The setting in which a sedimentary rock forms is called the
sedimentary 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, but also on the environment itself.[40]
A 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 m
below the water surface. 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 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 reefs, where the sediment consists mainly of the calcareous skeletons of larger organisms.[41]
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 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 radiolarians) are not as soluble and still deposit. 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 slopes, the sedimentary cover can become unstable, causing turbidity currents.
Turbidity currents are sudden disturbances of the normally quite deep
marine environment and can cause the geologically speaking instantaneous
deposition of large amounts of sediment, such as sand and silt. The
rock sequence formed by a turbidity current is called a turbidite.[42]
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 flats and shoals 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, 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 sediment (in contrast to chemical).
A sedimentary rock formed on land has a continental sedimentary environment. Examples of continental environments are lagoons, lakes, swamps, floodplains and alluvial fans.
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 in
continental environments also be transported by wind or glaciers.
Sediment transported by wind is called aeolian and is always very well sorted, while sediment transported by a glacier is called glacial till and is characterized by very poor sorting.[43]
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 of the Touchet Formation, and thus provided evidence of
the events that intervened over time among the forty-one layers that
were deposited.[44]
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