The impact of Quaternary sea-level and climatic change on coastal alluvialfans in the Cabo de Gata ranges, southeast Spain

Conventionally, a fall in base level is seen as stimulating incision into t he distal zones of alluvial fans, In the Cabo de Gata ranges of southeast S pain evidence exists to the contrary. Two sets of Quaternary coastal alluvi al fans demonstrate the interaction between climatically-driven variations in the supply of sediment and eustatically-driven changes in base level, Th e fans are supplied from Miocene volcanic terrain within which no evidence can be found for major tectonic deformation during the period of fan develo pment. The evolution of the east-coast fans has been affected by variations in sediment supply and changes in sea level. The west-coast fans were buff ered from the effects of changes in sea level by coastal barriers. Three ph ases of past sedimentation can be identified on the fans. These can be diff erentiated on the basis of field observations of soil profiles (particularl y colour of the B horizons and accumulation of CaCO3), and laboratory analy ses of sequential iron oxide extractions and magnetic mineral properties, T he two earlier (major) sedimentation phases were coincident with global gla cials (> ca. 135 ka and ca, 85-10 ka, based on the stratigraphy and uranium /thorium dating of the coastal sediments). High sea levels during the inter vening interglacial and during the Holocene caused erosion of the distal zo nes of the east-coast fans which led to channel incision into the fan surfa ces. On the west-coast fans no such incision occurred, simply proximal inci sion by small fanhead trenches. The youngest (relatively minor) phase of fa n sedimentation has occurred during the Holocene. These contrasting context s have produced differing styles of fans, with telescopic fan morphology on the east-coast and stacked morphology on the west-coast fans, The differen ces are reflected in the fan profiles, with Steeper gradients dominating th e east-coast fans, and extensive lower gradient distal surfaces on the west -coast fans. Fan morphometry, based on analysis of the residuals from drain age area to fan area and gradient regressions, also differentiates between the fan contexts. The fan building phases appear to be controlled proximall y by climatically-driven pulses of sediment supplied to the fans. These occ urred during global glacials coincident with low sea levels, and caused fan progradation onto the exposed foreshore. The intervening global interglaci als were times of little fan sedimentation, and on the east coast, where hi gh sea levels were able to erode the fan toes, deep through-fan dissection ensued. (C) 1999 Elsevier Science B.V. All rights reserved.