The Sedimentology of the Holkerian Rocks of South Wales

Abstract

In this thesis the Holkerian age rocks of South Wales are reviewed In four main areas Pembrokeshire, Gower, the North Crop and the Vale of Glamorgan. This is the first basinwide study of a Dinantian stage in the U.K. The sedimentology of the Holkerian rocks is interpreted using a scheme of fourteen lithofacies and four lithofacies associations. The latter include Open Shelf, Oolitic Barrier, Marginal Back Barrier and Back Barrier Lagoon associations. The Open Shelf association represents the deposits of a muddy outer shelf area, which was prone to storm redeposition of bioclastic sands. The Oolitic Barrier association represents a broad shallow area characterised by active ooid shoals at its seaward edges and elsewhere by the deposition of oolitic aggregate grains under conditions of early cementation and low rates of sedimentation. The Marginal Back Barrier association represents the areas immediately shoreward of the Oolitic Barrier. The lithofacies association is heterolithic and represents the deposits of micritic tidal flats within a background sediment of subtidal peloid sands. The tidal flats were channeltsed and oncoids were formed in the mouths of these channels. The tidal flats were also rimmed by low energy beaches. The Back Barrier Lagoon association represents an area marginal to the Holkerian shoreline. This was characterised by a mosaic of environments including a widespread subtidal peloid sand, micritic tidal flats, ooid shoals, reworked quartz-sands, carbonate bank/flood tidal delta developments and complex Intertidal shorelines. The evolution of the carbonate shelf is described using the terminology of. Read (1985). The South Wales Shelf developed through Holkerian times from a ramp with sheet sands, to a ramp with oolitic shoal barrier and back barrier lagoon complex, to an accretionary rimmed shelf by the end of the Holkerian. Controls on this evolution are split into structural and sedimentological controls. The main structural controls on shelf morphology were uplift on the Usk Axis in the east, flexure on a broadly east/west shelf hinge (now the Cefn Bryn Thrust, Gower) to accommodate high rates of sedimentation and uplift on the Ritec Fault in Pembrokeshire. The main sedimentological control was the progradation of the oolitic shoal barrier complex. This created the tripartite division of the South Wales Shelf into Outer Shelf, Oolitic Barrier and Back Barrier areas.