A Lower Cretaceous oyster reef (Cinder Member, Purbeck Limestone Group, Dorset, southern England): biotic response to co-seismic sea-level rise?

The Berriasian (Lower Cretaceous) Cinder Member (Durlston Formation, Purbeck Limestone Group) in south Dorset, southern England (Portland–Wight Basin) is an oyster-rich brackish-water horizon deposited as a biostromal reef complex within linked half-grabens immediately south of the Purbeck–Ridgeway fault system. It is well exposed in coastal sections between Durlston Bay, Swanage and Durdle Door, where its relative thickness and abrupt appearance above lacustrine lithofacies indicate rapid deepening and marine transgression, promoting reef establishment close to the northern margin of the Purbeck lagoon. We suggest that the marked increase in accommodation space demonstrated by this outcrop was due to movement on the Purbeck Fault, which was evidently active during Berriasian times. Cinder Member developments documented elsewhere in the southern English Wessex and Weald basins are potentially asynchronous, though deposited against a common backdrop of substantial marine influence within a basin-wide lagoonal complex, close to sea level.