Alluvial fans and fluvial distributary systems, Oligo-Miocene, northern Spain; contrasting processes and products

Abstract

Sediments of alluvial fans and distributive fluvial sediments were deposited by contrasting processes during the Oligo-Miocene in the north-central Ebro Basin, northern Spain. The coeval depositional systems can be separated on the basis of lithofacies criteria. A number of localized deposits, each covering only a few square kilometers and lying adjacent to the south Pyrenean thrust front, are made up of beds of poorly sorted conglomerate, sandstone, and mudstone that show a radial decrease in bed thickness and grain size. These deposits are interpreted as the products of sheet-flood events on alluvial fans that formed at the basin margin. Covering much larger areas of the basin (several thousand square kilometers) are conglomerate, sandstone, and mudstone as lensoid and irregular sheet bodies incised into thin-bedded sheets of sandstone and mudstone. These are interpreted as the deposits of fluvial systems that are considered to be distributary in form because the channel size and grain size of channel-fill deposits decrease down paleoflow and because there is a radial pattern of paleocurrents. At the distal margins, the fluvial facies interdigitate with ephemeral lacustrine and alluvial-plain deposits. The deposits of the alluvial fans and the fluvial distributary systems are petrographically distinct, reflecting separate source areas. Important components of the alluvial-fan deposits are limestone clasts derived from the adjacent thrust front. The deposits of the fluvial systems are mainly sand and subordinate gravel derived from rocks of the axial zone of the Pyrenean mountain belt. The deposits of these systems interdigitate, and there are no transitional forms between the two within this stratigraphic package.