An alluvial megafan is a large cone or fan-shaped deposit built up by complex deposition patterns of stream flows originating from a single source point known as an apex. Megafans differ from
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 a ...
s in their sheer size. Due to their larger size, they may be formed by different geomorphic processes. The criterion of what differentiates megafans from typical alluvial fans is an artificial one of scale. The scale divide varies in the literature, with the most common being a 100-km apex-to-toe length. Alternative values as little of 30-km apex-to-toe length have been proposed, as well as alternative metrics like coverage areas of greater than 10,000 square-km.
Formation
The flow source from the apex occupies a portion of the fans apron, building up that portion with depositions. Through complex processes like avulsion, the stream changes course over time occupying different areas of the fan. Over long periods of time, sediment builds up creating a fan formation. Generally the three-dimensional architecture of megafan deposits consists of multi-storied sandsheets, gravel in upper reaches, interbedded with overbank muddy layers, thickness and facies distribution vary from upstream to downstream reaches
Rivers forming large fans happen in various settings around the world most notably, in foreland settings (e.g. Kosi, Gandak, Pastaza), intracratonic basins (e.g. Pantanal, Taquari, Cuiaba), and in complex settings like in the case of Pilcomayo. Although the distribution of megafans occur throughout many environments, the Tropics are home of the largest megafans of the world in the Chaco Plain, with the largest being the Pilcomayo.
Examples of Megafans
*Pilcomayo - Chaco Plain
*Grande - Chaco Plain
*Bermejo - Argentina
*Kosi - Gangetic plain of India
*Gandak - Gangetic plain of India
*
Okavango - the Okavango delta in
Botswana
Botswana (, ), officially the Republic of Botswana ( tn, Lefatshe la Botswana, label=Setswana, ), is a landlocked country in Southern Africa. Botswana is topographically flat, with approximately 70 percent of its territory being the Kalahar ...
*Taquari
Gallery
Image:Alluvial fan, Taklimakan Desert, XinJiang Province, China, NASA, ASTER.jpg, A 60 km long Megafan across the desolate landscape
A landscape is the visible features of an area of land, its landforms, and how they integrate with natural or man-made features, often considered in terms of their aesthetic appeal.''New Oxford American Dictionary''. A landscape includes the ...
between the Kunlun
The Kunlun Mountains ( zh, s=昆仑山, t=崑崙山, p=Kūnlún Shān, ; ug, كۇئېنلۇن تاغ تىزمىسى / قۇرۇم تاغ تىزمىسى ) constitute one of the longest mountain chains in Asia, extending for more than . In the bro ...
and Altun mountain range
A mountain range or hill range is a series of mountains or hills arranged in a line and connected by high ground. A mountain system or mountain belt is a group of mountain ranges with similarity in form, structure, and alignment that have arise ...
s that form the southern border of the Taklamakan Desert
The Taklimakan or Taklamakan Desert (; zh, s=塔克拉玛干沙漠, p=Tǎkèlāmǎgān Shāmò, Xiao'erjing: , dng, Такәламаган Шамә; ug, تەكلىماكان قۇملۇقى, Täklimakan qumluqi; also spelled Taklimakan and Te ...
in Xinjiang
Xinjiang, SASM/GNC: ''Xinjang''; zh, c=, p=Xīnjiāng; formerly romanized as Sinkiang (, ), officially the Xinjiang Uygur Autonomous Region (XUAR), is an autonomous region of the People's Republic of China (PRC), located in the northwest ...
. The left side is the active part of the fan, and appears blue from water flowing in the many small streams
Image:Pantanal Basin.jpg, Taquari Megafan in the Pantanal Basin
See also
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Alluvium
Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...
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Alluvial plain
An alluvial plain is a largely flat landform created by the deposition of sediment over a long period of time by one or more rivers coming from highland regions, from which alluvial soil forms. A floodplain is part of the process, being the sma ...
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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 a ...
*
Avulsion
*
Floodplain
A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...
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Placer deposit
In geology, a placer deposit or placer is an accumulation of valuable minerals formed by gravity separation from a specific source rock during sedimentary processes. The name is from the Spanish word ''placer'', meaning "alluvial sand". Placer min ...
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River delta
A river delta is a landform shaped like a triangle, created by deposition (geology), deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, res ...
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Subaqueous fan A subaqueous fan is a fan-shaped deposit formed beneath water (similar to deltas or terrestrial alluvial fans), and are commonly related to glaciers and crater lakes.
Subaqueous fan deposits are generally described as coarse to fine gravel and/or s ...
References
{{DEFAULTSORT:Megafan
Sedimentology
Fluvial landforms
Geography terminology