Euan Soutter , Ander Martinez-Donate , Ian Kane, Miquel Poyatos-Moré, William J. Taylor, David Hodgson, Max Bouwmeester, Stephen S. Flint

Depositional and erosional bedforms can be used to reconstruct sedimentary processes and aid paleoenvironmental interpretations. Using exhumed deep-marine strata in the Eocene Aínsa Basin, Spain, we document a 3-dimensional package of dunes; a relatively rarely identified bedform in deep-marine environments. Our analysis shows that the dunes have curvilinear crests in planform, with smaller superimposed dunes and ripples deflected across the dune stoss sides. Beds containing these dunes have two main internal divisions: a lower inversely-graded (fine-to-coarse sandstone) and predominantly structureless division, and an upper coarse-grained sandstone division with well-developed cross-stratification, which is scoured and mantled with mudclasts and coarse-grains on the stoss-side. Following recently reported direct measurements of natural turbidity currents, we interpret the basal division as recording deposition from the dense basal head of a high-velocity turbidity current, followed by the development of dunes beneath the more sustained but still relatively high-velocity flow body that reworked the initial sandy deposit into downstream migrating dunes and scours. These dune-forming beds have been identified in different deep-water environments in the Aínsa Basin, including channel overbank and channel mouth settings and scour-fills. This indicates that the dunes are intimately tied to high-velocity flows that bypass through channel axes before becoming depositional during flow expansion across the channel overbank or at the channel mouth. Preservation of these dunes in the Aínsa Basin was likely enhanced by tectonically-forced lateral migration of channels, which prevented cannibalisation of bypass-zones, high aggradation rates dues to confinement, or by periodic sourcing of flows from a particularly clay-poor entry point. Where identified at outcrop or in the subsurface, these deposits are, therefore, diagnostic of substantial and contemporaneous sediment bypass downslope and are important for predicting the timing of sediment delivery to deep-water basins.