Abstract
Summary Aggradation and dissection on alluvial fans influence the coupling between sediment source areas and main channels. Previous alluvial fan models emphasize the influence of drainage area and sediment character on overall fan morphometry and proximal-distal sedimentary variations. A model is proposed for the morphology and sedimentology of dry-region alluvial fans, based on crusted and non-crusted Quaternary fans in SE Spain, differentiating between aggradational and dissectional tendencies. Many fans show fan-head trenching, with the channel emerging on the fan surface at a mid-fan intersection point. Distally either aggradation or headcut development may occur, with headcut development and trenching leading ultimately to the total dissection of the fan surface. Three sets of factors determine intersection-point behaviour: fan surface crusting, discrepancy between fan and channel slopes, and channel morphology. Multiple regression models are developed to account for variations in fan and channel slopes and channel morphology, taking into account source area and sediment variations. Intersection point trenching is most common on steep crusted fans, where the discrepancy between fan slope and channel slope is greatest, and where channel widths are limited. Arid region fans from the American Southwest are contrasted with the Spanish fans. Neither the non-crusted nor the crusted American fans examined show any tendency towards intersection-point trenching. Their fan slope/channel slope discrepancies are less and their channels are wider, the result of both differing Quaternary sequences and contemporary environments between Spain and the American Southwest. The implications are important for the continuity of dry-region fluvial systems and for the stratigraphy of alluvial fan sequences. Simple fans with distal area aggradation show a simple stacked stratigraphy, but fans with a propensity for mid-fan and distal trenching may show a complex inset stratigraphy.
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