The Bude Formation, SW England—a three-dimensional, intra-formational Variscan imbricate stack?

The deformation of the Upper Carboniferous Bude Formation, SW England, is re-interpreted in terms of a sedimentary multilayer sequence, which represents a compromise between mechanically competent and mechanically incompetent behaviour. The chevron folding that characterises the sandstone–shale multilayer sequence is shown to be only a part of a more complex, regional scale, polyphase deformational history that developed at high structural levels in the Late Variscan orogeny. The major tectonic features are thrust faults, which are responsible for the imbrication of the original sedimentary pile. Six distinct thrust sheets are recognised in the ∼5 km coastal section studied. Northerly directed thrusts pre-date chevron folding, whilst structurally necessary southerly propagating back-thrusts both pre- and post-date folding. Where they post-date folding, they impose a southerly shear onto the rocks, leading to modification of existing folds. The significance of these thrust structures has previously been ignored due to their tendency to develop parallel to bedding and to be overprinted by flexural slip features during chevron folding. A model is presented that views the Bude Formation as a three-dimensional intra-formational imbricate stack, which is, at any single locality, an instantaneous representation of an orogenic front propagating into its own foreland basin.