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Supercritical Morphodynamic Successions in the Deposits of Steep, Sandy Submarine Fans: Ainsa and Tabernas Basins, Spain


Outcrops of submarine fan deposits from steep, sandy depositional settings have been examined in order to define and identify morphodynamic successions that capture the mode and variation of lithofacies and stratal architecture. A morphodynamic succession is a set of genetically-related strata that record the evolution of flow conditions deduced from these units, meaning that the strata were deposited as a result of the interaction of a sediment-laden flow or flows with the bed that imparted a characteristic texture, fabric, stratal architecture, and depositional geometry. Strata at the deepwater channel-lobe transition zones (CLTZ) in outcrops in the Ainsa and Tabernas basins are characterized by lithofacies assemblages that have a distinct grain size trends, sedimentary structures, stacking pattern, and stratal geometry. These assemblages have a sharp, erosional basal surface, often exhibiting soft-sediment deformation features (flame structures). The basal surface is erosional and the sediment above often contains locally-derived rip-up clasts. The basal portion is coarsest-grained, and typically is massive, or exhibits contorted crude bedding. Where thickest, the basal portion commonly is characterized by large-scale backset cross bedding that dip counter to flow direction. The strata that make up the backsets are comprised of cm to dm scale crude stratification, with grain sizes that alternate between coarser (imbricate cobbles to coarse sand) and finer components. Locally-derived rip-up clasts may be aligned/imbricated along the backset bedding and fine down-current. Crudely stratified or backset strata are overlain by finer-grained inclined planar/spaced stratification and wavy to hummocky-like cross stratification. These lithofacies packages thin down-current at the expense of the lower units, and may consist solely of these fine spaced and/or wavy strata at the distal end of the CLTZ, or grade into other deepwater lobe lithofacies. These lithofacies assemblages are interpreted to be morphodynamic successions deposited by aggrading supercritical beforms, like cyclic steps, formed at the CLTZ. Erosion and soft-sediment deformation at the base of the succession formed by scour and fluidization of underlying sediments, while overlying crude and crude backset bedding encroached up-current into the scour as traction carpets. Sediment down-current of the scour is deposited as traction carpets and aggrading antidunes and supercritical dunes and ripples.