Introduction to sedimentary environments

MAHBOOB AHMED

Depositional environments

Landscapes form and constantly change due to weathering and sedimentation. The area where sediment accumulates and is later buried by 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 depositional settings 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 are

indicative of the conditions that created them. These are known as sedimentary structures. They may provide information about water depth, 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.

Continental/terrestrial depositional environments

Continental environments Continental environments are those in which sediments are deposited on land or in fresh water.

Fluvial environmentsIn fluvial system sediments deposited byStreams and rivers.Fluvial deposits include cross-bedded and rippled river sandstones and parallel or cross-bedded floodplain contains mudstones (siltstones and clay shales).

Fluvial bed forms

Braided and meandering streams

Fluvial environments include braided and meandering river and stream systems. River channels, bars, levees, and floodplains 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.

Braided riversRivers with a high proportion of sediments , sand or gravel in thechannel 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.

Alluvial environments

Alluvial fans Alluvial fans are fan shaped deposits formed at the base of mountains due to fast flowing stream, which are flattens, slows, and spreads typically onto a flatter plain. These are characterized by poorly sorted, boulder and gravel dominated, debris flow conglomerates.

Deserts environmentsAeolian Deposited 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.

Lacustrine environmentsSediments are deposited in lack .Lack deposits may be large or small, shallow or deep, and filled with terrigenous, carbonate, or evaporitic sediments. Fine sediment and organic matter settling in some lakes produced laminated oil shales and coal form.

Glacial environmentsSediments deposited by a glacier.Sediments which are deposited by a Glacier are poorly sorted ,angular sediments .

Deltaic environmentsDeltas are fan-shaped deposits 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.

Marginal marine environmentsMarginal marine environments 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 marine environments.

Beach and barrier islandsThese are shoreline deposits exposed 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.

Lagoonal environmentsLagoons are coastal bodies of water 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.

Estaurine environmentsAn 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 seawater when there is arelative 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.

Tidal flatsTidal flat are formed when mud 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 deposited by a tidal flat.

Marine environmentsMarine environments are in the seas or oceans. Marine environments include reefs, the continental shelf, slope, rise, and abyssal plain.

Continental shelfThe continental shelfis 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), and may be up to hundreds of miles wide. Continental shelves are exposed to waves, tides, and currents, and are covered by sand, silt, mud, and gravel.

Reef depositsReef 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.

Continental slope depositThe 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 riseContinental rise located between 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 submarine fans, 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.

Abyssal plainAbyssal plain is the deep ocean floor. It is basically flat, and is covered by very fine-grained sediment, consisting primarily of clay and the shells of microscopic organisms (such as foraminifera, radiolarians, and diatoms).Abyssal plain sediments may include chalk, diatomite, and shale, deposited over the basaltic ocean crust.