Shelf to slope sedimentation processes and the impact of Plio–Pleistocene glaciations in the northeast Atlantic, west of Ireland

Abstract

The eastern margin of the Rockall Trough, between 54°30′ and 56°30′N, straddles the southern limit of the glaciated North Atlantic margin. A complex system of canyons, channels and slope failure features on the steep margins of the trough is resolved on the TRIM (TOBI Rockall Irish Margins Project) deep-towed sidescan survey. The canyons consist of a complex headwall gullied region, produced by multiple slope failure events, and interconnect to form a cauliform pattern in plan view. Individual gullies are U-shaped in cross-section in the upper parts of the margin and are connected downslope to single, deeply-incised, V-shaped channels. These channels feed extensive aprons of sediment on the lower slope supplied by failure of the headwall regions and sediment transport along the shelf into the canyon headwalls. The change in the canyon morphology systems reflects a more abundant supply of glaciomarine sedimentary debris from Pleistocene ice sheets, whose southerly limit lay along this part of the eastern Rockall margin. A good correlation between acoustic sidescan backscatter and more deeply penetrating 3.5 kHz profiler data suggests that sediment transport pathways were stable through time. Slope failure escarpments occur in association with the canyon systems along the upper part of the slope. Canyons are absent in the north of the area where the slope gradient is lowest. This is interpreted as the result of progradation of sheet-like flows of glaciomarine debris across the southern part of the Donegal Fan. A poor correlation between acoustic backscatter and sub-bottom profile data indicates that sediment transport pathways varied through time in the region. Evidence for strong northeast-flowing bottom currents is present along the upper slope and shelf. These currents transport and sort sediments along the entire eastern margin of the Rockall Trough from the south of the Porcupine Bank to the Hebrides Terrace Seamount.