Sedimentary features and processes in the Nazar\ e and Set\ ubal submarine canyons, west Iberian margin

Here we present part of the first complete sidescan sonar dataset of the Nazar\ e and Set\ ubal Canyons, west Iberian margin, which, in combination with multibeam bathymetry, shallow seismic profiles and precise piston coring of intra-canyon targets, are used to characterise the sedimentary dynamics of these deep-sea settings. The results show that Nazar\ e and Set\ ubal Canyons are highly complex environments. They display a range of sedimentary features and processes that reflect changes in downslope canyon geometry and a transition from erosive proximal to more depositional distal sections. The proximal (upper) sections of both canyons are characterised by a deeply incised, narrow, V-shaped thalweg, flanked by small gullies and terraces. Numerous small and localised intra-canyon landslides and rock avalanches occur in this section, triggered by instability processes that are preconditioned by the steep topography. Sequences of stacked thin-bedded, fine-grained turbidites occur locally on intra-canyon terraces, and are interpreted to be the result of small-volume, possibly river flood-generated turbidity currents that do not appear to reach the lower canyon. In Nazar\ e Canyon these turbidites are associated with abundant coalified organic fragments. Part of the upper section in Nazar\ e Canyon has acted as an apparent depocenter through the Holocene, with very high sedimentation rates related to enhanced nepheloid layer activity. Much larger slope failures are sourced from the shelf break and canyon head and are the result of occasional releases of large volumes of sediment, likely related to earthquake activity. These failures rapidly evolve into large-volume, high-energy, sand-rich turbidity currents that flush the entire canyon and dominantly deposit in the lower canyon and distal abyssal plains. This episodic turbidity current activity results in highly heterogeneous sediment distribution across the distal (lower) U-shaped floor of both canyons, with abundant erosional scours and depositional bedforms such as sediment waves. Our results highlight the complex interplay of sedimentary processes operating within major canyon systems, and have potential implications for efficient characterisation of hydrocarbon reservoirs, accurate spatial and temporal prediction of geohazards, and the distribution of benthic ecosystems in canyon environments.