Submarine crevasse lobes controlled by lateral slope failure in tectonically-active settings: an exhumed example from the Eocene Aínsa depocentre (Spain)

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 deposits of low-sinuosity and narrow (2-3 km wide) channel-belts confined laterally by tectonically-controlled, fine-grained slopes. The studied succession (111 m-thick) is exposed along a 1.5 km long SE-NW trending depositional dip section and is documented here by 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 this structure promoted the breaching of confining channel walls and slope material 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 overbank succession. These crevasse lobes are characterized by structureless thick and medium beds that form < 5 m thick packages in proximal parts and thin abruptly over 1 km across strike (NE) and along downdip (NW) into structured thin beds, similar to the heterolithic dominated overbank deposits. Although the 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 margin of a foreland basin rather than on the opposing, more stable and gentler inactive margin. We discuss the mechanism for forming these crevasse deposits, which exploited the accommodation generated by submarine landslides derived from the tectonically-active compressional margin.