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Sedimentary rock
Sedimentary rock
Middle Triassic Triassic marginal marginal marine sequence of siltstones (below) and limestones (above), Virgin Formation, southwestern Utah Utah.. Sedimentary rock is one of the three main rock main rock types types (the others being igneous being igneous and and metamorphic metamorphic rock). rock). Sedimentary rock is formed by deposition and consolidation of mineral and organic material and from precipitation of minerals from solution. The processes that form sedimentary rock occur at the surface of the Earth and within bodies of water. Rock formed from sediments sediments covers covers 75-80% of the Earth’s land area, and includes common types such as limestone, limestone, chalk, chalk, dolostone, dolostone, sandstone, sandstone, conglomerate,, some types of breccia, conglomerate breccia, and shale and shale..[1] Sedimentary rocks are classified by the source of their sediments, and are produced by one or more of: • clastic rock rock formed formed from fragments broken off from parent rock, by • weathering in weathering in situ or • erosion by erosion by water, ice or wind followed by transportation by transportation of of sediments, to the place of deposition; deposition; • biogenic activity; biogenic activity; or • precipitation from precipitation from solution solution.. The sediments are then compacted and converted to rock by the process of lithification. lithification .
Formation Sedimentary rocks Sedimentary rocks are are formed because of the overburden pressure as pressure as particles of sediment are sediment are deposited deposited out out of air, ice, wind, gravity, or water flows carrying the particles in suspension. suspension. As sediment deposition builds up, the overburden (or ’lithostatic’) pressure squeezes the sediment into layered solids in a process known as lithification (’rock lithification (’rock formation’) and the original connate fluids are fluids are expelled. The term diagenesis is used to describe all the chemical, physical, and biological changes, including cementation including cementation,, undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface weathering.
Sedimentary-rock formation, Karnataka Karnataka , , India Sedime Sedimenta ntary ry rocks rocks are laid laid down down in layers layers called called beds or beds or strata strata.. That new rock layers are above older rock layers is stated in the principle of superposition. superposition . There are usually some gaps in the sequence called unconformities.. These represent periods in which no new sediformities ments were being laid down, or when earlier sedimentary layers were raised above sea level and eroded away. Sedimentar Sedimentaryy rocks contain contain important important informati information on about the history the history of Earth. Earth. They contain fossils contain fossils,, the preserved remains of ancient plants and animals animals.. Coal is considered a type of sedimentary rock. The composition of sediments provides us with clues as to the original rock. rock. Differ Differenc ences es betwee between n success successive ive layers layers indica indicate te changes to the environment which have occurred over time. Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remains. The sedimentary rock cover of the continents of the Earth’s crust Earth’s crust is is extensive, but the total contribution of sedimentary rocks is estimated to be only 5% of the total. As such, the sedimentary sequences we see represent only a thin veneer over a crust consisting consisting mainly mainly of igneous and metamorphic rocks.
Classification Sedime Sedimenta ntary ry rocks rocks are classi classifie fied d into into three three groups groups.. These groups are clastic, chemical precipitate and biochemical or biogenic.
Clastic Clastic sedimentary rocks are composed of discrete fragment me ntss or clas clasts ts of ma mate teri rial alss deri derive ved d from from othe otherr
From Wikipedia, the free encyclopedia minerals. They are composed largely of quartz with other common minerals including feldspar, amphiboles, clay minerals, and sometimes more exotic igneous and metamorphic minerals. 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 0.002 mm, siltstone being a little bigger with particles between 0.002 to 0.063 mm, and sandstone being coarser still with grains 0.063 to 2 mm, and conglomerates and breccias being more coarse with grains 2 to 263 mm. Breccia has sharper particles, while conglomerate is categorized by its rounded particles. Particles bigger than 263 mm are termed blocks (angular) or boulders (rounded). Lutite, Arenite and Rudite are general terms for sedimentary rock with clay/silt-, sand- or conglomerate/breccia-sized particles. The classification of clastic sedimentary rocks is complex because there are many variables involved. Particle size (both the average size and range of sizes of the particles), composition of the particles, the cement, and the matrix (the name given to the smaller particles present in the spaces between larger grains) must all be taken into consideration. Shales, which consist mostly of clay minerals, are generally further classified on the basis of composition and bedding. Coarser clastic sedimentary rocks are classified according to their particle size and composition. Orthoquartzite is a very pure quartz sandstone; arkose is a sandstone with quartz and abundant feldspar; 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 when exposed to mechanical and chemical weathering at the Earth’s surface.
Sedimentary rock Mechanical weathering is the breakdown of rock into particles without producing changes in the chemical composition of the minerals in the rock. Ice is the most important agent of mechanical weathering. Water percolates into cracks and fissures within the rock, freezes, and expands. The force exerted by the expansion is sufficient to widen cracks and break off pieces of rock. Heating and cooling of the rock, and the resulting expansion and contraction, also aids the process. Mechanical weathering contributes further to the breakdown of rock by increasing the surface area exposed to chemical agents. 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. Air and water are both involved in many complex chemical reactions. 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 agents reduce the size of the particles, sort them by size, and then deposit them in new locations. The sediments dropped by streams and rivers form alluvial fans, flood plains, deltas, and on the bottom of lakes and the sea floor. The wind may move large amounts of sand and other smaller particles. Glaciers transport and deposit great quantities of usually unsorted rock material as till. These deposited particles eventually become compacted and cemented together, forming clastic sedimentary rocks. Such rocks contain inert minerals which are resistant to mechanical and chemical breakdown such as quartz, zircon, rutile, and magnetite. Quartz is one of the most mechanically and chemically resistant minerals.
Organic
Lower Antelope Canyon was carved out of the surrounding sandstone by both mechanical weathering and chemical weathering. Wind, sand, and water from flash flooding are the primary weathering agents.
Organic sedimentary rocks contain materials generated by living organisms, and include carbonate minerals created by organisms, such as corals, mollusks, 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 biochemical sedimentary rock, a form
From Wikipedia, the free encyclopedia
Sedimentary rock oversaturated and precipitate. Sedimentary rocks from this process can include the evaporite minerals halite (rock salt), sylvite, barite and gypsum.
See also • Back-stripping • Deposition (sediment) • Dunham classification • erosion • List of minerals • List of rock types • Transportation (sediment) Outcrop of Ordovician oil shale ( kukersite ), northern Estonia.
Footnotes
of limestone), and coal and oil shale (derived from the remains of tropical plants and subjected to heat).
[1]
Chemical
References
Chemical sedimentary rocks form when minerals in solution become oversaturated and precipitate. In marine environments, this is a method for the formation of limestone. Another common environment in which chemical sedimentary rocks form is a body of water that is evaporating. Evaporation decreases the amount of water without decreasing the amount of dissolved material. Therefore, the dissolved material can become
"Sediment and Sedimentary Rocks." Sedimentary Rocks. Retrieved on July 29, 2007.
• Blatt, Harvey and Robert James Tracy, 1994, Petrology: Igneous, Sedimentary and Metamorphic , Freeman, 2nd Ed. ISBN 0-7167-2438-3 • Folk, R.L., 1965, Petrology of sedimentary rocks PDF version. Austin: Hemphill’s Bookstore. 2nd ed. 1981, ISBN 0-914696-14-9 • Basic Sedimentary Rock Classification
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