DEPOSITIONAL ENVIRONMENTS WITH THEIR RELATED FACIES

BY
MUHAMMAD FAHAD MAQBOOL (01-161161-055)

SUBMITTED TO

SIR MUYASSAR HUSSAIN

Department of Earth and Environmental Sciences Bahria University, Islamabad

2019
DEPOSITIONAL ENVIRONMENTS
A depositional environment is a specific type of place in which sediments are deposited, such as a
stream channel, a lake, or the bottom of the deep ocean. They are sometimes called sedimentary
environments. The layers of sediment that accumulate in each type of depositional environment
have distinctive characteristics that provide important information regarding the geologic history
of an area. Depositional environments are important for reconstructing earth history, understanding
earth processes, and helping humans survive and prosper on earth. The characteristics of a
sedimentary rock that are affected by its depositional environment are its sedimentary lithology
(the minerals and texture of the rock), its sedimentary structures, and its fossils. Sedimentary rocks
contain sedimentary structures that were formed as the sediments were being deposited. Many
sedimentary rocks also contain fossils, which are our main source of information of the history of
life on earth. Sedimentary structures, and fossils, are best found and and examined in outcrops,
where whole beds of sedimentary rocks are exposed in their undisturbed geological setting. The
structures and fossils in sedimentary rocks reveal what was happening on the earth at the place and
time the sediments were being deposited.

Figure 1 Schematic diagrams showing the main sedimentary depositional environment

TYPES OF DEPOSITIONAL ENVIRONMENT

1. CONTINENTAL ENVIRONMENT
The continental environments are those environments which are present on the
terrestrial plains of continents. They are alluvial fans, fluvial environments (rivers), lacustrine
environments (lakes), aeolian or eolian environments (deserts), and paludal environments
(swamps).

a) Fluvial environment deposition:

The Fluvial processes include the motion of sediment and erosion or deposition on the
river bed. Sediment in rivers is transported as either bed load (the coarser fragments which move
close to the bed) or suspended load (finer fragments carried in the water). There is also a
component carried as dissolved material.Fluvial environments include braided and meandering
river and stream systems. Fluvial environments include braided and meandering river and stream
systems. River channels, bars, levees and floodplains are parts (or sub-environments) of the
fluvial environment. Channel deposits consist of coarse, rounded gravel, and sand. Bars are made
of sand or gravel. Levees are made of fine sand or silt. Floodplains are covered by silt and clay.

b) Alluvial deposits:
Alluvial fans are sedimentary deposits that typically form at the margins of a dry basin. They
typically contain coarse boulders and gravels and are poorly sorted. Fine-grained sand and silt
may be deposited near the margin of the fan in the valley, commonly in hallow lakes. These
lakes may periodically dry, and evaporate deposits may result. It consists of silt, sand, clay, and
gravel, as well as much organic matter.

and Huang (Yellow) rivers. They contain most of the world’s supply of tin ore, as well as,
gold, platinum, and gemstones.
c) Eolian deposits:
Wind is an effective sorting agent and will selectively transport sand. Gravel is left
behind and dust-sized particles are lifted high into the atmosphere and transported great
distances. Windblown sand forms dunes that are characterized by well-sorted grains showing
large-scale cross bedding. Eolian dust sediments, which in many cases have given rise to
loesses and blanket loams, occur almost everywhere. Eolian deposits are found primarily in
arid regions (sands, loesses), but they also occur in other natural zones. When wind energy is
not sufficient to transport sand and dust particles, they fall out of the air and accumulate, often
in front of orographic barriers. When precipitating from the air, sometimes with rain droplets or
snow Dune sands are well sorted, well rounded, and frosted or polished, without associated gravel
or clay. Cross-bedding is common.

d) Lacustrine depostis:
Lacustrine Plains are lakes that get filled by incoming sediment. These deposits get exposed by
elevation of old lakebeds. Lacustrine deposits are very well sorted, devoid of coarse particles such
as coarse sand or gravels, and are characterized by thin layers that reflect annual deposition of
sediments. Lakes (lacustrine environments) are diverse. They may be large or small, shallow or
deep, fresh water or salt water, and filled with terrigenous, carbonate, or evaporite sediments.
Mudcracks, wave ripples, laminations, and varves may be present in lakes. The soil of the plain
left behind may constitute fertile and productive farm land, due to the previous accumulation of
lacustrine sediments (algal and bacterial).
e) Paludal:
The Swamps often form in low-lying areas in which parallel layered, organic-rich black
shales and coal form. Swamps are areas of standing water with trees. Decaying plant matter
accumulates to form peat, which may eventually become coal. The high amount of biological
activity, so rocks of high organic content are abundant. The mixed muds & coal/peat/lignite,
with some fluvial sediment are often associated with other environments (deltaic, fluvial,
coastlines, etc.) and low energy. The swamp deposits plant-choked, periodically inundated
environments. The swamps are organic-rich shale and sandstone or coal deposits with thin
stringers of silstone and shale. The plant fossils are common in all stages of preservation.

2. TRANSITIONAL ENVIRONMENT:
The Transitional environments are those environments at or near where the land meets
the sea. Transitional sedimentary environments include deltas, beaches and barrier islands,
lagoons, salt marshes, and tidal flats. The tidal flats are low-lying areas that are alternately
covered by water and exposed to the air each day. The Transition zone between continents
and marine environments.
Sub-environments include Deltaic, Estuarine, Lagoonal and Beach environmental
depositions.

Figure 2 showing Transitional depositional environment

a) Deltaic:
The Deltas are large accumulations of sediment that are deposited where a river empties into
a standing body of water. They are one of the most significant environments of sedimentation
and include a number of sub environments such as stream channels, flood plain beaches, bars,
and tidal flats. The deposit as a whole consists of a thick accumulation of sand, silt, and mud.
The deltas are fan-shaped deposits of sediment, formed where a river flows into a standing body
of water, such as a lake or sea. The coarser sediment (sand) tends to be deposited near
the mouth of the river; finer sediment is carried seaward and deposited in deeper water. The
alluvial-deltaic boundary occurs where the river begins to bifurcate and form distributaries.

b) Esturine:
An estuary is the marine-influenced portion of a drowned valley. A drowned valley is
the seaward portion of a river valley that becomes flooded with sea water when there is a
relative rise in sea level. They are regions of mixing of fresh and seawater. Sediment supply
to the estuary is from both river and marine sources, and the processes that transport and
deposit this sediment are a combination of river and wave and/or tidal processes.
c) Lagoonal:
The lagoons are shallow, often elongated bodies of water separated from a larger body of
water. The 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. They contain finer
sediment than the beaches (usually silt and mud). The lagoons are also present behind reefs,
or in the center of atolls. The lagoons are coastal bodies of water that have very limited
connection to the open ocean. The lagoons generally develop along coasts where there is a
wave-formed barrier and are largely protected from the power of open ocean waves. A
lagoonal succession is typically mudstone often organic-rich, with thin, wave-rippled sand
beds.
d) Beach:
The Beaches are the most recognizable transitional environments. The beaches form
where wave energy washes silt and clay away, leaving larger sand particles behind. The silt
and clay may then be deposited in other low energy transitional environments. They termed
as tidal flats or in deep marine environments. The beaches and barrier islands are shoreline
deposits exposed to wave energy and dominated by sand with a marine fauna. The barrier
islands are separated from the mainland by a lagoon. They are commonly associated with
tidal flat deposits.

4. MARINE ENVIRONMENT:
The Marine environments are in the seas or oceans. The marine environments include
reefs, the continental shelf, slope, rise, and abyssal plain. The reefs are wave-resistant,
mound-like structures made of the calcareous skeletons of organisms such as corals and
certain types of algae. The most modern reefs are in warm, clear, shallow, tropical seas,
between the latitudes of 30ºN and 30ºS of the equator. Thick accumulations of sand, silt and
6
mud form in several sub environments, including stream channels, flood plain, beaches, tidal
flats, and sand bars. From the beach outward, well sorted, clean, shoreface sandstones
accumulate in the wave-dominated near shore environment.

Further classification includes Shallow marine clastic, Continental Shelf, Continental

Slope and Deep marine environmental depositions.

Figure 3 showing continental margin

a) Shallow marine clastic:

Nearly flat, gentle sloping edge of the continent that extends under the
ocean. The regions near the mouths of rivers are usually clastic dominated, because the
critters that secrete CaCO3 tend to have trouble living in muddy water. In closed basins,
salinity levels of the water bodies may increase above that of seawater. As a result of excess
evaporation concentrating the salts in solution. The Sediments wash off of the continent onto
the shelf. The size may range from coarse sand
closest to the continent fining to clays. In some areas coral reefs or carbonate muds dominate
the shelf.
b) Continental shelf:
The continental shelf is the flooded edge of the continent. It is relatively flat (with a
slope of less than 0.1º), shallow (less than 200 m or 600 ft deep). It may be up to hundreds of
miles wide. The continental shelves are exposed to waves, tides, and currents. They are
covered by sand, silt, mud, and gravel. The flooding of the edges of the continents occurred
when the glaciers melted at the end of the last Ice Age, about 10,000 years ago. The
continental shelf is the shallow ocean surrounding the continent. The depth at the edge of the
shelf is usually not more than 100 to 150 meters. Some sand and mud are carried to the edge
of the continental shelf via submarine canyons which are like undersea river valleys.

c) Continental slope:
The continental slope and continental rise are located seaward of the continental shelf.
The continental slope is the steep (5- 25º) "drop-off" at the edge of the continent. The
continental slope passes seaward into the continental rise, which has a more gradual slope.
The continental rise is at the base of the continental slope, where thick accumulations of
sediment are deposited. The sediments build up at the edge of the shelf. When too much has
accumulated these flows down the continental slope and rise as turbidity currents. The
resulting deposits, called turbidites. They contain some chaotic, poorly sorted coarse layers at
their base and then finer layers on top. The repeated sequences of turbidites indicate
deposition on the continental slope and continental rise. The large landslides have occurred
down the continental slope. The landslide deposits at the base of the slope are part of the
continental rise. Such a landslide could trigger a tsunami.

d) Deep marine:
The abyssal plain is the deep ocean floor. It is basically flat, and is covered by very
fine-grained sediment. It consists primarily of clay and the shells of microscopic organisms
(such as foraminifera, radiolarians, and diatoms). The abyssal plain sediments may include
chalk, diatomite, and shale, deposited over the basaltic ocean crust. In the deep sea, out on the
abyssal plains, the depth to the seafloor varies from about 2.5 to 6 km (2500 to 6000 meters)
or more below sea level. The abyssal plains receive very little sediment from the continents.
The biogenic oozes: Calcareous oozes from deposits of single-cell, microscopic organisms
with calcite shells result in finely laminated limestone. The siliceous oozes from single-cell,
microscopic organisms with silica shells form finely laminated chert (silica) layers.
Furthermore, the limestones indicate warm water; limestone dissolves in cold water. The
chert indicates high biological productivity and cool water.

5. OTHER TYPES:
In this category there are about two types, they are Evaporite and Glacial deposition
environments.

Evaporites:
Evaporite is a name for water-soluble mineral sediment. This results the
concentration and crystallization by evaporation from an aqueous solution. There are two
types of evaporate deposits: marine, which can also be described as ocean deposits, and non-
marine, which are found in standing bodies of water such as lakes. Evaporites are considered
sedimentary rocks.
Marine Evaporites:
The marine evaporites tend to have thicker deposits and are usually the focus of more
extensive research. They also have a system of evaporation. The most common minerals that
are generally considered to be the most representative of marine evaporates are are Calcite,
gypsum and anhydrite, halite, sylvite, carnallite, langbeinite, polyhalite and kainite. However,
there are approximately 80 different minerals that have been reported found in evaporite
deposits.
Non-marine Evaporites:
The non-marine evaporites are usually composed of minerals that are not common in
marine environments. The water from which non-marine evaporite precipitates have
proportions of chemical elements different from those found in the marine environments. The
common minerals that are found in these deposits include blodite, borax, epsomite,
gaylussite, glauberite, mirabilite, thenardite and trona.

Glacial:
The glaciers are flowing streams of ice. They may be huge continental ice sheets or
small alpine (mountain) glaciers. All glaciers scrape up sediments and incorporate them into
the base of the ice sheet. The sand, gravel, and large boulders polish and gouge the surface of
the bedrock that they are dragged over leaving glacial striations. The glaciers do not sort
sediments as flowing water and wind do. The poorly sorted glacial sediments are known
as till. The large boulders often lie in a matrix of sand and silt (matrix-supported
conglomerate). At the end of a glacier, where ice is melting as fast as it is being supplied
from upstream. The sediments are deposited in a terminal moraine, a ridge of poorly-sorted
glacial till. The thinner depostits of glacial sediments called a ground moraine or till plain are
found behind the terminal moraine. The sediments that were deposited by melting ice or by
glacial streams are called Fluvio-glacial. The range of sediment sizes and processes of
deposition produce a wide range of landforms.

Important
Depositional
Environment Transport Typical Sediment Types
Environments
Processes

Terrestrial Environments
 gravity, moving ice, valleys, plains, glacial till, gravel, sand, silt,
Glacial
moving water streams, lakes and clay

Colluvial Gravity steep-sided valleys coarse angular fragments

Fluvial moving water streams gravel, sand, silt, and OM*

deserts and coastal

Aeolian Wind sand, silt
regions

Lacustrine moving water lakes sand, silt, clay, and OM*

Evaporite moving water lakes in arid regions salts, clay

Marine Environments

Deltaic moving water deltas sand, silt, clay, and OM*

waves, longshore beaches, spits, sand

Beach gravel, sand
currents bars
 Tidal tidal currents tidal flats silt, clay

waves and tidal reefs and adjacent

Reefs carbonates
currents basins

carbonates (in tropical

Shallow water waves and tidal shelves and slopes,
climates); sand/silt/clay
marine currents lagoons
(elsewhere)

carbonates (in tropical

Lagoonal little transportation lagoon bottom
climates)

underwater gravity continental slopes and

Submarine fan gravel, sand, mud
flows abyssal plains

Deep water deep-ocean abyssal clay, carbonate mud, silica

ocean currents
marine plains mud

CONCLUSION:
The depositional environments are reflected by the types of sediment that accumulate
and thus their potential to contain resources. The environments include continental,
transitional (coastal) and marine settings. The deposition are composed of particular
sedimentary rock type accumulates over a period of time. These are vital in all aspects of
generating and storing hydrocarbons.
REFERENCES

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 https://wiki.aapg.org/Depositional_environments#Definition_of_Depositional_Env
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