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With this contribution we use a pair of overlapping 3D seismic surveys and two exploration wells to document the response of long-lived slope channels to the onset of bottom currents sweeping the lower slope in the NE Rockall Basin, offshore Ireland. Downslope gravity current activity, linked to a phase of uplift, prevailed throughout the Eocene and led to the formation of multiple channels, most notably a large-scale sinuous channel complex (Channel 4 Complex) tied to a persistent sediment entry point on the margin. Channels fed lobes on the floor of the basin, with increased axial tilting forcing gravity currents to flow parallel to the base of slope. A phase of margin-wide differential subsidence and basin deepening in the Late Eocene then activated bottom current circulation across the basin. Northward-flowing bottom currents first erosionally refashioned the lower slope creating a prominent unconformity with contourites that then initially forming infill drifts. Bottom currents swept along and obliquely upslope, building plastered drifts that straddled the lower sections of the still active channels. The drifts modified and amplified the spurs separating the active channels, with interaction between along and downslope processes accreting sediment that built and maintained channel relief and allowed the channel mouths to extend further basinward over and across the earlier lobes. Contourite-forced channel extension on account of lower slope depositional re-profiling represents another manifestation of the interplay between slope channels and bottom currents. In the case of NE Rockall, the accreted base of slope wedge and mounded geometry resembles similar features elsewhere along the eastern Rockall margin. Those have previously been ascribed to mass-transport and base-up channel initiation, however, the similarities are so striking that we propose that these also are the result of lower slope re-profiling by bottom currents.

The eastern flank of the Rockall Bank is dominated by the scarps of a large slide complex with unusual low aspect ratio (120 km wide and 150 km long). With the use of an extensive 2D seismic dataset, re-processed high-resolution multibeam bathymetry, a TOBI mosaic and one piston core we demonstrate that the headwall of the Rockall Bank Slide Complex (previously referred to as the Rockall Bank Mass Flow) may have occurred as a multiphase slope collapse in at least three episodes of instability since 16 ka BP, with each of the events generating a slide with a more typical elongate morphology. Reconstructions of the pre-slide seafloor suggest a volume of collapsed sediments in the range265–765 km3. Seismic facies investigation indicates that the slope and failed sediments comprise almost exclusively contourites that are prone to failure due to their well sorted character and high water content. A systematic examination of the slope morphology, seismic facies and sub-seafloor structures demonstrates a direct relationship between the RBSC seafloor scarps and the basement morphology. Focused fluid flow along basement-bounding faults, and/or differential compaction across the scarps, are considered to have had an important role in slope failure. This study highlights the importance of deeply-buried structures in the stability of slopes. It also shows how detailed examination of high-resolution data across large areas of slope instability can reveal smaller-scale multiple events. We believe that where contour-swept slopes are steep, the resulting intensification of near-bottom velocities prevents healing slope processes and as a result steep slope gradients remain steep and thus unstable.

The continental margin offshore of western Ireland offers an opportunity to study the effects of glacial forcing on the morphology and sediment architecture of a mid-latitude margin. High resolution multibeam bathymetry and backscatter data, combined with shallow seismic and TOBI deep-towed side-scan sonar profiles, provide the basis for this study and allow a detailed geomorphological interpretation of the northwest Irish continental margin. Several features, including submarine mass failures, canyon systems and escarpments, are identified in the Rockall Trough for the first time. A new physiographic classification of the Irish margin is proposed and linked to the impact of glaciations along the margin. Correlation of the position and dimensions of moraines on the continental shelf with the level of canyon evolution suggests that the sediment and meltwater delivered by the British–Irish Ice Sheet played a fundamental role in shaping the margin including the upslope development of some of the canyon systems. The glacial influence is also suggested by the variable extent and backscatter signal of sedimentary lobes associated with the canyons. These lobes provide an indirect measurement of the amount of glaciogenic sediment delivered by the ice sheet into the Rockall Trough during the last glacial maximum. None of the sedimentary lobes demonstrates notable relief, indicating that the amount of glaciogenic sediment delivered by the British–Irish Ice Sheet into the Rockall Trough was limited. Their southward disappearance suggests a more restricted BIIS, which did not reach the shelf edge south of 54°23′ N. The various slope styles observed on the Irish margin represent snapshots of the progressive stages of slope development for a glacially-influenced passive margin and may provide a predictive model for the evolution of other such margins.