Large sediment waves over the Gulf of Roses upper continental slope (NW Mediterranean)

Abstract

Large sediment waves have been observed over the Gulf of Roses (GoR) continental slope (NW Mediterranean), developed between ~200 and ~400 m water depth. Geometric parameters computed from the acquired swath bathymetry revealed mean wave lengths of ~2000 m, and maximum wave heights of ~60 m. Single-channel reflection seismic profiles provided information on the sediment wave internal structure and the Quaternary stratigraphic architecture of the GoR outer shelf and slope. Seven main seismic units could be identified, with continuous development of sediment waves over the outer continental shelf and upper slope, showing differences in wave height and length. The seismic units are differentiated by erosional surfaces that can be followed from the outer shelf down the slope, and which have been correlated with Pleistocene eustatic oscillations. Sediment cores were collected over the sediment wave crests and troughs, and grain size distribution and sediment accumulation rates were analysed. Results show a dominant fraction of fine sediments, allowing classifying the observed bedforms as mud waves. Calculated sediment accumulation rates ranged between 0.08 and 0.18 cm/y, with no clear sedimentation pattern (e.g. differential sediment deposition rates) observed between wave crests and troughs. Nevertheless, the presence of thick surface mixed layers and the increase of the sand fraction in the upper sections of the cores indicate that the surface sediments are affected by bottom trawling activities, since the area is highly impacted by this human activity. The sediment waves observed over the GoR slope are most likely to be formed by bottom currents generated by overflows of dense water originated in the Gulf of Lions shelf, which cascade downslope in an oblique angle with respect the main bathymetric contours. This study offers new insights on the role of dense shelf water cascading processes and associated off-shelf sediment transport reshaping the morphology of the open-slope regions.