INTRA- AND EXTRABASINAL CONTROLS ON FLUVIALLY DOMINATED ALLUVIAL FAN SEQUENCES IN MID-TERTIARY DEPOSITS OF THE SOUTHERN PYRENEES

Oligocene through mid-Miocene strata crop out extensively along the southern margin of the Spanish Pyrenees. These deposits accumulated in a variety of basins within a large-scale foreland setting and within sub-basins that formed on top of migrating thrusts. As such, there was significant geomorphic partitioning of sub-basins resulting in differential styles of sediment fills. The Bernues Conglomerate and overlying Uncastillo Formation within the west Jaca Basin record the intra-play of debris-flow and fluvial processes. Deposition near the orogenic front initially was controlled by debris flow-dominated systems interrupted by minor, through-flowing rivers in distal fan areas. The sub-basins however, record deposition within alluvial fan complexes where fluvial processes were the controlling mechanisms. Sheetflood deposits dominate and are characterized by poor- to horizontally stratified conglomerates with rare imbrication, and low-angle stratified sandstones; both are indicative of high velocity, short-lived flooding across lower portions of fans. Their sheet-like character attests to unconfined flow in a rapidly fluctuating fluvial system. Subordinate channel-fill conglomerates and sandstones represent dissection and fill of incised channels on the fan surfaces. Braided stream deposits of the main axial drainage system intertongue with sheetflood deposits of the fans. This interplay is the result of structural adjustments within the Internal Sierra, as indicated by progressive unconformities, and local faulting in the sub-basins. The dominance of sheetflood deposits is likely the result of more humid conditions within the Jaca Basin. The progradation of fans into and across the basins resulted in interruption of local drainage patterns. Discrimination between sheetflood and other water-lain deposits of alluvial fans from river deposits is useful in evaluating both intra- and extrabasinal controls on sedimentation within these thrust-top basins.