Variscan facies and structure in central SW England

Abstract

The character and distribution of middle Devonian to upper Carboniferous facies in central SW England is described. These may be interpreted in terms of the simple model of a shelf extending from a continent to the north, across most of SW England in earliest Devonian times. Within it a major E–W trending basin developed during the middle Devonian, possibly as a half graben structure with an active northern hinge. On this shelf in mid and late Devonian times, argillite successions accumulated to the north and a carbonate complex on the now starved platform to the south of the basin. In the early Carboniferous, basinal conditions extended northwards over the shelf, but to the south, at first flysch and then paralic sediments with a southern origin were ponded against the carbonate complex. Flysch penetrated through and across the platform into the intra-shelf basin. This flysch prograded northward to cover the whole area by early late Carboniferous times. The western part of the main basin suffered structural inversion in late Visean times and semiconsolidated sediments spread northward across its margin. The eastern extension of the basin was not deformed until the late Namurian, at which time successions were folded and disrupted into nappe structures showing northward transport. The most far-travelled nappes are represented at the highest structural levels. The stratigraphy of the coastal section of N Cornwall, which has been used to devise a structural model involving a major autochthonous south-facing overfold, is reassessed. The nappe structures and successions recognized inland are identified on the coast, and the overfold model is disproved. The whole of central SW England forms part of southerly derived thrust and nappe terrane.