The geological results of seismic refraction surveys in the Bristol Channel, 1970-1973

Abstract

Results are presented of eighteen refraction traverses in the Bristol Channel. Estimates based on refraction and CSP surveys indicate a Mesozoic sequence about 2000 m thick in the Bristol Channel syncline north of Devon. The Central Bristol Channel fault zone in the northern limb of the syncline has an overall southerly downthrow of over 1000 m. Up to 270 m of Mesozoic strata occur in the outer part of Swansea Bay and the Helwick syncline contains about 700 m of Mesozoic strata SW of Gower. There is evidence for an igneous intrusion at shallow depth west of Lundy in the vicinity of a positive Bouguer anomaly. The Tertiary basin E. of the Sticklepath fault in Barnstaple Bay is at least 340 m thick. A high velocity refractor 2500 m below Carmarthen Bay is interpreted as lower Palaeozoic or Precambrian volcanic rocks. The surveys suggest that Cenozoic, Mesozoic and upper Palaeozoic strata can normally be differentiated on the basis of velocity but the results are of limited use in mapping upper Palaeozoic rocks and, in particular, provide no evidence for the major thrust postulated to subcrop beneath the Mesozoic strata of the Bristol Channel syncline.