Bottom-current sediment waves and mass-movement deposits on the sines contourite drift, SW Iberia

Large-scale sediment waves and mass movement deposits have been recently discovered, based on multichannel seismic reflection data, associated to the Sines Contourite Drift, on the middle continental slope along the Southwest Portuguese Margin. The sedimentary waves expand over 30 km2 between 750 m and 1850 m water depth, having sigmoid shapes with wavelengths from 500 to 2150 m, wave heights between 40 and 63 m and wave crests subparallel to the bathymetric contours. A basal discontinuity marks the onset of sediment wave development and upslope migration towards east since the Early Quaternary (2.5 Ma ago). In addition, numerous mass-movement deposits are seen mostly concentrated in the west of the Sines Drift, associated with the steep Pereira de Sousa Scarp. The mass-movements are characterized by amphitheater shapes, head scarps and occur staggered till the slope, accumulating at the base as thick deposits.
The morphology and location of the sedimentary waves suggests a close relation to the circulation of the Mediterranean Outflow Water (MOW). This water mass exits the Strait of Gibraltar and flows northward along the continental slope, due to the Coriolis Force. Climatic-eustatic fluctuations in the Quaternary caused several enhancements and deepening of the Mediterranean Outflow Water and, subsequently, interaction with the seafloor. With this work we propose that the dynamics between the MOW and the continental slope is responsible for generating the sediment waves, but also the presence and morphology of these waves encouraged the development of mass-movement deposits, especially when the setting corresponds to an unstable and oversteepening continental slope. This research is a good example of the interaction between along- and across-slope processes and it could be useful for a better understanding of the implications of bottom currents in controlling the continental margin morphology.