Mass-Transport Complexes (MTCs), and Tectonic Control on Confined Basin-Floor Submarine Fans, Middle Eocene, South Spanish Pyrenees

Kevin T. Pickering and Nicole Bayliss
UCL (University College London, Department of Earth Sciences), London, United Kingdom

Three types of MTC are recognized in the deep-marine systems of the Ainsa basin, Spanish Pyrenees, each of which appears characteristic of stratigraphic position in relation to the evolution of individual depositional sequences. Falls in relative base level began with the large-scale collapse of the basin slope (type I MTCs) that formed much of the seafloor topography and contributed to the lateral confinement of sands. The uppermost slope and any shelf edge then collapsed, redepositing unconsolidated sands and gravels (type II MTCs). These basal coarse clastics are overlain by mainly amalgamated and confined sands with major erosional events associated with channel development, as pebbly mudstones and sandstones rich in angular intraclasts (type III MTCs). The channelized sands pass up into 10s m of less confined, non-amalgamated, finer grained sands / marls; representing the phase of most active sand input, initially by erosional channel development, sediment bypass, and backfill (in several cycles), giving way to non-channelized, fine-grained sandy deposition, interpreted as due to the flushing out of the coarser clastics from coastal and near-coastal fluvial systems. During this stage and when sediment accumulation rates remained high, the degraded submarine slope was regraded and healed by fine-grained depositional events. In the upper few meters, a thinning-and-fining-upwards indicates a return to background marl deposition (abandonment). Many sequences are overlain by intraformational sediment slides (typically type I MTCs but, rarely type II), reflecting the increasing seafloor gradients due to regrading and healing of the slope. The geometric attributes of the MTCs are reviewed (thickness, lateral continuity, maximum clast size, composition) and the applicability of this data to other depositional systems evaluated.