Bottom-current sediment waves, sediment drifts and contourites in the northern Rockall Trough

Abstract

Four types of large-scale sediment bedform occur in the northeast Rockall Trough: broad sheeted drifts, elongate drifts, sediment waves and thin contourite sheets. A large sheeted drift occupies the northern basin; its crest can be traced for 40 km perpendicular to the bottom-current flow direction. Elongate drifts occur adjacent to steep areas of continental slope, separated from the break of slope by a moat. Drift crests are oriented alongslope, parallel to bottom-current flow. Elongate drifts are asymmetric, with the steeper flank facing upslope; sedimentation rates are higher on the downslope flank. Sediment waves, with a wavelength of 1–2 km and wave height up to 20 m, occur locally on the flanks of sheeted and elongate drifts; larger waves (wavelength of 3–4 km and wave height of 50–100 m) occur on the flank of the Wyville–Thomson Ridge. Wave crests are oblique to bathymetric contours, to drift crests and to bottom-current flow. The association of sediment waves and drifts, their orientation oblique to regional slopes, and the presence of contourite sediments in cores, indicate that these are bottom-current rather than turbidity-current waves. A contourite sheet, typically 10–25 cm in thickness and composed of well sorted sand, covers the present-day seafloor along the lower slope. Bedform evolution through time has involved a complex interaction between bottom currents of variable intensity, sea level change and sediment input. The present sea level highstand is characterised by minimal sediment input and redistribution of sediments by strong bottom currents, giving active bedform growth and contourite development. Variability of sediment input is the main factor affecting sedimentation during sea level lowstands. Where input was high, as when ice sheets reached the shelf edge, downslope sediment transport dominated, overwhelming any bottom currents that were active. When input was low, such as during the last lowstand, some evidence for active currents can be observed, although these were less energetic than those during highstands.