Sorting describes the distribution of grain size of sediments, either in unconsolidated deposits or in sedimentary rocks. The degree of sorting is determined by the range of grain sizes in a sediment deposit and is the result of various transport processes (rivers, debris flow, wind, glaciers, etc.). This should not be confused with crystallite size, which refers to the individual size of a crystal in a solid. Crystallite is the building block of a grain.
The terms describing sorting in sediments – very poorly sorted, poorly sorted, moderately sorted, well sorted, very well sorted – have technical definitions and semi-quantitatively describe the amount of variance seen in particle sizes.Very poorly sorted indicates that the sediment sizes are mixed (large variance); whereas well sorted indicates that the sediment sizes are similar (low variance). In the field, sedimentologists use graphical charts to accurately describe the sorting of a sediment using one of these terms. [1]
Tangential sorting is the result of sediment being deposited in same direction as flow. Normal tangential sorting results in a gradient of sediment sizes deposited from largest to finest as they travel downstream. [2] When sediments are deposited from smallest to largest as they travel downstream, this is referred to as reverse sorting. [2]
Rocks derived from well sorted sediments are commonly both porous and permeable, while poorly sorted rocks have low porosity and low permeability, particularly when fine grained.
Sediment sorting is influenced by: grain sizes of sediment, processes involved in grain transport, deposition, and post-deposition processes such as winnowing. [3] As a result, studying the degree of sorting in deposits of sediment can give insight into the energy, rate, and/or duration of deposition, as well as the transport process responsible for laying down the sediment. [3]
In reference to windblown sediment, a wide range of conditions such as distance and height of transport and varying wind patterns at the sediment source can affect grain size, rate of transport and distribution of sediment. [3] Windblown sediment travels one of three ways--rolling, saltation or suspension in the air.
Loess that is reworked by fluvial processes tends to have more poorly sorted sediment as compared to sediment sorted by only Aeolian processes because loess particles become mixed with preexisting sediment of varying grain sizes within bodies of water. [3]
Sandstone is a clastic sedimentary rock composed mainly of sand-sized silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation; if buried, they may eventually become sandstone and siltstone through lithification.
A braided river consists of a network of river channels separated by small, often temporary, islands called braid bars or, in British English usage, aits or eyots.
Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil or as sediment mixed in suspension with water. Silt usually has a floury feel when dry, and lacks plasticity when wet. Silt can also be felt by the tongue as granular when placed on the front teeth.
In geography and geology, fluvial sediment processes or fluvial sediment transport are associated with rivers and streams and the deposits and landforms created by sediments. It can result in the formation of ripples and dunes, in fractal-shaped patterns of erosion, in complex patterns of natural river systems, and in the development of floodplains and the occurrence of flash floods. Sediment moved by water can be larger than sediment moved by air because water has both a higher density and viscosity. In typical rivers the largest carried sediment is of sand and gravel size, but larger floods can carry cobbles and even boulders. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvioglacial is used, as in periglacial flows and glacial lake outburst floods. Fluvial sediment processes include the motion of sediment and erosion or deposition on the river bed.
Deposition is the geological process in which sediments, soil and rocks are added to a landform or landmass. Wind, ice, water, and gravity transport previously weathered surface material, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of sediment.
Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.
Aeolian processes, also spelled eolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth. Winds may erode, transport, and deposit materials and are effective agents in regions with sparse vegetation, a lack of soil moisture and a large supply of unconsolidated sediments. Although water is a much more powerful eroding force than wind, aeolian processes are important in arid environments such as deserts.
Conglomerate is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts. A conglomerate typically contains a matrix of finer-grained sediments, such as sand, silt, or clay, which fills the interstices between the clasts. The clasts and matrix are typically cemented by calcium carbonate, iron oxide, silica, or hardened clay.
Parent material is the underlying geological material in which soil horizons form. Soils typically inherit a great deal of structure and minerals from their parent material, and, as such, are often classified based upon their contents of consolidated or unconsolidated mineral material that has undergone some degree of physical or chemical weathering and the mode by which the materials were most recently transported.
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks, and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic to refer to sedimentary rocks and particles in sediment transport, whether in suspension or as bed load, and in sediment deposits.
In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The original depositional layering is tilted, such tilting not being the result of post-depositional deformation. Cross-beds or "sets" are the groups of inclined layers, which are known as cross-strata.
In geology, ripple marks are sedimentary structures and indicate agitation by water or wind.
In geology, a graded bed is a bed characterized by a systematic change in grain or clast size from bottom to top of the bed. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into progressively finer ones. Such a bed is also described as fining upward. Normally graded beds generally represent depositional environments which decrease in transport energy as time passes, but these beds can also form during rapid depositional events. They are perhaps best represented in turbidite strata, where they indicate a sudden strong current that deposits heavy, coarse sediments first, with finer ones following as the current weakens. They can also form in terrestrial stream deposits.
Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural systems where the particles are clastic rocks, mud, or clay; the fluid is air, water, or ice; and the force of gravity acts to move the particles along the sloping surface on which they are resting. Sediment transport due to fluid motion occurs in rivers, oceans, lakes, seas, and other bodies of water due to currents and tides. Transport is also caused by glaciers as they flow, and on terrestrial surfaces under the influence of wind. Sediment transport due only to gravity can occur on sloping surfaces in general, including hillslopes, scarps, cliffs, and the continental shelf—continental slope boundary.
In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock record. In most cases, the environments associated with particular rock types or associations of rock types can be matched to existing analogues. However, the further back in geological time sediments were deposited, the more likely that direct modern analogues are not available.
Channel patterns are found in rivers, streams, and other bodies of water that transport water from one place to another. Systems of branching river channels dissect most of the sub-aerial landscape, each in a valley proportioned to its size. Whether formed by chance or necessity, by headward erosion or downslope convergence, whether inherited or newly formed. Depending on different geological factors such as weathering, erosion, depositional environment, and sediment type, different types of channel patterns can form.
Roundness is the degree of smoothing due to abrasion of sedimentary particles. It is expressed as the ratio of the average radius of curvature of the edges or corners to the radius of curvature of the maximum inscribed sphere.
Niveo-aeolian or cryo-aeolian deposition is the process by which fine-grained sediments are transported by wind and deposited on or mixed with snow or ice. The wind sweeps the snow and sand grains into aeolian landforms such as ripples, and further sorts the snow and ice grains into distinct layers. When snow melts or sublimates, the sediments are redeposited onto the surface below., forming patterns known as denivation features.
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