Analluvial fan is an accumulation ofsediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from anescarpment. They are characteristic of mountainous terrain in arid tosemiarid climates, but are also found in more humid environments subject to intense rainfall and in areas of modernglaciation. They range in area from less than 1 square kilometer (0.4 sq mi) to almost 20,000 square kilometers (7,700 sq mi).
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 indeposition of sediments. The flow can take the form of infrequentdebris flows like in alandslide, 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 analluvial 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 theGreat Basin of western North America, in theNew Red Sandstone of southDevon, or all across the major population centers ofXinjiang in theTaklamakan Desert andJunggar 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 onMars andTitan, showing that fluvial processes have occurred on other worlds.
Some of the largest alluvial fans are found along theHimalaya mountain front on theIndo-Gangetic Plain. A shift of the feeder channel (anodal avulsion) can lead to catastrophic flooding, as occurred on theKosi River fan in 2008.
An alluvial fan is an accumulation of sediments that fans out from a concentrated source of sediments, such as a narrow canyon emerging from anescarpment. This accumulation is shaped like a section of a shallowcone,[1] with itsapex at the source of sediments.[2]
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.[1] Some giant alluvial fans have areas of almost 20,000 square kilometres (7,700 sq mi).[3] The slope measured from the apex is generally concave, with the steepest slope near the apex (theproximal fan[4] orfanhead[5]) and becoming less steep further out (themedial fan ormidfan) and shallowing at the edges of the fan (thedistal fan orouter 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.[6][7]
When there is enough space in thealluvial 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.[8]
Wave or channelerosion of the edge of the fan (lateral erosion) sometimes produces a "toe-trimmed" fan, in which the edge of the fan is marked by a small escarpment.[9] Toe-trimmed fans may record climate changes or tectonic processes, and the process of lateral erosion may enhance theaquifer orpetroleum reservoir potential of the fan.[10] Toe-trimmed fans on the planet Mars provide evidence of past river systems.[11]
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 abajada orpiedmont alluvial plain.[12][13]
Alluvial fans usually form where a confined feeder channel exits a mountain front[14][15] or a glacier margin.[6] 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.[15]
Flow in the proximal fan, where the slope is steepest, is usually confined to a single channel[6] (afanhead trench[3]), which may be up to 30 meters (100 ft) deep.[6] This channel is subject to blockage by accumulated sediments ordebris flows, which causes flow to periodically break out of its old channel (nodal avulsion) and shift to a part of the fan with a steeper gradient, where deposition resumes.[15] 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.[6]
Alluvial fans can be dominated by debris flows (debris flow fans) or stream flow (fluvial fans).[4][16][17] 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.[18]
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 wetconcrete. 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.[19]
Debris flow fans occur in all climates but are more common where the source rock ismudstone or matrix-richsaprolite rather than coarser, more permeableregolith. The abundance of fine-grained sediments encourages the initial hillslope failure and subsequent cohesive flow of debris.[20] Saturation of clay-richcolluvium by locally intense thunderstorms initiates slope failure. The resulting debris flow travels down the feeder channel and onto the surface of the fan.[21]
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 developdesert varnish or develop a soil profile fromeolian dust deposition, on time scales of 1,000 to 10,000 years.[22] 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.[23] 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.[24] Debris-flow-dominated fans tend to be steep and poorly vegetated.[25]
Fluvial fans (streamflow-dominated 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.[14]
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.[23] This results insheetfloods on the alluvial fan, where sediment-laden water leaves its channel confines and spreads across the fan surface. These may includehyperconcentrated 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.[25] As the flood recedes, it often leaves a lag of gravel deposits that have the appearance of a network of braided streams.[23]
Where the flow is more continuous, as with spring snow melt,incised-channel flow in channels 1–4 meters (3–10 ft) high takes place in a network of braided streams.[25] Such alluvial fans tend to have a shallower slope but can become enormous.[23] 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 asmegafans.[26] Here, continued movement on theMain Boundary Thrust over the last ten million years has focused the drainage of 750 kilometres (470 miles) of mountain frontage into just three enormous fans.[3]
Alluvial fans are common in the geologic record, but may have been particularly important before the evolution of land plants in the mid-Paleozoic.[27] They are characteristic of fault-bounded basins and can be 5,000 meters (16,000 ft) or thicker due to tectonic subsidence of the basin and uplift of the mountain front. Most are red from hematite produced bydiagenetic alteration of iron-rich minerals in a shallow, oxidizing environment. Examples of paleofans include theTriassic basins of eastern North America and the New Red Sandstone of south Devon,[23] theDevonianHornelen Basin of Norway, and the Devonian-Carboniferous in theGaspé Peninsula of Canada.[27] Such fan deposit likely contain the largest accumulations of gravel in the geologic record.[28]
Several kinds of sediment deposits (facies) 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-developedimbrication with the pebbles dipping towards the apex.[23] Fan deposits typically show well-developedreverse 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.[27] 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.[23][29]
Debris flow deposits are common in the proximal and medial fan.[23] These deposits lack sedimentary structure, other than occasional reverse-graded bedding towards the base, and they are poorly sorted.[30] 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.[30]Conglomerate originating as debris flows on alluvial fans is described asfanglomerate.[31]
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.[25] In the distal fan, where channels are very shallow and braided, stream flow deposits consist of sandy interbeds with planar and trough slanted stratification.[32] 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 radialpaleomorphology in a piedmont setting.[33]
Alluvial fans are characteristic of mountainous terrain in arid tosemiarid climates,[34][6] but are also found in more humid environments subject to intense rainfall[7] and in areas of modern glaciation.[6] They have also been found on other bodies of theSolar System.[35][36]
Alluvial fans are built in response to erosion induced bytectonic uplift.[37] The upwards coarsening of the beds making up the fan reflects cycles of erosion in the highlands that feed sediments to the fan. However, climate and changes inbase level may be as important as tectonic uplift. For example, alluvial fans in the Himalayas show older fans entrenched and overlain by younger fans. The younger fans, in turn, are cut by deep incised valleys showing twoterrace levels. Dating viaoptically stimulated luminescence suggests a hiatus of 70,000 to 80,000 years between the old and new fans, with evidence of tectonic tilting at 45,000 years ago and an end to fan deposition 20,000 years ago. Both the hiatus and the more recent end to fan deposition are thought to be connected to periods of enhanced southwestmonsoon precipitation. Climate has also influenced fan formation inDeath Valley,California, US, where dating of beds suggests that peaks of fan deposition during the last 25,000 years occurred during times of rapid climate change, both from wet to dry and from dry to wet.[38]
Alluvial fans are often found indesert areas, which are subjected to periodicflash floods from nearbythunderstorms in local hills. The typical watercourse in an arid climate has a large, funnel-shaped basin at the top, leading to a narrowdefile, which opens out into an alluvial fan at the bottom. Multiplebraided streams are usually present and active during water flows.[34]Phreatophytes (plants with long taproots capable of reaching a deepwater table) are sometimes found in sinuous lines radiating from arid climate fan toes. Thesefan-toe phreatophyte strips trace buried channels of coarse sediments from the fan that have interfingered with impermeableplaya sediments.[39]
Alluvial fans also develop in wetter climates when high-relief terrain is located adjacent to low-relief terrain.[37] InNepal, theKoshi River has built amegafan covering some 15,000 km2 (5,800 sq mi) below its exit fromHimalayan foothills onto the nearly level plains where the river traverses intoIndia before joining theGanges. Along the upper Koshi tributaries, tectonic forces elevate theHimalayas several millimeters annually. Uplift is approximately in equilibrium with erosion, so the river annually carries some 100 million cubic meters (3.5×10^9 ft3) of sediment as it exits the mountains. Deposition of this magnitude over millions of years is more than sufficient to account for the megafan.[40]
InNorth America, streams flowing intoCalifornia'sCentral Valley have deposited smaller but still extensive alluvial fans, such as that of theKings River flowing out of theSierra Nevada.[41] Like the Himalayan megafans, these are streamflow-dominated fans.[42]
Alluvial fans are also found onMars. Unlike alluvial fans on Earth, those on Mars are rarely associated with tectonic processes, but are much more common on crater rims.[43][44] The crater rim alluvial fans appear to have been deposited by sheetflow rather than debris flows.[45]
Three alluvial fans have been found inSaheki Crater. These fans confirmed past fluvial flow on the planet and further supported the theory that liquid water was once present in some form on the Martian surface.[46] In addition, observations of fans inGale crater made by satellites from orbit have now been confirmed by the discovery offluvial sediments by theCuriosity rover.[47] Alluvial fans in Holden crater have toe-trimmed profiles attributed to fluvial erosion.[11]
The few alluvial fans associated with tectonic processes include those at Coprates Chasma and Juventae Chasma, which are part of theValles Marineris canyon system. These provide evidence of the existence and nature of faulting in this region of Mars.[48]
Alluvial fans have been observed by theCassini-Huygens mission onTitan using the Cassini orbiter'ssynthetic aperture radar instrument. These fans are more common in the drier mid-latitudes at the end of methane/ethane rivers where it is thought that frequent wetting and drying occur due to precipitation, much like arid fans on Earth. Radar imaging suggests that fan material is most likely composed of round grains of water ice or solidorganic compounds about two centimeters in diameter.[49]
Alluvial fans are the most important groundwater reservoirs in many regions. Many urban, industrial, and agricultural areas are located on alluvial fans,[50] including theconurbations ofLos Angeles, California;Salt Lake City, Utah; andDenver, Colorado, in the western United States, and in many other parts of the world.[51] However, flooding on alluvial fans poses unique problems for disaster prevention and preparation.[52]
The beds of coarse sediments associated with alluvial fans form aquifers that are the most important groundwater reservoirs in many regions.[50] These include both arid regions, such as Egypt[53] or Iraq,[54] and humid regions, such as central Europe[55] or Taiwan.[56]
Alluvial fans are subject to infrequent but often very damaging flooding, whose unusual characteristics distinguish alluvial fan floods from ordinary riverbank flooding. These include great uncertainty in the likely flood path, the likelihood of abrupt deposition and erosion of sediments carried by the flood from upstream sources, and a combination of the availability of sediments and of the slope and topography of the fan that creates extraordinary hazards. These hazards cannot reliably be mitigated by elevation on fill (raising existing buildings up to a meter (three feet) and building new foundations beneath them[57]). At a minimum, major structuralflood control measures are required to mitigate risk, and in some cases, the only alternative is to restrict development on the fan surface. Such measures can be politically controversial, particularly since the hazard is not obvious to property owners.[58] In the United States, areas at risk of alluvial fan flooding are marked as Zone AO onflood insurance rate maps.[59]
Alluvial fan flooding commonly takes the form of short (several hours) but energeticflash floods that occur with little or no warning. They typically result from heavy and prolonged rainfall, and are characterized by high velocities and capacity for sediment transport. Flows cover the range from floods through hyperconcentrated flows to debris flows, depending on the volume of sediments in the flow. Debris flows resemble freshly poured concrete, consisting mostly of coarse debris. Hyperconcentrated flows are intermediate between floods and debris flows, with a water content between 40 and 80 weight percent. Floods may transition to hyperconcentrated flows as they entrain sediments, while debris flows may become hyperconcentrated flows if they are diluted by water.[60] Because flooding on alluvial fans carries large quantities of sediment, channels can rapidly become blocked, creating great uncertainty about flow paths that magnifies the dangers.[58]
Alluvial fan flooding in theApennine Mountains of Italy have resulted in repeated loss of life. A flood on 1 October 1581 atPiedimonte Matese resulted in the loss of 400 lives. Loss of life from alluvial fan floods continued into the 19th century, and the hazard of alluvial fan flooding remains a concern in Italy.[61]
On January 1, 1934, record rainfall in a recently burned area of theSan Gabriel Mountains,California, caused severe flooding of the alluvial fan on which the towns ofMontrose andGlendale were built. The floods caused significant loss of life and property.[62]
TheKoshi River in India has built up a megafan where it exits the Himalayas onto theGanges plain. The river has a history of frequently and capriciously changing its course, so that it has been called theSorrow of Bihar for contributing disproportionately to India's death tolls in flooding. These exceed those of all countries exceptBangladesh.[63] Over the last few hundred years, the river had generally shifted westward across its fan, and by 2008, the main river channel was located on the extreme western part of the megafan. InAugust 2008, highmonsoon flows breached the embankment of theKoshi River. This diverted most of the river into an unprotected ancient channel and flooded the central part of the megafan. This was an area with a highpopulation density that had been stable for over 200 years.[64] Over a million people were rendered homeless, about a thousand lost their lives and thousands of hectares of crops were destroyed.[65][66][67]
Buried alluvial fans are sometimes found at the margins ofpetroleum basins. Debris flow fans make poor petroleum reservoirs, but fluvial fans are potentially significant reservoirs. Though fluvial fans are typically of poorer quality than reservoirs closer to the basin center, due to their complex structure, the episodic flooding channels of the fans are potentially lucrative targets for petroleum exploration.[68] Alluvial fans that experience toe-trimming (lateral erosion) by an axial river (a river running the length of an escarpment-bounded basin) may have increased potential as reservoirs. The river deposits relatively porous, permeable axial river sediments that alternate with fan sediment beds.[69]
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