Sedimentary rocks

Sedimentary rocks form at or near the earth's surface from the weathered remains of pre-existing rocks or organic debris. The term sedimentary rock applies both to consolidated, or lithified sediments (bound together, or cemented) and unconsolidated sediments (loose, like sand ). Although there is some overlap, most sedimentary rocks belong to one of the following groups: clastic, chemical, or organic.

Mechanical weathering breaks up rocks, while chemical weathering dissolves and decomposes rocks. Weathering of igneous, metamorphic, and sedimentary rocks produces rock fragments, or clastic sediments, and mineral-rich water , or mineral solutions. After transport and laying down, or deposition, of sediments by wind , water, or ice , compaction occurs due to the weight of overlying sediments that accumulate later. Finally, minerals from mineral-rich solutions may crystallize, or precipitate, between the grains and act as cement. Cementation of the unconsolidated sediments forms a consolidated rock. Clastic rocks are classified based on their grain size. The most common clastic sedimentary rocks are shale (grains less than 1/256 mm in diameter), siltstone (1/256–1/16 mm), sandstone (1/16–2 mm), and conglomerate (greater than 2 mm).

Chemical or crystalline sedimentary rocks form from mineral solutions. Under the right conditions, minerals precipitate out of mineral-rich water to form layers of one or more minerals, or chemical sediments. For example, suppose ocean water is evaporating from an enclosed area , such as a bay, faster than water is flowing in from the open ocean. Salt deposits will form on the bottom of the bay as the concentration of dissolved minerals in the bay water increases. This is similar to putting salt water into a glass and letting the water evaporate; a layer of interlocking salt crystals will precipitate on the bottom of the glass. Due to their interlocking crystals, chemical sediments always form consolidated sedimentary rocks. Chemical rocks are classified based on their mineral composition. Rock salt (composed of the mineral halite, or table salt), rock gypsum (composed of gypsum), and crystalline limestone (composed of calcite) are common chemical sedimentary rocks.

Organic sedimentary rocks form from organically derived sediments. These organic sediments come from either animals or plants and usually consist of body parts. For example, many limestones are composed of abundant marine fossils , so these limestones are of organic rather than chemical origin. Coal is an organic rock composed of the remains of plants deposited in coastal swamps. The sediments in some organic rocks (for example, fossiliferous limestone) undergo cementation; other sediments may only be compacted together (for example, coal). Geologists classify organic rocks by their composition.

The origin (clastic, chemical, or organic) and composition of a sedimentary rock provide geologists with many insights into the environment where it was deposited. Geologists use this information to interpret the geologic history of an area, and to search for economically important rocks and minerals.

See also Depositional environments; Lithification; Mineralogy; Sedimentation; Stratigraphy

sedimentology, sediments, and sedimentary rocks Sedimentology is the study of the nature and origin of sediments. It includes the production, dispersal, and deposition of sediments as well as their conversion into rocks—sedimentary rocks.

Sediments are the products of reactions between the surface rocks of the Earth's crust and the surrounding envelope of water, air, organisms, and plants. They are accumulations of crystals or mineral grains that were deposited in layers from a fluid (water or air) under normal pressures and temperatures at the Earth's surface. Some sediments consist of particles derived from the breakdown and alteration of earlier rocks; these, technically termed terrigenous detrital, clastic, or siliciclastic deposits, include sandstones, silts, and mudstones. Other sediments form by precipitation or biological extraction of calcium carbonate; they are termed limestones. Chemical precipitation produces other deposits, such as evaporites, ironstones, and phosphates; and the accumulation of organic matter results in the formation of coal, oil shales, and hydrocarbons.

Sediments form in many environments, ranging from terrestrial situations to the deepest parts of the ocean. The analysis of sedimentary sequences and their associated floral and faunal remains can make it possible to reconstruct the changing conditions of the Earth's surface in the geological past.

Sediments are of great economic importance as raw materials (coal, ironstone, building materials) as well as containing, in the voids between grains, valuable accumulations of water, oil, gas, and economic minerals.

G. Evans

sedimentary rock Type of rock formed by deposition of sediment derived from pre-existing rocks, which may have been sedimentary, igneous or metamorphic. Most sediment accumulates on the bed of the sea, having been dumped there by rivers, or having accumulated as dead sea creatures fall to the seafloor. This accumulated sediment is consolidated and compressed. Earth movements uplift the sediments, and they may be tilted, folded or faulted. The resulting rocks are sedimentary, and their type depends on their composition. Sedimentaries consisting of land sediment are clastic rocks, and are gravels, sands, silts or clays, according to the size of the particles. Other types of sedimentary rock include: limestone, which consists of fragments of dead sea creatures; coal, which is accumulated vegetation; coralline, which contains large quantities of coral; and chalk, which is a pure form of limestone, with very little land sediment. See also fault; fold

sedimentary rock Rock formed by the deposition and compression of mineral and rock particles, but often including material of organic origin and exposed by various agencies of denudation. Sedimentary rocks may be classified as terrigenous (i.e. derived from the breakdown of pre-existing rocks exposed on the land), organic (i.e. produced either directly or indirectly by organic processes such as shell production or peat formation), chemical (i.e. produced by precipitation from water, e.g. some carbonates and all evaporites), or volcanogenic (pyroclastic, e.g. tuffs and bentonites). They may also be described according to their chemical properties and behaviour and their environmental deposition, and each scheme complements the others.

sedimentary rock Rock formed by the deposition and compression of mineral and rock particles, but often including material of organic origin, and exposed by various agencies of denudation.

sedimentary rock see rock ; sediment .