Abstract

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.