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

In many deep-marine clastic systems, mass-transport complexes (MTCs) constitute a major component of the stratigraphy and represent an integral part of the evolution and depositional style. In tectonically active basins, as is the case in our study of the syntectonic deep-marine Ainsa submarine fans (Eocene of the Ainsa basin, Spanish Pyrenees), organized, predictable vertical sedimentary sequences provide a testable generic model for submarine fan development in other basins where tectonic processes provide a first-order driver on fan growth. In the Ainsa basin, the linear submarine fans were confined by lateral thrust ramps and influenced by intra-basinal growth anticlines. This study represents an integration of outcrop data from sedimentary logs and mapping, with core data from eight wells and micropaleontological and palynomorph analyses. Mapping shows the lateral stepwise migration of sandy channelized submarine fans, as a foreland-propagating clastic wedge. The stepwise foreland migration of each fan (away from the deformation front), and also the time scales involved (many hundreds of thousands of years for each fan), are interpreted within the context of a primary tectonic control. Three distinct types of MTC are recognized, each of which appears to be characteristic of stratigraphic position in relation to the evolution of individual fans. The deep-marine expression of the inferred tectonic pulses began with the large-scale basin-slope collapse as sediment slides and debris flows (type I MTCs) that formed much of the seafloor topography for each fan and contributed to their lateral confinement. The uppermost slope and any shelf edge, including the narrow shelf, then collapsed, redepositing unconsolidated sands and gravels into deep water (type II MTCs). The basal coarse clastics are overlain by an interval of mainly channelized and amalgamated sandy deposits, with major erosional events, including reincision processes associated with channel development, being characterized by pebbly mudstones and sandstones rich in angular, locally derived intraclasts (type III MTCs). The channelized sands pass up into several tens of meters of less confined, non-amalgamated, medium- and thin-bedded, fine-grained sands and marls. These deposits represent the phase of most active fan growth, initially by erosional channel development, sediment bypass, and backfill (in several cycles), giving way to nonchannelized, fine-grained sandy deposition, interpreted as a response to the flushing out of the coarser clastics from the coastal and near-coastal fluvial systems. During this latter stage in active fan growth and when sediment accumulation rates probably remained high, the degraded submarine slope was regraded and healed by fine-grained depositional events. The high amount of woody material and the high nonmarine palynomorph signal in these sandy deposits suggest direct river input as both turbidity currents and hyperpycnal flows for the silty marls. In the upper few meters, a thinning and fining-upward sequence shows a return to background marl deposition, representing fan abandonment. Many sequences are overlain by intraformational sediment slides (typically type I MTCs but, rarely type II MTCs) that attest to the increasing seafloor gradients associated with the regrading and healing stage in slope development. Our explanation for these vertical sequences provides a readily testable depositional model for other deep-marine clastic systems associated with a tectonically active hinterland and basin slopes.