--> Large Fluvial Fans: Aspects of the Attribute Array

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Large Fluvial Fans: Aspects of the Attribute Array

Abstract

In arguing for a strict definition of the alluvial fan (coarse-grained with radii <10 km, in mountain-front settings), Blair and McPherson (1994) proposed that there is no meaningful difference between the largest fans (large fluvial fans—LFF) and floodplains, as the building blocks of both are the channel-levee-overbank suite of deposits. Sediment bodies at the LFF scale (>100 km long, fan-shaped in planform), of which >160 are now identified globally, are relatively unstudied. The following perspectives suggest that their significance needs to be reconsidered. (1) LFF-formed land surfaces and sediment bodies: Large areas covered by single (up to 200,000 km2) and nested LFF (750,000 km2 contiguous LFF surfaces in S America alone) show that such surfaces are significant at continental scales—though often unrecognized, especially when located far from mountain fronts. Since LFF are a major component of modern Distributive Fluvial Systems (DFS—fanlike forms >30 km), their role in the evolution of buried fluvial strata holds specific interest. (2) Drainage patterns: a—Diverging channel patterns over distances >102 km characterize not only coastal deltas, but also LFF situated hundreds of km from coastlines. b—Rivers in marginal depressions between neighboring LFF tend to be the best developed sectors of lowland, non-axial river systems due to significantly higher episodic drainage discharge. (3) LFF cascade: First-tier LFF (apexed at the upland margin) can give rise in large enough basins to a second tier of downstream derived LFF, the first-tier with distinct conicality, the derived being flatter with alluvial ridges as the most prominent topography. (4) Stratigraphic record: The sheer size of LFF surfaces reduces the rate of surface reworking accomplished by the avulsing river. Combined with relatively higher infiltration capacities LFF are likely to hold more complete sedimentary and pedologic records than those held by the more frequently reworked floodplain surfaces confined between valley walls. (5) Applied aspects: Recognition of a relict LFF in Namibia allowed reinterpretation of the dimensions of two aquifers—as orders of magnitude larger than those implied by the floodplain model. Such reinterpretations can be expected elsewhere. Hydrocarbon exploration can benefit from understanding the architectures and more realistic paleogeographic reconstructions implied in 2 and 1 above. LFF thus warrant classification as a discrete type of fluvial sediment body.