Glacimarine slope sedimentation, contourite drifts and bottom current pathways on the Barra Fan, UK North Atlantic margin

The sedimentary record of a 30-m core (MD95-2006) from the Barra Fan in the eastern Rockall Trough has been correlated with high-resolution seismic profiles obtained across sediment drifts and large mass flow deposits. A series of sediment drifts, featuring upslope migrating wavy bedforms, has been identified with deposition focussed along topographic steps created by glacigenic debrite lobes. The most extensive drift accumulation, termed the Barra Fan Drift, is observed on the distal fringe of the fan where an 80-m-thick sequence of aggrading to migrating sediment waves onlap a mega-debrite scarp. Prior to 26 ka (14C yr) and during the late glacial to Holocene transition silty-muddy contourites were deposited by the northward-flowing Deep Northern Boundary Current. Between 26 and 18 ka and around 14 ka contourite deposition was replaced by distal glacimarine sedimentation featuring thin-bedded sandy turbidites that were triggered during shelf-edge advances of the British Ice Sheet. During the last glacial period sediments accumulated at rates of more than 40 cm/ka, as a consequence of the high flux of sediments from the shelf margin and winnowing of exposed mass flow deposits by along-slope currents. In contrast the Holocene is represented by a condensed interval of silty-sandy mud due to vigorous bottom circulation and a low terrigenous sediment supply. Seismic seafloor signatures suggest that the present morphology of the Barra Fan is shaped by two pathways of bottom currents, probably both related to the Deep Northern Boundary Current. One branch of this water mass follows the lower slope, causing pronounced erosion on the distal part of the Barra Fan Drift, while the other is directed across the debrite topography of the fan bulge.