Sedimentary rock is a 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.
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 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 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 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.
Sedimentary rocks are classified into three groups. These groups are clastic, chemical precipitate and biochemical (or biogenic).
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 limestone 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 (in sandstones, this includes quartz arenites, arkoses, and lithic sandstones), 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.
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, 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 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 calcium carbonate 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 sedimentary rocks form when minerals in solution become supersaturated 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.
Sedimentary rocks are formed when sediment is deposited out of air, ice, wind, gravity, or water flows carrying the particles in suspension. This sediment often formed when weathering and erosion break down a rock into loose material in a source area. The material will then be transported from the source area to the area of deposition. The type of sediment that is transported to a place depends on the geology of the hinterland (the source area of the sediment). However, some sedimentary rocks, like evaporites, are composed of material that formed at the place of deposition. The nature of a sedimentary rock therefore not only depends on sediment supply, but also on the sedimentary depositional environment in which it formed.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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. 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.
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 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 quiet place. Other sedimentary environments are dominated by normal, ongoing sedimentation.
In some sedimentary environments, sedimentation only occurs in some places. In a desert, for example, the wind will deposit siliciclastic material (sand or silt) in some spots, or catastrophic flooding of a wadi can see the sudden deposition 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. Most deposited sediment will shortly after deposition be removed by erosion.
The term diagenesis is used to describe all the chemical, physical, and biological changes, including cementation, undergone by a sediment after its initial deposition, exclusive of surface weathering. Some of these processes cause the sediment to consolidate: a compact, solid substance forms out of loose material. Young sedimentary rocks, especially those of Quaternary age (the most recent period of the geologic time scale) are often still unconsolidated. As sediment deposition builds up, the overburden (or lithostatic) pressure rises and a process known as lithification takes place.
Sedimentary rocks are often saturated with seawater or groundwater, in which minerals can dissolve or from which minerals can precipitate. Precipitating minerals reduce the pore space in a rock, a process called cementation. Due to the decrease in pore space, the original connate fluids are expelled. The precipitated minerals form a cement and make the rock more compact and competent. In this way, loose clasts in a sedimentary rock can become "glued" together.
When sedimentation continues, an older rock layer becomes buried deeper as a result. The lithostatic pressure in the rock will increase due to the weight of the overlying sedimentary burden. This causes compaction, a process in which grains will reorganize themselves by mechanical means. Compaction is for example an important diagenetic process in clay, which can initially consist of 60% water. During compaction, this interstitial water is pressed out of the rock. Compaction can also be due to chemical processes, such as pressure solution. Pressure solution means material is going into solution at areas under high stress. The dissolved material precipitates again in open pore spaces, which menas there is a nett flow of material into the pores. However, in some cases a certain mineral will dissolve and not precipitate again. This process is called leaching and will increase the pore space in the rock.
Some biochemical processes, like the activity of bacteria, can affect minerals in a rock and are therefore seen as part of diagenesis. Fungi and plants (by their roots) and various other organisms that live beneath the surface can also influence diagenesis.
Burial of rocks due to ongoing sedimentation will lead to an increase in pressure and temperature, which stimulates certain chemical reactions. An example is the reactions by which organic material becomes lignite or coal. When temperature and pressure increase still further, the realm of diagenesis makes way for metamorphism, the process which forms a metamorphic rock.
The colour of a sedimentary rock is often mostly determined by iron, an element which has two major oxides: iron(II) oxide and iron(III) oxide. Iron(II) oxide only forms under anoxic circumstances and gives the rock a grey or greenish colour. Iron(III) oxide is often in the form of the mineral hematite 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 called red beds. However, a red colour does not necessarily mean the rock formed in a continental environment or arid climate.
The presence of organic material can colour a rock black or grey. Organic material is in nature formed from dead organisms, mostly plants. Normally such material will eventually decay by oxidation or the activity of bacteria. Under anoxic circumstances however organic material cannot decay and a dark sediment, rich in organic material forms. This can for example be the case at the bottom of deep seas and lakes. There is little current in the water in such environments, so that oxygen from surface waters will not be brought down and the deposited sediment will normally be a fine dark clay. Dark rocks rich in organic material are therefore often shales.
The size, form and orientation of clasts or minerals in a rock is called its texture. The texture is a small-scale property of a rock, but determined many of its large-scale properties, such as the density, porosity or permeabililty.
Clastic rocks have a 'clastic texture', which means they consist of clasts. The 3D orientation of these clasts is called the fabric of the rock. Between the clasts the rock can be composed of a matrix or a cement (the latter can consist of crystals of one or more precipitated minerals). The size and form of clasts can be used to determine the amount and direction of current in the sedimentary environment where the rock was formed. Fine calcareous mud will only settle in quiet water, while gravel can only deposited by water with large currents. The grain size of a rock is usually expressed with the Wentworth scale, though alternative scales are used sometimes. The grain size can be expressed as a diameter or a volume, and is always an average value - a rock is composed of clasts with different sizes. The statistical distribution of grain sizes is different for different rock types and is described in a property called the sorting of the rock. When all clasts are more or less of the same size, the rock is called 'well-sorted', when there is a large spread in grain size, the rock is called 'poorly sorted'.
The form of clasts can reflect the origin of the rock. 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:
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.
Most sedimentary rocks contain either quartz (especially siliciclastic rocks) or calcite (especially carbonate rocks). In contrast with igneous and metamorphic rocks, a sedimentary rocks usually contains very few different major minerals. However, the origin of the minerals in a sedimentary rock is often more complex than those in an igneous rock. Minerals in a sedimentary rock can have formed by precipitation during sedimentation or diagenesis. In the second case, the mineral precipitate can have grown over an older generation of cement. A complex diagenetic history can be studied by optical mineralogy, using a petrographic microscope.
Carbonate rocks dominantly consist of carbonate minerals like calcite, aragonite or dolomite. Both cement and clasts (including fossils and ooids) of a carbonate rock can consist of carbonate minerals. The mineralogy of a clastic rock is determined by the supplied material from the source area, the manner of transport to the place of deposition and the stability of a particular mineral. The stability of the major rock forming minerals (their resistance to weathering) is expressed by Bowen's reaction series. In this series, quartz is most stable, followed by feldspar, micas and other less stable minerals that will only be present when little weathering occurred. 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 there. In most sedimentary rocks, mica, feldspar and less stable minerals will have reacted to clay minerals like kaolinite, illite or smectite.
Structures in sedimentary rocks can be divided in 'primary' structures (formed during deposition) and 'secondary' structures (formed after deposition). Unlike textures, structures are always large-scale features that can easily be studied in the field. Sedimentary structures can tell something about the sedimentary environment or can serve to tell which side was originally facing up in case sedimentary layers have been tilted or overturned by tectonics.
Sedimentary rocks are laid down in layers called beds or strata. A bed is defined as a layer of rock that has a uniform lithology 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 called bedding. Single beds can be a couple of centimetres to several meters thick. Finer, less pronounced layers are called laminae and the structure it forms in a rock is called lamination. Laminae are usually less than a few centimetres thick. 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 called cross-bedding. Cross-bedding forms when small-scale erosion occurs during deposition, cutting off part of the beds. Newer beds will then form at an angle with older ones.
The opposite of cross-bedding is parallel lamination, where all sedimentary layering is parallel. With laminations, differences are generally caused by cyclic changes in the sediment supply, caused for example by seasonal changes in rainfall, temperature or biochemical activity. Laminae which represent seasonal changes (like tree rings) are called varves. Some rocks have no lamination at all, their structural character is called massive bedding.
Graded bedding is a structure in which 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 will settle first; smaller clasts follow later. Though graded bedding can form in many different environments, it is characteristic for turbidity currents.
The bedform (the surface of a particular bed) can be indicative for a particular sedimentary environment too. Examples of bed forms are scour marks, tool marks and ripple marks. Scour marks are hollow traces in the surface where sediment particles were taken into suspension by the flow. Tool marks are tracks of larger clasts rolling over the sedimentary surface in the direction of the flow. Both are often elongated structures and can be used to establish the direction of the flow during deposition.
Ripple marks also form in flowing water. There are two types: asymmetric wave ripples and symmetric current ripples. Environments where the current is in one direction, such as rivers, produce asymmetric ripples. The longer flank of such ripples is oriented opposite to the direction of the current. Wave ripples occur in environments where currents occur in all directions, such as tidal flats.
Another type of bed form are mud cracks, caused by the dehydration of sediment that occasionally comes above the water surface. Such structures are commonly found at tidal flats or point bars along rivers.
Sedimentary rocks are the only type of rock that can contain fossils, the remains or imprints of dead organisms. In nature, dead organisms are usually quickly removed by scavengers, bacteria, rotting and erosion. In some exceptional circumstances a carcass is fossilized because these natural processes are unable to work. The chance of fossilisation is higher when the sedimentation rate is high (so that a carcass is quickly buried), in anoxic environments (where little bacterial activity exists) or when the organism had a particularly hard skeleton. Larger, well-preserved fossils are relatively rare. Most sedimentary rocks contains fossils, though with many the fact only becomes apparent when studied under a microscope (microfossils) or with a loupe.
Fossils can both be the direct remains or imprints of organisms and their skeletons. Most commonly preserved are the harder parts of organisms such as bones, shells, woody tissue of plants. Soft tissue has a much smaller chance of being preserved and fossilized and soft tissue of animals older than 40 million years is very rare. Imprints of organisms made while still alive are called trace fossils. Examples are burrows, foot prints, etc.
Being part of a sedimentary rock, fossils will experience the same diagenetic processes as the rock. A shell consisting of calcite can for example dissolve, while a cement of silica then fills the cavity. In the same way, precipitating minerals can fill cavities formerly occupied by blood vessels, vascular tissue or other soft tissues. This preserves the form of the organism but changes the chemical composition, a process called permineralisation. The most common minerals in permineralisation cements are carbonates (especially calcite), forms of amorphous silica (chalcedony, flint, chert) and pyrite. In the case of silica cements, the process is called lithification.
At high pressure and temperature, the organic material of a dead organism will experience chemical reactions in which volatiles like water and carbon dioxide are expulsed. The fossil will in the end consist of a thin layer of pure carbon or its mineralized form, graphite. This form of fossilisation is called carbonisation. It is particularly important for plant fossils. The same process is responsible for the formation of fossil fuels like lignite or coal.
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 the 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.
These types of rock formations are generally porus, in that fluids can flow in between the rock particles and the binding matrics. This is the basis of ground water as opposed to a cavity within a rock substrate.
All types of sediment are classed as unconsoldated in that they can move indivdually and are not bound to one another. There are three different classifications of sediment and these are classed according to size in millimeters in diameter, with sub-categories.
Transportation in the geological sense is the mass movement of unconsolidated material, using the force of gravity the material can be shifted from from a high elevation to a lower elevation, and sometines in the reverse. This movement is not precise and smaller fragments can be moved a greater distance than larger material. This is because the energy required to move a large piece of say rock is greater and can be overcome quicker than the energy required to move silt.
A fragment of weathered rock would in the right circumstance be transported via a river system, glacier or waves down to a lower elevation. During this transportation the size of the rock would be sorted into the size categorization above. When the transportation body has more energy the size and amount of the material carried would increase.
During the process, the material goes through a process called Rounding. This is the grinding away of sharp edges of a rock to form rounded corners. The process involves a rock material hitting another equally hard or harder rock surface. This occurs in sand or gravel.
Sorting is the process where the sediment is selected and seperated into different size, grain shape and specific gravity. Shallow marine shoreline systems are the most effective at sorting material, as the sediment is continuously reworked. High energy shoreline systems can move larger sediemnt and may therefore produce rock, pebble and sand beaches. Sediment from beach environments are characteristically well sorted, well rounded and are composed of 90% or more quartz. Glaciers are poor at sorting the material because of their high viscosity and manner of flow, as all size rocks are trasnported together till the terminus of glacier is reached and once it has retreated form terminal moraine
When a transporting fluid body like wind, a river, a wave or a glacier looses momentum, it looses energy. Once the kinetic energy decreases below that required to sustain a certain weight and/or friction, a stone over the energy weight to sustain its' suspention in the transporting body will drop out of circulation down to surface level. Also deposition can refer to organic matter, as it falls and accumulates onto a surface area. Another type of deposition is the precipitation os salts out of a solution. There is deposition that depends on the shape or specific gravity of the object, that can also be affected by kinetic energy of the transporting body.
This is the group of processes that turns loose sediment into sedimentary rock. First there is the deposition of loose sedimentary grains, during this process the material is compacted as more sediment is put on top. The initial pore space for slow moving sediment can be about 40-50 percent of the sediment, in faster moving sediment this is reduced. Compaction reduces this area by decreasing the pore space as it squeezes water or air from the matrix. After compaction cementation usually occurs and makes the sedimentary rock harder and stable. The most prominant cement is calcite (CaCO3) which may be produced from chemical weathering of the sediment itself, or transported from a distant site from water percolating through the rock.
Clastic rocks are grained rock which may or may not be filled with a matrics. The largest of these include coarse grainedw:Breccia and w:Conglomerate types of rocks, usually filled in with 15% or less of matrics of sand and finer particles. As with the nature and size of the material, the source rock was close to the locality of deposition. For Breccia as there is little rounding of the rock fragments, the source material was in the immediate vacinity. Deposition could have been the base of a cliff or another locality where transportation from the immediate area is not likely to occure.
For conglomerates the material has suffered rounding indicating transportation. As the size of the material is quiet large, the area of deposition is still close to the area of erosion, generally no more than a few kilometres.
w:Sandstone rocks are made of mainly quartz and/or feldspar minerals. Because of the size of the material, there is consderable transportion from the parient material. This material is more dominate as sediment in slow flowing rivers, lakes, streams the that have a lower energy factor. But the sand dominance depends on the energy contained within the transporting water course. Sand is also a major consitution of beach material along with organic rocks.
Major types of rock inclue those made by the accumulation of w:foraminifera, w:coral, w:bivalves and W:brachiopods, w:crinoids just to name a few. Generally vertebrate remains do not consolidate into rock alone, usually sand, clay etc or other organism makes up the rest of the matrixs and/or cement.
A chemical rock is one that has been precipitated from a solution, these include rock salts and some limestones. As a water body has a reduced inflow of water, through friction and evaporation it tends to loose that water. As the concentration of suspended chemicals and other material increases it tends to perculate out of the solution if concentrations of that chamical become oversaturated. During this chemical formation into crystals, some other material eg,. plant remains, shell or silt can become trapped within the crystals that are forming. This accumulation of crystals and other material over a long period, or even if it only occured once and covered with another type sedimentary material can form a layer within a rock formation.
Gravel (largest fragment in this photo is about 4 cm) from Thirasia, Santorini, Greece.
Close-up of sand from a beach in Vancouver, showing a surface area of (approximately) between 1-2 square centimetres.
Quaternary clay in Estonia (400 000 years old).
Sediments are usually formed from matter which falls to the bottom of oceans and lakes. The matter includes tiny pieces of other rocks, and dead animals, plants and microorganisms. Also, inorganic chemicals may be precipitated from solution in the water.
Squeezed and compressed over time, the sediments become 'consolidated' (made solid) into layers of rock.
Otherwise, rocks may have the water squeezed out, yet remain for long periods with their character unchanged. There are many quarries of unconsolidated deposits where sand and pebbles are removed for the construction industry. Such quarries may be taking out sand which has lasted as sand since the Jurassic period. But consolidated sand, sandstone, can be a very hard rock.
Consolidated rocks mady be changed some more, by water seeping through, or by heat and extreme pressure.
Sediments are formed by three processes: 1. Solids swept down from the land. 2. Bits & pieces laid down in water, e.g. shells. 3. Chemicals in solution, which may be precipitated. These sedimentary processes – weathering, transportation, and deposition – reach three final end products: quartz sand, shale (from clay), and limestone (CaCO3). Most sedimentary rocks are variations on this general pattern.
Chalk, limestone, and dolomite are all basically made from calcium carbonate. This comes from a mixture of minerals and pieces of animals (especially animal shells). They are mostly formed in oceans. Shales, sandstones, and conglomerates are all clastic rocks. They are made from pieces of other rocks. The pieces may have come from erosion by water, ice or wind. Coal is made from ancient plants; oil and natural gas is also organic in origin.
Sedimentary rocks may be found anywhere on Earth. When sedimentary rocks are heated and squeezed, they become metamorphic rocks. Igneous rocks have a volcanic origin. Over a very long time, rocks get recycled, in two ways. When marine sediments are raised above sea level, they get weathered, and the pieces carried down to the sea. On a much longer time-scale continental plates may collide. Then one plate goes under the other (is subducted), and all its material is recycled, emerging much later.
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