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Sedimentary rock
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Sedimentary rock

overlaid by limestone. Cumberland Plateau, Tennessee.]]

Sedimentary rock is one of the three main rock groups (along with igneous and metamorphic rocks) and is formed in three main ways—by the deposition of the weathered remains of other rocks (known as clastic sedimentary rocks); by the deposition of the results of biogenic activity; and by precipitation from solution.

Table of contents
1 Formation
2 Classification
3 Other information
4 See also


Sedimentary rockss are formed from overburden pressure as particles of sediment are deposited out of air, ice, or water flows carrying the particles in suspension. As sediment deposition builds up, the overburden (or lithostatic) pressure squeezes the sediment into layered solids in a process known as lithification ("rock formation") and the original connate fluids are expelled.

Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remnants.


Clastic sedimentary rocks

Clastic sedimentary rocks are composed of discrete clasts of materials derived from other rocks. They are composed largely of
silica (i.e. quartz), with other common minerals including feldspars, amphiboles, clay minerals and sometimes more exotic igneous minerals.

Clastic sedimentary rocks may be regarded as falling along a scale of grain size, with shale being the finest, siltstone being intermediate, and sandstone being coarser still, and congolmerates being the coarsest.

Biogenic sedimentary rocks

Biogenic sedimentary rocks contain materials generated by living organisms, and include carbonate minerals created by organisms, such as corals, molluscs, and foraminifera, which cover the ocean floor with layers of calcite which can later form limestone. Other examples include stromatolites, and the flint nodules found in chalk (which is itself a biogenic sedimentary rock, a form of limestone).

Precipitate sedimentary rocks

Precipitate sedimentary rocks form when mineral solutions, such as sea water, evaporate. Examples include the evaporite minerals halite and gypsum.

Other information

Sedimentary rocks are economically important in that they can be used as construction material. In addition, sedimentary rocks often form porous and permeable reservoirs in sedimentary basins in which petroleum and other hydrocarbons can be found.

It is believed that the relatively low levels of carbon dioxide in the Earth's atmosphere, in comparison to that of Venus, is due to large amounts of carbon being trapped in limestone and dolomite sedimentary layers. The flux of carbon from eroded sediments to marine deposits is known as the carbon cycle.

The shape of the particles in sedimentary rocks has an important effect on the ability of micro-organisms to colonize them. This interaction is studied in the science of geomicrobiology. One measure of the shape of these particles is the roundness factor, also known as the Krumbein number after the geologist W. C. Krumbein.

See also