Geophysical surveys over the South Irish Sea and Nymphe Bank

Abstract

Geophysical surveys using seismic profiler, seismic refraction and gravity methods have been made between 1964 and 1969 to explore the geology of the South Irish Sea and the Nymphe Bank. The results of these surveys are interpreted independently and when combined together form a consistent body of evidence of the physical nature of the rocks and the structural pattern in the area. A sedimentary basin has been found to underlie Cardigan Bay which contains gently folded sediments with low seismic velocities and which gives rise to a gravity anomaly of –70 mgal. This and a subsidiary basin underlying Tremadoc Bay are structurally controlled by north-east to south-west trending faults connected with lineaments recognised in the Palaeozoic succession of North Wales. Fold axes within the basin follow the general Caledonoid trends found in Wales. The depth to basement in the centre of the Cardigan Bay basin is estimated to be at least 6 km. The interpretation suggests that much of the sediment in the basin is Permo-Trias and Jurassic in age but that Chalk is absent. Unconformably overlying these in the centre of the basin is a succession up to 2 km thick of gently folded marine sediments interpreted as of Palaeogene age which are in turn unconformably overlain by horizontally stratified sediments which beneath parts of St. George's channel have a thickness of at least 200 m. The Cardigan Bay basin is attenuated sharply to the south-west at a line approximating to the position of the Hercynian front between South Wales and Ireland and appears to bifurcate to form two shallower synclinal structures, one along the south coast of Ireland and the other trending south to the Bristol Channel approaches. Between these structures, Chalk has been recognised to cover an extensive area south of Ireland, the geophysical evidence being supported by core samples from the sea bed. Direct sampling is impossible in the South Irish Sea without deep drilling because the older rocks there are covered by a layer of drift at least 30 m thick. The drift is limited to the region north of the line from St. David's Head to Carnsore Point. Across the South Irish Sea, therefore, the geological interpretation has been made using the physical characteristics determined from the geophysical surveys and referring these to the geological log from the Mochras borehole which penetrates the Tremadoc Bay basin, to outcropping formations in Wales and Ireland and to core samples taken from the sea bed south of Ireland. It is recognised that the geological history of the region is much as envisaged by O. T. Jones (1955) who predicted a sedimentary basin beneath Cardigan Bay rather like the Cheshire Basin. However, the extent of Tertiary subsidence appears to be greater than realised hitherto. It is argued that the Tertiary represented a period of rifting in the Irish Sea, following upon considerable post-Cretaceous erosion, and the subsidence within the basins may still be continuing to the present. The view that the Irish Sea formed a Tertiary rift structure is supported by similarities with other rift areas, such as the Rhine graben.