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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
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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.
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