Contourite distribution on the continental slope off NW Ireland, Rockall Trough (NE Atlantic Ocean): genesis and evolution

Continental slopes are commonly shaped by ocean currents, forming contourite systems, especially in areas where submarine canyons are not very abundant. Although contourites have been identified in different environments, it is still unclear how near bottom circulation controls the deposition and erosion of sediment in contourite systems and shapes the slope due to the limited direct oceanographic observations. The continental slope located on the eastern Rockall Trough offshore NW Ireland is dominated by numerous plastered drifts, which are genetically linked with contourite terraces. Terraces and plastered drifts generally grow synchronously to form terrace-plastered drift systems (T-D system). This research focuses on the development of T-D systems and the effects of oceanographic processes through a multidisciplinary approach that combines geophysical, sedimentological and oceanographic observations. T-D systems develop on the upper and middle slopes at 500–1500 m water depths under the influence of the Eastern North Atlantic Water (ENAW). They started to form during the Late Oligocene. Direct measurements show that strong along-slope bottom currents (up to 50 cm/s) and breaking internal waves control the development of T-D systems. Internal wave activity causes the formation of high-turbidity nepheloid layers above the T-D systems. The extension of the observed contourite systems is confined by downslope gravitational processes, with submarine canyons limiting their lateral extension and mass-wasting processes dominating the lower slope. This study provides a better understanding of the conditions under which bottom currents control the morphology of continental slopes worldwide. • Terraces and plastered drifts (T-D systems) dominate NW Ireland continental slope. • T-D systems are formed by bottom currents and breaking internal waves. • Downslope gravitational processes limit the extensions of T-D systems.