Latest Jurassic–Early Cretaceous sedimentary cyclicity and events (Wessex Basin, southern England): A case of pulsed mantle convection?

Abstract

A pulsed mantle convection model has been proposed for regional third-order cyclic sea level changes during time intervals for which robust evidence of extensive polar ice caps is lacking. One such interval, the Late Jurassic–Early Cretaceous, is represented in southern England's Wessex Basin by the marine to non-marine Portland and Purbeck Limestone groups and Wealden Group and Supergroup. Third-order cyclicity in this succession is reviewed against the mantle convection model, involving pulsed growth of a regional uplift centred on the Cornubian Massif, bordering the Wessex Basin. Supporting evidence for the model is forthcoming from cyclicities in the Upper Tithonian marine Portland Group and Hauterivian–Lower Aptian alluvial–lacustrine–lagoonal Weald Clay Group. Third-order cycles in the Berriasian–Valanginian alluvial–lacustrine Hastings Group reflect fault reactivation along the Anglo-Brabant Massif, conventionally linked to North Atlantic opening. Credible cyclicities on comparable scales have not been identified in the Upper Tithonian–Lower Valanginian lagoonal–lacustrine Purbeck Limestone Group or the Valanginian–Barremian fluvial–alluvial Wessex Formation. Purbeck lithofacies are defined largely by climatic and local tectonic signals; cyclicities were potentially masked in the Wessex Formation by dynamic fluvial processes in proximity to the massif. Certain biotic events in the Purbeck Limestone Group and fluvial events in the Wessex Formation possibly reflect vertical movements of the Cornubian uplift.