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Sedimentary rock

Middle Triassic marginal marine sequence of siltstones (below) and limestones (above), Virgin Formation, southwestern Utah.

Middle Triassic marginal marine sequence of siltstones (below) and limestones (above), Virgin Formation, southwestern Utah.
Sedimentary rock is the type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles (detritus) to settle and accumulate or minerals to precipitate from a solution. Particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, sediment was formed by weathering and erosion in a source area, and then transported to the place of deposition by water, wind, mass movement or glaciers.

Though sedimentary rocks form just a small part of the Earth's crust, they cover the largest part of the Earth's surface. Sedimentary rocks are deposited in strata that form a structure called bedding. The study of sedimentary rocks and rock strata provides information about the subsurface that is useful for civil engineering, for example in the construction of roads, houses, tunnels canals or other constructions. Sedimentary rocks are also important sources of natural resources like coal, fossil fuels, drinking water or ores.

The study of the sequence of sedimentary rock strata is the main source for scientific knowledge about the Earth's history, including paleogeography, paleoclimatology and the history of life.

The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology. Sedimentology is both part of geology and physical geography and overlaps partly with other disciplines in the Earth sciences, such as pedology, geomorphology, geochemistry or structural geology.

Contents


Classification

Sedimentary rocks are classified into three groups. These groups are clastic, chemical precipitate and biochemical or biogenic.

Clastic

Clastic sedimentary rocks are composed of discrete fragments or clasts of materials derived from other 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.

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.

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.
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. Quartz is one of the most mechanically and chemically resistant minerals. Highly weathered sediments can contain several heavy and stable minerals, best illustrated by the ZTR index.

Organic

Outcrop of Ordovician oil shale (kukersite), northern Estonia. 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 of limestone), and coal and oil shale (derived from the remains of tropical plants and subjected to heat).

Chemical

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 oversaturated and precipitate. Sedimentary rocks from this process can include the evaporite minerals halite (rock salt), sylvite, barite and gypsum.

Formation

Sedimentary-rock formation, Karnataka, India

Sedimentary-rock formation, Karnataka, India

Sedimentary rocks are formed because of the overburden pressure as particles of sediment are deposited out of air, ice, wind, gravity, 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. The term diagenesis is used to describe all the chemical, physical, and biological changes, including cementation, undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface weathering.

Sedimentary rocks are laid down in layers called beds or strata. That new rock layers are above older rock layers is stated in the principle of superposition. There are usually some gaps in the sequence called unconformities. These represent periods in which no new sediments were being laid down, or when earlier sedimentary layers were raised above sea level and eroded away.

Sedimentary rocks contain important information about the history of Earth. They contain fossils, the preserved remains of ancient plants and animals. Coal is considered a type of sedimentary rock. The composition of sediments provides us with clues as to the original rock. Differences between successive layers indicate changes to the environment which have occurred over time. Sedimentary rocks can contain fossils because, unlike most igneous and 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 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 mainly of igneous and metamorphic rocks.

Sedimentary environments

The setting in which a sedimentary rock forms is called the sedimentary environment. Every environment has a characteristic combination of geologic processes and circumstances. The type of sediment that is deposited is not only dependent on the sediment that is transported to a place, but also on the environment itself.[1]

A marine environment means the rock was formed in a sea or ocean. Often, a distinction is made between deep and shallow marine environments. Deep marine usually refers to environments more than 200 m below the water surface. Shallow marine environments exist adjacent to coastlines and can extend out to the boundaries of the continental shelf. The water in such environments has a generally higher energy than that in deep environments, because of wave activity. This means coarser sediment particles can be transported and the deposited sediment can be coarser than in deep environments. When the available sediment is transported from the continent, an alternation of sand, clay and silt will be deposited. When the continent is far away, the amount of such sediment brought in may be small, and biochemical processes will dominate the type of rock that is formed. Especially in warm climates, shallow marine environments far offshore will mainly see the deposition of carbonate rocks. The shallow, warm water is an ideal habitat for many small organisms that build carbonate skeletons. When these organisms die their skeletons sink to the bottom, forming a thick layer of calcareous mud that may lithify into limestone. Warm shallow marine environments also are ideal environments for coral reefs, where the sediment consists mainly of the calcareous skeletons of larger organisms.[2]

In deep marine environments, the water current over the sea bottom is small. Only fine particles can be transported to such places. Typically sediments depositing on the ocean floor are fine clay or small skeletons of micro-organisms. At 4 km depth, the solubility of carbonates increases dramatically (the depth zone where this happens is called the lysocline). Calcareous sediment that sinks below the lysocline will dissolve, so no limestone can be formed below this depth. Skeletons of micro-organisms formed of silica (such as radiolarians) still deposit though. An example of a rock formed out of silica skeletons is radiolarite. When the bottom of the sea has a small inclination, for example at the continental slopes, the sedimentary cover can become unstable, causing turbidity currents. Turbidity currents are sudden disturbances of the normally quite deep marine environment and can cause the geologically speaking instantaneous deposition of large amounts of sediment, such as sand and silt. The rock sequence formed by a turbidity current is called a turbidite.[3]

The coast is an environment dominated by wave action. At the beach, dominantly coarse sediment like sand or gravel is deposited, often mingled with shell fragments. Tidal flats and shoals are places that will sometimes fall dry as a result of the tide. They are often cross-cut by gullies, where the current is strong and the grain size of the deposited sediment is larger. Where along a coast (either the coast of a sea or a lake) rivers enter the body of water, deltas can form. These are large accumulations of sediment transported from the continent to places in front of the mouth of the river. Deltas are dominantly composed of clastic sediment.

A sedimentary rock formed on the land has a continental sedimentary environment. Examples of continental environments are lagoons, lakes, swamps, floodplains and alluvial fans. In the quite water of swamps, lakes and lagoons, fine sediment is deposited, mingled with organic material from dead plants and animals. In rivers, the energy of the water is much higher and the transported material consists of clastic sediment. Besides transport by water, sediment can in continental environments also be transported by wind or glaciers. Sediment transported by wind is called aeolian and is always very well sorted, while sediment transported by a glacier is called glacial and is characterized by very poor sorting.[4]

Sedimentary facies

Sedimentary environments usually exist alongside each other in certain natural successions. A beach, where sand and gravel is deposited, will usually be bounded by a deeper marine environment a little offshore, where finer sediments are deposited at the same time. Behind the beach, there can be dunes (where the dominant deposition is well sorted sand) or a lagoon (where fine clay and organic material is deposited). Every sedimentary environment has its own characteristic deposits. The typical rock formed in a certain environment is called its sedimentary facies. When sedimentary strata accumulate through time, the environment can shift, forming a change in facies in the subsurface at one location. On the other hand, when a rock layer with a certain age is followed laterally, the lithology (the type of rock) and facies will eventually change.[5]

Shifting sedimentary facies in the case of transgression (above) and regression of the sea (below). Facies can be distinguished in a number of ways: the most common ways are by the lithology (for example: limestone, siltstone or sandstone) or by fossil content. Coral for example only lives in warm and shallow marine environments and fossils of coral are thus typical for shallow marine facies. Facies determined by lithology are called lithofacies; facies determined by fossils are biofacies.[6]

Sedimentary environments can shift their geographical positions through time. Coastlines can shift in the direction of the sea when the sea level drops, when the surface rises due to tectonic forces in the Earth's crust or when a river forms a large delta. In the subsurface, such geographic shifts of sedimentary environments of the past are recorded in shifts in sedimentary facies. This means that sedimentary facies can change either parallel or perpendicular to an imaginary layer of rock with a fixed age, a phenomenon described by Walther's facies rule.[7]

The situation in which coastlines move in the direction of the continent is called transgression. In the case of transgression, deeper marine facies will be deposited over shallower facies, a succession called onlap. Regression is the situation in which a coastline moves in the direction of the sea. With regression, shallower facies will be deposited on top of deeper facies, a situation called offlap.[8]

The facies of all rocks of a certain age can be plotted on a map to give an overview of the palaeogeography. A sequence of maps for different ages can give an insight in the development of the regional geography.

Sedimentary basins

Places where large-scale sedimentation takes place are called sedimentary basins. The amount of sediment that can be deposited in a basin depends on the depth of the basin, the so called accommodation space. Depth, shape and size of a basin depend on tectonics, movements within the Earth's lithosphere. Where the lithosphere moves upward (tectonic uplift), land will in due course rise above sea level, so that and erosion will remove material and the area becomes a source for new sediment. At places where the lithosphere moves downward (tectonic subsidence) a basin will form where sedimentation can take place. When the lithosphere keeps subsiding, new accommodation space keeps being created.

A type of basin formed by the moving apart of two pieces of a continent is called a rift basin. Rift basins are elongated, narrow and deep basins. Due to divergent movement, the lithosphere is stretched and thinned, so that the hot asthenosphere will rise and heat the overlying rift basin. Apart from continental sediments, rift basins normally also have part of their infill consisting of volcanic deposits. When the basin grows due to continued stretching of the lithosphere, the rift will grow and the sea can enter, forming marine deposits.

When a piece of lithosphere that was heated and stretched cools again, its density will rise, causing isostatic subsidence. If this subsidence continues long enough the basin is called a sag basin. Examples of sag basins are the regions along passive continental margins, but sag basins can also be found in the interior of continents. In sag basins, the extra weight of the newly deposited sediments is enough to keep the subsidence going in a vicious circle. The total thickness of the sedimentary infill in a sag basins can thus exceed 10 km.

A third type of basin exists along convergent plate boundaries - places where one tectonic plate moves under another into the asthenosphere. The subducting plate will bend and as a result a fore-arc basin forms in front of the overriding plate, an elongated, deep asymmetric basin. Fore-arc basins are filled with deep marine deposits and thick sequences of turbidites. Such infill is called flysch. When the convergent movement of the two plates results in continental collision, the basin will become shallower and develop into a foreland basin. At the same time, tectonic uplift forms a mountain belt in the overriding plate, from which large amounts of material are eroded and transported to the basin. Such erosional material of a growing mountain chain is called molasse and has either a shallow marine or a continental facies.

At the same time, the growing weight of the mountain belt can cause isostatic subsidence in the area of the overriding plate on the other side to the mountain belt. The basin type resulting from this subsidence is called a back-arc basin and is usually filled by shallow marine deposits and molasse.[9]

Cyclic alternation of competent and less competent beds in the Blue Lias at Lyme Regis, southern England.

Cyclic alternation of competent and less competent beds in the Blue Lias at Lyme Regis, southern England.

Influence of astronomical cycles

In many cases facies changes and other lithological features in sequences of sedimentary rock have a cyclic nature. This cyclic nature was caused by cyclic changes in sediment supply and the sedimentary environment. Most of these cyclic changes are caused by astronomic cycles. Short astronomic cycles can be the difference between the tides or the spring tide every two weeks. On a larger time-scale, cyclic changes in climate and sea level are caused by Milankovitch cycles: cyclic changes in the orientation and/or position of the Earth's rotational axis and orbit around the Sun. There are a number of Milankovitch cycles known, lasting between 10,000 and 200,000 years.[10]

Relatively small changes in the orientation of the Earth's axis or length of the seasons can be a major influence on the Earth's climate. An example are the ice ages of the past 2.6 million years (the Quaternary period), which are assumed to have been caused by astronomic cycles.[11] Climate change can influence the global sea level (and thus the amount of accommodation space in sedimentary basins) and sediment supply from a certain region. Eventually, small changes in astronomic parameters can cause large changes in sedimentary environment and sedimentation.

Sedimentation rates

The rate at which sediment is deposited differs depending on the location. A gully in a tidal flat can see the deposition of a few metres of sediment in one day, while on the deep ocean floor each year only a few millimetres of sediment accumulate. A distinction can be made between normal sedimentation and sedimentation caused by catastrophic processes. The latter category includes all kinds of sudden exceptional processes like mass movements, rock slides or flooding. Catastrophic processes can see the sudden deposition of a large amount of sediment at once. In some sedimentary environments, most of the total column of sedimentary rock was formed by catastrophic processes, even though the environment is usually a quite place. Other sedimentary environments are dominated by normal, ongoing sedimentation.[12]

In some sedimentary environments, sedimentation only occurs in some places. In a desert for example, the wind will deposit siliclastic material (sand or silt) in some spots, or catastrophic flooding of a wadi can see the sudden deposition of large quantities of detritic material, but in most places (aeolian) erosion dominates. The amount of sedimentary rock that forms is not only dependent on the amount of supplied material, but also on how well the material consolidates ("fossilizes"). Most deposited sediment will shortly after deposition be removed by erosion.[12]

See also

Footnotes

References

External link

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