Sedimentary structures reflecting the flow and depositional regime of the 8.15 ka yr BP Storegga submarine landslide tsunami at the Montrose Basin, North-East Scotland

An anomalous sand unit preserved within an outcrop section at the south-west margin of the Montrose Basin (north-east Scotland) is investigated using a multi-proxy approach comprising lithostratigraphic analysis, particle size analysis, loss on ignition, geochemistry, and experimental modelling. This enables detailed reconstruction of the flow and depositional regime of the 8.15 ka yr BP Storegga Submarine Landslide Tsunami. The sand deposit (micaceous silty sand) is bounded by uplifted Carse Clay (sandy silt) and contrasts with this estuarine sediment due to its sharp contacts, inverse-normal and ungraded sequences, shell fragments, and soft-sediment deformation structures (SSDS). These include antiforms: flame structures (FS) and asymmetric flame structures (AFS); and synforms: simple load casts (SLC), pendulous load casts (PLC), and detached pseudonodules (DP), which are occasionally described in tsunami deposits. Particle size data suggests deposition occurred during the inundation wave phase. SSDS at the lower boundary formed through load deformation, with the development of antiforms aided by buoyancy-induced diapirism. AFS identified at this boundary indicate high basal shear stress orientated in the direction of palaeoflow during inundation. These structures result from both ductile and load deformation during flow and deposition. Experimental modelling supports the syn-sedimentary origin of AFS and the formation of FS, SLC, and PLC under high shear stress and subsequent sediment loading. Post-depositional processes, particularly seismic-induced liquefaction, likely contributed to the formation of upper boundary SSDS. The lower structures may have been influenced by uneven loading or downslope movement of sediment post-deposition. The structures, especially AFS in the direction of palaeoflow, enhance understanding of tsunami dynamics and expand the proxy toolkit for identifying palaeotsunami deposits.