A fan-shaped, low-angle depositional landform of predominantly fluvial origin, characterised by a distributary pattern of channels, and composed of alluvial deposits (see alluvium). Alluvial fans, also known as alluvial cones, tend to be located in the piedmont zone at the foot of a mountain or upland where rivers and streams deposit bedload at a distinct break of slope, commonly fault controlled. Unlike mountain-front river systems, alluvial fans are oriented transverse to the mountain front. They are well developed at the mouths of narrow valleys or canyons, where flow expansion occurs, leading to deposition. They typically show a concave longitudinal profile, a convex cross-profile and down-fan fining of sediment grain size. Fluvially dominated alluvial fans (fluvial fans) differ from colluvial fans dominated by debris flows (gravity-flow fans) in that the former tend to be larger, lower-angled and associated with larger, less rugged drainage basins.
Many classic examples of alluvial fans occur in arid and semi-arid settings, such as Death Valley in California. They typically have a radius of a few kilometres to tens of kilometres from apex to toe, and their deposits may be several hundred metres thick. Adjacent fans along a mountain front may coalesce to form a bajada. Alluvial fans in humid settings (humid fans) include the Kosi Fan in northern India, with a radius of 150 km from apex to toe. Alluvial fans in proglacial settings are sometimes known as outwash fans, in contrast to outwash plains (see sandur). Some river systems terminate inland at a terminal fan due to infiltration or evaporation. Alluvial fans that advance into a body of standing water are sometimes described as fan deltas.
Alluvial fan deposits are well represented in the stratigraphical record where they are an important component of the fill of many fault-controlled sedimentary basins. Their interpretation provides important sedimentological evidence of environmental change. The sedimentary facies of alluvial fan deposits can be confused with those of braided river deposits (see braiding), and the term fanglomerate has been used to describe coarse-grained sedimentary deposits attributed to alluvial fan deposition.
[See also megafan, submarine fan]
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1. a partially submerged alluvial fan at the mouth of a river. Plural: fan deltas
1. a fan-shaped alluvial deposit formed by a stream where its velocity is abruptly decreased, as at the mouth of a ravine or at the foot of a mou
Alluvium is the general term used to describe all the various sediment types that were deposited in river systems. The component material is predomi