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Basement tectonics and their relationship to Mesozoic megasequences in the Celtic Seas and Bristol Channel area

Abstract A synthesis of new and pre-existing data on the deep geological structure of the Celtic Sea and Bristol Channel basins shows similarities to published models of deep structure from the onshore Wessex Basin to the east. A series of Variscan thrust planes, reactivated in the Mesozoic as normal faults, underlie the study area. Beneath the Bristol Channel, one such southward-dipping thrust may be equated with the ‘Variscan Front’. This structure is offset by NW-SE orientated transcurrent faults, structures thought to be active from the late Palaeozoic to the present-day. Mesozoic stratigraphic successions from the North Celtic Sea Basin and South Celtic Sea; Bristol Channel, and northern Wessex Basins suggest uplift and erosion of the latter areas in the late Jurassic. This is contrasted with increased subsidence at the same time in the North Celtic Sea. This uplift may have been facilitated by reactivation of the ‘Variscan Front’ thrust as a thrust in the east of the area, and by similar thrusts beneath the Pembrokeshire Ridge to the west. This reactivation and uplift during North Atlantic opening is suggested to have controlled the preservation of Upper Jurassic-Lower Cretaceous sedimentary megasequences.