Sedimentary basins formed and carried piggyback on active th rust sheets G. G. Ori lstituto di Geologia e Paleontologia, Universita di Bologna, Via Zamboni 67, 40127 Bologna, Italy
P. F. F. Friend Department of Earth Sc Sciences, University of Cambr i Cambr idge , Downing Street, Cambridge CB2 3EQ , England ABSTRACT
Small sedimentary sedimentary basins that formed formed on the Apennine (southern) (southern) margin of of the Po basin complex, northern Italy, have been studied using borehole and seismic-reflection information, and their evolution can be dated using the largely marine biostratigraphy of the Miocene to Quaternary sediments. These studies show that the basins formed and were filled while being carried on moving thrust sheets , and they are therefore named piggyback basins. Each of these basins corresponds to the active phase of a different thrust-front ramp. On the Pyrenean (northern) margin of the Ebro basin c omplex, northern Spain, similar piggyback piggyback basins formed between Eocene and mid-Miocene time. They have since been uplifted and eroded and may therefore be studied in outcrop. In northern Spain, much of the basin-fill sediment was deposited in nonmarine environments.
INTRODUCTION
Folding, faulting, and sedimentation have occurred together in many places in the margi nal parts of sedimentary basins (Miall, 1978) and provide evidence of tectonic movement of the basins with respect to their surrounding s . Evidence of the transport of entire active sedi mentary basins as a direct result of movements in the upper crust is much less common . We direct attention to two areas where complexes of basins (the Po in Italy and the Ebro in Spain) had one major compressional margin in which thrust sheets formed and transported actively filling sub-basins. In both the Po and Ebro complexes, there is evidence that the thrust sheets formed by "pig gyback thrust propagation" (Dahlstrom, 1970; Butler , 1982). This occurs when the high-angle frontal ramp of a thrust sheet becomes inactive and a new active ramp forms in front of the old one. Continuing movement of the thrust sheet carries the old ramp, piggyback piggyback , incorporated in the thrust sheet. We propose that the term "piggyback basin" be used to describe all sedi mentary basins that formed on active thrust sheets.. sheets The type of evidence available from the two areas differs. The Po complex is still subsiding, and information about the structure of the Mi ocene to Quaternary deposits that lie below the Po plain comes largely from borehole and seismic work. In contrast, the Ebro complex was filled mainly in Eocene, Eocene , Oli gocene, gocene, and Miocene time and was subsequently uplifted, so that extensiv extensivee outcrops are locally available available..
of the Apennine orogenic belt. During this pe riod the Apennines have become a source area for the sediment of the complex, and various depositional cycles have been distinguished (Cremoninii and Ricci Lucchi, (Cremonin Lucchi, 1982). A program of subsurface analysis, carried out by the AGIP AGIP (Azienda Generale Italiana Pe troli) oil company, has shown three major areas of thrust sheets extending into the Po basin from the Apennines. The AGIP structura l map of the Po basin (Pieri and Groppi, Groppi, 1981) 1981),, re drawn in this paper (Fig. (Fig. IA), shows that the
depocenters (areas of greatest thickness) of the Pliocene-Quaternary Pliocene-Quatern ary sequence occur either within the thrust arcs (Figs. (Figs. l, 2) or just in front of them. them. The depocenters depocenters within the arc areas are interpreted as pig gyback basins, filled by sediment poured from the Apennine chain and dammed by the thrust-s thrust-sheet heet ramps . The thrust sheets are arc-shaped in plan (Fig. 2).. Their leading edges, the tips of the frontal 2) ramps, are higher than the edges of the lateral ramps.. This is a direct result of the form and ramps movementt of the thrust sheet. A strike-slip movemen
PO BASIN COMPLEX
The Miocene to Quaternary Po basin com plex ple x can be regarded as the the youngest foredeep foredeep GEOLOGY , v. 12, p. 475- 478, 478, August 1984
'--+-- Anticline ..-.,
Thrusl
B
Figure 1. Comparisons (on same scale) of Po (A) and Ebro (B) foredeep basin complexes. Depo centers correspond to both main basins and piggyback basins. Sec tion lines show location of Figures 3, 4, and 5. 5. Modi fied from (A) Pieri and Groppi (1981); (B) Mapa Tectonico de la Peninsula lberica y Baleares, lnsli tuto Geologico Geologico y Minero Minero de Espana, Madrid . 475
component of movement of the lat eral ramps is an inevitable local result of the advance of these arc-shaped thrust sheets (Butler, 1982). The different influence of the frontal and lat eral ramps of the thrust sheets on the sediment
Subsurface :
infill is clear, as shown in Figure 2: the iso pachs of the Quaternary sediment show a pro nounced decrease in thickness against the frontal ramps of the thrust sheets, but no strong variation across the lateral ramps. The Quater-
Thrust influencini Quaternary
\
Quaternary isopod, contour interval 500m
-......._ Thrust not influencing Quaternary Outcrops:
External Pliocene-Pleistocene deposits Oligocene-Miocene internal basins (allochthonous) Allochthonous sheet
r-:::::1 Pliocene-Pleistocene internal basin
tJ "Marnosa-Arenacea," Miocene D Holocene alluvium
Figure 2. Larger scale map of Ferrara thrust area and adjacent Apennines . Quaternary centers li,e on backs of major thrust faults. Internal basin sediments of outcropping area could be re garded as older piggyback basins, external deposits (Oligocene-Miocene ones are negligible at this scale and thus are not shown) repres1 nt foredeep sediments. lsopachs have been drawn from seismic sections and well data o,f AGIP. Subsurface structures are modified from Pieri and Groppi (1981). See Figure 1 for location.
Outer thrust arc
Inner thrust belt _
P'--'iggyback bo,.,._,,·s in,
N l
nary deposits define an example of a piggyback basin that is particularly clear because it is un deformed and displays a simple relationship of the fill to the deformation. This is not the case for the older basins (depocenters of Fig. l) be cause their thicknesi,es have been further modi fied by postdepositional deformation. Figures 3 and 4 show cross sections of young piggyback basins (Pieri and Groppi, 1981). In Figure 3, the main piggyback basin of Quater nary age extends as far south as the margin of the Apennine hills, where (Ori, 1982, Fig. 2) there is no evidence: of the synsedimentary thrusting that is typical of compressional faulted margins (Riba, 1976; Miall, 1978). The north ern edge of the basin, and of the basins shown in Figure 4, shows onlap (overlap) of the sedi ments against the major "outer" thrust s heets, which must have been emergent mountain ridges during most of the deposition. Whereas the youngest deposits of these pig gyback basins appear to have accumulated while the basins w1:re relatively static, the older sediments record the history of thrust-sheet movement. In the Ferrara area (Fig. 3), the outer thrust arc wa.s emergent in l atest Miocene time, as shown by conglomerates (Dondi et al., 1982). Pliocene deposits were cut by thrusting, and yet, by the end of Pliocene time, deposition had largely mantled the thrusts. In the Emilia area (Fig. 4), Miocene and Pliocene strata were increasingly tilted, presumably on the backs of the moving thrust sheets. No similar major tilt ing occurred in th1: main b asin to the north. The general environment of sedimentation in the piggyback basins (AGIP, 1982) varied from relatively deep-se.i, to alluvial, higher in the suc cessions. Detailed study of outcrops in the foothills of the Apennines has shown that the marginal deposits, of Pliocene and early Qua ternary age, formed in fan deltas (Ricci Lucchi et al., 1981; Farabegoli, 1983), in deep-sea fans, and on submarine slopes. Holocene depos its extend across the whole basin immediately below the surface. The sediment supply to the piggyback basins came predominantly from the Apennine cha.in; the outer thrust sheets do not seem to have played an important role as producers of detritus.
-2 -·
EJl QUATERNARY i
·.:·!MIDDLE-LATE
EARLY PLIOCENE PLIOCENE
D
MELANGE-LIKE TERRANES
I<<,:j PALEOGENE-MIOCENE
MESOZOIC
IOKm
Figure 3. Section of southern Po basin complex from AGIP seismic profile (after Pieri and Groppi, 1981). Piggyback basin consists of middle Pliocene to Quaternary deposits. Older pre-Mesozoic deposits can be regarded as ancient main-basin sediments afterward involved in thrusting. They form basement of piggyback basin. See Figure 1 for location. 476
EBRO BASIN The sedimentary fill in the Ebro basin com plex is largely continental (nonmarine), al though the earliest episodes were marine. The northern (Pyrenean) margin consists of a s eries of arcuate thrusts separating several distinct piggyback basim, (e.g., Jaca, Graus, etc., Fig. I). The non marine nature of the basin fills means that their ages cannot be determined as precisely as those of the Po basin complex. However, the fills of both the piggyback basins and the main (Ebro) basin can be regarded as belonging to broadly the same elastic complex GEOLOGY, August 1984
South
North
0-
"' 4>
E
-
2
>-
0
I 0
4
Km
Courtesy AG IP
Figure 4. Seismic-reflection profile across Emilia thrust area (Fig. 1) (reproduce d by permission of AGIP oil company; Pieri and Groppi, 1981). Stratigraphic surfaces distinguished are t ops or late and middle Pliocene (LMP), early Pliocene (EP), late Miocene (LM), middle Miocene (MM), and early Miocene (EM). Positions of wells serve to locate crests of two thrust-sheet ramps. Both ramps form northern margins of localpiggy back basins, and Pliocene sediments onlap against ramp ridges, which must have been emergent and moving during Pliocene time. Pliocene sediment in m ain basin, to north, is relatively undeform ed.
s
Inner belt
Outer thrust arc
N Km -I
Figure 5. Section through Graus basin and marginal area of Ebro basin. Onlap and bypassing have been observed in field; section is modified from Seguret (1972). See Figure 1 for location.
_-2 -3
_-4
10 Km
ITITffil EOCENE (EVAPORITES)
F f B OLIGOCENE MIOCENE
§§:§ CARBONIFEROUS-TRIASSIC
LJ
MESOZOIC-EOCENE
[/ (:/ /.j HERCYNIAN BASEMENT
( WITH EVAPORITES)
because they rest on the same basement and have comparable sedimentary facies. The age range of the sediments extends from latest Eo cene (on the basis of the age of the lower ma rine formations) to Miocene (on the b asis of the scattered occurrence of mammalian faunas) . The emergence of the outer arc defining the Graus basin (Figs. I, 5, 6) is shown by the coarse sedimentary breccias to the north and south of its present out crop. Sedimentation later overtopped the outer arc, (Fig. 5), and the Graus basin was united with the main Ebro basin, at least in the west of the outcrop. As in the piggyback basins of the Po plain, the north ern margin of the Graus basin shows no evi dence of the compressive deformation that occurs in the margins of adjacent piggyback basins (Riba, 1976). The Graus basin may have GEOLOGY, August 1984
NORTH
SOUTH
PIGGYBACK BASIN+ OUTER ARC
+
EBRO BASIN
BASEMENT
Terrigenous
,£:5 t CALCAREOUS
BRECCIA
Calcareous
. :: :. : : ·; FANGLO MERATE
l it FANGLOMERATE
WITH LIMESTONE CLASTS ALLUVIAL PLAIN DEPOSITS
..
Figure 6. Section from field observation showing sedimentary infill of western tip of Graus basin. Arrows show paleocurrents. See text for explanation. See Figure 1 for location.
477
Figure 7. Depositional en11ironments in marine pig !Jyback basins similar to those of Po basin 1:omplex.
MAIN
BASIN
PIGGYBACK BASIN
--'----011\0I'
- --'c,,.,..--lSequence contemporaneous with thrusting
not to scale
,--:.-::=I FAN DELTA
communicated with the main basin by bypass ing local valleys, but t here was a major connec tion through its western margin that probably corresponded to a lower lateral ramp.
DEVELOPMENT AND ROLE OF PIGGYBACK BASINS Piggyback basins of the sort that have formed in the Po and Ebro areas were initiated when new thrust-sheet ridges formed in front of the previously active thrust margins and within the areas of the previous main basins. The continued movement of the thrust sheets is commonly marked by stratigraphic wedging and unconformities within the new piggyback basins and by deformation and unconformitics in the margins of the main basins. In many cases there was a later phase of passive fill of the piggyback basins without further tectonic activity, and during this phase, sediment on lapped (overlapped) against the thrust-sheet ridge. Most of the sediment transport betwee:n 1:he piggyback basins and their main basins oc curred across the lateral ramps of the thrust sheets, although minor conduits traversed the outer ridges in some places. There is also a distinct trend in the evolution of sedimentary environments within the piggy back basins. In the Po basin complex where marine environments predominated (Fig. 7) warse-grained fan deltas built out from the Apennines and fed deep basin fans and the basin plains. The slopes of the ridges, in con trast, tended to be starved of sediment. Some of the piggyback marine basins were filled with sediment and became nonmarine. In most of the Ebro basin complex, nonmarine environ ments were predominant, although marine sed imentation was dominant in the earlier stages. Piggyback basin growth appears to have been the dominant process in these mountain systems, both of which belong to the continen478
lI
SUBMARINE FAN
SLOPE
g;;;; 1 BASIN PLAIN
tal collision type (Bally et al., 1966). Pieri and Groppi (1981) noted that the Apennines are composed throughout of similar thrust arc structures, and the Spanish part of the Pyrenees is also dominated by these st ructures (Sole Sugranes, 1978). In the Apennines, internal basins of Pliocene and Pleistocene age, and perhaps older (Fig. 2), contrast with marginal remnants of main-basin sediments and have some features of piggyback basins. They have now been uplifted by the further growth of the Apennines and occur as relicts in t he more in ternal zone of the Apennines. The depocenter of the main (foredeep) basin tends to have mi grated in front of the thrusts , whereas the pig gyback basins rest on the main b asin and have been uplifted and eroded. An abundant supply of sediment from a rising internal zone seems to be necessary to allow the preservation of pig gyback basins. REFERENCES CITED
AGIP (Azienda Generale ltaliana Petroli), 1982, Les sico delle Formazioni
Printed in U.S.A.
Pianura Padana, in Cremonini, G., and Ricci Lucchi, F., eds., Guida alla geologia de! margine appenninico-padano: Societa Geologica ltaliana, Guida Geologica Regionale, p. 47-58. Farabegoli, E., 1983, Note illustrative alla Carta Geo logica Regionale Cesena-Sagliano: Carta Geo logica Regionale, Regione Emilia-Romagna, Tecnoprint. Miall, A.D., 1978, Tectonic setting and syndeposi tional deformation of molasse and other non marine-paralic sedimentary basins: Canadian Journal of Earth Sciences, v. I 5, p. 1613-I 632. Ori, G.G., 1982, Braided to meandering channel patterns in humid-region alluvial fan deposits, River Reno, Po Plain (northern Italy): Sedimen tary Ge ology, v. 31, p. 231-248. Pieri, M., and Groppi, G., 1981, Subsurface geologi cal structure of the Po Plain, Italy: Progetto Fi nalizzato Geodinamica, Consiglio N azionale delle Ricerch,:, no. 414, 23 p. Riha, 0., 1976, S}ntectonic unconformities of the Alto Cardener, Spanish Pyrenees: A genetic in terpretation: Sedimentary Geology, v. I 5, p. 213-233. Ricci Lucchi, F., Colella, A., Ori, G.G., Ogliani, F., and Colalongo, M.L., 1981, Pliocene fan deltas of the Intra-Apenninic basin, Bologna: Interna tional Association of Sedimentologists, 2nd Eu ropean Regional Meeting, Excursion Guidebook, p. 81-164. Seguret, M., 1972, Etude tectonique des nappes et series decollfos de la partie central du versant sud des Pyrenees [Ph.D. thesis]: Montpellier, France, Univ,:rsity of Montpellier. Sole-Sugranes, L., 1978, Gravity and compressive nappes in the central southern Pyrenees (Spain): American Jo·Jrnal of Science, v. 278, p. 609-637. ACKNOWLEDGMENTS
We thank F. Ricci Lucchi, A. G. Smith, N. H. Woodcock, and G. J. Nichols for commenting on various drafts of this paper. The AGIP company con tributed fundamentally t o this paper by allowing us the use of their da1a. Cambridge Earth Sciences Con tribution No. 450. Manuscript received November 22, 1983 Revised manuscript received April I3, 1984 Manuscript accepted April 26, 1984 GEOLOGY, August 1984
