Editing Sedimentary rock (section)

==Depositional environments==
[[File:Main_depositional_environments.svg|thumb|upright=1.6|Common types of [[depositional environment]]s]]
[[File:Sediment off the Yucatan Peninsula.jpg|thumb|The swirls of tan, green, blue, and white are sediment in the shallow waters of the [[Gulf of Mexico]] off the [[Yucatan Peninsula]]. The blue-green cloud in this image roughly matches the extent of the shallow [[continental shelf]] west of the peninsula. This is a perfect example of a shallow marine [[depositional environment]].]]

The setting in which a sedimentary rock forms is called the [[depositional environment]]. Every environment has a characteristic combination of geologic processes, and circumstances. The type of sediment that is deposited is not only dependent on the sediment that is transported to a place ([[Provenance (geology)|provenance]]), but also on the environment itself.<ref>For an overview of different sedimentary environments, see {{harvnb|Press|Siever|Grotzinger|Jordan|2003}} or {{harvnb|Einsele|2000|loc=part II}}.</ref>

A [[Oceanic basin|marine]] environment means that the rock was formed in a [[sea]] or [[ocean]]. Often, a distinction is made between deep and shallow marine environments. Deep marine usually refers to environments more than 200&nbsp;m below the water surface (including the [[abyssal plain]]). Shallow marine environments exist adjacent to coastlines and can extend to the boundaries of the [[continental shelf]]. The water movements in such environments have a generally higher energy than that in deep environments, as [[Wind wave|wave activity]] diminishes with depth. This means that coarser sediment particles can be transported and the deposited sediment can be coarser than in deeper environments. When the sediment is transported from the continent, an alternation of [[sand]], [[clay]] and [[silt]] is deposited. When the continent is far away, the amount of such sediment deposited may be small, and biochemical processes dominate the type of rock that forms. Especially in warm climates, shallow marine environments far offshore mainly see deposition of carbonate rocks. The shallow, warm water is an ideal habitat for many small organisms that build carbonate skeletons. When these organisms die, their skeletons sink to the bottom, forming a thick layer of calcareous mud that may lithify into limestone. Warm shallow marine environments also are ideal environments for [[coral reef]]s, where the sediment consists mainly of the calcareous skeletons of larger organisms.<ref>For a definition of shallow marine environments, see {{harvnb|Levin|1987|p=63}}</ref>

In deep marine environments, the water current working the sea bottom is small. Only fine particles can be transported to such places. Typically sediments depositing on the ocean floor are fine clay or small skeletons of micro-organisms. At 4&nbsp;km depth, the solubility of carbonates increases dramatically (the depth zone where this happens is called the [[lysocline]]). Calcareous sediment that sinks below the lysocline dissolves; as a result, no limestone can be formed below this depth. Skeletons of micro-organisms formed of [[silica]] (such as [[radiolarian]]s) are not as soluble and are still deposited. An example of a rock formed of silica skeletons is [[radiolarite]]. When the bottom of the sea has a small inclination, for example, at the [[continental slope]]s, the sedimentary cover can become unstable, causing [[turbidity current]]s. Turbidity currents are sudden disturbances of the normally quiet deep marine environment and can cause the near-instantaneous deposition of large amounts of sediment, such as sand and silt. The rock sequence formed by a turbidity current is called a [[turbidite]].{{sfn|Tarbuck|Lutgens|1999|pp=452-453}}

The coast is an environment dominated by wave action. At a [[beach]], dominantly denser sediment such as sand or gravel, often mingled with shell fragments, is deposited, while the silt and clay sized material is kept in mechanical suspension. [[Tidal flat]]s and [[shoal]]s are places that sometimes dry because of the [[tide]]. They are often cross-cut by [[gullies]], where the current is strong and the grain size of the deposited sediment is larger. Where rivers enter the body of water, either on a sea or lake coast, [[River delta|deltas]] can form. These are large accumulations of sediment transported from the continent to places in front of the mouth of the river. Deltas are dominantly composed of clastic (rather than chemical) sediment.

A continental sedimentary environment is an environment in the interior of a continent. Examples of continental environments are [[lagoon]]s, lakes, [[swamp]]s, [[floodplain]]s and [[alluvial fan]]s. In the quiet water of swamps, lakes and lagoons, fine sediment is deposited, mingled with organic material from dead plants and animals. In rivers, the energy of the water is much greater and can transport heavier clastic material. Besides transport by water, sediment can be transported by wind or glaciers. Sediment transported by wind is called [[aeolian processes|aeolian]] and is almost always [[sorting (sediment)|very well sorted]], while sediment transported by a glacier is called [[glacial till]] and is characterized by very poor sorting.<ref>For an overview of continental environments, see {{harvnb|Levin|1987|pp=67–68}}</ref>

Aeolian deposits can be quite striking. The depositional environment of the [[Touchet Formation]], located in the [[Northwestern United States]], had intervening periods of aridity which resulted in a series of [[rhythmite]] layers. Erosional cracks were later infilled with layers of soil material, especially from [[aeolian processes]]. The infilled sections formed vertical inclusions in the horizontally deposited layers, and thus provided evidence of the sequence of events during deposition of the forty-one layers of the formation.{{sfn|Baker|Nummedal|1978}}

=== Sedimentary facies ===
The kind of rock formed in a particular depositional environment is called its [[sedimentary facies]]. Sedimentary environments usually exist alongside each other in certain natural successions. A beach, where sand and gravel is deposited, is usually bounded by a deeper marine environment a little offshore, where finer sediments are deposited at the same time. Behind the beach, there can be [[dune]]s (where the dominant deposition is well sorted sand) or a [[lagoon]] (where fine clay and organic material is deposited). Every sedimentary environment has its own characteristic deposits. When sedimentary strata accumulate through time, the environment can shift, forming a change in facies in the subsurface at one location. On the other hand, when a rock layer with a certain age is followed laterally, the [[lithology]] (the type of rock) and facies eventually change.{{sfn|Tarbuck|Lutgens|1999|pp=158-160}}

[[File:Offlap & onlap EN.svg|thumb|upright=1.35|Shifting sedimentary facies in the case of [[transgression (geology)|transgression]] (above) and [[Marine regression|regression]] of the sea (below)]]

Facies can be distinguished in a number of ways: the most common are by the lithology (for example: limestone, siltstone or sandstone) or by [[fossil]] content. [[Coral]], for example, only lives in warm and shallow marine environments and fossils of coral are thus typical for shallow marine facies. Facies determined by lithology are called [[Facies|lithofacies]]; facies determined by fossils are [[biofacies]].{{sfn|Reading|1996|pp=19-20}}

Sedimentary environments can shift their geographical positions through time. Coastlines can shift in the direction of the sea when the [[sea level]] drops ([[Marine regression|regression]]), when the surface rises ([[transgression (geology)|transgression]]) due to tectonic forces in the Earth's crust or when a river forms a large [[River delta|delta]]. In the subsurface, such geographic shifts of sedimentary environments of the past are recorded in shifts in sedimentary facies. This means that sedimentary facies can change either parallel or perpendicular to an imaginary layer of rock with a fixed age, a phenomenon described by [[Walther's Law]].{{sfn|Reading|1996|pp=20-21}}

The situation in which coastlines move in the direction of the continent is called [[transgression (geology)|transgression]]. In the case of transgression, deeper marine facies are deposited over shallower facies, a succession called [[onlap]]. [[Marine regression|Regression]] is the situation in which a coastline moves in the direction of the sea. With regression, shallower facies are deposited on top of deeper facies, a situation called [[offlap]].<ref>For an overview over facies shifts and the relations in the sedimentary rock record by which they can be recognized, see {{harvnb|Reading|1996|pp=22–33}}.</ref>

The facies of all rocks of a certain age can be plotted on a map to give an overview of the [[palaeogeography]]. A sequence of maps for different ages can give an insight in the development of the regional geography.

====Gallery of sedimentary facies====
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File:Facies_migration2.jpg | A regressive facies shown on a stratigraphic column

</gallery>