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

Abstract

Conventionally, a fall in base level is seen as stimulating incision into the distal zones of alluvial fans. In the Cabo de Gata ranges of southeast Spain evidence exists to the contrary. Two sets of Quaternary coastal alluvial fans demonstrate the interaction between climatically-driven variations in the supply of sediment and eustatically-driven changes in base level. The fans are supplied from Miocene volcanic terrain within which no evidence can be found for major tectonic deformation during the period of fan development. 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 buffered from the effects of changes in sea level by coastal barriers. Three phases of past sedimentation can be identified on the fans. These can be differentiated on the basis of field observations of soil profiles (particularly colour of the B horizons and accumulation of CaCO 3), and laboratory analyses of sequential iron oxide extractions and magnetic mineral properties. The two earlier (major) sedimentation phases were coincident with global glacials (>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 intervening interglacial and during the Holocene caused erosion of the distal zones of the east-coast fans which led to channel incision into the fan surfaces. On the west-coast fans no such incision occurred, simply proximal incision by small fanhead trenches. The youngest (relatively minor) phase of fan sedimentation has occurred during the Holocene. These contrasting contexts have produced differing styles of fans, with telescopic fan morphology on the east-coast and stacked morphology on the west-coast fans. The differences are reflected in the fan profiles, with steeper gradients dominating the east-coast fans, and extensive lower gradient distal surfaces on the west-coast fans. Fan morphometry, based on analysis of the residuals from drainage area to fan area and gradient regressions, also differentiates between the fan contexts. The fan building phases appear to be controlled proximally by climatically-driven pulses of sediment supplied to the fans. These occurred during global glacials coincident with low sea levels, and caused fan progradation onto the exposed foreshore. The intervening global interglacials were times of little fan sedimentation, and on the east coast, where high sea levels were able to erode the fan toes, deep through-fan dissection ensued.