The role of base-level change in the dissection of alluvial fans: case studies from southeast Spain and Nevada

Abstract

Tectonics, climate and base level are the underlying controls of alluvial fan development. On many fans base level is stable, but where base-level change occurs it may itself be a response to tectonics or climatic change. Tectonically induced base-level change will be temporally independent of climatic change, and will show spatial variability depending on the rate of propagation of tectonically induced regional dissection. This is illustrated by the Tabernas fans, southeast Spain. The timing of climatically induced base-level change follows the timing of climatic change itself, and will show more consistency of spatial relationships within groups of fans affected. Alluvial fans which toe out in coastal zones or at lake shorelines may be influenced by climatically controlled base-level changes. Conventionally, a fall in base level would be expected to cause dissection of the distal fan zone. However, the reverse may occur, with fan progradation taking place in response to a base-level fall, and dissection occurring in response to a base-level rise. Two contrasted case studies are considered, based in part on previously published work, from dry-region alluvial fan zones, the coastal Cabo de Gata fans in southeast Spain and the Stillwater and Cold Springs fans in Nevada, USA. In these areas, base-level changes occurred in response to late Quaternary eustatic changes of sea level, and changes in the levels of pluvial lakes, respectively. Within both areas there are fans not affected by base-level change (Cabo de Gata, west-coast and inland fans; Cold Springs fans), and others where base-level change has been important in fan evolution (Cabo de Gata, east-coast fans; Stillwater fans). The differences in geomorphic regime between those fans influenced by base-level changes and those not are expressed in morphometric contrasts and different fan profile and plan characteristics. Within those fans influenced by base-level changes, the different mechanisms have influenced the location and elevation of the zone of incision. The differences can be attributed in part to the temporal interactions of base-level change with climatically induced changes in sediment supply, and in part to differences in gradients of the foreshore exposed by the falling sea or lake base levels.