Depositional processes and stratigraphic architecture of an exhumed retrogradational to progradational submarine slope succession (Eocene, Aínsa Basin, Spain)

Sedimentary successions recording the initiation and evolution of submarine slopes have rarely been documented in outcrop studies. Consequently, the temporal and spatial distribution of subaqueous slope deposits at sub-seismic resolutions are poorly constrained. Furthermore, many models of submarine slope initiation and evolution are based on subsurface datasets from tectonically passive margins, with a consequent limitation of their application to active settings. This study documents the facies architecture and depositional processes of the ≤ 800-m-thick Eocene Fosado System (Hecho Group, Aínsa Basin, Spanish Pyrenees), that records the initiation, evolution, and burial of a submarine slope in a tectonically active epicontinental basin margin. Facies mapping reveals that synsedimentary thrust propagation and differential subsidence across an oblique ramp system initiated a shelf–slope profile. The slope underwent two distinct phases of deposition. Initially, thrusting and folding led to recurrent slope over-steepening, collapse, and healing, resulting in an out-of-grade retrogradational slope. Tectonic deformation and the emplacement of mass-transport complexes generated an irregular seafloor topography that promoted accumulation of coarser grained slope pond- and channel-fills. Later, as the locus of thrusting propagated basinward, and the slope was incorporated into a thrust hanging wall, accommodation and instability on the slope were reduced, leading to a progradational, in-grade phase of slope deposition. This in-grade slope was dominated by unconfined mud deposition, with markedly less mass wasting and pond- and channel-fill sand deposition. Hyperpycnal flows, derived from shelf-edge deltas, were also common. These results are inconsistent with passive-margin models that link shelf-margin retrogradation (and degradation) to transgression, and slope mudstone deposition, and progradational shelf-margins to regression with associated delivery of coarser-grained sediment to the slope. This study shows that the opposite can be true in tectonically active settings.