Alluvial Fans

Abstract

Alluvial fans are sedimentary bodies of broadly conical form, deposited where transporting power suddenly decreases, as at mountain-front or tributary junction sites. If they “toe out” in water they are known as fan deltas. Their sedimentology and geomorphology depend on the sediment transport mechanism, which commonly ranges from debris flow to fluvial transport. Smaller, steeper catchments tend towards debris-flow transport and larger less steep catchments towards fluvial transport. On deposition, debris cones are steeper than fluvially dominant fans. Fan morphologies, in relation to catchment characteristics have often been studied by statistically-based morphometric analysis. Alluvial fans are common in arid-zone mountain areas. Much of the earlier literature on fans dealt with the American southwest. However fans also occur in humid-climate mountain areas. The morphological and sedimentological evolution of fans tends to reflect Pleistocene climatic sequences. For Holocene fans, the controls are the Holocene climatic and human-induced sequence of environmental change. Fans often exhibit fanhead trenches in their apex areas, and sometimes complete through-fan trenching, caused either by base-level related toe dissection or by reductions in sediment supply. One of the implications of through-fan trenching is that such fans allow “coupling” through the catchment sediment system, whereas non-trenched fans “buffer” the sediment system. A cautionary word! Most of the above relates to relatively small (up to several km-scale), Quaternary fans, their morphology and sedimentology. There are studies in the stratigraphic literature relating to much larger features: megafans. Their controls are primarily long-term regional tectonics at the major mountain system/sedimentary basin scale, rather than the shorter term process interactions controlling Quaternary fan development.