of the Cenozoic are shown in Fig. 107. The Paleogene consists of the Paleocene, Eocene, and Oligocene epochs and is terminated by the Chattian stage. The nummulites and some other foraminifers disappear during this time. The Neogene begins with the first appearance of the Miogypsinidae and Globigerinoides. It is subdivided into Miocene and Pliocene. The Quaternary begins traditionally at about 1.8 m.y. ago with the Calabrian stage and the Olduvai paleomagnetic event. It is also the beginning of a transgression characterized in the Mediterranean by a cold-water fauna (Arctica islandica)19. The Quaternary is subdivided into the Pleistocene and the Holocene, the latter epoch being also called the "Recent" with a duration of about the last 10 millenia. 19 In 1984, the Pliocene-Pleistocene boundary was raised by international agreement about 200000 years above the Olduvai magnetozone.
156
•
The Major Stages of Earth History
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3.2.2 Major Geodynamic Stages 3.2.2.1 Paleogene: Initial Frontal Collision Between Eurasia and Parts of Gondwana; Complete Opening of the North Atlantic (Fig, 108)
At many locations, the Laramide orogenic phase continued into the Paleocene or even the Eocene, the latter epoch being particularly active tectonically.
The Mesozoic and Cenozoic: Breakup of Pangea
157
Fig. 108. Paleogeography of 1 the Lower Eocene and 2 the Upper Oligocene about 50 and 25m.y. ago: Complete opening of the North Atlantic (Daly 1984, modified after Irving 1977). Dashed lines indicate boundaries between oceans and epicontinental seas
Tethyan Domain. Increased tectonic activity resulted from the contacts between Eurasia, the Arabian-African block, and India, initiating the final phase of the closure of the Tethys. Until the Eocene, the convergence of Africa and Europe continued at an accelerating pace (Fig. 97), provoking the formation of the Rhodope Volcanic Arc in Iran, and ended in the Upper Eocene with a general collision. Principal consequences were a tectonic phase in the Betic Cordillera and North Africa, and mountain building from the internal Alps (origin of the ophiolitic and "schistes lustres" nappes) to central Iran. Very schematically it could be said that in the west, Africa was overriding Europe, particularly the Apulian block which continued its northern overthrusting, while from the Balkans to Indonesia Gondwana was being overridden by Eurasia (Aubouin and Debelmas 1980). The movement of microplates between the two continents can also explain certain tectonic features of the Tethyan chain. For example, after sliding to the east and moving away from Europe during the Cretaceous, the Iberian block moved back again with a SE-NW movement beginning in the Maestrichtian. The result of this was the closure of a trench by subduction and the underthrusting of the Iberian plate beneath the European plate in the western
158
The Major Stages of Earth History
Pyrenees, as well as an activation of the North Iberian margin adjacent to the Gulf of Gascogne (Boillot et al. 1984). This important tectonic phase, called the Pyrenean-Proven<;al phase, culminated at the end of the Eocene in a N-S compression traceable as far as the Massif Central and the Jura. It caused, also, the uplift of the Pyrenean chain and deformation in Provence, the western Alps, and western Italy. Other events were related to the eastern extremity of the Tethys, which began to close significantly in the Paleocene, resulting in the obduction of ophiolites onto the North Indian margin. In the Upper Eocene, India and Asia collided, reducing the Tethys to the ophiolitic suture of the Indus Zangbo. This collision zone is marked by an accretionary prism forming the system of nappes in the Lower Indus. Atlantic Domain. The Norwegian sea opened from the Paleocene to the Lower Eocene with the separation of Greenland and Scandinavia. Th~s established a definite connection between the North Atlantic and the Arctic Sea while inducing deformation and volcanicity in the North Sea. Peri-Pacific and Caribbean Domain. Volcanism and plutonism were dominant in this region, especially in the West American Cordillera from Canada to the Central Andes (3000m of basalt in the Colombian plateau) and at the east and west extremities of the Caribbean plate, where the arcs of the Lesser Antilles and Central America were formed. Some deformation and uplift, however, did take place, including the stacking of thrust sheets in the Rocky Mountains and of south-verging nappes in the Caribbean chain, the latter probably resulting from the convergence of the two American plates beginning in the Lower Eocene. There was also the Incasic phase at the end of the Eocene in the Central Andes, where a number of separate intermontane basins were developed. Australia and Antarctica separated in the Upper Eocene. The Indo-Australian plate, drifting to the north, made contact in a complex way with the Asiatic, Pacific and Philippine plates, resulting in the obduction of the ophiolites of the peri-Australian belt onto New Caledonia and New Guinea in the Upper Eocene. Finally, in the Bering sea region a dynamic of transtension was established between the North American and Eurasian plates, following a phase of compression which had continued since the Maestrichtian.
3.2.2.2 Oligocene Significant compression continued in the peri-Mediterranean chains (Internal Alps with a cover of sliding nappes, Dinarides, Hellenides, and North Africa, where it was dominant) and in Iran. In the Upper Oligocene, a certain relaxation of orogenic activity can be noted worldwide, but especially in western Europe where this period is called a phase of relaxation, following a brief episode of dextral transcurrent faulting affecting Africa from east to west between 35 and 20m.y. (Fig. 97). Distension, therefore, followed compression and was marked by the appearance of N-S rift valleys, some-
The Mesozoic and Cenozoic: Breakup of Pangea
159
times accompanying the volcanism (Limagne, Bresse, Rhenan Trough, Ales Trough, Haute-Provence and Bohemia20 ), or of uplifts (Caledonides, Scandinavia, Internal Alps) and diapirism (subalpine domain). Also during the Oligocene, the Uralian Straits, which until that time separated Asia from Europe, were closed. In the Andes, uplifts, volcanism and plutonism were again active, but less so in the Himalayas where the collision of India and Asia made a pause during the Upper Oligocene. At the end of the Paleogene, the East Pacific having been totally consumed by subduction under North America, the East Pacific Rise passed beneath the continent. The subduction then stopped from the Gulf of California to Cape Mendocino, north of San Francisco, giving way to transcurrent faulting and oblique accretion, while extension regimes appeared more to the east in the Cordilleran domain and in the Mexican Sierra Madre. 3.2.2.3 Neogene and Quaternary: Climax of the Alpine Movements The Neogene repeated locally the Paleogene cycle with intense tectonic activity in the Miocene and Lower Pliocene, related apparently to an accelerated ocean expansion, and then some relaxation in the Upper Pliocene. The Quaternary was dominantly a period of reactivation of tectonic movements. During this time, the present paleogeography gradually took shape, for instance, with the formation of the modern Mediterranean and the mountain chains as we now know them. The Miocene. This was the most eventful epoch, and included the following:
1. Ocean openings and closures. These resulted in the formation of the Red Sea2l , the Algerian-Provencal basin in the Mediterranean, following an anticlockwise rotation of 30 of the Corso-Sardinian block [Ligurian Sphenochasm (Fig. 109)], the Tyrrhenian basin behind the Calabrais arc, the Sea of Japan, and the Okinawa basin 22 . An opening was also initiated in East Africa (Great Lakes Rift). Conversely, the renewal of an active convergence between the African and European plates (Fig. 97) could explain the closures in the Lower Miocene and the emplacement of the Aegean Arc by subduction of the African plate under the European plate, and the Tyrrhenian arc in the East Mediterranean. These activities are still taking place today. In addition, the Tethys was cut off from the Indo-Pacific domain at the end of the Lower Miocene by the approach of Arabia to Eurasia. In the Messinian, the isolation of the Tethys became complete when a slight rotation of Africa cut it off from the Atlantic. 0
2°The origin of most of these grabens goes back to the Upper Eocene when they were associated more with the dynamics of transtension than extension. 21 Resulting in a sinistral movement of Arabia in the Middle East (grabens) and the formation of the Antilebanon chain. The accretion of oceanic crust in the centre of the Red Sea began only in the Pliocene. 22 A mega pull-apart appeared in the Upper Oligocene in a shear zone between the North American and Eurasian plates.
160
The Major Stages of Earth History
Fig. 109. Opening of the Provence basin between 23-24 and 19m.y. Consequent rotation of the Corso-Sardinian block. Arrows indicate sense of movement on transform faults; stippled zones are magnetic anomalies (After Burrus 1984)
~ Siwallks
IT2J Gangetic Quaternary
[s:;:J
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II:II High Country series and nappes Issued from the Indian margin IIlDIDJ
B
Nappes Issued Irom the Indian margin loot Fore-arc series
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am
_
km
~ Tibet Slab
Ophiolitic nappes
Nappes Issued Irom the subduction complex
Series 01 southern Tibet block
l±:±l Leucogranltes lI!J
Volcanic rocks
Fig. 110. Interpretive structural section of the Himalayas (after Mascle 1985). 1 SubHimalayas; 2 Low Himalayas; 3 Upper Himalayas; 4 Trans-Himalayas
There followed a salinity crisis (see below) which did not end until the beginning of the Pliocene with the opening of the Straits of Gibraltar. 2. Major deformations. These mainly concern the Tethyan chains. Large nappes, which may have been transported more than 100 km, were emplaced in North Africa (where they originated from internal zones, now submerged, of the eastern Mediterranean chains), in the Hellenides, and in the Himalayas where the main thrusting was to the south from the Upper onto the Lower Himalayas (Fig. 110). These emplacements were accompanied by uplifts, by metamorphism and by the deposition of molasse. Folding and thrusting also affected the Internal and External Alps (the latter affected also by metamorphism), the Jura, the Appenines, the Middle East, and Southeast Asia. In the Carpathians there was also some volcanism, still active in the Pliocene. 3. Folding, volcanism, and/or plutonism characterize the major part of the peri-Pacific belt: in New Zealand, where the movement on the Alpine Fault was beginning; the Aleutians; the West Pacific Island Arc (collision
The Mesozoic and Cenozoic: Breakup of Pangea
161
between the volcanic arc of Luzon and the Chinese passive margin); Indonesia (continuation of the collision between the Indian, Eurasia, and Pacific plates); the Coast Ranges; and the Andes. The latter region was affected by a latest Miocene phase (Sub andean thrust sheets of Bolivia and Peru, and the East Cordillera of Colombia). A reduced activity (uplifts, extension and volcanism defining the "Basin and Range" phase) occurred in the West American Cordillera (Great basin, Rocky Mountains, and Mexican Sierra Madre). In the Plio-Pleistocene, the compression between the Arabian-African block and Eurasia continued, welding Arabia firmly to Asia and forming the Zagros Chain. In the Lower Pliocene, significant deformation involving the molasse basins was localized in the Apennines and in the External Alps. The squeezing of India against Asia is still active (5 cm/year), producing the most external overthrusts of the Himalayas (the Lower Himalayas onto the Siwaliks and the latter onto the Indo-Pakistan Shield; Fig. 110). In total, 2000 km of shortening has resulted from the collision between India and Asia. In the Upper Pliocene, the Alpine and Maghrebian domains were subjected to a regime of extension. Uplifts resulted in the Alps, for example those of the external crystalline massifs, causing some folding and thrusting in the external zones. The Jura was also uplifted, including the Bresse region. Conversely, the Po Plain subsided. These vertical movements, still discernible today, also affected areas more distant, as seen in the rejuvenation of the French Central Hercynian Massif and the basaltic volcanism of that region 23 • However, the extension affected particularly the Mediterranean domain with NW-SE and NE-SW directions, giving it the following essential characteristics: those of an intermontane sea created partly by major rifting (for example the Tyrrhenide sea, with the formation of the modern Western Mediterranean), which appears oblique to the Alpine structures. Zones of compression remain, however, as seen in the Tyrrhenian and Aegean Arcs, implying a subduction and destruction of oceanic crust (Fig. 111). In the Pliocene, vertical and strike-slip movements occurred also in the peri-Pacific belt, with very noticeable effects in the western Americas. These include basaltic outpourings in the Rockies, the Canadian Cordillera, and British Columbia; transcurrent movement of the San Andreas Fault System, one result of which was the northern drift of the peninsula of Baja California, until then joined with Mexico (Tardy et al. 1986); uplift in the Rockies, the Great Basin, and the Andes, where interior grabens were formed. In the Quaternary, a compressive neotectonism, initiated in the terminal Pliocene, has deformed recent deposits of the Mediterranean rim. The 23The latest manifestations of which occurred between 35000 and 40000 years ago.
The Major Stages of Earth History
162
Ophiolitic scar •
•
Thrusting front
-A-...t:- Subduction zone
Fig. 111. Mediterranean chains resulting from the collision of Africa and Eurasia (after Aubouin 1984). 1 West Mediterranean chain; 2 Middle Mediterranean chain (with hypercollision of the eastern Alps); 3 East Mediterranean chain; a Tyrrenian arc; b Aegean arc
oceanic ridges increased their activity everywhere, leading to renewed ocean spreading of the Red Sea and the Gulf of California for example, and this was sometimes accompanied by changes in direction of spreading (Pacific). Other consequences were folding in the Coast Ranges, rifting in the Rio Grande (Rocky Mountains), folding and uplift in the Andes, and folding and volcanism in the West-Pacific Arc, the Aleutians, New Zealand, and Indonesia, which collided with New Guinea and Australia. In addition, the Caribbean chains in Venezuela attained essentially their present altitude of about 5000m in the Pleistocene, the Calabrian was involved there in overthrusting, and the northern coast of Venezuela is being uplifted today at about 2 cm/year. This neotectonism is found also in Mexico, in California (San Andreas Fault with an average movement of 2 cm/year) , on the Pacific border of the Andes, in New Zealand, and in the Himalayas (uplifting at lcm/year).
3.2.3 Sedimentation and Environment Sedimentation in the Cenozoic was more detrital, on a worldwide scale, than it was in the Mesozoic, for two major reasons: 1. The orogenies of the Alpine cycle, which were particularly numerous, intense, and widespread. 2. The climatic cooling, which, added to the tectonic activity, reduced carbonate sedimentation while keeping sea level relatively low, which increased erosion on the continents.
The Mesozoic and Cenozoic: Breakup of Pangea
163
I \
',-, --·'--x I
"/
'
."
... ::':0'
.:"
Fig. 112. Western Europe in the Upper Eocene (inspired by Pomero11973; stereographic support from Smith and Briden 1977). Horizonta/lines, land; dotted lines, zones of open communication at other times (between the North Sea and the Paris basin in the Middle Eocene, and between the North Sea and the Tethys via the Rhenan trough in the Oligocene)
3.2.3.1 Platforms In the Paleogene, the last major marine transgressions in global history encroached upon the emerged continents of the terminal Cretaceous. Western Europe was the scene of numerous marine invasions from the permanent seas of the Tethys, the North Sea, covering Denmark at that time and the North Atlantic. These transgressions and regressions were largely controlled by deformations affecting the platforms, due to the activities of orogenic belts nearby. The three oceans were able to communicate temporarily (Fig. 112) across the Paris Basin, the Channel, the Rhenan Trough, the peri-Alpine depression (in the Oligocene only), North Germany, Poland and the Russian platform. The marine facies were diverse: sands, shales, marls and limestones generally organized into sedimentary cycles related to marine oscillations, and always rich in fauna (molluscs and
164
The Major Stages of Earth History
foraminifers, including nummulites). Environments were deeper and cooler in England, and especially in the actively subsiding North Sea where the nummulites were not able to exist. The Tertiary here totals more than 3000 m as a predominantly shaly facies. Among lagoonal and continental deposits, evaporites and lacustrine limestones were well represented. The evaporites, of proven marine origin, were often deposited in rift basins of Upper Eocene to Oligocene age, for example the Rhenan and Rhodanian troughs and the Bresse basin. In the Neogene, the major transgressions stopped at the same time as did the subsidence. Only a few gulfs penetrated far onto the continent, in the Paris Basin, the Aquitaine Basin, England, and northern Europe, depositing sand and shale often very rich in shells (faluns). Africa also experienced in the Paleocene its last marine transSaharan transgression, linking the Tethys with the Gulf of Guinea and extending over the sands of the "Terminal Continental". On the margins of this continent, in Tunisia, Egypt, Morocco, Senegal, and Togo, important deposits of sedimentary phosphates were formed between the Maestrichtian and the Middle Eocene. In the Pliocene, major rifts accumulated lacustrine sediments in the east. The two Americas and Australia remained emergent except for a few gulfs. Around the modern Gulf of Mexico, large petroleum deposits were formed. The centre of each of the three continents was the locus of a dominantly terrigenous sedimentation fed by adjacent mountains: in North America there were the Rocky Mountains, still actively forming at the beginning of the Tertiary, then rejuvenated, with the Appalachians, by an uplift of about 1000 m from the Miocene to the Quaternary. In the Green River Basin in the Western Rockies, bituminous shales with a huge potential reserve of gas and oil were deposited in Eocene lakes. In South America, the Andes, strongly uplifted in the Pliocene, were the source of continental sediments. 3.2.3.2 Atlantic Margins To the south of the Gulf of Guinea, the Paleogene is a complex of sand and shale, alternately marine and continental, deposited when the margin was still undergoing significant thermal subsidence. In the Neogene the margin stabilized, the sea level dropped considerably, and a prograding sedimentary prism constructed the essential form of the modern continental shelf (Fig. 105). On the other side of the Atlantic, the South American margins had a similar history, well illustrated to the north of Brazil where the Amazon has deposited 14km of sediment since the Middle Miocene. After the formation and stabilization of the North American margin in the Upper Cretaceous, the Cenozoic sediments built the sedimentary prism which was the foundation of the continental shelf. In the Bahamas and Florida the thick carbonate cover, already consisting of 2000 m of Cretaceous sediments, was augmented by 500-600 m of Tertiary carbonates. Elsewhere, deposits were sandy or shaly and particularly abundant at the mouths of major rivers
The Mesozoic and Cenozoic: Breakup of Pangea
165
(6000 m of Tertiary and almost 5000 m of Quaternary in the Mississippi Delta). 3.2.3.3 Circum-Pacific Active Margins Thick detrital sequences derived from island arcs and continental highlands were deposited in marginal basins. Seven thousand metres of Paleogene sediments have been recognized on the Japan margin, and 8000m of essentially detrital Tertiary, sometimes flyschoid, in the North American Cordilleran domain. With a strong similarity to the Alpine facies, the Oligocene at the foot of the Sierra Nevadas is of the red molasse type (see below) with sand, shale, and conglomerate channel fill. In the Coast Ranges, the recent deposits are thick by virtue of an active neotectonism. 3.2.3.4 Tethyan Domain The paleogeography and sedimentation reflect conditions of extreme mobility due to the plate collisions and orogenies characterizing this area during the Cenozoic. The Tethys first underwent sufficient closure to isolate it from other oceans, then a further reduction ending today with an almost total disappearance. In the Miocene for example, after a general transgression in the Nummulitic, particularly noticeable in the Alps, it communicated with the Atlantic only via two narrow passages between Africa and Spain, the South-Betic and North Rif straits. To the east it was not long (about 16.2m.y. ago) before it was cut off from the Indopacific region; then for 1.5m.y., beginning in the Messinian, its connections with the Atlantic were cut for both tectonic (see above) and eustatic reasons. The consequent isolation resulted in a "salinity crisis" during which about 22 x 103 km 3 of gypsum and halite were precipitated on the floor of the Mediterranean of that time extending to the Caspian and the Yemen. The salinity of the world's oceans was probably decreased as a result. Opinions are very divided on the subject of a total drying up or not of the Mediterranean during this episode. However, the quantity of salt precipitated would certainly exclude a total separation from the Atlantic or from the Indian Ocean via the Red Sea. After the Nummulitic transgression, the Tethyan waters withdrew to the peripheries of the orogens. In the Miocene they collected in a northern part, the Paratethys, consisting of several large basins (Pannonic, Dacic, Pontic, and Aralo-Caspian), and in a southern part corresponding partly to the modern Mediterranean (Fig. 113). Between these two regions were narrow passages such as the peri-Alpine molasse trough, the Aegean straits, and the Dinaric straits. All the basins of the Paratethys gradually emerged and in the Messinian became the sites of lakes. After aquiring a new configuration in the Pliocene, the Mediterranean was once more filled with ocean waters when the Straits of Gibralter reopened, thereby creating the modern Mediterranean surrounded by high relief, the erosion of which has led to the deposition of about 2000 m of mud in the Pleistocene alone. Only the eastern part of this sea represents a more
166
=== Temporary connections
The Major Stages of Earth History
o
400km
'----'
Fig. 113. Europe in the Tortonian (lOm.y.). 1 Perialpine depression; 2 Pannonian basin; 3 Dacian basin; 4 Pontic basin; 5 Aralo-Caspian basin (Inspired by Dercourt et al. 1985)
or less directly inherited part of the Tethys24. The Paratethys evolved rapidly into a restricted environment, brackish, then fresh, as represented in the modern Lake Balaton, the Black Sea (opening into the Mediterranean in the Quaternary), the Caspian Sea, and the Aral Sea. Two major types of depositional environments existed in the Tethys during the Cenozoic: 1. Stable environments. Sediments were generally shallow and calcareous, and nummulites flourished (examples in northern Italy, Istria, Atlas, Libya, and Egypt). In the Middle East, they contain the largest reserves of hydrocarbons in the world. 2. Environments marked by nearby active tectonism. Two major types of sediment are characteristic. Flysch sediments occurred relatively early in the history of the Alpine chains, especially during the Eocene and Oligocene, and generally in the internal zones (e.g. Alps, Carpathians, Pyrenees, Hellenides, Dinarides, Appennines, and in the Rif and Tell domains). They indicate generally deeper-water environments as in a trough or foredeep, and result from the erosion of uplifted highlands, island arcs and coastal mountain chains. Molasse sediments are related to the uplift of massifs during their last major tectonic phases. Thus the uplift and emplacement of nappes in the Alpine Arc in the Upper Oligocene resulted in the deposition of 6000 m of Oligocene-Miocene 24The Levant area probably represents a passive margin created in the Upper Trias when an ocean opened between the Levant and the Taurides block.
The Mesozoic and Cenozoic: Breakup of Pangea
167
molasse in marine and fresh water in the peri-Alpine foredeep in Switzerland. Some molasse was overthrust during the Miocene by the Helvetic nappes. Similarly, the continental Stampian formation, called "red molasse", deposited in the Subalpine extension graben, was partly derived from uplifted internal zones. The molasse was deposited until the Lower Pliocene in a compressive regime, the youngest often being designated as postorogenic. It indicates emerged or very shallow environments fluctuating between continental and marine and rates of subsidence and sedimentation in equilibrium. Evidence for this is seen in the intercalations of coal, lacustrine limestones and evaporites. A very similar type of sedimentation, but in a paralic environment, occurred in Sumatra in response to Miocene and Pliocene tectonic phases. This continues today and is conspicuous for its important deposits of coal.
3.2.4 Climatic Events A glacial epoch reappeared in the Cenozoic, similar to those that the Earth had experienced already several times during its history. Traditionally this epoch is associated with the Quaternary, on the assumption that a rapid and sudden change in climate occurred at about 2m.y. In fact, there were glaciations earlier than this, beginning already in the Early Cenozoic. 3.2.4.1 Paleogene The global cooling at the end of the Cretaceous was succeeded by a sequence of contrasting climates, with cold periods in the Middle Paleocene, Middle Eocene, and at the Eocene-Oligocene boundary, and warm humid periods in the Lower and Upper Eocene, the latter periods notable for the deposits of laterite and the absence of polar ice. Distinct climatic belts were established with an Antarctic zone, an Arctic-tertiary temperate zone extending over the nordic countries, and a tropical zone, between 500 Nand S, including in particular Europe. The first detectable general cooling occurred at the Eocene-Oligocene boundary. This was a significant event, well recorded because of its abrupt nature, by the evolution of the mammals (the "Great break" of Stehlin), by an important renewal of marine microorganisms, and by a lowering of the CCD, indicating a reduction in general ocean fertility including the platform regions. Several competing causes have been suggested: 1. The beginning of an important exchange of waters between the North Atlantic and the Arctic and Antarctic regions. This is placed generally at 35 m.y. when the Walvis-Rio Grande and Iceland barriers disappeared. 2. The formation of the first glaciers and the first sea ice in the Antarctic region, as well as the establishment of the great circum-polar current carrying cold deep waters (5-6°C) to the Pacific and Atlantic.
168
The Major Stages of Earth History
3. The arrival of arctic waters in the North Atlantic and part of the Pacific. 4. The reduction in exchange of waters between the Tethys and the Atlantic by weakening of its E-W current. 5. The appearance of the mountain chains, the Alps and the Himalayas, at the end of the Eocene, the result being a drop of 4°C in average ocean temperature and a drier climate during the Oligocene. This climate was warm-temperate in Europe (evaporites in the fault troughs). In the Upper Oligocene, a decrease in the 0 180 of the oceans indicates a warming followed by the melting of a certain quantity of ice. 3.2.4.2 Miocene The average elevation of the emerged lands increased. The Antarctic ice cap formed in the Lower or Middle Miocene, while the total opening of the Drake Straits between South America and Antarctica allowed unhindered ciculation of the cold circumpolar current, thus thermally isolating Antarctica. The temperature of the ocean bottom waters dropped from an average of 9 to 4°C. The lowering of the Greenland-Faroes-Scotland Ridge in the Upper Miocene also allowed a regular flow of Arctic water into the North Atlantic. The first glaciers were formed in Alaska. Finally, from the end of the Miocene to the beginning of the Pliocene the isthmus of Panama was emergent, increasing the flow of the Gulf Stream and the warmth and humidity associated with it, towards the north where rain and snowfall intensified. This led to the first glaciers in Greenland (first traces of icebergs are about Sm.y. in the Baffin Sea). In Europe, the climate became warm and wet with an average annual temperature of lS-20°C. A first important glacio-eustatic drop in sea level was responsible, at least partly, for the isolation of the West Mediterranean (see above). 3.2.4.3 Pliocene In the orogenic belts the relief continued to increase. The average global temperature dropped again, reaching 12-15°C in France. At about 3m.y., the Arctic ice caps were formed at the same time as the peri-Antarctic ice pack. Already the glacial-interglacial cycles were beginning, with an alternation of temperate warm stages and mild winters with stages more humid, seasons more contrasting and winters dry and severe. 3.2.4.4 Terminal Pliocene-Quaternary This was the true period of glaciations. Ice caps were formed in northern Europe, in North America, and in the Alps, while mountain glaciers developed in the Andes, Africa, Australia, and New Zealand (Fig. 114). Six major glacial periods (Biber, Donau, Gunz, Mindel, Riss, and Wiirm) are traditionally accepted for the last 2.4m.y. Each period contains cold phases and warmer interstages within it, and each is separated by interglacial periods, these in turn divisible into alternating warm and cold subperiods, during which the climate was often as warm or warmer than it is today. The
169
The Mesozoic and Cenozoic: Breakup of Pangea ,'
.........
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Fig. 114. The major Wiirm glaciations 18000 years ago (after Lorius and Duplessy 1977). Note the differences in continental contours from today due to a drop of sea level of 120m
pluvial and dry alternations in the tropics and variations in glaciation at high altitudes may be related, but it has not yet been clearly established25 . The Riss marked the furthest advance (as far as Lyon) of the Alpine glaciers and of the northern European ice sheets which covered the London Basin, Holland, and Germany. In the Wiirm, the European and American ice sheets reached their largest size; 8 x 106 km 3 for an area of 6 x 106 km2 in Europe, 30 x 106 km 3 for an area of 12 x 106 km 2 in North America. Sea level was 120 m below the present and icebergs travelled as far as PortugaL The consequences of the glaciations were significant and varied. The ocean temperature fell by 2 to 3°C and the lowered sea level resulted in increased fluvial erosion on the continents and the buildings of large submarine fans at the base of the continental margins. Many submarine canyons also date from this period. The coastlines and the drainage patterns were appreciably altered (Fig. 115). Cryoturbations, deposition of loess and the development of cold steppes and tundras were also characteristic of these cold periods. The disappearance of Wiirm ice resulted in a recent acceleration of the rate of rotation of the Earth and an isostatic readjustment of regions 25 During the Upper Pliocene and Pleistocene pluvial phases of Africa, the Nile discharged enormous volumes of fresh water into the eastern Mediterranean, causing a stratification of waters, the formation of deep anoxic water, and the deposition of sapropels (Sect. 2.2.4, Chap. 3).
The Major Stages of Earth History
170
Fig. 115. Emergence of the Franco-British continental platform in the Wiirm (After Le Danois and Mathieu, in Bellair and Pomerol 1968)
to the unloading of ice. This occurred in the Baltic of Scandinavia, where in the last 10 000 years an alternation of marine and lacustrine conditions shows well the delicate balance between rising sea level and isostatic uplift of the crust, both due to the melting of ice. This uplift is continuing today in the Gulf of Bothnia. Among the possible causes of the climatic events in the Cenozoic, those directly involving plate mobility have so far been preferred. The change in characteristics of ocean circulation, the positioning of a continental mass over the South Pole, the slow drift of the North Hemisphere to higher latitudes, and the formation of high terrestrial relief have all been invoked as possible reasons. However, other phenomena could explain the cooling in the Quaternary: increase in density of atmospheric dust (more active volcanism 26 , fall of giant meteorites), changes of the magnetic field, passage of the Earth through a cloud of cosmic dust, or variations in solar activity. As far as the glacial-interglacial cycles are concerned, they owe their origin without doubt to the cyclic fluctuations of the Earth's orbit (Milankovitch cycles, see above). This phenomenon, present throughout the Earth's history but barely apparent during the warm periods (e. g., Mesozoic), was expressed in the Quaternary, and also in the Upper Carboniferous (Heckel 1986), as a succession of advances and retreats of the ice caps.
3.2.5 Biological Phenomena The massive disappearance of species at the Cretaceous-Tertiary transition created a new situation with a large number of ecologic niches available at the beginning of the Cenozoic. Many groups also experienced adaptive radiations from which emerged a large number of new forms typical of modern
26 According to Arthur (1979), the number of volcanic ash beds deposited per 1000-year interval increased from the Oligocene to the Quaternary.
The Mesozoic and Cenozoic: Breakup of Pangea
171
faunas. Biological evolution was also affected by climatic changes which led to an increase in provincialism. In the seas the protists, pelecypods, gastropods, and echinoids thrived and diversified actively. On the continents the flora, particularly the angiosperms, as shown by pollen analyses, underwent a rapid adaptation to the different environments. The mammals, particularly, characterize the Cenozoic, their evolution being much more dependent on the continental drift than was the case for the reptiles. Their climax was in the Neogene and they began to decline at the end of the Pliocene. The Quaternary glaciations had prolonged repercussions on continental faunas, some species disappearing, others adapting. The adaptations to the cold seen in some large species are among the more spectacular. Nevertheless, many large mammals of the Pleistocene became extinct between 10 000 and 5000 years ago, for reasons not really known. Man, in contrast, appearing about 2m.y. ago, has proliferated.
3.3 Conclusions on the Mesozoic and Cenozoic The beginning of the last Wilson cycle corresponded to a breakup of the Permian Pangea due to several ocean openings. One, latitudinal, affected the Tethys and resulted in the separation and sinistral offset of the two large continental masses Gondwana and Laurasia between the Trias and Upper Jurassic. A continuous seaway was thus established from the permanent Tethys to the east to the Caribbean, and perhaps even the Pacific, in the west. Another, meridian and crossing the first, resulted from the Cretaceous to the Tertiary in the opening of the Atlantic, carrying the two Americas westwards to overthrust the Pacific plate, including its mid-ocean ridge. Others, to the southwest of Gondwana, separated India, Australia, Madagascar, and Antarctica. From the Cretaceous, the two latter systems were more active than the first. The Tethys then closed, causing the approach and then collision of the Gondwana elements (India, ArabianAfrican block) and Eurasia. The Alpine chains arose from this encounter, sometimes taking the form (eastern Alps, Himalayas) of a hypercollision with gigantic intracrustal thrust sheets with dimensions of hundreds of kilometers. These took place in the Neogene. Today the N-S collision is continuing between the Australia-New Guinea and Indonesia blocks. The opening of the Atlantic was the cause of significant modifications in ocean current patterns and climate, leading, beginning in the Oligocene, to glaciation. In effect, the E-W Tethyan current system, important carrier of heat during the Mesozoic, gave way to a N-S Atlantic system distributing deep cold water to all latitudes. The sedimentation reflects both the tectonic and climatic changes, being dominated by terrigenous products from the Cretaceous to the present day. This double influence is also found in the eustatic cycles from the Trias to the Quaternary, with high sea levels in
172
The Major Stages of Earth History
the Mesozoic accompanying the fragmentation of Pangea, then falling sea levels responding to the Alpine tectonism and the gradual glaciation of the poles. From the biological point of view it is remarkable how abruptly the modern faunas replaced the Mesozoic faunas, still somewhat archaic, at the Cretaceous-Tertiary transition. The mammals, rid of competition from the reptiles, were the principal beneficiaries of this revolution. However, from about 2 m.y. ago, one species in particular evolved, capable of modifying the terrestrial environment and setting foot on other planets.
General Conclusions
Apart from correlation and dating, stratigraphy strives today to create a language common to all geologists of the international community, by means of conventions recognized and adopted by all and symbolized by a standard stratigraphic scale. The recognition of global phenomena facilitates this.
1 Characteristics of the History of the Earth It appears that regular cycles have always been and still are a part of the
Earth's history. They affect orogenies, the dispersion of the continental crust (Wilson cycles of 400-500m.y.), sedimentation, climate, and biological evolution. These cycles lend some credibility to the idea that the Earth's system is in permanent disequilibrium about a mean state. They also allow predictions to be made about the Earth's history. Certain events, the equivalents of which can be observed today, have occurred repeatedly. These include the formation of black shales in marine environments (Upper Cambrian, Lower Ordovician, Lower Silurian, Devonian, Toarcian, CallovoOxfordian, and Middle Cretaceous), the precipitation of abundant evaporites, significant deposition of coal (Carboniferous, Permian, Cretaceous, and Eocene), and successions of orogenies in the same locations, especially near the sites of major N-S collisions (between Gondwana and Eurasia) and E-W collisions (between Africa, Europe, and America). The volumes and surface areas of continental and oceanic crust also seem to have been stable, at least for 2500m.y., which is contrary to the theory of an expanding crust. In short, uniformitarianism is still a valid concept for the Earth's history. Nevertheless, shifts in the values of certain parameters through geologic time make the evolution of the Earth irreversible. They affect the length of the day, the chemical composition of the oceans and the atmosphere, and the thermal flux. For example, during the Precambrian, ferrous iron and siliceous deposits of the platform were irreversibly replaced by red beds and carbonates. In addition, there are the events due to chance, more or less abrupt, related to climate (glaciations), geomagnetism, chemistry of seawater
174
The Major Stages of Earth History
(isotopic ratios), ocean openings, ocean currents, and the biological extinctions observed in the Eocambrian, at the end of the Lower Cambrian, and in the Ordovician, Permian, Trias, and Cretaceous. These events make the Earth's history nongradual and noncontinuous as believed by uniformitarianists. In summary, therefore, it is a complex history which the entrenched dogmas of former times are not able to explain.
2 Interdependence of the Major Driving Forces of Earth's History Setting aside those events which appear totally unpredictable and totally contingent, the history of the Earth appears as a logical sequence of interdependent events. This has become very apparent from the study of global tectonics. For instance, the internal geodynamic forces of the Earth influence the external geodynamics and both influence biological processes. Volcanism plays a major role in these relationships in the following ways: 1. By ejecting material into the atmosphere, it can modify the climate (cooling, if sulphate aerosols, which reflect the sun's rays, are introduced into the high atmosphere, warming if the atmosphere is enriched in CO2), and, therefore, the ocean dynamics and sedimentation. Even life can be affected, for example the acid rain resulting from sulphate aerosol fallout can cause deforestation. In contrast, an excess of CO2 stimulates the productivity of marine plankton and terrestrial plants, thereby increasing the deposition of organic matter. 2. By controlling the volume of oceanic ridges, volcanism is also a determining factor in eustasy. Transgressions cause warming and humidification, while promoting faunal exchanges and adaptive radiations. In contrast, cooling, dry climates, isolation of marine populations, speciation, and sometimes extinction are associated with regressions.
The movements and deformation of the plates are also a primary cause underlying relationships between the Earth's internal and external geodynamics and biological phenomena. The amalgamation of cratons into supercontinents results in a lowering of sea level, the creation of continental relief (therefore possible barriers for the migration of faunas), and a cooling trend which could initiate glaciation. In addition, marine areas are reduced and CO2 in the atmosphere decreases. Geological history has included four or five such glacial eras lasting from 20-200m.y. and generally coinciding with the major orogenies. During times of continental dispersion, the opposite effects are produced. Among others, the possibilities for circulation and exchange of faunas are increased in marine environments. Another notable interaction occurs between the processes in the Earth's core and
General Conclusions
175
the dynamics of the biosphere. The perturbations of the magnetosphere induced by the geomagnetic polarity inversions facilitate the penetration of mutagenic cosmic radiation, causing an acceleration of the rate of evolution at the time of the inversions (Dercourt et al. 1986).
3 The Search for Fundamental Causes Since it is not always possible to precisely define these causes, attempts are being made to at least localize them. The cyclic nature of the phenomena and their interdependence have allowed some progress in this direction. For example, the correlation between periods of magnetic calm and periods of active ocean expansion during the Earth's history suggests that the origin of variations in the activity of the ocean ridges is connected with the boundary between the mantle and the core, or even the core itself. The variations of solar heat flux reaching the Earth's surface are responsible for the climatic cycles, well illustrated in the Quaternary and recorded in the sediments. These variations depend on the oscillations of the Earth's axis of rotation and characteristics of its orbit. Life, which is undoubtedly the most sensitive and fragile system of the Earth, has also probably been influenced by more distant phenomena. The passage of the sun and its planets across the galactic plain, every 32 m.y., for instance, may be the cause of biological crises, especially mass extinctions. During these periods, cosmic radiation is intensified and the frequency of meteorite impacts increases, thereby generating a screen of dust which temporarily lessens solar radiation at the Earth's surface. Biological evolution, therefore, may have depended not only on the Earth itself but also on its environment near and far.
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Subject Index
Abundance Zone 30 Acadian Orogeny 116, 124 Active Margins 104, 122, 153, 165 Alkenones 55 Alleghanian Orogeny 118 Allochronology 25 Alpine Chain 132, 153, 157 -161 - Cycle 17, 132, 162 American-Mexican Block 137 Ammonitico-Rosso 147 Anagenesis 25 Andean Phase 137 Andes 140, 141, 158-164 Appalachians 104, 106, 108, 116, 121, 164 Appenines 147, 153, 160, 161 Apulian block 136, 142, 157 Archean 83, 84, 90, 94, 98, 99 Arctic Ocean 139, 143, 158, 168 Ardennian Phase 108, 113 Armorican Sandstones 111 Arvinche Phase 143 Assyntic Unconformity 102 Asturian Phase 121 Atlantic 69, 137-139, 141, 158, 163, 167, 168 Atmosphere 87, 88, 95, 96, 102 Aulacogen 91, 94 Autochronology 25 Barrovian metamorphism 120 Basin and Range Phase 161 Biofacies 65 Biohorizon 31, 35 Biological extinction 5, 129, 154 Biozones 10, 29, 30 Black Shale 55, 147 Boreal Province 67, 145 Bradytely 11 Break 15,38 Breton Phase 120, 124 Buller Chain 105 Cadomian Orogeny 92, 95, 103 Caledonian Orogeny 100, 105
Caledonides 106, 110, 121 Cambrian 84, 102, 106, 109, 11 1- 113 Carboniferous 117, 124, 130 Caribbean Chain 158, 162 Carpathians 141, 153 Castile Sea 128 Catskill Delta 11 6, 128 Celtic Ocean 92 Chemostratigraphy 50 Chronohorizon 49 Chronozone 36 Cimerian Continent 135 - Phase 136 Cinerite 41, 48, 49 Circum-Caribbean Phase 141 Cladogenesis 25 Climate 27,73,79, 112, 113, 143, 145, 154, 162, 167-170 Climatic zonation 96 Coal 125, 130, 131, 149 Coal-Bearing Series 115, 130, 149 Coast Ranges 140, 153, 161, 162 Composite log 41 Continental intercalations 149 Core 87 Correlation 3-5,42,43,46-48,51,53,55,56 Corso-Sardinian Block 159 Cretaceous 79, 134, 138, 144 Crust 87, 90, 93, 94 Culm 118, 127 Cycle of Punctuated Aggradation 44 Cyclogram 46, 47 Dakota Group 149 Dating 21, 23, 29 Death assemblage 27 Devonian 115, 124, 130 Diachronism 4, 9, 16, 36, 37, 42, 43 Dinarides 147, 153, 158 Discontinuity 9, 13, 17,33,43,85 Discordance 3, 16, 17 Ediacara 100 Electrofacies 41
Subject Index
186
Ensialic tectonism 91, 121 Environment 5,26,65,66 Eocambrian 82, 84 Era 17 Erosion 77, 78 Eustasy 43, 44, 72, 78-81, 109, 143, 168 Evaporite 55, 96, 99, 111-113, 124, 125, 127, 128, 130, 146-149, 164 Event 1,3-5, 15, 16,55 Evolutive cartography 75 Facies 11, 12 Faunal barriers 27, 104, 105 - migration 5, 11 - Province 11, 67, 105 - renewal 17, 133 Fenno-Sarmatia 103 Flysch 92, 99, 106, 166 Formation 7, 8 Franciscan Basin 137, 139, 154 Geochronology 3, 12, 20 Glaciation 77,80,96,97,113,123-125, 129, 130, 167, 170 Glauconite 22 Global map 72 - Revolution 13, 35 Glossopteris 129, 131 Gneissic dome 94 - 95 Gondwana 92, 115, 129-131, 142, 148 Greenstone Belt 90 Grenville Belt 90, 92, 93 Griotte Marble 120 Group 7 Heat Flow 76, 77, 93, 100 Hellenides 147, 153, 158, 160 Hercynian Orogeny 100 Hercynides 119, 121 Himalaya 121, 159, 162 Hydrosphere 87, 90 Hypostratotype 33, 34 Iapetus 103 Ibero-Corso-Sardinian block 142, 157 Index fossil 10, 26 Indian Ocean 137 Indosinian Suture 135 Intercontinental bridge 72 Interprovincial exchange 78 Interval Zone 31 Iridium 143 Isochronism 4, 16, 43, 44 Isopic Map 65 Isostatic adjustment 77, 169, 170 Isotopic Age 21
- Stage 51 Israelski principle 43 Jasper 99 Jura 158, 160, 161 Jurassic 79, 134, 145 Karroo Formation
149
Lance Formation 149 Laramide Phase 80, 133, 140, 143, 156 Lateral continuity 3 Life assemblage 27 Liguro-Piemontais Ocean 136, 137 Lithoclinal sequence 42 Lithofacies 65 Lithohorizon 3 Lithomarker 35 Lower Continental beds 130 Maghrebids 148, 153 Magnetic anomaly 58-61 - reversal 56-58 Magnetozone 58 Mantle 88, 89 Marathon Mountains 122 Marl-Limestone alternation 44, 47 Mauritanides 118, 121 Mediterranean 159, 160, 161, 165, 166 Melanges 104, 107 Member 7 Meteorite 62, 82, 87, 94, 170 Michigan Basin 111 Microcontinent 106, 116 Middle Cretaceous Crisis 151 Mid-Oceanic ridge 77, 80 Milankovitch cycle 170 Mineral markers 47-49 Molasse 98, 127, 165, 166 Morrison Formation 148 Neocimerian Phase 138 Neogene 155, 159, 164 Neotectonism 164 Neotethys 135, 136 Nevadian Phase 80, 134, 137 New Red Sandstones 125, 130 North Sea 139, 158, 163 Nubian Sandstones 148 Numerical Age 31 Old Red Sandstones 114, 118, 124, 127, 130 Ophiolite 92, 120, 137, 141, 158 Ophite 136 Orbital Cycles 46 Ordovician 102, 107, 109-112
187
Subject Index Orogenic cycles 84 - Phase 15 Overlapping Range Zone
30
Pacific Ocean 137, 143, 158, 159, 168 Palatine Phase 133 Paleobiogeography 64, 67, 105 Paleocirculation 73, 75 Paleogene 156, 163, 167 Paleogeography 3 Paleomagnetism 72, 81, 102 Paleotethys 103, 118, 122, 127, 135 Palinspastic Map 70 Pan-African Orogeny 92, 95 Pangea 16, 76, 80, 92, 105, 114, 115, 128, 129 Paradox Basin 122 Paratethys 165 Passive margins 104, 152, 164 Permian 115, 128, 130 Permo-lrias 134 Phanerozoic 10, 17,83,88 Phyletic gradualism 31 Phylozone 30 Planets 82, 88, 89 Plate-tectonics 5, 87, 92, 100, 102, 121, 122, 158 Polar Wandering 81,82, 103, 123-125, 145 Principle of superposition 2, 85 Proterozoic 83, 91, 92, 94 Protoatlantic 103 -105, 108, 115 Protogondwana 102 Provincialism 11, 129, 131 Purbeckian 144 Pyrenean-Provencal Phase 158 Pyrenees 153, 158 Quaternary
155, 159, 168
Radiochronology 20-23, 85 Radiometric Age 21 Range zone 30 Rangitata Phase 138 Red Beds 86, 96, 99, 112 - Sandstones 65 - Sea 159 Reefs 111, 112, 127, 145 Remanent magnetism 56-58 Rhythm stratigraphy 42 Rocky Mountains 140, 158, 161, 164 Saalian phase
121
Saamian Orogeny 90 Salinity Crisis 160, 165 Sapropel 55, 66 Sardinian Phase 108, 113 Sedimentary rhythm 20, 37 Sedimentation 77, 78 Seismic method 38, 43 Sequence Stratigraphy 42-44 Shield 85, 86 Shoreline 70-72 Silinic Disturbance 108 Silurian 102, 108, 109, 111, 112 Stage 17 Stratotype 7, 13, 33-35 Stromatolite 85, 99, 110, 111 Subsidence 76 Sudetic Phase 121 Sundance Sea 144, 148 Suspect Thrranes 140 Synsedimentary tectonics 16 System 17 'Thchytely 10 Thconic Phase 107, 109, 113 Thphrostratigraphy 48 Thrminal Continental 164 Tethyan Province 67, 145 Tethys 12,132,135-139,141,144-146, 153, 158, 159, 163, 168 Tigillite Sandstones 111 Tillite 96-98 Time correlation 10 Tonstein 41, 48, 130 Torridonian 105 nace elements 51 lrias 3,48,65, 135, 145-147 Unconformable surface 41 Uniformitarianism 35, 49, 85, 86 Unitary association 32 Urals 115 Varve 20,47 Vocontian Basin 45-47, 151 Volcanism 108, 129, 135, 143, 155, 158, 160, 170 Wealdian 144, 151 Well logging 39-42 Wilson cycle 76, 79, 92 Zagros Chain Zechstein Sea
161 127, 128
