Gravity measurements in the north-eastern part of the Irish Sea

Abstract

During the summer of 1961 124 new gravity measurements were made in the north (mainly north-east) Irish Sea with an underwater gravity meter. In north-west England, south-west Scotland, and Northern Ireland a regional rise in the Bouguer anomaly is observed on approaching the north Irish Sea; a high regional Bouguer anomaly of more than 40mgal is also characteristic of the Isle of Man. The author considers that this reflects either a crustal thinning of about 3 km or a denser crust (by about0 03g/cm 3 ) beneath the Irish Sea; denser basement rocks may in part be responsible. Over most of the sea area surveyed negative gravity anomalies of 20mgal to 40mgal amplitude are superimposed on the high regional Bouguer anomaly. Both the shape of the anomalies and their correlation with the surrounding coastal geology suggest that they are essentially caused by large sedimentary basins. The two main negative anomalies thus interpreted are ( a ) between the Point of Ayr, the Solway Firth, and Wigtown Bay (thought to be at least 2130 m [7000ft] deep), and (b) between south Cumberland and the Isle of Man (probably at least 2440m [8000ft] deep). Coastal geology suggests that Carboniferous and New Red Sandstone rocks form the major 'fill', although sediments of Old Red Sandstone or post-Triassic age cannot be ruled out. A negative anomaly of nearly 20mgal amplitude forms the south-eastern continuation into Luce Bay of the negative anomaly over the Stranraer New Red Sandstone basin. It closes to the south-east near the mouth of Luce Bay. As on land, it is interpreted as an asymmetrical basin of New Red Sandstone and earlier sedimentary infill with a steep (? faulted) northeastern margin, reaching a maximum depth of at least 1340 m (4400 ft). A single gravity traverse westwards from Peel reveals a sharp gravity drop of about 15mgal immediately off shore; this is interpreted as a sedimentary basin lying off the west coast of the Isle of Man. The Isle of Man thus appears to be an uplifted horst-like region of Lower Palaeozoic rocks with the northwestern and south-eastern coastlines controlled by the margins of steep-sided sedimentary basins of post-Lower Palaeozoic age.