MiddleTriassic marginal marine sequence of siltstones (reddish layers at the cliff base) andlimestones (brown rocks above),Virgin Formation, southwesternUtah, U.S.
Sedimentary rocks are types ofrock formed by thecementation ofsediments—i.e. particles made ofminerals (geologicaldetritus) ororganic matter (biological detritus)—that have been accumulated or deposited atEarth's surface.Sedimentation is any process that causes these particles to settle in place. Geological detritus originates fromweathering anderosion 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 ormass movement, which are called agents ofdenudation. Biological detritus is formed by bodies and parts (mainly shells) of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies (marine snow). Sedimentation may also occur when dissolved minerals precipitate fromwater solution.
The sedimentary rock cover of the continents of the Earth's crust is extensive (73% of the Earth's current land surface),[1] but sedimentary rock is estimated to be only 8% of the volume of the crust.[2] Sedimentary rocks are only a thin veneer over a crust consisting mainly ofigneous andmetamorphic rocks. Sedimentary rocks are deposited in layers asstrata, forming a structure calledbedding. Sedimentary rocks are often deposited in large structures calledsedimentary basins. Sedimentary rocks have also been found onMars.
Sedimentary rocks can be subdivided into four groups based on the processes responsible for their formation: clastic sedimentary rocks, biochemical (biogenic) sedimentary rocks, chemical sedimentary rocks, and a fourth category for "other" sedimentary rocks formed by impacts,volcanism, and other minor processes.
Clastic sedimentary rocks are composed of rock fragments (clasts) that have been cemented together. The clasts are commonly individual grains ofquartz,feldspar,clay minerals, ormica. However, any type of mineral may be present. Clasts may also belithic fragments composed of more than one mineral.
Clastic sedimentary rocks are subdivided according to the dominant particle size. Most geologists use theUdden-Wentworth grain size scale and divide unconsolidated sediment into three fractions:gravel (>2 mm diameter),sand (0.06 to 2 mm diameter), andmud (60 μm diameter). Mud is further divided intosilt (60 to 4 μm diameter) andclay (4 μm diameter). The classification of clastic sedimentary rocks parallels this scheme;conglomerates andbreccias are made mostly of gravel,sandstones are made mostly ofsand, andmudrocks are made mostly of mud. This tripartite subdivision is mirrored by the broad categories ofrudites,arenites, andlutites, respectively, in older literature.
The subdivision of these three broad categories is based on differences in clast shape (conglomerates and breccias), composition (sandstones), or grain size or texture (mudrocks).
Breccias are dominantly composed of angular gravel in agroundmass (matrix),[3] while conglomerates are dominantly composed rounded gravel.
Sandstones
Sedimentary rock with sandstone inMalta, southern EuropeLower Antelope Canyon was carved out of the surroundingsandstone by both mechanical weathering and chemical weathering. Wind, sand, and water fromflash flooding are the primary weathering agents.
Sandstone classification schemes vary widely, but most geologists have adopted the Dott scheme,[4] which uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of a muddy matrix between the larger grains.
Composition of framework grains
The relative abundance of sand-sized framework grains determines the first word in a sandstone name. Naming depends on the dominance of the three most abundant components quartz, feldspar, or the lithic fragments that originated from other rocks. All other minerals are considered accessories and not used in the naming of the rock, regardless of abundance.
Quartz sandstones have >90% quartz grains
Feldspathic sandstones have <90% quartz grains and more feldspar grains than lithic grains
Lithic sandstones have <90% quartz grains and more lithic grains than feldspar grains
Abundance of muddy matrix material between sand grains
When sand-sized particles are deposited, the space between the grains either remains open or is filled with mud (silt and/or clay sized particle).
"Clean" sandstones with open pore space (that may later be filled with matrix material) are called arenites.
Muddy sandstones with abundant (>10%) muddy matrix are called wackes.
Six sandstone names are possible using the descriptors for grain composition (quartz-, feldspathic-, and lithic-) and the amount of matrix (wacke or arenite). For example, a quartz arenite would be composed of mostly (>90%) 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[4] is widely used by sedimentologists, common names likegreywacke,arkose, and quartz sandstone are still widely used by non-specialists and in popular literature.
Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles. These relatively fine-grained particles are commonly transported byturbulent flow in water or air, and deposited as the flow calms and the particles settle out ofsuspension.
Most authors presently use the term "mudrock" to refer to all rocks composed dominantly of mud.[5][6][7][8] 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.[5][6] Most authors use "shale" as a term for afissile mudrock (regardless of grain size) although some older literature uses the term "shale" as a synonym for mudrock.
Coal, formed from vegetation that has removedcarbon from the atmosphere and combined it with other elements to build their tissue, this vegetation gets compressed by overlying sediments and undergoes chemical transformation.[3]
Deposits ofchert formed from the accumulation of siliceous skeletons of microscopic organisms such asradiolaria anddiatoms.
This fourth miscellaneous category includes volcanictuff andvolcanic breccias formed by deposition and later cementation of lava fragments erupted by volcanoes, andimpact breccias formed afterimpact events.
Carbonate sedimentary rocks are composed ofcalcite (rhombohedralCaCO 3), aragonite (orthorhombicCaCO 3), dolomite (CaMg(CO 3) 2), and other carbonate minerals based on theCO2− 3 ion. Common examples includelimestone and the rockdolomite.
Evaporite sedimentary rocks are composed of minerals formed from the evaporation of water. The most common evaporite minerals arecarbonates (calcite and others based onCO2− 3),chlorides (halite and others built onCl− ), andsulfates (gypsum and others built onSO2− 4). Evaporite rocks commonly include abundant halite (rock salt),gypsum, andanhydrite.
Sedimentaryrocks are formed whensediment isdeposited out of air, ice, wind, gravity, or water flows carrying the particles insuspension. This sediment is often formed whenweathering anderosion break down a rock into loose material in a source area. The material is thentransported from the source area to the deposition area. The type of sediment transported depends on the geology of thehinterland (the source area of the sediment). However, some sedimentary rocks, such asevaporites, 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 thesedimentary depositional environment in which it formed.
Transformation (Diagenesis)
Pressure solution at work in aclastic rock. While material dissolves at places where grains are in contact, that material may recrystallize from the solution and act as cement in open pore spaces. As a result, there is a net flow of material from areas under high stress to those under low stress, producing a sedimentary rock that is harder and more compact. Loose sand can become sandstone in this way.
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 includescompaction andlithification of the sediments.[10] Early stages of diagenesis, described aseogenesis, take place at shallow depths (a few tens of meters) and is characterized bybioturbation and mineralogical changes in the sediments, with only slight compaction.[11] The redhematite that givesred bed sandstones their color is likely formed during eogenesis.[12][10] Somebiochemical processes, like the activity ofbacteria, can affect minerals in a rock and are therefore seen as part of diagenesis.[13] Another example of sedimentary diagenesis is thedolomitzation of rocks such as limestone.[14]
Deeper burial is accompanied bymesogenesis, during which most of the compaction and lithification takes place. Compaction takes place as the sediments come under increasingoverburden (lithostatic) pressure from overlying sediments. Sediment grains move into more compact arrangements, grains of ductile minerals (such asmica) are deformed, and pore space is reduced. Sediments are typically saturated withgroundwater or seawater when originally deposited, and as pore space is reduced, much of theseconnate fluids are expelled. In addition to this physical compaction, chemical compaction may take place viapressure 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.[10] The increased pressure and temperature stimulate further chemical reactions, such as the reactions by whichorganic material becomeslignite or coal.[15]
Lithification follows closely on compaction, as increased temperatures at depth hasten theprecipitation of cement that binds the grains together. Pressure solution contributes to this process ofcementation, 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 andcompetent.[10]
Unroofing of buried sedimentary rock is accompanied bytelogenesis, the third and final stage of diagenesis.[11] As erosion reduces the depth of burial, renewed exposure tometeoric water produces additional changes to the sedimentary rock, such asleaching of some of the cement to producesecondary porosity.[10]
At sufficiently high temperature and pressure, the realm of diagenesis makes way formetamorphism, the process that formsmetamorphic rock.[16]
The color of a sedimentary rock is often mostly determined byiron, an element with two major oxides:iron(II) oxide andiron(III) oxide. Iron(II) oxide (FeO) only forms under low oxygen (anoxic) circumstances and gives the rock a grey or greenish colour. Iron(III) oxide (Fe2O3) in a richer oxygen environment is often found in the form of the mineralhematite and gives the rock a reddish to brownish colour. In arid continental climates rocks are in direct contact with the atmosphere, and oxidation is an important process, giving the rock a red or orange colour. Thick sequences of red sedimentary rocks formed in arid climates are calledred beds. However, a red colour does not necessarily mean the rock formed in a continental environment or arid climate.[17]
The presence of organic material can colour a rock black or grey. Organic material is formed from dead organisms, mostly plants. Normally, such material eventuallydecays by oxidation or bacterial activity. Under anoxic circumstances, however, organic material cannot decay and leaves a dark sediment, rich in organic material. This can, for example, occur at the bottom of deep seas and lakes. There is little water mixing in such environments; as a result, oxygen from surface water is not brought down, and the deposited sediment is normally a fine dark clay. Dark rocks, rich in organic material, are therefore often shales.[17][18]
Texture
Diagram showingwell-sorted (left) and poorly sorted (right) grains
Thesize, form and orientation of clasts (the original pieces of rock) in a sediment is called itstexture. The texture is a small-scale property of a rock, but determines many of its large-scale properties, such as thedensity,porosity orpermeability.[19]
The 3D orientation of the clasts is called thefabric of the rock. The size and form of clasts can be used to determine the velocity and direction ofcurrent in the sedimentary environment that moved the clasts from their origin; fine,calcareous mud only settles in quiet water while gravel and larger clasts are moved only by rapidly moving water.[20][21] The grain size of a rock is usually expressed with the Wentworth scale, though alternative scales are sometimes used. The grain size can be expressed as a diameter or a volume, and is always an average value, since a rock is composed of clasts with different sizes. Thestatistical distribution of grain sizes is different for different rock types and is described in a property called thesorting of the rock. When all clasts are more or less of the same size, the rock is called 'well-sorted', and when there is a large spread in grain size, the rock is called 'poorly sorted'.[22][23]
The form of the clasts can reflect the origin of the rock. For example,coquina, a rock composed of clasts of broken shells, can only form in energetic water. The form of a clast can be described by using four parameters:[24][25]
Surface texture describes the amount of small-scale relief of the surface of a grain that is too small to influence the general shape. For example,frosted grains, which are covered with small-scale fractures, are characteristic of eolian sandstones.[26]
Rounding describes the general smoothness of the shape of a grain.
Sphericity describes the degree to which the grain approaches asphere.
Grain form describes the three-dimensional shape of the grain.
Chemical sedimentary rocks have a non-clastic texture, consisting entirely of crystals. To describe such a texture, only the average size of the crystals and the fabric are necessary.
Mineralogy
Global collage of sand samples. There is one square centimeter of sand on every sample photo. Sand samples row by row from left to right: 1. Glass sand from Kauai, Hawaii 2. Dune sand from the Gobi Desert 3. Quartz sand with green glauconite from Estonia 4. Volcanic sand with reddish weathered basalt from Maui, Hawaii 5. Biogenic coral sand from Molokai, Hawaii 6. Coral pink sand dunes from Utah 7. Volcanic glass sand from California 8. Garnet sand from Emerald Creek, Idaho 9. Olivine sand from Papakolea, Hawaii.[1]
Most sedimentary rocks contain either quartz (siliciclastic rocks) orcalcite (carbonate rocks). In contrast to igneous and metamorphic rocks, a sedimentary rock usually contains very few different major minerals. However, the origin of the minerals in a sedimentary rock is often more complex than in an igneous rock. Minerals in a sedimentary rock may have been present in the original sediments or may formed by precipitation during diagenesis. In the second case, a mineral precipitate may have grown over an older generation of cement.[27] A complex diagenetic history can be established byoptical mineralogy, using apetrographic microscope.
Carbonate rocks predominantly consist ofcarbonate minerals such as calcite,aragonite ordolomite. Both the cement and the clasts (including fossils andooids) of a carbonate sedimentary rock usually consist of carbonate minerals. The mineralogy of a clastic rock is determined by the material supplied by the source area, the manner of its transport to the place of deposition and the stability of that particular mineral.
The resistance of rock-forming minerals to weathering is expressed by theGoldich dissolution series. In this series, quartz is the most stable, followed byfeldspar,micas, and finally other less stable minerals that are only present when little weathering has occurred.[28] The amount of weathering depends mainly on the distance to the source area, the local climate and the time it took for the sediment to be transported to the point where it is deposited. In most sedimentary rocks, mica, feldspar and less stable minerals have been weathered toclay minerals likekaolinite,illite orsmectite.
Among the three major types of rock, fossils are most commonly found in sedimentary rock. Unlike most igneous and metamorphic rocks, sedimentary rocks form at temperatures and pressures that do not destroy fossil remnants. Often these fossils may only be visible undermagnification.
Dead organisms in nature are usually quickly removed byscavengers,bacteria,rotting and erosion, but under exceptional circumstances, these natural processes are unable to take place, leading to fossilisation. The chance of fossilisation is higher when the sedimentation rate is high (so that a carcass is quickly buried), inanoxic environments (where little bacterial activity occurs) or when the organism had a particularly hard skeleton. Larger, well-preserved fossils are relatively rare.
Fossils can be both the direct remains or imprints of organisms and their skeletons. Most commonly preserved are the harder parts of organisms such as bones, shells, and the woodytissue of plants. Soft tissue has a much smaller chance of being fossilized, and the preservation of soft tissue of animals older than 40 million years is very rare.[29] Imprints of organisms made while they were still alive are calledtrace fossils, examples of which areburrows,footprints, etc.
As a part of a sedimentary rock, fossils undergo the samediagenetic processes as does the host rock. For example, a shell consisting of calcite can dissolve while a cement of silica then fills the cavity. In the same way, precipitating minerals can fill cavities formerly occupied byblood vessels,vascular tissue or other soft tissues. This preserves the form of the organism but changes the chemical composition, a process calledpermineralization.[30][31] The most common minerals involved in permineralization are various forms ofamorphous silica (chalcedony,flint,chert),carbonates (especially calcite), andpyrite.
At high pressure and temperature, theorganic material of a dead organism undergoes chemical reactions in whichvolatiles such aswater andcarbon dioxide are expulsed. The fossil, in the end, consists of a thin layer of pure carbon or its mineralized form,graphite. This form of fossilisation is calledcarbonisation. It is particularly important for plant fossils.[32] The same process is responsible for the formation offossil fuels like lignite or coal.
Structures in sedimentary rocks can be divided intoprimary structures (formed during deposition) andsecondary structures (formed after deposition). Unlike textures, structures are always large-scale features that can easily be studied in the field.Sedimentary structures can indicate something about the sedimentary environment or can serve to tellwhich side originally faced up where tectonics have tilted or overturned sedimentary layers.
Sedimentary rocks are laid down in layers calledbeds orstrata. A bed is defined as a layer of rock that has a uniformlithology and texture. Beds form by the deposition of layers of sediment on top of each other. The sequence of beds that characterizes sedimentary rocks is calledbedding.[33][34] Single beds can be a couple of centimetres to several meters thick. Finer, less pronounced layers are called laminae, and the structure a lamina forms in a rock is calledlamination. Laminae are usually less than a few centimetres thick.[35] Though bedding and lamination are often originally horizontal in nature, this is not always the case. In some environments, beds are deposited at a (usually small) angle. Sometimes multiple sets of layers with different orientations exist in the same rock, a structure calledcross-bedding.[36] Cross-bedding is characteristic of deposition by a flowing medium (wind or water).
The opposite of cross-bedding is parallel lamination, where all sedimentary layering is parallel.[37] Differences in laminations are generally caused by cyclic changes in the sediment supply, caused, for example, by seasonal changes in rainfall, temperature or biochemical activity. Laminae that represent seasonal changes (similar totree rings) are calledvarves. Any sedimentary rock composed of millimeter or finer scale layers can be named with the general termlaminite. When sedimentary rocks have no lamination at all, their structural character is called massive bedding.
Graded bedding is a structure where beds with a smaller grain size occur on top of beds with larger grains. This structure forms when fast flowing water stops flowing. Larger, heavier clasts in suspension settle first, then smaller clasts. Although graded bedding can form in many different environments, it is a characteristic ofturbidity currents.[38]
The surface of a particular bed, called thebedform, can also be indicative of a particular sedimentary environment. Examples of bed forms includedunes andripple marks. Sole markings, such as tool marks and flute casts, are grooves eroded on a surface that are preserved by renewed sedimentation. These are often elongated structures and can be used to establish the direction of the flow during deposition.[39][40]
Ripple marks also form in flowing water. There can be symmetric or asymmetric. Asymmetric ripples form in environments where the current is in one direction, such as rivers. The longer flank of such ripples is on the upstream side of the current.[41][42][43] Symmetric wave ripples occur in environments where currents reverse directions, such as tidal flats.
Mudcracks are a bed form caused by the dehydration of sediment that occasionally comes above the water surface. Such structures are commonly found at tidal flats orpoint bars along rivers.
Secondary sedimentary structures are those which formed after deposition. Such structures form by chemical, physical and biological processes within the sediment. They can be indicators of circumstances after deposition. Some can be used asway up criteria.
Organic materials in a sediment can leave more traces than just fossils. Preserved tracks andburrows are examples oftrace fossils (also called ichnofossils).[44] Such traces are relatively rare. Most trace fossils are burrows ofmolluscs orarthropods. This burrowing is calledbioturbation by sedimentologists. It can be a valuable indicator of the biological and ecological environment that existed after the sediment was deposited. On the other hand, the burrowing activity of organisms can destroy other (primary) structures in the sediment, making a reconstruction more difficult.
Secondary structures can also form bydiagenesis or the formation of asoil (pedogenesis) when a sediment is exposed above the water level. An example of a diagenetic structure common in carbonate rocks is astylolite.[45] Stylolites are irregular planes where material was dissolved into the pore fluids in the rock. This can result in the precipitation of a certain chemical species producing colouring and staining of the rock, or the formation ofconcretions. Concretions are roughly concentric bodies with a different composition from the host rock. Their formation can be the result of localized precipitation due to small differences in composition or porosity of the host rock, such as around fossils, inside burrows or around plant roots.[46] In carbonate rocks such as limestone orchalk,chert orflint concretions are common, while terrestrial sandstones sometimes contain iron concretions. Calcite concretions in clay containing angular cavities or cracks are calledseptarian concretions.
After deposition, physical processes candeform the sediment, producing a third class of secondary structures. Density contrasts between different sedimentary layers, such as between sand and clay, can result inflame structures orload casts, formed by inverteddiapirism.[47] While the clastic bed is still fluid, diapirism can cause a denser upper layer to sink into a lower layer. Sometimes, density contrasts occur or are enhanced when one of the lithologies dehydrates. Clay can be easily compressed as a result of dehydration, while sand retains the same volume and becomes relatively less dense. On the other hand, when thepore fluid pressure in a sand layer surpasses a critical point, the sand can break through overlying clay layers and flow through, forming discordant bodies of sedimentary rock calledsedimentary dykes. The same process can formmud volcanoes on the surface where they broke through upper layers.
Sedimentary dykes can also be formed in a cold climate where the soil is permanently frozen during a large part of the year. Frost weathering can form cracks in the soil that fill with rubble from above. Such structures can be used as climate indicators as well as way up structures.[48]
Density contrasts can also cause small-scalefaulting, even while sedimentation progresses (synchronous-sedimentary faulting).[49] Such faulting can also occur when large masses of non-lithified sediment are deposited on a slope, such as at the front side of adelta or thecontinental slope. Instabilities in such sediments can result in the deposited material toslump, producing fissures and folding. The resulting structures in the rock are syn-sedimentaryfolds and faults, which can be difficult to distinguish from folds and faults formed bytectonic forces acting on lithified rocks.
The setting in which a sedimentary rock forms is called thedepositional 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 (provenance), but also on the environment itself.[50]
Amarine environment means that the rock was formed in asea orocean. 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 (including theabyssal plain). Shallow marine environments exist adjacent to coastlines and can extend to the boundaries of thecontinental shelf. The water movements in such environments have a generally higher energy than that in deep environments, aswave activity diminishes with depth. This means that coarser sediment particles can be transported and the deposited sediment can be coarser than in deeper environments. When the sediment is transported from the continent, an alternation ofsand,clay andsilt is deposited. When the continent is far away, the amount of such sediment deposited may be small, and biochemical processes dominate the type of rock that forms. Especially in warm climates, shallow marine environments far offshore mainly see 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 forcoral reefs, where the sediment consists mainly of the calcareous skeletons of larger organisms.[51]
In deep marine environments, the water current working 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 thelysocline). Calcareous sediment that sinks below the lysocline dissolves; as a result, no limestone can be formed below this depth. Skeletons of micro-organisms formed ofsilica (such asradiolarians) are not as soluble and are still deposited. An example of a rock formed of silica skeletons isradiolarite. When the bottom of the sea has a small inclination, for example, at thecontinental slopes, the sedimentary cover can become unstable, causingturbidity currents. Turbidity currents are sudden disturbances of the normally quiet deep marine environment and can cause the near-instantaneous deposition of large amounts of sediment, such as sand and silt. The rock sequence formed by a turbidity current is called aturbidite.[52]
The coast is an environment dominated by wave action. At abeach, dominantly denser sediment such as sand or gravel, often mingled with shell fragments, is deposited, while the silt and clay sized material is kept in mechanical suspension.Tidal flats andshoals are places that sometimes dry because of thetide. They are often cross-cut bygullies, where the current is strong and the grain size of the deposited sediment is larger. Where rivers enter the body of water, either on a sea or lake coast,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 (rather than chemical) sediment.
A continental sedimentary environment is an environment in the interior of a continent. Examples of continental environments arelagoons, lakes,swamps,floodplains andalluvial fans. In the quiet 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 greater and can transport heavier clastic material. Besides transport by water, sediment can be transported by wind or glaciers. Sediment transported by wind is calledaeolian and is almost alwaysvery well sorted, while sediment transported by a glacier is calledglacial till and is characterized by very poor sorting.[53]
Aeolian deposits can be quite striking. The depositional environment of theTouchet Formation, located in theNorthwestern United States, had intervening periods of aridity which resulted in a series ofrhythmite layers. Erosional cracks were later infilled with layers of soil material, especially fromaeolian processes. The infilled sections formed vertical inclusions in the horizontally deposited layers, and thus provided evidence of the sequence of events during deposition of the forty-one layers of the formation.[54]
Sedimentary facies
The kind of rock formed in a particular depositional environment is called itssedimentary facies. Sedimentary environments usually exist alongside each other in certain natural successions. A beach, where sand and gravel is deposited, is usually bounded by a deeper marine environment a little offshore, where finer sediments are deposited at the same time. Behind the beach, there can bedunes (where the dominant deposition is well sorted sand) or alagoon (where fine clay and organic material is deposited). Every sedimentary environment has its own characteristic deposits. 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, thelithology (the type of rock) and facies eventually change.[55]
Shifting sedimentary facies in the case oftransgression (above) andregression of the sea (below)
Facies can be distinguished in a number of ways: the most common are by the lithology (for example: limestone, siltstone or sandstone) or byfossil 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 calledlithofacies; facies determined by fossils arebiofacies.[56]
Sedimentary environments can shift their geographical positions through time. Coastlines can shift in the direction of the sea when thesea level drops (regression), when the surface rises (transgression) due to tectonic forces in the Earth's crust or when a river forms a largedelta. 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 byWalther's Law.[57]
The situation in which coastlines move in the direction of the continent is calledtransgression. In the case of transgression, deeper marine facies are deposited over shallower facies, a succession calledonlap.Regression is the situation in which a coastline moves in the direction of the sea. With regression, shallower facies are deposited on top of deeper facies, a situation calledofflap.[58]
The facies of all rocks of a certain age can be plotted on a map to give an overview of thepalaeogeography. A sequence of maps for different ages can give an insight in the development of the regional geography.
Gallery of sedimentary facies
A regressive facies shown on a stratigraphic column
Places where large-scale sedimentation takes place are calledsedimentary basins. The amount of sediment that can be deposited in a basin depends on the depth of the basin, the so-calledaccommodation space. The depth, shape and size of a basin depend ontectonics, movements within the Earth'slithosphere. Where the lithosphere moves upward (tectonic uplift), land eventually rises above sea level and the area becomes a source for new sediment aserosion removes material. Where the lithosphere moves downward (tectonic subsidence), a basin forms and sediments are deposited.
A type of basin formed by the moving apart of two pieces of a continent is called arift basin. Rift basins are elongated, narrow and deep basins. Due to divergent movement, the lithosphere isstretched and thinned, so that the hotasthenosphere rises and heats the overlying rift basin. Apart from continental sediments, rift basins normally also have part of their infill consisting ofvolcanic deposits. When the basin grows due to continued stretching of the lithosphere, therift grows and the sea can enter, forming marine deposits.
When a piece of lithosphere that was heated and stretched cools again, itsdensity rises, causingisostatic subsidence. If this subsidence continues long enough, the basin is called asag basin. Examples of sag basins are the regions alongpassivecontinental 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 avicious circle. The total thickness of the sedimentary infill in a sag basin can thus exceed 10 km.
A third type of basin exists alongconvergent plate boundaries – places where onetectonic plate moves under another into the asthenosphere. Thesubducting plate bends and forms afore-arc basin 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 calledflysch. When the convergent movement of the two plates results incontinental collision, the basin becomes shallower and develops into aforeland basin. At the same time, tectonic uplift forms amountain 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 calledmolasse 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 aback-arc basin and is usually filled by shallow marine deposits and molasse.[59]
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 byastronomic cycles. Short astronomic cycles can be the difference between the tides or thespring tide every two weeks. On a larger time-scale, cyclic changes in climate and sea level are caused byMilankovitch 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.[60]
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 theice ages of the past 2.6 million years (theQuaternaryperiod), which are assumed to have been caused by astronomic cycles.[61][62] 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 channel 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 likemass movements,rock slides orflooding. 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 quiet place. Other sedimentary environments are dominated by normal, ongoing sedimentation.[63]
In many cases, sedimentation occurs slowly. In adesert, for example, the wind deposits siliciclastic material (sand or silt) in some spots, or catastrophic flooding of awadi may cause sudden deposits of large quantities of detrital material, but in most places eolian 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. Erosion removes most deposited sediment shortly after deposition.[63]
Sedimentary rock are laid down in layers called beds or strata, each layer is horizontally laid down over the older ones and new layers are above older layers as stated in theprinciple of superposition. There are usually some gaps in the sequence calledunconformities which represent periods where no new sediments were laid down, or when earlier sedimentary layers were raised above sea level and eroded away.[3][64]
Unconformities can be classified based on the orientation of the strata on either sides of the unconformity:[3][65]
Angular unconformity when the earlier layers are tilted and eroded while the later layers are horizontally laid.
Nonconformity if the early layers have no bedding in contrast to the later layers, ie. they are igneous or metamorphic rocks.
Disconformity if both the early beds and the later beds are parallel to each other.
Sedimentary rocks contain important information about thehistory of the Earth. They contain fossils, the preserved remains of ancientplants andanimals. 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 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.
Provenance is the reconstruction of the origin of sediments. All rock exposed at Earth's surface is subjected to physical or chemicalweathering and broken down into finer grained sediment. All three types of rocks (igneous, sedimentary andmetamorphic rocks) can be the source of sedimentary detritus. The purpose of sedimentary provenance studies is to reconstruct and interpret the history of sediment from the initial parent rocks at a source area to final detritus at a burial place.[66]
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