Distribution and palaeoenvironmental significance of molluscs in the late Jurassic-early Cretaceous Purbeck Formation of Dorset, southern England: a review

Knowledge of the stratigraphic distribution of Purbeck molluscs in the largely lacustrine to marginal-marine Purbeck Formation of Dorset, southern England is summarized. The gastropod faunas are well known but the bivalves await detailed documentation. The shell concentrations are largely a result of wave and/or current processes although bivalves are locally preserved in life position. Comparison with Recent marginal-marine settings suggests that the stratigraphic distribution of Purbeck molluscs reflects long-term palaeoenvironmental change. Some bivalves are interpreted as strongly euryhaline, environmentally-tolerant opportunists. Others, notably those of marine aspect, probably tolerated only restricted salinity fluctuations. Dinosaur tracks throughout much of the succession indicate no significant changes in water depth. Hence salinity excursions are attributed mainly to fluctuations in runoff and evaporation, as well as changing barrier porosities and inflow-channel configurations. Amongst the near-marine phases, the Cinder Member provides evidence for attendant sea-level rise. Many upper Middle-Upper Purbeck shell concentrations resemble those of the younger Wealden lacustrine-lagoonal successions and attest to upfaulting of intrabasinal highs and/or of the Cornubian massif in latest Berriasian or earliest Valanginian times.