Sedimentary rock
Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals or organic matter —that have been accumulated or deposited at Earth's surface. Sedimentation is any process that causes these particles to settle in place. Geological detritus originates from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus is formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur when dissolved minerals precipitate from water solution.
The sedimentary rock cover of the continents of the Earth's crust is extensive, but sedimentary rock is estimated to be only 8% of the volume of the crust. Sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks. Sedimentary rocks are deposited in layers as strata, forming a structure called bedding. Sedimentary rocks are often deposited in large structures called sedimentary basins. Sedimentary rocks have also been found on Mars.
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 structures. Sedimentary rocks are also important sources of natural resources including coal, fossil fuels, drinking water and ores.
The study of the sequence of sedimentary rock strata is the main source for an understanding of the Earth's history, including palaeogeography, paleoclimatology and the history of life. The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology. Sedimentology is part of both geology and physical geography and overlaps partly with other disciplines in the Earth sciences, such as pedology, geomorphology, geochemistry and structural geology.
Classification based on origin
Sedimentary rocks can be subdivided into four groups based on the processes responsible for their formation: clastic sedimentary rocks, biochemical sedimentary rocks, chemical sedimentary rocks, and a fourth category for "other" sedimentary rocks formed by impacts, volcanism, and other minor processes.Clastic sedimentary rocks
Clastic sedimentary rocks are composed of rock fragments that have been cemented together. The clasts are commonly individual grains of quartz, feldspar, clay minerals, or mica. However, any type of mineral may be present. Clasts may also be lithic fragments composed of more than one mineral.Clastic sedimentary rocks are subdivided according to the dominant particle size. Most geologists use the Udden-Wentworth grain size scale and divide unconsolidated sediment into three fractions: gravel, sand, and mud. Mud is further divided into silt and clay. The classification of clastic sedimentary rocks parallels this scheme; conglomerates and breccias are made mostly of gravel, sandstones are made mostly of sand, and mudrocks are made mostly of mud. This tripartite subdivision is mirrored by the broad categories of rudites, arenites, and lutites, respectively, in older literature.
The subdivision of these three broad categories is based on differences in clast shape, composition, or grain size or texture.
Conglomerates and breccias
Breccias are dominantly composed of angular gravel in a groundmass, while conglomerates are dominantly composed rounded gravel.Sandstones
Sandstone classification schemes vary widely, but most geologists have adopted the Dott scheme, which uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of a muddy matrix between the larger grains.Six sandstone names are possible using the descriptors for grain composition and the amount of matrix. For example, a quartz arenite would be composed of mostly quartz grains and have little or no clayey matrix between the grains, a lithic wacke would have abundant lithic grains and abundant muddy matrix, etc.
Although the Dott classification scheme is widely used by sedimentologists, common names like greywacke, arkose, and quartz sandstone are still widely used by non-specialists and in popular literature.
Mudrocks
Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles. These relatively fine-grained particles are commonly transported by turbulent flow in water or air, and deposited as the flow calms and the particles settle out of suspension.Most authors presently use the term "mudrock" to refer to all rocks composed dominantly of mud. Mudrocks can be divided into siltstones, composed dominantly of silt-sized particles; mudstones with subequal mixture of silt- and clay-sized particles; and claystones, composed mostly of clay-sized particles. Most authors use "shale" as a term for a fissile mudrock although some older literature uses the term "shale" as a synonym for mudrock.
Biochemical sedimentary rocks
Biochemical sedimentary rocks are created when organisms use materials dissolved in air or water to build their tissue. Examples include:- Most types of limestone are formed from the calcareous skeletons of organisms such as corals, mollusks, and foraminifera.
- Coal, formed from vegetation that has removed carbon from the atmosphere and combined it with other elements to build their tissue, this vegetation gets compressed by overlying sediments and undergoes chemical transformation.
- Deposits of chert formed from the accumulation of siliceous skeletons of microscopic organisms such as radiolaria and diatoms.
Chemical sedimentary rocks
Other sedimentary rocks
This fourth miscellaneous category includes volcanic tuff and volcanic breccias formed by deposition and later cementation of lava fragments erupted by volcanoes, and impact breccias formed after impact events.Classification based on composition
Alternatively, sedimentary rocks can be subdivided into compositional groups based on their mineralogy:- Siliciclastic sedimentary rocks, are dominantly composed of silicate minerals. The sediment that makes up these rocks was transported as bed load, suspended load, or by sediment gravity flows. Siliciclastic sedimentary rocks are subdivided into conglomerates and breccias, sandstone, and mudrocks.
- Carbonate sedimentary rocks are composed of calcite, aragonite, dolomite, and other carbonate minerals based on the ion. Common examples include limestone and the rock dolomite.
- Evaporite sedimentary rocks are composed of minerals formed from the evaporation of water. The most common evaporite minerals are carbonates, chlorides, and sulfates. Evaporite rocks commonly include abundant halite, gypsum, and anhydrite.
- Organic-rich sedimentary rocks have significant amounts of organic material, generally in excess of 3% total organic carbon. Common examples include coal, oil shale as well as source rocks for oil and natural gas.
- Siliceous sedimentary rocks are almost entirely composed of silica, typically as chert, opal, chalcedony or other microcrystalline forms.
- Iron-rich sedimentary rocks are composed of >15% iron; the most common forms are banded iron formations and ironstones.
- Phosphatic sedimentary rocks are composed of phosphate minerals and contain more than 6.5% phosphorus; examples include deposits of phosphate nodules, bone beds, and phosphatic mudrocks.
Deposition and transformation
Sediment transport and deposition
Sedimentary rocks are formed when sediment is deposited out of air, ice, wind, gravity, or water flows carrying the particles in suspension. This sediment is often formed when weathering and erosion break down a rock into loose material in a source area. The material is then transported from the source area to the deposition area. The type of sediment transported depends on the geology of the hinterland. However, some sedimentary rocks, such as evaporites, are composed of material that form at the place of deposition. The nature of a sedimentary rock, therefore, not only depends on the sediment supply, but also on the sedimentary depositional environment in which it formed.Transformation (Diagenesis)
As sediments accumulate in a depositional environment, older sediments are buried by younger sediments, and they undergo diagenesis. Diagenesis includes all the chemical, physical, and biological changes, exclusive of surface weathering, undergone by a sediment after its initial deposition. This includes compaction and lithification of the sediments. Early stages of diagenesis, described as eogenesis, take place at shallow depths and is characterized by bioturbation and mineralogical changes in the sediments, with only slight compaction. The red hematite that gives red bed sandstones their color is likely formed during eogenesis. Some biochemical processes, like the activity of bacteria, can affect minerals in a rock and are therefore seen as part of diagenesis. Another example of sedimentary diagenesis is the dolomitzation of rocks such as limestone.Deeper burial is accompanied by mesogenesis, during which most of the compaction and lithification takes place. Compaction takes place as the sediments come under increasing overburden pressure from overlying sediments. Sediment grains move into more compact arrangements, grains of ductile minerals are deformed, and pore space is reduced. Sediments are typically saturated with groundwater or seawater when originally deposited, and as pore space is reduced, much of these connate fluids are expelled. In addition to this physical compaction, chemical compaction may take place via pressure solution. Points of contact between grains are under the greatest strain, and the strained mineral is more soluble than the rest of the grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact. The increased pressure and temperature stimulate further chemical reactions, such as the reactions by which organic material becomes lignite or coal.
Lithification follows closely on compaction, as increased temperatures at depth hasten the precipitation of cement that binds the grains together. Pressure solution contributes to this process of cementation, as the mineral dissolved from strained contact points is redeposited in the unstrained pore spaces. This further reduces porosity and makes the rock more compact and competent.
Unroofing of buried sedimentary rock is accompanied by telogenesis, the third and final stage of diagenesis. As erosion reduces the depth of burial, renewed exposure to meteoric water produces additional changes to the sedimentary rock, such as leaching of some of the cement to produce secondary porosity.
At sufficiently high temperature and pressure, the realm of diagenesis makes way for metamorphism, the process that forms metamorphic rock.