Consolidated deposits composed of material laid down by water, wind, ice, or gravity, or by chemical precipitation. They are generally classified into three groups: Clastic rocks are made up of fragments of pre-existing rocks or minerals, and bound together by a cementing medium which is formed after deposition, eg shales, sandstones, and conglomerates. Organic rocks are composed largely of the remains of living organisms, eg coal and fossiliferous limestone. Inorganic rocks are formed by chemical precipitation from supersaturated solutions, eg some limestones and evaporite deposits.
Sedimentary rock is one of the three main rock groups (along with igneous and metamorphic rocks) and is formed in four main ways:
by the deposition of the weathered remains of other rocks (known as 'clastic' sedimentary rocks);Sedimentary rocks include common types such as chalk, limestone, sandstone, clay and shale. Sedimentary rocks cover 75% of the Earth's surface. Four basic processes are involved in the formation of a clastic sedimentary rock: weathering (erosion)caused mainly by friction of waves, transportation where the sediment is carried along by a current, deposition and compaction where the sediment is squashed together to form a rock of this kind.
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.
Sedimentary rocks are laid down in layers called beds or strata.
Sedimentary rocks contain important information about the history of the Earth. Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remnants.
The sedimentary rock cover of the continents of the Earth's crust is extensive, but the total contribution of sedimentary rocks is estimated to be only five percent of the total. As such, the sedimentary sequences we see represent only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks.
Classification
Sedimentary rocks can be classified as clastic, biogenic, or precipitate.
Clastic
Clastic sedimentary rocks are composed of discrete fragments or clasts of materials derived from other rocks.
Clastic sedimentary rocks, such as breccia or sandstone, were formed from rocks that have been broken down into fragments by weathering, which then have been transported and deposited elsewhere.
Clastic sedimentary rocks may be regarded as falling along a scale of grain size, with shale being the finest with particles less than 0.004 mm, siltstone being a little bigger with particles between 0.004 to 0.06 mm, and sandstone being coarser still with grains 0.06 to 0.2 mm, and conglomerates and breccias being the coarsest with grains 2 to 256 mm. Arenite is a general term for sedimentary rock with sand-sized particles.
The classification of clastic sedimentary rocks is complex because there are many variables involved.
Coarser clastic sedimentary rocks are classified according to their particle size and composition. greywacke is a sandstone with quartz, clay, feldspar, and metamorphic rock fragments present, which was formed from the sediments carried by turbidity currents.
All rocks disintegrate slowly as a result of mechanical weathering and chemical weathering.
Mechanical weathering is the breakdown of rock into particles without producing changes in the chemical composition of the minerals in the rock.
Chemical weathering is the breakdown of rock by chemical reaction. In this process the minerals within the rock are changed into particles that can be easily carried away. The minerals in igneous rocks may be unstable under normal atmospheric conditions, those formed at higher temperatures being more readily attacked than those which formed at lower temperatures. Igneous rocks are commonly attacked by water, particularly acid or alkaline solutions, and all of the common igneous rock forming minerals (with the exception of quartz which is very resistant) are changed in this way into clay minerals and chemicals in solution.
Rock particles in the form of clay, silt, sand, and gravel, are transported by the agents of erosion (usually water, and less frequently by ice and wind) to new locations and redeposited in layers, generally at a lower elevation.
These deposited particles eventually become compacted and cemented together, forming clastic sedimentary rocks.
Biogenic
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, the flint nodules found in chalk (which is itself a biogenic sedimentary rock, a form of limestone), and coal (derived from the remains of tropical plants and subjected to pressure).
Precipitate
Precipitate sedimentary rocks form when mineral solutions, such as sea water, evaporate.
Other information
Sedimentary rocks are economically important in that they can easily be used as construction material because they are soft and easy to cut. In addition, sedimentary rocks often form porous and permeable reservoirs in sedimentary basins in which petroleum and other hydrocarbons can be found (see Bituminous rocks).
The shape of the particles in sedimentary rocks has an important effect on the ability of micro-organisms to colonize them.
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