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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 transitional and several dozen of subenvironments

But, can clastic sediments form in all these sedimentary environments? No, Why? Because each type of

sediments accumulates within a

specific environment (i.e. Depositional 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

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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 characteristics which indicate

a particular depositional

environment.

Physical & Chemical

factors

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• On a continental 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” composed of well sorted, rounded, 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 environment

� physical (i.e. 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 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

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Walther’s Law Sedimentary environments that started out side-by-side will end up

overlapping one another over time due to transgressions and regressions.

LLiimmeessttoonnee SShhaallee SSiillttssttoonnee SSaannddssttoonnee

RReeeeff LLaaggoooonn NNeeaarr SShhoorree BBeeaacchh

EEnnvviirroonnmmeenntt

FFaacciieess

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This is how a transgressive sequence is formed

Landward Migration of Shoreline = transgression

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Facies changes due to rising sea level - water getting deeper everywhere

Facies and General characteristics of the Main Depositional

Environments

RiverDirection of migration

of shoreline, and landward shift of sedimentary facies

Shoreline at

time B

Shoreline at time A

Time B

Time A Sea level rising

Deposited at time A

Deposited

at time B

Shallowmarine

Beach River

Deepmarine

Deep marine

Shallow

marine

Beach

Shallow

marine

Comparison of sediments deposited

REMEMBER: the facies follow the shoreline

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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

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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

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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 Trough 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.

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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.

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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: o 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

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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.

o 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.

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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 Planar cross bedding

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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.

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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 ridge-like deposits called moraines

DDeellttaa ddeeppoossiittss

WWiitthh hhiigghh ppoorroossiittyy aanndd ppeerrmmeeaabbiilliittyy

CChhaannnneell ffiillll ffaacciieess

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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:

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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-

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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 top

Delta front

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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

o 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.

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o 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

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

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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

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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

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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.

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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.

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� 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 Graded bedding

� Estuary Facies

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� 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

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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.

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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.

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• 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.

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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.

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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

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C. Marine Depositional Environments

1. Shallow Marine Continental Shelf Environments

2. Deep Marine Environments

• Continental Slope • Deep Basin

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General Environmental Factors that Control the Deposition in Marine

Environments I. Physical Factors

1. Depth of water

2. Temperature of water

3. Currents

4. Bottom topography 5. Turbulence

III. 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 bed-

forms, or may contain banks, islands, and shoals near its offshore edge.

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Attached

Terrigenoussedimentsource

Detached Detached

Terrigenoussediment

source

Current Circulation within the Shelf

Circulation decreases

Circulation increases

Continental Linkage of the Shelf

wide > 10 km

narrow < 10 km

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Morphology of the Shelf

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

Ramp

Homoclinal Distally steepened

Attached Detached

Rimmed Platform

Flat-Topped Platform

Attached Detached

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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,

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• 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 cross-stratified 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.

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• 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.

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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

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• 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

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• 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.

Other 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

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Reservoir Potential of turbidite facies

• Turbidites could have significant source potential because of their

origin in shallow water areas of high organic productivity.

• In addition, their porous facies (e.g. coarse grained sandstone of the

fan lobes) may act as reservoirs or as conduits for migrating oil.