Sedimentary EnvironmentBY

ABDUL BASIT

UNIVERSITY OF THE PUNJAB

Sedimentary Environment

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

The characteristics that can be observed and measured in a sedimentary rock to deduce its depositional environment include its lithology (which is essentially its rock type), its sedimentary structures, and any fossils it may contain

WHY ARE DEPOSITIONAL ENVIRONMENTS

IMPORTANT?

Knowledge of depositional environments is important for reconstructing earth history, understanding earth processes, and helping humans survive and prosper on earth

Reconstructing earth history

By analyzing a sedimentary rock, a geologist can deduce what was happening on earth at the place and time the sediment was originally being deposited

Types of depositional environments

Four types of Sedimentry Environments

1. Continental

2. Transitional

3. Marine

4. Others

Continental sedimentryenvironment

It has further four types

a) Alluvium

b) Aeolian

c) Fluvial

d) Lacustrine

Alluvium

Alluvium is drived from Latin, alluvius, which means"towash against“

Alluvium is loose, unconsolidated (not cemented together into a solid rock) soil or sediments, which has been eroded, reshaped by water in some form, and redeposited in a non-marine setting

Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel.

When this loose alluvial material is deposited or cemented into a lithological unit, or lithified, it is called an alluvial deposit

Aeolian

Aeolian processes, also spelled eolian or æolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth (or other planets)

Winds may erode, transport, and deposit materials and are effective agents in regions with sparse vegetation, a lack of soil moisture and a large supply of unconsolidated sediments

Although water is a much more powerful eroding force than wind, aeolian processes are important in arid environments such as deserts

Fluvial

Fluvial is a term used in and geology to refer to the processes associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvioglacial is used

Fluvial processes include the motion of sediment and erosion or deposition on the river bed

Lake

A lake is an area of variable size filled with water, localized in a basin, that is surrounded by land, apart from any river or other outlet that serves to feed or drain the lake

Lakes lie on land and are not part of the ocean (except for sea lochs in Scotland and Ireland), and therefore are distinct from lagoons, and are also larger and deeper than ponds, though there are no official or scientific definitions

Lakes can be contrasted with rivers or streams, which are usually flowing. However most lakes are fed and drained by rivers and streams

Natural lakes are generally found in mountainous areas, rift zones, and areas with ongoing glaciation

Other lakes are found in endorheic basins or along the courses of mature rivers

Lake

In some parts of the world there are many lakes because of chaotic drainage patterns left over from the last Ice Age

All lakes are temporary over geologic time scales, as they will slowly fill in with sediments or spill out of the basin containing them

Marine Sedimentary Environments

Marginal Marine Environments

Marine Environments

Marginal Marine SedimentryEnvironment

Marginal marine environments lies along the boundary between continetal and marine depositional environments

A wide varity of sediments including Conglomerates, Sand stones, Shales Carbonates and evaporites can accumulate in these various marginal marine environments

Beach & Barrier Islands

These are shoreline deposits exposed to wave energy and dominated by sand with marine fauna

Barrier islands are separated from margined from the mainland by a Lagoon

The are commonly associated with tidal flat deposits

Lagoon Environment

Lagoons are coastal bodies of water that have very limited connection to the open ocean

Lagoons generally develop along the 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

Estuarine Environments

An estuary is the margined influenced portion of a drowned valley

A drowned valley is the seaward portion of a river valley that becomes flooded with seawater when there is a relative rise in sea level

They are regions of mixing of fresh water and seawater

Sediment supply to the estuary is from both river and marine sources, and the processes that transport and deposit sediments are a combination of river and wave or tidal processes

Tidal Flats

Tidal flats are formed when mud is deposited 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 may be deposited by a tidal flat

Marine Environments

Marine environments are in the seas or oceans

Marine environments include reefs, the continental shelf, slope, rise and abyssal plain

Continental Shelf

The continental shelf is flooded edge of the continent

It is relatively flat with a slope of less than 0.1*, shallow less than 200m and may be up to hundreds of kilometers wide

Continental shelves are exposed to waves tides and currents and are covered by sand silt mud and gravel

Reef Deposits

Reefs are wave resistant, mound like structures made of calcareous skeletons of organisms such as corals and certain types of algae

Most modern reefs are in warm, clear, shallow, tropical seas between latitudes of 30o N and 30o S of equator

Continental Slope Deposits

The continental slope 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

Continental Rise

Continental rise located between the continental slope and the abyssal plain

The continental rise is the site of deposition of thick accumulations of sediments much of which is in submarine fans

Deposited by turbidity currents at the base of continental rise

Turbidity current deposits are called turbidities are characterized by graded bedding

Abyssal Plain

Abyssal plain is the deep ocean floor

It is basically flat, and is covered by very fine grained sediments consisting primarily of microscopic organisms e.g. radiolarians and diatoms

Abyssal plain sediments may include chalk diatomite and shale deposited over basaltic oceanic crust