Alluvial fan
An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but are also found in more humid environments subject to intense rainfall and in areas of modern glaciation. They range in area from less than to almost.
Alluvial fans typically form where a flow of sediment or rocks emerge from a confined channel and are suddenly free to spread out in many directions. For example, many alluvial fans form when steep mountain valleys meet a flat plain. The transition from a narrow channel to a wide open area reduces the carrying capacity of flow and results in deposition of sediments. The flow can take the form of infrequent debris flows like in a landslide, or can be carried by an intermittent stream or creek.
The reduction of flow is key to the formation of alluvial fans. If a river exits a mountain valley without any reduction in flow, it is more common to see the formation of an alluvial plain. The steepness of an alluvial formation depends on how much flow decreases when entering flat ground as sediment will be deposited further away from its source if river flow is high.
Alluvial fans are most commonly found at the foot of desert mountains, such as in the Great Basin of western North America, in the New Red Sandstone of south Devon, or all across the major population centers of Xinjiang in the Taklamakan Desert and Junggar Basin.
Alluvial fans are not unique to Earth, as they are simply a result of gravity and geometry, and thus have also been found abundantly on Mars and Titan, showing that fluvial processes have occurred on other worlds.
Some of the largest alluvial fans are found along the Himalaya mountain front on the Indo-Gangetic Plain. A shift of the feeder channel can lead to catastrophic flooding, as occurred on the Kosi River fan in 2008.
Description
An alluvial fan is an accumulation of sediments that fans out from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. This accumulation is shaped like a section of a shallow cone, with its apex at the source of sediments.Alluvial fans vary greatly in size, from only a few meters across at the base to as much as 150 kilometers across, with a slope of 1.5 to 25 degrees. Some giant alluvial fans have areas of almost. The slope measured from the apex is generally concave, with the steepest slope near the apex and becoming less steep further out and shallowing at the edges of the fan. Sieve deposits, which are lobes of coarse gravel, may be present on the proximal fan. The sediments in an alluvial fan are usually coarse and poorly sorted, with the coarsest sediments found on the proximal fan.
Image:Gigantic Alluvial Fan Being Uplift by New Fault.jpg|thumb|Large alluvial fan in Death Valley showing a "toe-trimmed" profile |alt=refer to caption
When there is enough space in the alluvial plain for all of the sediment deposits to fan out without contacting other valley walls or rivers, an unconfined alluvial fan develops. Unconfined alluvial fans allow sediments to naturally fan out, and the shape of the fan is not influenced by other topological features. When the alluvial plain is more restricted, so that the fan comes into contact with topographic barriers, a confined fan is formed.
Wave or channel erosion of the edge of the fan sometimes produces a "toe-trimmed" fan, in which the edge of the fan is marked by a small escarpment. Toe-trimmed fans may record climate changes or tectonic processes, and the process of lateral erosion may enhance the aquifer or petroleum reservoir potential of the fan. Toe-trimmed fans on the planet Mars provide evidence of past river systems.
When numerous rivers and streams exit a mountain front onto a plain, the fans can combine to form a continuous apron. This is referred to as a bajada or piedmont alluvial plain.
Formation
Alluvial fans usually form where a confined feeder channel exits a mountain front or a glacier margin. As the flow exits the feeder channel onto the fan surface, it is able to spread out into wide, shallow channels or to infiltrate the surface. This reduces the carrying power of the flow and results in deposition of sediments.Image:Alluvial fan, Taklimakan Desert, XinJiang Province, China, NASA, ASTER.jpg|thumb|right|Alluvial fan in the Taklamakan Desert in Xinjiang showing active left and inactive right sectors |alt=refer to caption
Flow in the proximal fan, where the slope is steepest, is usually confined to a single channel, which may be up to deep. This channel is subject to blockage by accumulated sediments or debris flows, which causes flow to periodically break out of its old channel and shift to a part of the fan with a steeper gradient, where deposition resumes. As a result, normally only part of the fan is active at any particular time, and the bypassed areas may undergo soil formation or erosion.
Alluvial fans can be dominated by debris flows or stream flow. Which kind of fan is formed is controlled by climate, tectonics, and the type of bedrock in the area feeding the flow onto the fan.
Debris flow
Debris flow fans receive most of their sediments in the form of debris flows. Debris flows are slurry-like mixtures of water and particles of all sizes, from clay to boulders, that resemble wet concrete. They are characterized by having a yield strength, meaning that they are highly viscous at low flow velocities but become less viscous as the flow velocity increases. This means that a debris flow can come to a halt while still on moderately tilted ground. The flow then becomes consolidated under its own weight.Debris flow fans occur in all climates but are more common where the source rock is mudstone or matrix-rich saprolite rather than coarser, more permeable regolith. The abundance of fine-grained sediments encourages the initial hillslope failure and subsequent cohesive flow of debris. Saturation of clay-rich colluvium by locally intense thunderstorms initiates slope failure. The resulting debris flow travels down the feeder channel and onto the surface of the fan.
Debris flow fans have a network of mostly inactive distributary channels in the upper fan that gives way to mid- to lower-level lobes. The channels tend to be filled by subsequent cohesive debris flows. Usually only one lobe is active at a time, and inactive lobes may develop desert varnish or develop a soil profile from eolian dust deposition, on time scales of 1,000 to 10,000 years. Because of their high viscosity, debris flows tend to be confined to the proximal and medial fan even in a debris-flow-dominated alluvial fan, and streamfloods dominate the distal fan. However, some debris-flow-dominated fans in arid climates consist almost entirely of debris flows and lag gravels from eolian winnowing of debris flows, with no evidence of sheetflood or sieve deposits. Debris-flow-dominated fans tend to be steep and poorly vegetated.
Fluvial
Fluvial fans receive most of their sediments in the form of stream flow rather than debris flows. They are less sharply distinguished from ordinary fluvial deposits than are debris flow fans.Fluvial fans occur where there is perennial, seasonal, or ephemeral stream flow that feeds a system of distributary channels on the fan. In arid or semiarid climates, deposition is dominated by infrequent but intense rainfall that produces flash floods in the feeder channel. This results in sheetfloods on the alluvial fan, where sediment-laden water leaves its channel confines and spreads across the fan surface. These may include hyperconcentrated flows containing 20% to 45% sediments, which are intermediate between sheetfloods having 20% or less of sediments and debris flows with more than 45% sediments. As the flood recedes, it often leaves a lag of gravel deposits that have the appearance of a network of braided streams.
Where the flow is more continuous, as with spring snow melt, incised-channel flow in channels high takes place in a network of braided streams. Such alluvial fans tend to have a shallower slope but can become enormous. The Kosi and other fans along the Himalaya mountain front in the Indo-Gangetic plain are examples of gigantic stream-flow-dominated alluvial fans, sometimes described as megafans. Here, continued movement on the Main Boundary Thrust over the last ten million years has focused the drainage of of mountain frontage into just three enormous fans.
Geologic record
Alluvial fans are common in the geologic record, but may have been particularly important before the evolution of land plants in the mid-Paleozoic. They are characteristic of fault-bounded basins and can be or thicker due to tectonic subsidence of the basin and uplift of the mountain front. Most are red from hematite produced by diagenetic alteration of iron-rich minerals in a shallow, oxidizing environment. Examples of paleofans include the Triassic basins of eastern North America and the New Red Sandstone of south Devon, the Devonian Hornelen Basin of Norway, and the Devonian-Carboniferous in the Gaspé Peninsula of Canada. Such fan deposit likely contain the largest accumulations of gravel in the geologic record.Depositional facies
Several kinds of sediment deposits are found in alluvial fans.Alluvial fans are characterized by coarse sedimentation, though the sediments making up the fan become less coarse further from the apex. Gravels show well-developed imbrication with the pebbles dipping towards the apex. Fan deposits typically show well-developed reverse grading caused by outbuilding of the fan: Finer sediments are deposited at the edge of the fan, but as the fan continues to grow, increasingly coarse sediments are deposited on top of the earlier, less coarse sediments. However, a few fans show normal grading indicating inactivity or even fan retreat, so that increasingly fine sediments are deposited on earlier coarser sediments. Normal or reverse grading sequences can be hundreds to thousands of meters in thickness. Depositional facies that have been reported for alluvial fans include debris flows, sheet floods and upper regime stream floods, sieve deposits, and braided stream flows, each leaving their own characteristic sediment deposits that can be identified by geologists.
Debris flow deposits are common in the proximal and medial fan. These deposits lack sedimentary structure, other than occasional reverse-graded bedding towards the base, and they are poorly sorted. The proximal fan may also include gravel lobes that have been interpreted as sieve deposits, where runoff rapidly infiltrates and leaves behind only the coarse material. However, the gravel lobes have also been interpreted as debris flow deposits. Conglomerate originating as debris flows on alluvial fans is described as fanglomerate.
Stream flow deposits tend to be sheetlike, better sorted than debris flow deposits, and sometimes show well-developed sedimentary structures such as cross-bedding. These are more prevalent in the medial and distal fan. In the distal fan, where channels are very shallow and braided, stream flow deposits consist of sandy interbeds with planar and trough slanted stratification. The medial fan of a streamflow-dominated alluvial fan shows nearly the same depositional facies as ordinary fluvial environments, so that identification of ancient alluvial fans must be based on radial paleomorphology in a piedmont setting.