ABSTRACT

Oligocene to Miocene conglomerate bodies interpreted as alluvial-fan deposits are well exposed along the northern margin of the Ebro Basin in Spain. The source area was the southern Pyrenean thrust front, which was tectonically active at that time. Eleven separate conglomerate bodies have been identified and interpreted as the deposits of individual alluvial fans. The smallest alluvial-fan deposit is less than 0.8 km radius, is about 460 m thick, and has a calculated volume of 0.1 km³; the largest is up to 5.5 km radius, is at least 500 m thick, and has a calculated volume of over 3 km³. Fan evolution was strongly controlled by the lithologies exposed by erosion of the emergent thrust front: the size of the fan bodies was determined by the nature and extent of the bedrock lithologies, which were in turn structurally controlled. Growth structures in the strata are common, because deformation continued along the thrust front during fan sedimentation. Stepwise tectonic reconstructions of one of the alluvial-fan conglomerate bodies and the adjacent thrust front suggest that the relief in the hinterland was around 500 m.