Taphonomy and origin of an accumulate of soft-bodied cephalopods in the Oxford Clay Formation (Jurassic, England)

An exceptionally well-preserved fauna from a new exposure in the Peterborough Member (Oxford Clay Formation) of southern England, equivalent to the famous and now inaccessible Lagerstätte at Christian Malford, Wiltshire, is described. It comes from a single bed and includes coleoid cephalopods with phosphatized soft tissues, and fully articulated fish. The level is unusual in that it lacks permanent benthos and is dominated by 'rain-out' from the upper water column. It was deposited on a sea floor that experienced prolonged periods of anoxia and which was overlain by a water column that was at least intermittently stratified. It is postulated that the coleoids formed large shoals that were killed en masse , together with other elements of the associated fauna, in one or more catastrophic mass mortality events that affected a significant area of the Peterborough Member sea. During the event(s), many of the coleoids preyed upon moribund fish and other coleoids, sometimes of the same species, before becoming overcome themselves. Crucially for the phosphatization of their soft tissues, the substrate surface was 'soupy' and allowed them to sink to a zone of rapid diagenesis where levels of dissolved phosphorus were greatly augmented by the large number of associated decaying carcasses.