Deep-marine seabed erosion and gravel lags in the northwestern Rockall Trough, North Atlantic Ocean

Abstract

A zone of active seabed erosion has been identified in the northwestern Rockall Trough using seismic reflection profiles and cores. The region from George Bligh Bank to Rockall Bank has been subject to vigorous bottom-current activity, for at least the last 35 Ma. Bottom currents originate from southward flowing North Atlantic Deep Water, in water depths of 500–2000 m. Acoustic character mapping reveals a zone of erosion extending over 8500 km 2 along the northwestern margins of the trough. The erosion surface is characterized by an acoustically reflective seabed, with occasional parallel to transparent reflectors. These are the result of strong bottom-currents exposing the underlying volcanic basement, drift sequences and fan sediments. The erosion surface is covered by a <10 m veneer of Quaternary sediment. The majority of the basin consists of well-stratified, parallel Quaternary drift and hemipelagite sequences. Along the western margin of the trough, these sediments form broad-sheeted drifts. Eocene sediments adjacent to George Bligh Bank have been exposed by strong bottom-currents for the last 35 Ma, compared with the flanks of Rockall Bank, where sedimentation was intermittent. Core sampling from George Bligh Bank and Rockall Bank recovered Quaternary gravelly–sandy muds interpreted as gravel lags, and muddy sandy contourites, overlying early–mid-Eocene aged sediments. The gravels represent the influence of a strongest North Atlantic Deep Water flow, winnowing coarse sediment into ‘lags’, commonly preserved within sandy contourite sequences.