Deposition

Not only do the agents of erosion—streams, wind, glaciers, waves, currents—wear away the land, they also transport the resulting rock debris, which eventually is deposited when their energy runs out and gravity takes over. The major sedimentary environments are eolian (wind), alluvial fans, glacial till and drift, fluvial (stream), and lacustrine (lake). Along the shore deposition takes place in evaporite basins, deltas, lagoons, and estuaries, and along barrier islands. Marine environments include continental shelves, reefs, submarine slopes, and the deep sea. Each one of these sedimentary environments has its own distinct chemical, physical, and biological conditions. This makes it easy to recognize the conditions under which ancient sedimentary rocks were deposited. The following are brief descriptions of representative examples.

The nature of fluvial deposits depends upon the source of the material, distance traveled, climatic regimes, and the amount of energy available—for example, velocity derived from slope steepness. These deposits contain particles that are angular to rounded and range from clay size to boulders. They are located in channels and on floodplains; climate determines the kinds of terrestrial animals and plants that live in and near the streams.

Lacustrine deposits form in low-energy freshwater environments. They usually contain fine particles, but in temperate and cold climates they could be varves—layers that form in pairs and are deposited yearly. Spores and pollen found within the deposits are tools helpful in determining past vegetation and climate. In deserts, where evaporation is brisk, water evaporates in the lake basin (playa lake), leaving salt-encrusted deposits.

Swamp and marsh environments contain slow-accumulating fine sediments, abundant plant life, and animals that are freshwater, estuarine, or marine. Most of the world's coal deposits were originally plant material that accumulated in freshwater swamps.

Eolian environments occur where windblown deposits accumulate, usually containing well-rounded, sorted fine sand that is cross-bedded. They form in deserts and along coast-lines—almost anywhere there is a supply of loose material and prevailing winds. Stable dunes host a variety of plants and associated animals.

Glacial environments of deposition contain large amounts of unsorted, angular to rounded particles, from the smallest to house-size, piled in mounds and ridges, as well as layered material that has been reworked by meltwater. Along the margins of the ice, cold climate prevails, determining the type of plants and animals, but older moraines may be located in a variety of climates.

Shallow marine environments are found in estuaries, deltas, and along the inner continental shelves, containing clay, sand, and pebbles; their layers are sometimes disturbed by waves and currents. The skeletons of floating and bottom-dwelling animals are often mixed with these sediments, and they occur in any climatic zone.

Low-energy marine environments include the outer continental shelves; bathyal and abyssal environments are generally low-energy environments with abundant organisms, including floaters, swimmers, and bottom dwellers. They can occur anywhere in the oceans.

Pelagic environments contain sediments dominated by skeletal debris, almost entirely micro-organisms that have settled slowly onto the seafloor and have mixed with small amounts of wind-blown dust, volcanic ash, and other material. They are called oozes, and the shells of the organisms settle from open water in very large numbers; oozes may be calcareous or siliceous in chemical composition

High-energy deep sea environments contain turbidities derived from continental shelves and slopes, and are rapidly dumped on the deep seafloor. They are usually motivated by gravity, earthquakes, or both.

Organic reef environments are tropical and composed of living and dead corals and associated fauna. An apron of broken skeletons is located on the sea side of the reef, and fine carbonate sediment with abundant organisms is found behind the reef in the protected lagoon. If adjacent to a shoreline, tidal flats contain algal mats, stromatolite mounds, and limy sediments that are often mud-cracked.

The term deposition also refers to the accumulation of chemical precipitates in caves, lakes, and the sea. Water in embayments along the margin of the sea and freshwater lakes contains dissolved materials that are deposited when the water evaporates. Stalactites and stalagmites in caves are composed of calcium carbonate (calcite); they occur when water containing the mineral evaporates. Travertine, a type of limestone and also composed of calcite, is precipitated from warm- and hot-water springs. Precipitation occurs as a result of changes in temperature, pressure, solution concentration, or chemistry.

—Sidney Horenstein

See also: Abyssal Floor; Erosion; Estuaries; Freshwater; Lagoons; Lakes; Oceans; Rivers and Streams

Bibliography

Mack, Walter, and Elizabeth A. Leistikow. 1996. "Sands of the World." Scientific American. 275 (2): 62-67; Press, Frank, and Raymond Siever. 2000. Understanding Earth, 3rd ed. New York: W. H. Freeman and Company; Prothero, Donald R. 1990. Interpreting the Stratigraphic Record. New York: W. H. Freeman and Company; Selley, R. C. 1996. Ancient Sedimentary Environments and Their Subsurface Diagnosis. London: Chapman and Hall.

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