What is a “Sedimentary Environment”? It is part of the earth surface which is characterized by specific chemical, physical and biological characteristics that differ than its surroundings. There are three main types of environments: marine , terrestrial and terrestrial and transitional and transitional and several dozen of subenvironments
But, can clastic sediments form in all these sedimentary environments? No , Why ? Because each type of sediments accumulates accumulates within a specific environment (i.e. Depositionall Environment Depositiona Environment). ). A depositional environment is defined as a part of the earth's surface characterized by a unique combination of physical, chemical, and biological processes that makes it suitable for the deposition of certain type (s) of sediments. It is obvious that each depositional environment is characterized with specific type (s) of sedimentary rocks
1
Physical & Chemical factors
What do we need in order to characterize the deposits of differing environments (i.e. state their facies attributes)? • Fabrics : grain characteristics & mineralogy • Sedimentary structures : type, size, distribution Paleocurrents: Trace fossils : • Vertical and lateral variations: (time & space) So, we can easily estimate the main environmental controls: • Physical factors: e.g. Water depth and type: river, lake, ocean, Topography: mountain, plain, shallow or deep ocean, currents • Biological activity: fauna, flora, ichnofossils • Chemical factors: Ph, PCO2, salinity, …
Sedimentary Facies What is a Facies? Sedimentary rocks or rock Sedimentary characteristics which indicate a particular depositional environment.
2
•
On a continenta continentall shelf, sand may accumulate in the high-energy nearshore environment While mud and carbonate deposition takes place at the same time in offshore low-energy environments Lithofacies The aspect, appearance, and characteristics of a rock unit, usually reflecting the conditions of its origin. Example: Mature sandstone lithofacies” “ Mature composed of well sorted, rounded, medium-sized medium-sized quartz particles
Different sediments may accumulate adjacent to one another at the same time. Sediment facies ( lithofacies ) differ in those characteristics which indicate the depositional depositional environment physical (i.e. sedimentological) sedimentological) chemical (salinity, pH, etc...) The changes between adjacent lithofacies tend to be gradual Lithofacies descriptions should include grain size composition (minerals, rocks, other sedimentary particles) fabric content (ooids, peloids, shells, plant remains, etc... ) and its physical condition sedimentary structures dimensions (particularly thickness) Environmental Facies It describes the environment environment or area in which a rock was formed. Example: “Dune facies” where the texture (i.e. sorting, roundness, size) indicate that the rock was formed in a dune environment
3
Walther’s Law Sedimentary environments that started out side-by-side will end up overlapping one another over time due to transgressions and regressions.
Facies Limestone
Reef
Shale
Siltstone
Lagoon Near Shore
Environment 4
Sandstone
Beach
This is how a transgressive sequence is formed
Landward Migration of Shoreline = transgression
5
Facies changes due to rising sea level - water getting deeper everywhere River
Direction of migration of shoreline, and landward shift of sedimentary facies
Time B
Shoreline at time B
Time A
Shoreline at time A
Shallow marine Beach
River
Sea level rising
Deep marine Deep marine
Shallow marine
Shallow marine
Beach
Deposited at time A Deposited at time B
REMEMBER: the facies follow the shoreline
Comparison of sediments deposited
Facies and General characteristics of the Main Depositional Environments
6
Clastic Depositional Environments
1. Continental – fluvial, desert, lake & glacial 2. Shoreline (transitional) – delta, eustary, beach & tidal flat 3. Marine – Continental shelf, continental margin, and deep ocean
A.
Continental Depositional Environments
1.Desert(Aeolian)Depositional Environments
Deserts (Aeolian or environments) usually contain vast areas where sand is deposited in dunes. Dune sands are generally cross-bedded, well sorted, and well rounded. Desert climate is not always arid but it may be (semi) arid, warm or cold (sub) polar climatic zones: glacial processes dominate, (semi) arid climatic zones: aeolian processes dominate Deserts (Aeolian environments) are vast areas not only contain sands but eroding mountains and stone flats. Only 20% of modern deserts are sandy: eroding mountains (40%) ‘stony’ deserts (10-20%) desert flats (10-20%)
Desert sedimentary environments A desert basin showing the association of alluvial fan, sand dune, and playa lake deposits
7
Alluvial fan Desert Deposits • Alluvial fans are fan-shaped deposits formed at the base of mountains along the margins of desert basins where streams and debris flows discharge from mountains onto a valley floor.
• Alluvial fans are most common in arid and semi-arid regions where rainfall is infrequent but torrential, and erosion is rapid. • The slope of alluvial fans average about 50. Size of fans ranges from less than a 100m to more than 150 km in radius (av. 10 km). • Sediments deposited relatively close to their source area via high-energy unidirectional fluid flows. • Sediments are typically very poorly sorted and poorly rounded, reflecting the short distance of transport. • Sediments are generally oxidized, characteristic colors are red, brown, yellow. • Sediments generally not containing many fossils except for scattered vertebrate bones and plant fragments. • Sediments generally containing a limited suite of sedimentary structures, most commonly medium- to large-scale cross-strata and planar stratification. Reservoir Potentiality Alluvial fan deposits are not generally reservoir rocks for petroleum because: 1. They fail to connect laterally to source rocks, generally do not contain facies that are good source rocks. 2. They are not very deeply buried, 3. They are not sufficiently extensive laterally, depositional bodies having a lenticular or wedge-shaped geometry and typically forming clastic wedges. 4. They do not have proper seals, 5. They have low permeability and porosities following diagenesis
8
Playa Lake Desert Deposits • The more central part of a desert basin might be the site of a temporary lake, a playa lake, in which laminated mud and evaporites accumulate Desert Dune Deposits Aeolian dune deposits are, generally, made up of texturally and compositionally mature sand Dune sands are cross-bedded, well sorted, and well rounded, without associated gravel or clay.
Aeolian sedimentary structures Sand Ripples
Planar cross-bedding
Aeolian Facies • Cross-bedded, well sorted, sandstones • Coarse grained (conglomerate) or finer grained (claystone) clastics are not common.
Aeolian Reservoir Potentiality The problem with the aeolian sediments as potential hydrocarbon reservoirs is: Although aeolian deposits appear homogeneous, they are, in fact, texturally and depositionally complex deposits.
9
Trough cross-bedding
Heterogeneity of Aeolian deposits: Dune a hill, mound or ridge of wind-blown sand; Interdune those sediments occuring in the relatively flat areas between dunes of a dune complex; Sand Sheet those eolian deposits occurring marginal to a dune complex that generally do not have definable dune forms (also called low-angle eolian deposits); and Extradune includes those sediments marginal to a dune field that are not eolian, but are related to dune sediments in time and by source
Although, Aeolian deposits have proven to be complex, heterogeneous hydrocarbon reservoirs with variable and complex porosity and permeability variations
Here are: Some common problems are recorded with aeolian reservoirs include: Lateral discontinuity of reservoir zones; Impermeable or less permeable flat-bedded units interspersed with more permeable cross-bedded units; Anisotropic permeabilities and related textural changes and cementation along individual laminae causing low transmissivity across laminae are problems in well log interpretations; Isolated reservoirs causing reduced well spacing
•
Moreover, •
•
Differences in cross-bedding result in different fluid flow properties when the dunes are lithified. Interdune deposits, which are commonly impermeable in ancient rocks further complicate fluid movement in eolian reservoirs. Excessive reliance on the concept of eolian rocks as thickly cross-bedded and homogeneous deposits has hampered recognition of more lithologically complex eolian rocks that are commonly intercalated with marine or noneolian continental deposits. This in turn has hampered petroleum exploration and production in rocks of eolian origin.
10
2. River (Fluvial) Depositional Environments River depositional environment refers to river and stream types and activities and to their deposits.Fluvial environments are complex systems of erosion, sediment transport and deposition which give rise to a great variety of landforms. They refer to river and stream activity and to their deposits.
Sedimentation pattern is mainly related to current speed: As the current speed decreases, the larger sediments settle out and fall to the river bottom. Fast mountain river, everything smaller than a boulder is carried down the current. Moderately flowing river – carries small sand grains and clay. Slow moving river carries only fine clay particles
Two important types of river streams are known: Meandering Streams (high sinuosity) When a stream flows in a region of flat topography, it will often meander. Meandered streams are characterized generally by low current velocity and low sediment load o
11
Meandering Stream Deposits Floodplains are covered by silt and clay. Channel deposits consist of coarse, rounded gravel and sand. Point Bars are made of sand or gravel. Levees are made of fine sand or silt.
Braided Streams (low sinuosity) Braided streams are formed in regions with a high sediment load and changing stream volume. Sediment supply is greater than the amount stream can support. Braided streams have multiple broad, shallow channels. At any one moment the active channels may account for only a small proportion of the area of the channel system, but essentially all is used over one season. o
12
Braided Stream Deposits Braided stream deposits consist of conglomerate, cross-bedded sandstone but mudstone is rare or absent.
River (Fluvial) Facies • Fluvial sediments range from the coarsest conglomerates through sandstones to mudrocks. • Fluvial sandstones are usually cross bedded & sharp based. • Fossils are not common and mostly consist of plant remains and fresh water skeletal fragments. The effect of river flood on the nature of river sediments
1. When the amount of river water e dramatically increases (such as after heavy precipitation or rapid snow melt), water will overflow the banks of the channel unto the floodplain 2. As water flows unto the floodplain, its current velocity is dramatically reduced. 3. Larger sediments such as sand are deposited along the banks of the channel, forming a natural levee. 4. Finer sediments such as clay and silt are deposited further out in the flood plain. 5. These finer sediments deposited along the floodplain provide excellent regions for agriculture.
Fluvial Sedimentary Structures Graded bedding
Asymmetric ripples
13
Planar cross bedding
River (Fluvial) Facies • Fluvial sediments range from the coarsest conglomerates through sandstones to mudrocks.
• Fluvial sandstones are usually cross bedded & sharp based. • Fossils are not common and mostly consist of plant remains and fresh water skeletal fragments. Reservoir Potentiality • •
• •
River facies provide a better understanding of the architecture and heterogeneity of hydrocarbon reservoirs. Fluvial sediments are commonly highly oxidized because of exposure to oxygenated water during early diagenesis. They are also located on the margins of basins, relatively far from marine source rocks. These considerations have tended to lessen exploration interest in fluvial sediments of many basins. In spite of this, both structurally and stratigraphically trapped hydrocarbons have been discovered in fluvial deposits.
Potentiality of Braided-River Deposits •
Braided-river deposits offer excellent reservoirs in many cases, but have little potential for stratigraphic traps because of their paucity of thick, continuous fine-grained sediment. • Thus, prospecting for structural traps would seem the optimum exploration strategy. • Because of the coarse-grained nature of channel fill fluvial sediments, they may form potentially good reservoir rocks for oil and gas. Sandstones from channel fills (with up to 30% average porosity and permeability of thousands of milli-darcys) have been reported) • Whilst abandoned channels and levee sediments have the lowest porosity and permeability.
14
Delta de osits
Channel fill facies With high porosity and permeability
Potentiality of Meandering River Deposits •
•
Meandering stream deposits, with their abundant impermeable floodplain shales and laterally restricted sand bodies, are most likely to form stratigraphic traps of limited size. Because fluvial sediments are commonly associated with plant material and coal, they are commonly considered more likely to contain gas than oil.
3.
Glacial Depositional Environments
All sediments deposited in glacial environments are collectively called drift. Till is poorly sorted, nonstratified drift deposited directly by glacial ice mostly in ridgelike deposits called moraines
15
Sedimentary Structures Glacial Striations • They represent the best indicators for the direction of ice advance. They are made of sand and gravel clasts adhering to the ice at the base of the glacier Glacial Facies Glacial Till: poorly sorted sediment consisting of both very large and very small clasts. Tillites - lithified tills with poorly sorted texture Erratic clasts – glacial rocks derived from distant outcrops Dropstones – large boulder, cobble, or pebble clasts set in a fine carbonate or clay matrix Diamictites:
4.
Lacustrine Environments
Lacustrine environments (or lakes) are diverse; they may be large or small, shallow or deep, and filled with terrigenous, carbonate, or evaporitic sediments.
Clastic Lacustrine sedimentation can be categorized by:
16
a. Shallow littoral sediments, mostly sandy deposits b. Pelagic sediments, muddy sediments at the deepest parts of the basin
Palustrine environments occur where lake margin sediments are subaerially exposed.
Sedimentary Structures Burrows
Stromatolites
Ripples
Lacustrine Facies -Fine grained sandstone and mudstone. Bioturbation may occur -Alternating parallel bedded very fine grained sandstone and mudstone
Hydrocarbon Potentiality •
•
•
Although sedimentary rocks formed in lacustrine depositional systems are common from much of the world, relatively few have been the focus of exploration for oil and/or gas. However, large accumulations of oil and gas trapped in rocks formed in ancient lake systems are known from the western part of the United States and from much of China. In addition, "shows" and oil and gas fields developed in strata of lacustrine origin are known from several other parts of the world. Siliciclastic rocks of lacustrine ori¬gin are known to contain oil, natural gas, and bitumens as well as oil shale. They are commonly interbedded with beds composed of saline minerals such as halite, trona. In addition, marginal-
17
• • •
•
lacustrine and related alluvial siliciclastic strata may contain uranium and coal. The best known petroleum-bearing lacustrine rocks are those of Utah, USA and of the giant oil fields of China. In these lacustrine depositional systems, the primary reservoirs are siliciclastic rocks. Although oil and gas are produced from lacustrine rocks therein, much is recovered from rocks that formed outside the ancient lakes in depositional settings at the fluctuating margin of the lake or in environments well removed from the lake. Hydrocarbons found in non-lacustrine beds are believed to have formed from lacustrine source rocks and migrated into beds of the peripheral depositional facies.
B.
Transitional Depositional Environments
1. Delta & Estuary Depositional Environments Delta
The delta begins where the river channel transforms itself into the delta channel or into a system of distributary channels. Deltas are made up of: o Delta top o Delta distributary channel system o Delta plain (overbank) o Delta front
Delta plain (overbank) Distributary delta channel
Delta front
Delta top
18
Delta Depositional Environments • A delta depositional environment forms when a river reaches the sea carrying more material than marine currents can redistribute.. • So, a significant deltaic accumulation necessarily requires the existence of a river system carrying substantial quantities of clastic sediment from an inland drainage basin to the coast where the deposits form the delta plain. • Delta depositional environments comprise very complex depositional systems embracing fluvial, terrestrial and marine environments. • Deltas consist of a subaerial (upper) delta plain , and a subaqueous delta front (lower and marginal deltaic plains) and prodelta • The delta slope is commonly 1-2° and consists of silty facies; the most distal prodelta is dominated by even finer muddy facies
Simple Deltas The simplest deltas are those in lakes and consist of topset beds, foreset beds and bottomset beds. and forms a vertical sequence of rocks that becomes coarser-grained from the bottom to top The bottomset beds may contain lake fossils, whereas the topset beds contain land fossils. o
•
19
Marine Deltas • Marine deltas rarely conform precisely to his simple threefold division because they are strongly influenced by one or more modifying processes: • When fluvial processes prevail a stream/river-dominated delta results • Strong wave action produces a wave dominated delta • Tidal influences result in tide-dominated deltas River-dominated Delta Delta channel and delta plain sediments are basically made up of fluvial deposits (i.e. sands & gravels) similar to those found in river streams River-dominated delta have long distributary channels extending far seaward and form a “bird’s foot" pattern • The "bird's foot" pattern of the Mississippi River delta is the result of: 1. the abundant sediment supply and large river discharge relative to 2. the much lower wave energy for sediment transport in the Gulf of Mexico o
Wave-influenced Delta • Forms where strong and persistent wave energy exceeds river or tidal energy • They have distributary channels but their seaward margin is modified by wave action • Bedload (sand or gravel) is reworked by waves and currents as quickly as it is deposited • Delta front sandbars and beach ridges are aligned parallel to shore
20
Tide-influenced Delta • Forms where the river meets a coast with a large tidal range (> 3 m) • Tidal channels on the delta front display reversal flow as the flood tide balances the downstream discharge • Sandbars are aligned perpendicular to the shoreline
21
Delta Sedimentary Structures
Planar cross bedding
Flat parallel lamination
Graded bedding
Delta facies • The complexity of environmental settings under which deltas exist results in a variety of vertical sequences that can form within the delta facies. • Delta facies is composed mainly of clastic deposits that form (either subaerial or subaqueous) by fluvial, waves and/or tidal processes. • Coarser sediment (sand) tends to be deposited near the mouth of the river. • Finer sediment is carried seaward and deposited in deeper water. Delta facies is composed mainly of clastic deposits that form (either subaerial or subaqueous) by fluvial and/or tidal processes. Coarser sediment (sand) tends to be 22
deposited near the mouth of the river. Finer sediment is carried seaward and deposited in deeper water. Delta facies include massive sandstone with thin vertical burrows, parallel and cross-bedded sandstone. Molasse Facies • In deltaic environment, with a oscillating relative sea level change, repeated sequence of sandstone, clay, coal, limestone (sometimes) and shale- from the bottom up- is formed.. This is called molasse facies.
23
Estuary Environment • Estuaries are transgressed, drowned river valleys where fluvial, tide, and wave processes interact. • An estuary is a semi-enclosed marginal marine body of water in which salinity is measurably diluted by fluvial discharge
Estuary It is a "drowned" river valley. It forms when an inlet of the sea reaches into a river valley as far as the upper limit of tidal rise Two types of channels exist: tidal channels , which extend well below the position of the lowest tides; and runoff channels , which are perched atop the flats. Near the inlet, sediments are generally sand: bars and tidal ridges.
24
Within the estuary, tidal channels may contain either longitudinal or oblique bars. In the upper reaches of the estuary sediments are "muddier"
•
Estuarine sediment typically consists of well-sorted fine sand and mud, two very different types of material. The sand may be introduced mostly from the ocean, while the mud is contributed primarily by river discharge. • The winnowing effects of waves and tidal currents typically diminish toward the upper reaches of an estuary. Moreover, the finest material may be selectively transported landward by the tides. In any event, sediment near the inlet tends to consist predominantly of sand, whereas that in upper parts of an estuary is mostly mud. • Commonly, the mud and well-sorted sand are interlayered in sharply contrasting strata, although intense bioturbation may mix the components into a muddy sand or sandy mud. • The fining-upward character of estuary fill resembles that of fluvial deposits • Estuarine deposits should typically be of limited geographic extent. Within this constraint, the best geologic evidence suggesting an estuarine environment is probably brackish water fauna. Estuary Sedimentary Structures
Planar cross bedding
Flat parallel lamination
Estuary Facies 25
Graded bedding
Sediments deposited in this setting are influenced by a complex combination of tides and tidal currents, oceanic waves, locally generated waves, river discharge, precipitation, temperature, and local flora and fauna. These factors differ markedly among the world's estuaries, and accordingly the sedimentary facies produced vary widely. Clastic estuarian facies include parallel bedded-rippled burrowed sandstone, Cross-bedded sandstone Tidal channel bedload may include shells and other large clasts (mainly sand) with current ripples Tidal flat deposits are bioturbated or laminar mud with thin sand sheets Hydrocarbon Potentiality • Ancient estuary-fill complexes should have excellent oil and gas source potential. • Sand deposits are typically very well sorted and should have excellent reservoir characteristics. • They are associated with muds rich in organic products; estuaries are among the most biologically productive sedimentary environments known. • The intercalation of supratidal and other mud with sand provides impermeable barriers necessary to the development of stratigraphic traps. 3. Beach Environments
Beach Clastic Facies Parallel bedded-rippled sandstone, Granular, well sorted conglomerate
26
3. Lagoon Environment and Barrier Sand Islands • Lagoons are bodies of water on the landward side of barrier islands. They are protected from the pounding of the ocean waves by the barrier islands or reefs. • They contain finer sediment than beaches (usually silt and bioturbated mud).
Sand Barriers Elongate sandy islands that parallel the shoreline and are separated from it by lagoons or marshes. • In contrast to river deltas, which result from interaction of fluvial and marine processes, barriers and strand plains are controlled entirely by marine processes. They are formed on coastlines where the wave processes are more important than tidal currents.
27
The environments of sand deposition include: (1) Beach and shoreface environments on the seaward side of barriers and strand plains; (2) Inlet channels and tidal deltas , separating barriers laterally; and (3) Washover fans on the landward or lagoonward side of barriers. Seaward or longshore migration of these environments results in facies sequences constituting much of the volume of many coastal sand bodies.
28
• Barrier islands migrate landward and seaward with sealevel changes. •They often form major components of shoreline regressive/transgressive sequences.
Sand Barrier Facies Distribution of facies, external geometry of sand bodies, and nature of associated facies are variable and depend on sediment supply and relative sea-level changes Cross- Laminated Sandstone Laminated Sandstone Bioturbated Sandstone Clean sandstone
Hydrocarbon Potentiality • Barrier Islands generate long thin sand bodies of excellent porosity & permeability within impermeable shale sequences. • Excellent reservoir rocks for petroleum.
29
4. Tidal Flat Environments • Tidal flats are vast areas regularly to rarely covered by water and dominated by weak currents & wave actions. • They involve areas within the intertidal zone (flooded by daily tides) and the supratidal zone (flooded by wind tides). • The tidal facies and the rate of capillary evaporation vary among the supratidal and intertidal zones.
Tidal Flat Sediments • • • •
Tidal flat sediments occur as widespread sheets that are often dissected by channels. Bedding is thin and even and contacts are sharp; but evaporites show irregular bedding and may be nodular. Collapse breccias of angular fragments are local. Tidal flat sediments range from cross bedded sands to bioturbated muds.
Tidal Flat Sedimentary Structures
Tidal flats are excellent sedimentary environments for the preservation of trace fossils (e.g. bioturbation) as well as physical sedimentary structures because of the alternating layers of sand and mud (e.g. parallel, lenticular and flaser bedding).
Tidal Flat Clastic Facies • Laminated or rippled clay, silt, and fine sand (either terrigenous or carbonate) may be deposited. Lenticular bedding (i.e. interbeded clay and sand Where mud dominates) is common. Intense burrowing is common. Sediments usually possess low porosity.
30
Tidal-flat deposits showing a prograding shoreline
Shallowing upward sequence from subtidal to tidal flat
• • •
Supratidal flats are dominated by burrowed mudstones with plant remains. Some swamps with coal and peat may exist. The intertidal flat displays a variety of intertidal sand and mud layers, including flaser, wavy, and lenticular bedding. Bioturbation is common. Large-scale cross-bedding, and associated small-scale current ripples and flaser bedding, are common in subtidal zone. Tidal flats developed under regressive or prograding conditions are characterized by a fining upward sequence, consisting of coarse sediments at the base and progressively finer sediments toward the top in an uninterrupted, vertical sequence. This reflects decreasing energy in a progression from subtidal to intertidal parts of the tidal flat. Hydrocarbon Potentiality • The relatively coarse grained cross bedded sandstones formed within the intertidal areas and tidal channels are recorded as good reservoirs. • Source rocks are mainly organic-rich marine clays. Factors that may control the production of the different coastal depositional environments: 1. Type of sediment supply (marine vs. fluvial) 2. The relative effect of tidal, waves and fluvial processes. 3. Eustatic sea level variations
31
C. Marine Depositional Environments
1. Shallow Marine Continental Shelf Environments 2. Deep Marine Environments • Continental Slope • Deep Basin
32
General Environmental Factors that Control the Deposition in Marine Environments I. 1. 2. 3. 4. 5. III. -
Physical Factors Depth of water Temperature of water Currents Bottom topography Turbulence Biological Factors Organic population Organic diversity
1. Shallow Marine Shelf Sediments
•
•
•
The continental shelf is that part of the sea floor between the shoreline and the shelf break, or upper edge of the continental slope (i.e. The gently sloping area adjacent to a continent is a continental shelf) However, an examination of bathymetric charts shows that both the shelf depth and the shelf width vary: The shelf depth may be as shallow as 18 m or as deep as 915 m and the shelf width may range from a few kilometers to more than 1,000 km. On the average, the shelf break occurs at 124 m depth, and the shelf width is 75 km. Morphologic and sedimentary characteristics of the shelf also vary considerably. The shelf surface may be smooth, covered by a variety of bedforms, or may contain banks, islands, and shoals near its offshore edge.
33
Current Circulation within the Shelf
wide > 10 km
Circulation decreases
narrow < 10 km Circulation increases
Continental Linkage of the Shelf Attached Terrigenous sediment source
Detached
Detached
34
Terrigenous sediment source
Morphology of the Shelf Ramp
Distally steepened
Homoclinal
Rimmed Platform Detached
Attached Flat-Topped Platform Attached
Detached
Types of Shallow marine shelf environments Shallow marine shelf environments include: 1. Peri-continental seas that occur along continental margins and have a shoreline-shelf-slope profile with barriers or rims; and 2. Epi-continental seas that exhibit ramp morphology with no barriers along the shelf or the shelf edge. Ramp Shelf • Gently sloping (<1°) Shelf with no continuous rim or barrier along platform edge • Ramp shelf may have high energy beach
35
Rimmed Shelf Margin • A shelf with pronounced break in slope • It is characterized with the presence of nearly continuous rim or barrier along platform edge • The barrier is a wave resistant structure consisting of either a reef or sand shoals • The landward side of rim/barrier is a low energy "lagoonal" area of variably restricted circulation. This lagoonal area commonly grades landward into tidal flat
Depositional Processes on Clastic Shelves • The sedimentary characteristics of the clastic shelf sediments change from one area to the next depending on differences in waves, currents, climatic conditions, and proximity to large sources of sediment. • For example, muddy shelves may contain nearly homogeneous sediments except for the presence of layers formed during storms. • Other shelves consist primarily of sand blanket dissected by valleys and then partly filled with either fine-or coarse-grained sediments. • In many areas, the overall smooth shelf has a sand blanket that has been molded into a variety of small and large bed-forms. • Complex interactions between such factors as tectonics, sea-level fluctuations, daily and seasonal wave and current dynamics, along with special events such as storms result in a mixture of detrital sediments within a variety of microenvironments. Depositional Settings within Shelves • Shallow marine shelves involve: • Upper high-energy shoreface depositional setting (periodically stirred up by waves and tidal currents) characterized by sandstones with current & wave structures, 36
• •
Lower shoreface depositional setting characterized by mudstone with thin cross-stratified sandstone beds and Offshore depositional setting below the storm wave base characterized by bioturbated mudstones and no current sedimentary structures
Detrital Shallow Marine Sediments • Texturally and compositionally mature sands • lower shoreface settings can be identified by mudstone with thin crossstratified sandstone beds • upper shoreface settings can be identified by sandstones with current & wave structures • bioturbation increases offshore and may obscure some primary sedimentary structures Shallow marine clastic facies is characterized by bioturbated and cross-bedded sandstones. Porosity varies although rocks that display good petrophysical characteristics (reservoir quality) are common Continental Platforms are the main offshore target for hydrocarbon exploration • Continental shelves of the World Ocean have become the primary regions for petroleum and natural gas exploration and drilling. • Hundreds of reservoirs of "black gold" are already developed in these areas. • There are over 3,000 steel platforms extracting of petroleum and natural gas, and tens of thousands of holes have been drilled into these reservoirs. • The potential for economic oil and gas accumulations in sandstone facies of ancient shelf deposits is high. • This setting, whether it be epicontinental or pericontinental, provides the four main ingredients for petroleum accumulations: (1) potential reservoirs; (2) potential hydrocarbon source rocks; (3) potential trapping situations; and (4) the time and depth of burial required to generate petroleum. • Reservoir potential is high because of marine sorting mechanisms that produce relatively clean, well-sorted sands.
37
•
•
Surrounding marine muds, representing relatively low rates of sediment accumulation under low energy conditions, may contain significant quantities of organic carbon compounds. These same muds become relatively impermeable mudrocks after compaction, forming seals for stratigraphic traps.
2. Deep Marine Depositional Environments • Much sediments derived from the continents crosse the continental shelf and is funneled into deeper water through submarine canyons • They eventually comes to rest on the continental slope as a series of overlapping submarine fans
Mass (Sediment) Transport Via Continental Slope • Subaqueous mass transport through canyons (mostly sediment gravity flows) involve: • Debris flows, which are commonly laminar and typically do not produce sedimentary structures • Turbidity currents, which are primarily turbulent; they commonly evolve from debris flows. A turbidity current is a gravity current of rapidly moving, sediment-laden water moving down a slope. • A turbidity current moves because it has a higher density than the fluid through which it flows. The sediments from the submarine canyon are then deposited on the deep-ocean floor.
38
Turbidites are usually graded bedded rocks The normal graded bedding formed mainly by turbidity currents: these currents can transport grains up to pebble size for long distance and when the velocity of the flow decreases, their load is finally deposited, it is graded according to specific gravity that the coarse-grained materials fall firstly, and followed by the finer fraction.
Deep Marine Fans
•
Submarine fans share several characteristics with deltas; they consist of a feeder channel that divides into numerous distributary channels bordered by natural levees (‘channel-levee systems’) and are subject to avulsions 39
• Proximal upper fan (trunk channel) • Medial fan (lobes) • Distal lower fan Bouma Sequence • A certain sequence of sediments may be generated by these turbidity currents and is named turbidites. • An ideal sequence of turbidites was first described by Bouma (1962) and is called Bouma sequence
The idealized Bouma sequence consists of divisions A-E, A: Rapidly deposited, massive sand and gravels B: Planar stratified sand C: Small-scale rippled, cross-stratified fine sand D: Laminated silt E: Homogeneous mud
40
• • • •
Other • • •
• •
Bouma cycles begin with an erosional contact of a coarse lower bed of pebble to granule conglomerate in a sandy matrix, and grade up through coarse then medium plane parallel sandstone; through cross-bedded sandstone; rippled cross-bedded sand/silty sand, and finally laminar siltstone and shale. This vertical succession of sedimentary structures, bedding, and changing lithology is representative of turbidity flow regime and currents and their corresponding sedimentation. Deep Basin Sediments Rather than turbidites, deep seafloor is nearly covered by: Calcareous pelagic muds Terrigenous muds Hemipelagic sediments consist of fine-grained (muddy) terrigenous material that is deposited from suspension. Eolian dust is an important component (~50%) of hemipelagic (and pelagic) facies Volcanogenic muds = <30% CaCO3+ Ash Black shales , with 1–15% organic-matter content, may form in anoxic bottom waters
Deep marine clastic facies Alternating parallel bedded very fine grained sandstone and mudstone, graded bedded sandstones, bedded siltstone and mudstone, Bioturbated mudstone Flysch or turbidite deep marine facies are Rhythmically bedded thin beds of shale alternating with graywacke deposited by submarine landslides from the continental shelf down to the deep ocean basin
41
