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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.
Formation and classification
Formation
Sedimentary rocks 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.
Classification
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
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