OLIGOMIOCENE ALLUVIAL-FAN EVOLUTION AT THE SOUTHERN PYRENEAN THRUST FRONT, SPAIN

Oligocene to Miocene conglomerate bodies interpreted as alluvial-fan d eposits are well exposed along the northern margin of the Ebro Basin i n Spain. The source area was the southern Pyrenean thrust front, which was tectonically active at that time. Eleven separate conglomerate bo dies have been identified and interpreted as the de posits of individu al alluvial fans. The smallest alluvial-fan deposit is less than 0.8 k m radius, is about 460 m thick, and has a calculated volume of 0.1 km( 3); the largest is up to 5.5 km radius, is at least 500 m thick, and h as a calculated volume of over 3 km(3). Fan evolution was strongly con trolled by the lithologies exposed by erosion of the emergent thrust f ront: the size of the fan bodies was determined by the nature and exte nt of the bedrock lithologies, which were in turn structurally control led. Growth structures in the strata are common, because deformation c ontinued along the thrust front during fan sedimentation. Stepwise tec tonic reconstructions of one of the alluvial-fan conglomerate bodies a nd the adjacent thrust front suggest that the relief in the hinterland was around 500 m.