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Shallow Marine Sequences |
Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK
Department of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK
At outcrop, the Portlandian Stage of the western Wessex Basin, southern England comprises a complex series of marine and non-marine carbonates deposited in moderate to shallow water depth, with the occasional development of siliciclastic deposits near to the basin margins. The strata record part of a marked overall long-term regression on which are superimposed four shorter term relative sea-level cycles. The base of each of these short-term cycles is marked by a major unconformity or its correlative conformity (sequence boundary) formed during the maximum rate of relative sea-level fall; these are denoted P1 to P4. The south Dorset exposures are each in slightly different tectonic settings within the Central Channel Sub-basin. The Isle of Purbeck sections are stratigraphically the most complete, but the sections on the Isle of Portland, previously assumed to be complete, contain at least two significant stratal gaps. The sections at Ringstead Bay and Dungy Head are thin, condensed and in part incomplete. Proximally, in the Vale of Wardour (Mere Sub-basin), Vale of Pewsey (Pewsey Sub-basin) and on the London Platform at Swindon, in Oxfordshire and in Buckinghamshire the succession becomes condensed, the major unconformities are frequently angular and there is at least one incised valley.
