The geological interpretation of a gravity survey of the Bristol channel

A Bouguer anomaly map of the Channel and surrounding land areas is presented and interpreted. The main features of the map are: (1) a negative anomaly approaching -20 mgal trending WNW across the southern half of the Channel and adjacent areas of west Somerset and north Devon; (2) a broad flanking area of weak positive anomaly in the northern part of the Channel which may extend southeastwards across the Cothelstone fault into Bridgwater Bay; and (3) a local positive anomaly of over +2omgal around Lundy Island. These anomalies are all superimposed on a strong regional gradient of +0.38 mgal km-1 to the southwest. The negative anomaly in the southern part of the Channel is due partly to Mesozoic strata in the Bristol Channel syncline but partly to underlying, low density, Upper Palaeozoic rocks which are probably preserved in a structural basin similar to the South Wales coal basin but overthrust by the Devonian succession of Exmoor. The parallel belt of positive anomaly in the northern half of the Channel is interpreted as overlying an anticlinorial zone, characterized by Old Red Sandstone at shallow depth, separating the South Wales coal basin to the north from the postulated basin to the south. The positive anomaly around Lundy Island is attributed to the effect of a large basic pluton of Tertiary age occurring at shallow depth, and the gravity field of Lundy is compared with that of other Tertiary igneous centres. A sketch structural section across the Bristol Channel is presented.