sedimentary rock

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

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 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.