Abstract

In a review of the geology of the sea area between 48° and 52° N., 5° and 14° W., the dominating structural trend is seen to be northeast-southwest with secondary features trending northwest. Interpretation depends mainly on geophysical data, as only a relatively small portion of the area has been sampled directly, and little information is as yet available from drilling. Seismic experiments indicate a uniform 28 km thick crust beneath most of the Celtic Sea, thinning southwestwards across the continental shelf edge to 16 km in the south of Porcupine Seabight. A detailed gravity map shows the area to be divided into two major sedimentary basins, the Celtic Sea Basin and the Western Approaches Basin. Some of the ENE-WSW trending gravity ‘lows’ are related to granite batholiths that extend southwestwards from the Cornish Peninsula and through Haig Fras, and which are intruded into a basement ridge that separates the two major basins. Other gravity ‘lows’ of similar trend relate to sediment filled troughs, confirmed from seismic surveys. In the Celtic Sea Basin, these include the Nymphe Bank trough south of Ireland, bounded to the northwest by a major flexure, the Carnsore-Fastnet line, and separated from the Celtic Sea trough to the southeast by a basement rise. The Celtic Sea trough is fault bounded to the southeast along the Lundy-Haig Fras line and extends southwestwards to about 8°W. Westwards of this is the Great Sole trough, with northeast-southwest trend centred on 50°N., 10°W. and south of this is another at 49°20′N., 9°30′W. These ‘lows’ abut to the west against a north trending gravity ‘high’ that marks the shelf edge. The Western Approaches Basin forms an ENE-WSW trending synclinal feature plunging gently westwards, structurally controlled by the Aldernay-Ushant line to the south and the granite intruded ridge to the north. Combined gravity and seismic interpretations indicate sediment thicknesses of 5 to 6 km in Nymphe Bank and Celtic Sea troughs and at least 4 km in Great Sole trough. Chalk is extensive across the seabed south of Ireland but is overlain by Tertiary to Recent sands and clays in the troughs. Westward the chalks thicken towards the shelf edge and in Porcupine Seabight where they form the upper part of a 2-3 km thick sequence. The Chalk seabed outcrop continues across the Lundy-Haig Fras line into the Western Approaches Basin. It appears that within the two basins, beneath a relatively thick Chalk cover, is a sequence of Lower Cretaceous sediments of deltaic character overlying Jurassic and Triassic beds possibly containing salt structures.

The structural history of the area is compared with that of the Newfoundland Shelf in relation to the evolution of the North Atlantic.