INTERACTIONS BETWEEN ALLUVIAL FAN AND FLUVIAL DISTRIBUTARY SYSTEMS, SPANISH PYRENEES

The West Jaca Basin occurs along the southwestern margin of the Spanish Pyrenees and contains thick sequences of Oligocene to early Miocene strata that accumulated in a series of geomorphically and structurally partitioned thrust-top basins. These deposits form part of a widespread molassic wedge that accumulated during numerous phases of southward progradation of sediments shed off the Spanish Pyrenees. Deposition occurred in a variety of alluvial fan and extensive fluvial distributary systems that were controlled by episodic movements of the south-verging Guarga thrust system and attendant thrust-top anticline development. The Bernues Conglomerate and overlying Uncastillo Formation represent the confluence of basin-margin fan and fluvial systems that developed within the western Jaca Basin during this time. Marginal fans spread into these sub-basins from adjacent highlands during periods of movement on localized faults and/or major climatic events. Sheetflood and subordinate debris flow deposits characterize fan sediments. The former form sheet-like units characterized by sub-horizontal and imbricated conglomerates and also low-angle cross-stratified conglomerates indicative of upper stage transport. Many of the cross-stratified conglomerates exhibit clasts oriented up flow suggesting deposition during extreme flow events. Sheetflooding events clearly were the dominant depositional motif. Debris-flow deposits characterized by poorly sorted conglomerates occur in proximal regions of the fan system. Subordinate channel-fill conglomerates and sandstones cap many of these successions and represent incised channels on the fan surface that formed during falling stage events and also during subsequent fan aggradation. Braided stream deposits intertongue with distal fan sediments and comprise lenticular, trough cross-stratified sandstones and minor mudstones. These deposits formed in axial drainage systems that truncated distal portions of the marginal fans. The progradation of alluvial fans into the Jaca Basin represent significant basin-margin adjustments associated with movement of thrust sheets that resulted in interruption of fluvial drainage patterns. Discrimiation between fan and fluvial deposits allows recognition of extrabasinal controls on sedimentation within the west Jaca Basin.