The geology of the Bristol Channel floor

Abstract
A reconnaissance survey has been made with side-scan sonar, Boomer and gravity corer of a sea-floor area extending from the mouth of the River Severn to the longitude of Hartland Point, and from the North Devon coast to the latitude of Porthcawl. The results include the recognition of two major WNW -ESE trending synclines arranged en echelon with a minor intervening pericline. The eastern syncline, traceable to the longitude of Watchet and preserving Upper Pliensbachian (Lower Jurassic) rocks in its core, is the seaward extension of the Glastonbury Syncline. The western syncline, of which little more than the eastern half could be investigated, includes a sequence of Jurassic rocks up to high Kimeridge Clay. This Bristol Channel Syncline is cut by a series of N W -SE trending tear-faults analogous to those in southwest England and has its southern m argin truncated by a strike fault with northerly transport. The Jurassic sequence is unusual in its abnormal thickness (in excess of 1600 m) and its predominantly argillaceous nature. Thick limestones are unknown, although cementstone bands occur, mainly in the Lower Lias. Sands and sandstones were found only in beds of Upper Oxfordian and, dubiously, of Portlandian (Middle Volgian) age. Elsewhere in the succession, and notably for the Aalenian-Lower Callovian interval, the correlatives of thick carbonate sequences of m ainland successions are thin sands and sandy clays or non-sequences. The Bristol Channel region may therefore have supported a lesser depth of water than other basins in southern England. Facies changes within the area are small. They offer no support to the hypothesis of a Welsh island, but suggest the possibility of a Cornubian source area for clastic sediments in early Upper Jurassic times.

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