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Glacigenic sediment fans recording shelf edge deposition from marine-terminating ice sheets have previously been recognised along the NW European continental margin from Svalbard to as far south as Donegal, off north-west Ireland. Here we present evidence of a previously unrecognised partially glacially-fed Plio-Pleistocene sediment wedge on the continental shelf west of central Ireland using 2D and 3D seismic reflection data correlated to a commercial borehole. The ‘Connemara Fan’ covers a shelf area of approximately 9000 km2 in water depths of 125–310 m, extending westwards into the Porcupine Seabight from the Irish Mainland Shelf. The wedge comprises up to 160 m of sediment that culminates in a prominent moraininc ridge at seabed and contains two discordant reflection surfaces (R1 and 2) that subdivide it into three seismic units (SU1–3). Stratigraphic boreholes 27/24-2 and 2A located on the inner shelf show that the lower unit (SU1) is composed of Pliocene marine sediments, while SU2 and 3 comprise glacially influenced facies of Quaternary age. Extracts from a 3D seismic data volume within the central part of the fan show channels within the Pliocene succession, while iceberg scours are observed on the R1 and R2 reflectors. The Connemara Fan is inferred to record sediment supply from central western Ireland, with Quaternary units probably recording at least two glacial advance-retreat cycles with ice sheets repeatedly grounding across the inner (Irish Mainland) shelf. Our findings extend the range of glacially-influenced grounding line depocentres southwards along the NW European continental margin.

The sediment-undersupplied eastern margin of the Rockall Trough, west of Ireland, is incised by numerous canyons and slope failure features. Swath multibeam bathymetry has been integrated with 2D seismic profiles to constrain the Neogene evolution of the slope and its canyons. The morphology varies along the margin, with canyon heads located at mid-slope depths in the south but extending onto the shelf in the north. West of Porcupine Bank, slope gullies connect with a distributative channel system on the trough floor, while north of Porcupine Bank the basin floor is flat and featureless. Draped fault-blocks and deep structures exerted an important influence on slope gradients, canyon extent and geometry. A ‘bottom driven’ upslope–retrogressive slope failure mechanism is inferred for canyon formation. They were initiated by failure localisation following widespread slope rotation and instability linked to differential subsidence that produced a latest Eocene–early Oligocene (C30) regional unconformity. In the NE Rockall, where the greatest density of canyons occurs, a large mass failure wedge directly overlies the C30 surface and the seabed canyons have incised the upper part of the wedge. Axial profile data indicate that canyons in the NE Rockall Trough formed in Mid-Cenozoic times but were locally reutilised as sediment conduits during Plio-Pleistocene slope progradation.

International audience; The offshore Niger Delta provides a spectacular example of gravity collapse tectonics, but the timing of shale mobilisation remains poorly understood. Here we present new information from the western Niger Delta, based on a detailed interpretation of a 3D seismic volume, calibrated with biostratigraphic data from exploration wells. The study area is underlain by mobile shales containing thrust-fold anticlines, overlain by >5 km thick Late Eocene to present succession of folded and faulted sediments with fluid migration features. Sedimentation rates estimated at one deep well increased during two phases, more than doubling during the Late Eocene to Serravallian (39.5–12.5 Ma), and by up to ten times during the Tortonian (9.5 Ma) to present. Thinning and onlapping geometries within the lowermost, Late Eocene-Burdigalian (39.5–18.5 Ma) interval are interpreted to record syn-depositional deformation. This indicates that shale tectonics in the offshore western Niger Delta initiated early in the evolution of the region, supporting previous interpretations that the active compressional zone of the Niger Delta was in the present continental slope before prograding to the outer-fold-thrust belt in the Pliocene. Stratal thinning above the crests of thrust-fold anticlines in the northeast and southwest of the study area suggests that shale tectonics persisted throughout the Neogene and Quaternary. A chaotic column that rises to seafloor from the crest of thrust-fold anticline, interfingering with the stratified succession is interpreted as a mud volcano edifice that has been active since at least the Burdigalian. This supports early shale mobilisation in response to generation of overpressure and also the fact that mud volcano system once formed, may act as conduits for pressure release throughout the history of a gravity-driven collapse system. The spatial association of seafloor mud volcanoes and pockmarks with deeper thrust-fold anticlines and normal faults suggests fluid migration from depth. In contrast, pockmarks overlying mass-transport deposits and submarine channels suggest fluid-flow from dewatering of more recent sediments. Our findings provide new insights into the Cenozoic tectono-stratigraphic evolution of the offshore western Niger Delta and the relation of shale mobility to fluid migration and escape during continental margin collapse.