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Related questions What is oxidation and how does it result in weathering? What is dissolution and how does this result in weathering? What is frost wedging? To be the provider of geoscience data globally Navigation Main content Bottom links. What is geology? Rocks and minerals. Fossils and dinosaurs. Geology around the world. Sediment grains smaller than sand are called silt.
Silt is unique; the grains can be felt with a finger or as grit between your teeth, but are too small to see with the naked eye. Sorting describes the range of grain sizes within sediment or sedimentary rock. It is important to note that soil A type of non-eroded sediment mixed with organic matter, used by plants. When reading the story told by rocks, geologists use sorting to interpret erosion or transport processes, as well as deposition energy. For example, wind-blown sands are typically extremely well sorted, while glacial deposits are typically poorly sorted.
These characteristics help identify the type of erosion process that occurred. Coarse-grained sediment and poorly sorted rocks are usually found nearer to the source of sediment , while fine sediments are carried farther away. In a rapidly flowing mountain stream you would expect to see boulders and pebbles. In a lake fed by the stream , there should be sand and silt deposits. If you also find large boulders in the lake, this may indicate the involvement of another sediment transport process, such as rockfall caused by ice- or root-wedging.
Rounding is created when angular corners of rock fragments are removed from a piece of sediment due to abrasion during transport. Well-rounded sediment grains are defined as being free of all sharp edges.
Very angular sediment retains the sharp corners. More rounded grains imply a longer erosion time or transport distance, or more energetic erosional process. Mineral hardness is also a factor in rounding. Composition describes the mineral components found in sediment or sedimentary rock and may be influenced by local geology, like source rock and hydrology.
Other than clay, most sediment components are easily determined by visual inspection see Chapter 3, Minerals. The most commonly found sediment mineral is quartz because of its low chemical reactivity and high hardness , making it resistant to weathering , and its ubiquitous occurrence in continental bedrock. Other commonly found sediment grains include feldspar and lithic fragments.
Lithic fragments are pieces of fine-grained bedrock , and include mud chips , volcanic clasts, or pieces of slate. This is because the local rock is composed almost entirely of basalt and provides an abundant source of dark colored clasts loaded with mafic minerals.
According to the Goldich Dissolution Series , clasts high in mafic minerals are more easily destroyed compared to clasts composed of felsic minerals like quartz. Geologists use provenance to discern the original source of sediment or sedimentary rock.
Provenance is determined by analyzing mineral composition and types of fossils present, as well as textural features like sorting and rounding. In quartz sandstone , sometimes called quartz arenite SiO 2 , provenance may be determined using a rare, durable clast mineral called zircon ZrSiO 4. Zircon , or zirconium silicate , contains traces of uranium, which can be used for age-dating the source bedrock that contributed sediment to the lithified sandstone rock see Chapter 7, Geologic Time.
Clastic rocks are classified according to the grain size of their sediment. Coarse-grained rocks contain clasts with a predominant grain size larger than sand. Typically, smaller sediment grains, collectively called groundmass or matrix, fill in much of the volume between the larger clasts, and hold the clasts together. Conglomerates are rocks containing coarse rounded clasts, and breccias contain angular clasts see figure.
Both conglomerates and breccias are usually poorly sorted. Medium-grained rocks composed mainly of sand are called sandstone , or sometimes arenite if well sorted. Sediment grains in sandstone can having a wide variety of mineral compositions, roundness, and sorting. Quartz sandstone contains predominantly quartz sediment grains. Sandstone that contains feldspar , which weathers more quickly than quartz , is useful for analyzing the local geologic history. Greywack e is a term with conflicting definitions.
Greywacke may refer to sandstone with a muddy matrix, or sandstone with many lithic fragments small rock pieces. Fine-grained rocks include mudstone , shale , siltstone , and claystone.
Mudstone is a general term for rocks made of sediment grains smaller than sand less than 2 mm. Rocks that are fissile , meaning they separate into thin sheets, are called shale. Rocks exclusively composed of silt or clay sediment , are called siltstone or claystone , respectively.
These last two rock types are rarer than mudstone or shale. Rock types found as a mixture between the main classifications, may be named using the less-common component as a descriptor. For example, a rock containing some silt but mostly rounded sand and gravel is called silty conglomerate.
Sand-rich rock containing minor amounts of clay is called clayey sandstone. Chemical sedimentary rocks are formed by processes that do not directly involve mechanical weathering and erosion. Chemical weathering may contribute the dissolved materials in water that ultimately form these rocks. Biochemical and organic sediments are clastic in the sense that they are made from pieces of organic material that is deposited, buried, and lithified; however, they are usually classified as being chemically produced.
Inorganic chemical sedimentary rocks are made of minerals precipitated from ions dissolved in solution , and created without the aid of living organisms. Inorganic chemical sedimentary rocks form in environments where ion An atom or molecule that has a charge positive or negative due to the loss or gain of electrons. Biochemical sedimentary rocks are formed from shells and bodies of underwater organisms.
The living organisms extract chemical components from the water and use them to build shells and other body parts. The components include aragonite, a mineral similar to and commonly replaced by calcite , and silica. Organic sedimentary rocks come from organic material that has been deposited and lithified, usually underwater.
The source materials are plant and animal remains that are transformed through burial and heat, and end up as coal Former swamp-derived plant material that is part of the rock record. Inorganic chemical sedimentary rocks are formed when minerals precipitate out of an aqueous solution , usually due to water evaporation.
The precipitate minerals form various salts known as evaporites. For example, the Bonneville Salt Flats in Utah flood with winter rains and dry out every summer, leaving behind salts such as gypsum and halite. The deposition order of evaporites deposit is opposite to their solubility order, i.
The deposition order and saturation percentages are depicted in the table, bearing in mind the process in nature may vary from laboratory derived values. Table after. Calcium carbonate - saturated water precipitates porous masses of calcite called tufa Porous variety of carbonate that form in relatively unheated water, sometimes as towers and spires. Waterfalls downstream of springs often precipitate tufa Porous variety of carbonate that form in relatively unheated water, sometimes as towers and spires.
Saline lakes concentrate calcium carbonate from a combination of wave action causing degassing, springs in the lakebed, and evaporation.
In salty Mono Lake in California, tufa Porous variety of carbonate that form in relatively unheated water, sometimes as towers and spires. Cave deposits like stalactites and stalagmites are another form of chemical precipitation of calcite , in a form called travertine. Calcite slowly precipitates from water to form the travertine , which often shows banding.
This process is similar to the mineral growth on faucets in your home sink or shower that comes from hard mineral rich water. Oxygenation of the atmosphere and oceans caused free iron ions, which are water-soluble, to become oxidized and precipitate out of solution. The iron oxide was deposited, usually in bands alternating with layers of chert. Chert , another commonly found chemical sedimentary rock, is usually produced from silica SiO 2 precipitated from groundwater.
Silica is highly insoluble on the surface of Earth, which is why quartz is so resistant to chemical weathering. Water deep underground is subjected to higher pressures and temperatures, which helps dissolve silica into an aqueous solution.
As the groundwater rises toward or emerges at the surface the silica precipitates out, often as a cementing agent or into nodules. For example, the bases of the geysers in Yellowstone National Park are surrounded by silica deposits called geyserite or sinter.
The silica is dissolved in water that is thermally heated by a relatively deep magma source. Chert can also form biochemically and is discussed in the Biochemical subsection. Chert has many synonyms, some of which may have gem value such as jasper, flint, onyx, and agate, due to subtle differences in colors, striping, etc.
Ooid refers to the sphere, oolite the rock with the spheres. When water is oversaturated with calcite , the mineral precipitates out around a nucleus, a sand grain or shell fragment, and forms little spheres called ooid Spheres of calcite that form in saline waters with slight wave agitation. As evaporation continues, the ooid Spheres of calcite that form in saline waters with slight wave agitation.
Biochemical sedimentary rocks are not that different from chemical sedimentary rocks; they are also formed from ions dissolved in solution. However, biochemical sedimentary rocks rely on biological processes to extract the dissolved materials out of the water.
Most macroscopic marine organisms use dissolved minerals , primarily aragonite calcium carbonate , to build hard parts such as shells. When organisms die the hard parts settle as sediment , which become buried, compacted and cemented into rock. This biochemical extraction and secretion is the main process for forming limestone , the most commonly occurring, non- clastic sedimentary rock. Solid calcite reacts with hydrochloric acid by effervescing or fizzing.
Dolomite only reacts to hydrochloric acid when ground into a powder, which can be done by scratching the rock surface see Chapter 3, Minerals. Limestone occurs in many forms, most of which originate from biological processes. Entire coral reef A topographic high found away from the beach in deeper water, but still on the continental shelf. Typically, these are formed in tropical areas by organisms such as corals. Fossiliferous limestone contains many visible fossils.
A type of limestone called coquina originates from beach sands made predominantly of shells that were then lithified. Coquina is composed of loosely-cemented shells and shell fragments. You can find beaches like this in modern tropical environments, such as the Bahamas.
Chalk contains high concentrations of shells from a microorganism called a coccolithophore. Micrite , also known as microscopic calcite mud, is a very fine-grained limestone containing microfossils that can only be seen using a microscope. Biogenetic chert forms on the deep ocean floor , created from biochemical sediment made of microscopic organic shells.
This sediment , called ooze, may be calcareous calcium carbonate based or siliceous silica-based depending on the type of shells deposited.
For example, the shells of radiolarians zooplankton and diatoms phytoplankton are made of silica, so they produce siliceous ooze. Under the right conditions, intact pieces of organic material or material derived from organic sources, is preserved in the geologic record.
Although not derived from sediment , this lithified organic material is associated with sedimentary strata and created by similar processes—burial, compaction , and diagenesis. C Deposits of these fuels develop in areas where organic material collects in large quantities. Lush swamplands can create conditions conducive to coal Former swamp-derived plant material that is part of the rock record. Shallow-water, organic material-rich marine sediment can become highly productive petroleum and natural gas deposits.
See Chapter 16, Energy and Mineral Resources, for a more in-depth look at these fossil -derived energy sources. In contrast to detrital sediment , chemical, biochemical , and organic sedimentary rocks are classified based on mineral composition.
Most of these are monomineralic, composed of a single mineral , so the rock name is usually associated with the identifying mineral. Chemical sedimentary rocks consisting of halite are called rock salt. Rocks made of Limestone calcite is an exception, having elaborate subclassifications and even two competing classification methods: Folk Classification and Dunham Classification.
The Folk Classification deals with rock grains and usually requires a specialized, petrographic microscope. The Dunham Classification is based on rock texture , which is visible to the naked eye or using a hand lens and is easier for field applications.
Most carbonate geologists use the Dunham system. Sedimentary rock identification chart. Clastic or detrital rocks are categorized based on their grain size i. Conglomerates are rounded, and breccias are angular. All chemical rocks are names based on composition i. Shale is the fissile , very fine grained sedimentary rock and splits easily into thin layers.
What is the most likely cause of a detrital sediment with highly rounded grains? The general rule of thumb is: the longer the transport distance, the more the rounding. Which of the following is a biochemical sedimentary rock? Coquina , chalk , and fossiliferous limestone are forms of biochemical rocks since their components are precipitated by organisms.
Shale and sandstone are detrital even if they include fossils , banded iron formation is chemical, and coal Former swamp-derived plant material that is part of the rock record. Sedimentary structures are visible textures or arrangements of sediments within a rock. Geologists use these structures to interpret the processes that made the rock and the environment in which it formed.
They use uniformitarianism to usually compare sedimentary structures formed in modern environments to lithified counterparts in ancient rocks. Below is a summary discussion of common sedimentary structures that are useful for interpretations in the rock record. The most basic sedimentary structure is bedding planes , the planes that separate the layers or strata in sedimentary and some volcanic rocks. Visible in exposed outcroppings, each bedding plane indicates a change in sediment deposition conditions.
This change may be subtle. For example, if a section of underlying sediment firms up, this may be enough to create a form a layer that is dissimilar from the overlying sediment. Each layer is called a bed A specific layer of rock with identifiable properties. As would be expected, bed A specific layer of rock with identifiable properties.
Technically, a bed A specific layer of rock with identifiable properties. A layer thinner than 1 cm 0. Varves are bedding planes created when laminae and bed A specific layer of rock with identifiable properties. Varves are valuable geologic records of climatic histories, especially those found in lakes and glacial deposits.
Graded bedding refers to a sequence of increasingly coarse- or fine-grained sediment layers. Graded bedding often develops when sediment deposition occurs in an environment of decreasing energy. A Bouma sequence is graded bedding observed in clastic rock called turbidite. Bouma sequence bed A specific layer of rock with identifiable properties.
These subsea density flows begin when sediment is stirred up by an energetic process and becomes a dense slurry of mixed grains. The sediment flow courses downward through submarine channels and canyons due to gravity acting on the density difference between the denser slurry and less dense surrounding seawater.
As the flow reaches deeper ocean basins it slows down, loses energy, and deposits sediment in a Bouma sequence of coarse grains first, followed by increasingly finer grains see figure.
In fluid systems, such as moving water or wind, sand is the most easily transported and deposited sediment grain. Smaller particles like silt and clay are less movable by fluid systems because the tiny grains are chemically attracted to each other and stick to the underlying sediment. Under higher flow rates, the fine silt and clay sediment tends to stay in place and the larger sand grains get picked up and moved.
Bedforms are sedimentary structures created by fluid systems working on sandy sediment. Grain size , flow velocity, and flow regime or pattern interact to produce bedforms having unique, identifiable physical characteristics. Flow regimes are divided into upper and lower regimes, which are further divided into uppermost, upper, lower, and lowermost parts.
The table below shows bedforms and their associated flow regimes. For example, the dune A large pile of sediment, deposited perpendicular to flow. Internal bedding in dunes dips toward flow direction i. Formed in the upper part of the lower flow regime. Plane bed A specific layer of rock with identifiable properties. The flat, parallel layers form as sandy sediment piles and move on top of layers below. Even non-flowing fluid systems, such as lakes, can produce sediment plane bed A specific layer of rock with identifiable properties.
They may look identical to lower-flow-regime bed A specific layer of rock with identifiable properties. Ripples are known by several names: ripple marks, ripple cross bed A specific layer of rock with identifiable properties. The ridges or undulations in the bed A specific layer of rock with identifiable properties. With the exception of dune A large pile of sediment, deposited perpendicular to flow. Occasionally, large flows like glacial lake outbursts, can produce ripples as tall as 20 m 66 ft.
First scientifically described by Hertha Ayrton, ripple shapes are determined by flow type and can be straight-crested, sinuous, or complex. Asymmetrical ripples form in a unidirectional flow. Symmetrical ripples are the result of an oscillating back-and-forth flow typical of intertidal swash zones. Climbing ripples are created from high sedimentation rates and appear as overlapping layers of ripple shapes see figure.
Cross bedding happens when ripples or dune A large pile of sediment, deposited perpendicular to flow. Desert sand dune A large pile of sediment, deposited perpendicular to flow. British geologist Agnold considered only Barchan and linear Seif dune A large pile of sediment, deposited perpendicular to flow.
Other workers have recognized transverse and star dunes as well as parabolic and linear dunes anchored by plants that are common in coastal areas as other types of dune A large pile of sediment, deposited perpendicular to flow.
The biggest difference between river dune A large pile of sediment, deposited perpendicular to flow. Some famous air-formed dune A large pile of sediment, deposited perpendicular to flow. As airflow moves sediment along, the grains accumulate on the dune A large pile of sediment, deposited perpendicular to flow. The angle of the windward side is typically shallower than the leeward downwind side, which has grains falling down over it.
This difference in slopes can be seen in a bed A specific layer of rock with identifiable properties. There are typically two styles of dune A large pile of sediment, deposited perpendicular to flow. In tidal locations with strong in-and-out flows, dune A large pile of sediment, deposited perpendicular to flow. This produces a feature called herringbone cross bedding. Another dune A large pile of sediment, deposited perpendicular to flow. These bed A specific layer of rock with identifiable properties.
Antidunes are so named because they share similar characteristics with dune A large pile of sediment, deposited perpendicular to flow. While dune A large pile of sediment, deposited perpendicular to flow.
Antidunes form in phase with the flow; in rivers they are marked by rapids in the current. Antidunes are rarely preserved in the rock record because the high flow rates needed to produce the bed A specific layer of rock with identifiable properties. Bioturbation is the result of organisms burrowing through soft sediment , which disrupts the bedding layers. These tunnels are backfilled and eventually preserved when the sediment becomes rock.
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