A sediment gravity flow is one of several types of sediment transport mechanisms, of which most geologists recognize four principal processes. These flows are differentiated by their dominant sediment support mechanisms, [1] [2] which can be difficult to distinguish as flows can be in transition from one type to the next as they evolve downslope. [3]
Sediment gravity flows are represented by four different mechanisms of keeping grains within the flow in suspension.
Although the deposits of all four types of sediment support mechanisms are found in nature, pure grain flows are largely restricted to aeolian settings, whereas subaqueous environments are characterized by a spectrum of flow types with debris flows and mud flows on one end of the spectrum, and high-density and low-density turbidity currents on the other end. It is also useful in subaqueous environments to recognize transitional flows that are in between turbidity currents and mud flows. The deposits of these transitional flows are referred to by a variety of names, some of the more popular being "hybrid-event beds (HEB)", linked debrites" and "slurry beds". [7] Powder snow avalanches and glowing avalanches (gas-charged flows of super heated volcanic ash) are examples of turbidity currents in non-marine settings.
Modern and ancient (outcrop) examples of deposits resulting from different types of sediment gravity flows.
Sediment gravity flows, primarily turbidity currents, but to a lesser extent debris flows and mud flows, are thought to be the primary processes responsible for depositing sand on the deep ocean floor. Because anoxic conditions at depth in the deep oceans are conducive to the preservation of organic matter, which with deep burial and subsequent maturation through the absorption of heat can generate oil and gas, the deposition of sand in deep ocean settings can ultimately juxtapose petroleum reservoirs and source rocks. In fact, a significant portion of the oil and gas produced in the world today is found in deposits (reservoirs) originating from sediment gravity flows. [9]
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.
Conglomerate is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts. A conglomerate typically contain 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.
A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean.
Greywacke or graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found in Paleozoic strata. The larger grains can be sand- to gravel-sized, and matrix materials generally constitute more than 15% of the rock by volume. The term "greywacke" can be confusing, since it can refer to either the immature aspect of the rock or its fine-grained (clay) component.
A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. Turbidity currents can also occur in other fluids besides water.
A way up structure, way up criterion, or geopetal indicator is a characteristic relationship observed in a sedimentary or volcanic rock, or sequence of rocks, that makes it possible to determine whether they are the right way up or have been overturned by subsequent deformation. This technique is particularly important in areas affected by thrusting and where there is a lack of other indications of the relative ages of beds within the sequence, such as in the Precambrian where fossils are rare.
The Bouma Sequence describes a classic set of sedimentary structures in turbidite beds deposited by turbidity currents at the bottoms of lakes, oceans and rivers.
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 with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.
Debris flows are geological phenomena in which water-laden masses of soil and fragmented rock rush down mountainsides, funnel into stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors. They generally have bulk densities comparable to those of rock avalanches and other types of landslides, but owing to widespread sediment liquefaction caused by high pore-fluid pressures, they can flow almost as fluidly as water. Debris flows descending steep channels commonly attain speeds that surpass 10 m/s (36 km/h), although some large flows can reach speeds that are much greater. Debris flows with volumes ranging up to about 100,000 cubic meters occur frequently in mountainous regions worldwide. The largest prehistoric flows have had volumes exceeding 1 billion cubic meters. As a result of their high sediment concentrations and mobility, debris flows can be very destructive.
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, a graded bed is one characterized by a systematic change in grain or clast size from one side of the bed to the other. 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.
A subaqueous fan is a fan-shaped deposit formed beneath water, and are commonly related to glaciers and crater lakes.
Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition.
A contourite is a sedimentary deposit commonly formed on continental rise to lower slope settings, although they may occur anywhere that is below storm wave base. Countourites are produced by thermohaline-induced deepwater bottom currents and may be influenced by wind or tidal forces. The geomorphology of contourite deposits is mainly influenced by the deepwater bottom-current velocity, sediment supply, and seafloor topography.
A dish structure is a type of sedimentary structure formed by liquefaction and fluidization of water-charged soft sediment either during or immediately following deposition. Dish structures are most commonly found in turbidites and other types of clastic deposits that result from subaqueous sediment gravity flows.
A powder snow avalanche is a type of avalanche where snow grains are largely or completely suspended and moved by air in a state of fluid turbulence. They are particle-laden gravity currents and closely related to turbidity currents, pyroclastic flows from volcanoes and dust storms in the desert. The turbulence is typically generated by the forward motion of the current along the lower boundary of the domain, the motion being in turn driven by the action of gravity on the density difference between the particle-fluid mixture and the ambient fluid. The ambient fluid is generally of similar composition to the interstitial fluid, and is water for turbidity currents and air for avalanches. These flows are non-conservative in that they may exchange particles at the lower boundary by deposition or suspension, and may exchange fluid with the ambient by entrainment or detrainment. Such flows dissipate when the turbulence can no longer hold the particles in suspension and they are deposited on the lower boundary. When the turbulence is strong enough to suspend new material from the bed or the underlying dense flow then current is said to be auto-suspending. Particle concentrations in the suspension cloud are usually sufficiently low that particle-particle interactions play a small or negligible role in maintaining the suspension. In powder snow avalanches, even at these low concentrations, the extra density of the suspended particles is large relative to that of air, so the Boussinesq approximation, where density differences are considered negligible in inertia terms, is invalid, so that the snow grains carry most of the flows momentum. This is in contrast to turbidity currents and laboratory experiments in water where the extra inertia of the particles can usually be neglected. Nonetheless, due to the extreme difficulty in estimating particle concentrations in natural flows there remains considerable uncertainty—and debate—concerning the particle loading in large submarine turbidity currents and the validity of the Boussinesq approximation.
Rip-up clasts are gravel-size pieces of clay or mud created when an erosive current flows over a bed of clay or mud and removes pieces of clayey sediment, and transports them some distance. Because clayey sediments can be quite cohesive, even when freshly deposited, large clasts of clayey sediment can be ripped up, transported and subsequently preserved when the eroding current finally deposits its sediment. After deposition and deep burial by the accumulation of additional sediments, diagenesis transforms the gravel-size pieces of clayey sediment into shale or mudstone rip-up clasts. Shale rip-up clasts are often found at the base of sandy turbidites, in lag deposits at the base of channelized sandstones, and associated with subaqueous dunes and bars.
The Lowe sequence describes a set of sedimentary structures in turbidite sandstone beds that are deposited by high-density turbidity currents. It is intended to complement, not replace, the better known Bouma sequence, which applies primarily to turbidites deposited by low-density turbidity currents.
A grain flow is a type of sediment-gravity flow in which the fluid can be either air or water, acts only as a lubricant, and grains within the flow remain in suspension due to grain-to-grain collisions that generate a dispersive pressure to prevent further settling. Grain flows are very common in aeolian settings as grain avalanches on the slip faces of sand dunes. By contrast, pure grain flows are rare in subaqueous settings, where the grains in a flow are generally held in suspension dominantly by traction, saltation, fluid turbulence and/or grain buoyancy when the grains are floating in the clay matrix of a mud flow. However, grain-to-grain collisions are very important as a contributing process of sediment support in subaqueous, sand-rich, high-density turbidity currents. The high concentrations of sand that develop at the base of high-density flows brings grains close enough together that frequent grain-to grain collisions are inevitable and result in layers of sediment that are inverse graded as the smaller grains are able to fall in between larger grains and settle out beneath them.
Liquefied flows are types of sediment-gravity flows in which grains within the flow are kept in suspension by the upward movement of fluid. They form in granular substances where the concentration of suspended mud is too low to develop cohesive forces within the flow. As grains at the base of the suspension settle out, fluid that is displaced upward by the settling generates pore fluid pressures that can help suspend grains in the upper part of the flow. Application of an external pressure to the suspension will initiate flow. This external pressure can be applied by a seismic shock, which may turn transform loose sand into a highly viscous suspension as in quicksand. Generally as soon as the flow begins to move, fluid turbulence results and the flow rapidly evolves into a turbidity current. Flows and suspensions are said to be liquefied when the grains settle downward through the fluid and displace the fluid upwards. By contrast, flows and suspensions are said to fluidized when the fluid moves upward through the grains, thereby temporarily suspending them. Most flows are liquefied, and many references to fluidized sediment gravity flows are in fact incorrect and actually refer to liquified flows. Because fluid is displaced upward in these types of flows, dewatering features such as dish structures, pillars, pipes and dikes are common.
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