A seismic study of deep geological structure in the Bristol Channel area, SW Britain |
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| Authors: | J Mechie M Brooks |
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| Institution: | Geophysikalisches Institut, Universität Karlsruhe, Hertzstrasse 16, D- 7500 Karlsruhe 21, Federal Republic of Germany;Department of Geology, University College, Cardiff, PO Box 78, Cardiff CF1 1XL, Wales |
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| Abstract: | Summary. Results from eight seismic refraction lines, 35–90 km long, in the Bristol Channel area are presented. The data, mostly land recordings of marine shots, have been interpreted mainly by ray-tracing and time-term modelling. Upper layer velocities through Palaeozoic rocks usually fall within the range 4.8–5.2 km s?1. Below the Carboniferous Limestone with a normal velocity of 5.1–5.2 kms?1, the Old Red Sandstone with a velocity of 4.7–4.8 kms?1 acts as a low velocity layer, as do parts of the underlying Lower Palaeozoic succession. In the central South Wales/Bristol Channel area and the Mendips, a 5.4–5.5 km s?1 refractor is correlated with a horizon at or near the top of the Lower Palaeozoic succession. Under the whole area, except for north Devon, a 6.0–6.2 km s?1 basal refractor has been located and is correlated with Precambrian crystalline basement rocks. In general, this refractor deepens southwards from a series of basement highs, which existed before the major movements of the Variscan orogeny in South Wales, resulting in a southerly thickening of the pre Upper Carboniferous supra-basement sequence. In north Devon, a 6.2 km s?1 refractor at shallow depth, interpreted as a horizon in the Devonian or Lower Palaeozoic succession, overlies a deep reflector that may represent the Precambrian crystalline basement. |
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