Peter F. Croker

The combined analysis of high‐resolution 2D seismics and an industrial 3D seismic data volume from the western Porcupine Basin, offshore SW Ireland, revealed an unusual picture of a buried sediment collapse and slope failure. A proportionally thin (≤ 85 m) but vast (> 750 km²) slab of consolidated sediments started to slide downslope, in the meantime breaking into hundreds of vertically undisturbed blocks, up to 500 m in diameter. The most probably overpressured underlying horizon seems to have liquefied and acted as a slide plane until the excess pore pressure had dissipated. Then – still very early in the slide development – the process stopped, freezing the failure at its initial stage.

The Irish Sea has long been known to contain extensive areas of shallow gas accumulation, particularly in the area southwest of the Isle of Man. More limited occurrences of seabed features associated with shallow gas migration, such as columnar disturbances, seabed doming and pockmarks, have also been reported. Until now this gas was generally assumed to be of biogenic origin. Recently acquired route survey data have provided new insights into the distribution of both gas accumulation and gas migration in this area, as well as identifying areas of active seepage. It is proposed that the gas originates as coalbed methane or thermogenic gas in the extensive Carboniferous deposits underlying the region. This has considerable significance for hydrocarbon exploration in the area.

The Porcupine basin is a north-south-aligned basin offshore western Ireland that contains more than 8 km of post-Paleozoic sediments. The Mesozoic and

Seabed samples, consisting of shale of Lower Liassic age and limestones of Upper Liassic age, have been recovered from the northwest flank of the St. George's Channel basin. These data have made possible the calibration of parts of lines 6 and 11 of the I.G.S. deep seismic survey. An updated and refined version of the geology of the south Irish Sea is presented which incorporates the new information and indicates the extent of the Precambrian and Lower Palaeozoic basement that is now thought to subcrop beneath the Pleistocene cover southwest of the Lleyn Peninsula. In addition the offshore extension of the Bala Fault is suggested to be an intra‐basin fault across Cardigan Bay and St. George's Channel.