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Introduction to sedimentary

environments

MAHBOOB AHMED

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Landscapes form and constantly change due to weathering andsedimentation. The area where sediment accumulates and is later buriedby other sediment is known as its depositional environment.

Depositional environments are often separated into three general types,or settings: terrestrial (on land), marginal marine (coastal), and marine

(open ocean). Examples of each of these three regional depositionalsettings are as follows:

terrestrial-alluvial fans, glacial valleys, lakes.

marginal marine- beaches, deltas, estuaries, tidal mud and sand flats.

marine-coral reefs, continental slope and deep marine deposition.

During deposition of sediments, physical structures form that areindicative of the conditions that created them. These are known assedimentary structures. They may provide information about waterdepth, current speed, environmental setting or a variety of other factors.Among the more common of these are: bedding planes, beds, channels,cross-beds, ripples, and mud cracks.

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Continental/terrestrial depositional

environments

Continental environments

Continental environments are those in

which sediments are deposited on land

or in fresh water.

Fluvial environments

In fluvial system sediments deposited by

Streams and rivers.

Fluvial deposits include cross-bedded

and rippled river sandstones and parallelor cross-bedded floodplain contains

mudstones (siltstones and clay shales).

Fluvial bed forms

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Braided and

meandering streams

Fluvial environments

include braided and

meandering river and

stream systems. River

channels, bars, levees, andfloodplains are parts 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.

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Braided rivers

Rivers with a high proportionof sediments , sand or gravel

in the

channel the flow is divided to

give the river a braided form.

The bars in a braided river

channel are exposed at low

flow stages.

The bars within the channel

may vary in shape lithology

and sizes.

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Alluvial environments

Alluvial fansAlluvial fans are fan shaped

deposits formed at the base

of mountains due to fast

flowing stream, which are

flattens, slows, and spreadstypically onto a flatter plain.

These are characterized by

poorly sorted, boulder and

gravel dominated, debris flow

conglomerates.

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Deserts

environments

AeolianDeposited by wind in deserts

Deposits.

Usually contain vast areas

where sand is deposited in

dunes. Dune sands are cross-bedded, well sorted, and well

rounded, without associated

gravel or clay.

Aeolian sandstones frequently

display large scale (1 to 3

meter) cross bed sets.

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Glacial environments

Sediments deposited by aglacier.

Sediments which are

deposited by a Glacier are

poorly sorted ,angular

sediments .

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Deltaic environments

Deltas are fan-shapeddeposits formed where a river

flows into a standing body of

water, such as a lake or sea .

Coarser sediment (sand)

tends to be deposited near

the mouth of the river; finer

sediment is carried seaward

and deposited in deeper

water.

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Marginal marine

environments

Marginal marineenvironments lies along the

boundary between

continental and marine

depositional

Environments.

A wide variety of sediments

including Conglomerates ,

sandstone s , shales

carbonates , and evaporites

can accumulate in these

various marginal marineenvironments.

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Beach and barrier

islands

These are shoreline depositsexposed to wave energy and

dominated by sand with a

marine fauna.

Barrier islands are separated

from the mainland by a

lagoon. They are commonly

associated with tidal flats

deposits.

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Lagoonal environments

Lagoons are coastal bodies ofwater that have very limited

connection to the open

ocean.

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.

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Estaurine

environments

An estuary is the marine-influenced portion of adrowned valley .

A drowned valley is the seawardportion of a river valley thatbecomes flooded with seawaterwhen there is a

relative rise in sea level.

They are regions of mixing offresh and seawater.

Sediment supply to the estuaryis from both river and marinesources, and the processes that

transport and deposit thissediment are a combination ofriver and wave and/or tidalprocesses.

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Tidal flats

Tidal flat are formed whenmud is deposits by tides or

rivers.

Tidal flats are the border of

lagoons and estuarine

environments.

Tidal flats are areas of low

relief, cut by meandering tidal

channels. Laminated or

rippled clay, silt, and fine sand

(either terrigenous or

carbonate) may be depositedby a tidal flat.

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Marine environmentsMarine environmentsare in the seas oroceans. Marineenvironments includereefs, the continentalshelf, slope, rise, andabyssal plain.

Continental shelfThe continental shelfis the flooded edge ofthe continent. It isrelatively flat (with aslope of less than0.1o), shallow (less

than 200 m or 600 ftdeep), and may be upto hundreds of mileswide. Continentalshelves are exposedto waves, tides, andcurrents, and are

covered by sand, silt,mud, and gravel.

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Reef deposits

Reef are wave-resistant,mound-like structures made

of the calcareous skeletons of

organisms such as corals and

certain types of algae. Most

modern reefs are in warm,

clear, shallow, tropical seas,between the latitudes of

30oN and 30oS of the

equator.

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Continental slope

deposit

The continental slope arelocated 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.

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Continental rise

Continental rise locatedbetween the continental

slope and the abyssal plain.

The continental rise is the site

of deposition of thick

accumulations of sediment,

much of which is in submarinefans, deposited by turbidity

currents at the base of

continental rise. Turbidity

current deposits are called

turbidities are characterized

By graded bedding.

Continental slope and

continental rise are show deep

marine deposition.

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Abyssal plain

Abyssal plain is the deepocean floor. It is basically flat,

and is covered by very fine-

grained sediment, consisting

primarily of clay and the shells

of microscopic organisms

(such asforaminifera,radiolarians, and diatoms).

Abyssal plain sediments may

include chalk, diatomite, and

shale, deposited over the

basaltic ocean crust.

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