Post-Cadomian erosion, deposition and basin development in the Channel Islands and northern Brittany

Abstract

Abstract Facies analysis, provenance, and palaeocurrent studies of the Rozel Conglomerate Formation (Jersey), Alderney Sandstone Formation and the Erquy—Fréhel Group (northern Brittany), have established patterns of ‘molasse-like’ sedimentation in a series of basins related to a phase of post-Cadomian tectonism. Although shallow marine sediments form minor constituents of the Alderney Sandstone Formation and Erquy-Fréhel Group, fluviatile sediments deposited by braided streams axially draining eastwards predominate. Palaeocurrents within alluvial fan deposits in Jersey, and similar tributary systems in Alderney and Brittany, indicate drainage orientated either transverse to the basin axes, or at a high angle to the main trunk streams. Gross sediment dispersal was towards the Cambrian shoreline that lay to the east. These basins and their equivalents in Normandy are elongate parallel to the east-west structural grain of the Cadomian Orogen. The basins opened, and the sediments were shed, in response to a phase of post-Cadomian extension. The geometries of these basins were possibly controlled by the re-activation of Cadomian thrust and strike-slip faults. The area to the west of Normandy, though probably mountainous in aspect, was one that was, for the most part, open to a marine connection. In Normandy itself, Cambrian sediments were deposited sometimes in continental environments, but more commonly in shallow seas.