Abstract

Quaternary alluvial fans occur throughout southeast Spain. Their development was characterised by early phases of aggradation and later dissection. This paper, based on field observations on 31 fans in Alicante, Murcia and Almeria provinces, deals with the sedimentology of the aggradation phases and its implications for fan morphology.

Four types of deposit are recognised: debris flow, sheet gravel, channel gravel and silt, their relative occurrence varying both vertically and laterally within fans as well as between fans. Within the fan sequences, there is a progressive vertical decrease in debris flow and increase in channel gravel deposition, and a downfall decrease in debris flow and increase in silt and channel gravel deposition.

Variations between fans reflect source area geology and relief. On the basis of their constitutent deposits fans are grouped into debris flow, intermediate and fluvial types. Small steep watersheds especially on massive sedimentary rocks favour debris flow fans and larger, less steep watersheds especially on metamorphic rocks favour fluvial fans.

The nature of sedimentary processes strongly influences fan morphology. Multiple regression analysis demonstrates the effect of watershed size and relief on fan slopes but regression residuals reflect fan sediment types, with debris flow fans having steeper slopes in relation to drainage basin characteristics than do intermediate and fluvial fans. The implications are important for aggradation and dissection behaviour in that steeper debris flow fans show more complex patterns of dissection than do less steep, simpler fluvial fans.