Zoe Adele Cumberpatch, Ian Kane, Euan Soutter , David Hodgson, Christopher Aiden-Lee Jackson , Ben Kilhams, Yohann Poprawski

Sediment gravity flow behaviour is influenced by seafloor topography associated with salt structures, which controls the depositional architecture of deep-water sedimentary systems. Typically, salt-influenced deep-water successions are poorly-imaged in seismic reflection data and exhumed systems are rare, hence the detailed sedimentology and stratigraphic architecture of these systems remains poorly understood.

The exhumed Triassic (Keuper) Bakio and Guernica salt bodies in the Basque-Cantabrian Basin, Spain were active during deep-water sedimentation. The salt diapirs grew reactively, then passively, during the Aptian-Albian, and are flanked by deep-water carbonate (Aptian-earliest Albian Urgonian Group) and siliciclastic (middle Albian-Cenomanian Black Flysch Group) successions. The study compares the deposition in two salt-influenced confined (Sollube basin) and partially-confined (Jata basin) minibasins by actively growing salt diapirs, comparable to salt-influenced minibasins in the subsurface. The presence of a well-exposed halokinetic sequence, beds that pinch out towards topography, soft sediment deformation, variable paleocurrents and intercalated mass transport deposits (MTDs) indicate that salt grew during deposition. Overall, the Black Flysch Group coarsens- and thickens-upwards in response to regional axial progradation, which is modulated by laterally-derived MTDs from halokinetic slopes. The variation in type and number of MTDs within the Sollube and Jata basins indicate the basins had different tectono-stratigraphic histories despite their proximity. In the Sollube basin, the routeing systems were confined between the two salt structures eventually depositing amalgamated sandstones in the basin’s axis. Different facies and architectures are observed in the Jata basin due to partial confinement.

The findings show exposed minibasins are individualised and that facies vary both spatially and temporally in agreement with subsurface salt-influenced basins. Salt-related, active topography and the degree of confinement are shown to be important modifiers of depositional systems, resulting in facies variability, remobilisation of deposits and ‘channelisation’ of flows. The findings are directly applicable to the exploration and development of subsurface energy reservoirs in salt basins globally, enabling better prediction of depositional architecture in areas where seismic imaging is challenging.