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.

Tectonic deformation and associated submarine slope failures modify seafloor relief, influencing sediment dispersal patterns and the resulting depositional architecture of deep-water systems. The exhumed Middle Eocene strata of the Banastón deep-water system in the Aínsa depocentre, Spain, allow the interplay between submarine slope confined systems, mass flow deposits, and syn-depositional compressional tectonics to be investigated. This study focuses on the Banastón II sub-unit, interpreted as low-sinuosity and narrow (2-3 km wide) channel-belt deposits confined laterally by opposing tectonically induced, fine-grained slopes. The studied succession (111 m-thick) is exposed along a 1.5 km long depositional dip-orientated (SE-NW) outcrop belt and documented here using facies analysis and physical correlation of 10 measured sections. Results show a stratigraphic evolution in which the channel axes migrated to the southwest, away from a growing structure in the northeastern part of the Aínsa depocentre. Uplift of the active margin promoted breaching of channel walls and confining slopes, with mass failures and the development of sand-rich crevasse scour-fills and crevasse lobes. We show that crevasse deposits form an important component of the overbank succession. These crevasse lobes are characterised by structureless thick and medium beds that form < 5 m thick packages in proximal parts and thin abruptly over 1 km into structured thin beds similar to the heterolithic dominated overbank deposits. Although development of crevasse lobes has been observed in multiple deep-water systems in ancient and modern systems, this study documents, for the first time, crevasse lobe development on the active compressional margins of a foreland basin rather than in the opposing and more stable and gentle margin. We discuss the mechanism for the formation of these crevasse deposits, which exploited the accommodation generated by the submarine landslides derived from the tectonically-active compressional margin.