A high-resolution shallow marine record of the Toarcian (Early Jurassic) Oceanic Anoxic Event from the East Midlands Shelf, UK

Abstract

The global deposition of organic rich-mudrocks, a mass extinction, and marked geochemical changes in the sedimentary rocks and fossils deposited during the early Toarcian indicates a period of extreme environmental change and an oceanic anoxic event. This study investigates at a high-resolution the environmental and biotic changes that occurred during the event in a shallow-marine area using rocks deposited on the East Midlands Shelf, UK. We present a new graphic log, geochemical data (δ13Corg, total organic carbon, CaCO3, total sulphur and total nitrogen), and benthic macroinvertebrate ranges from North Quarry, Holwell, Leicestershire, UK.Comparison of the ammonite ranges between the Cleveland Basin (Yorkshire) and East Midland Shelf successions in the UK shows that there is a hiatus on the shelf across the Dactylioceras semicelatum–Cleviceras exaratum Subzone boundary. The new high-resolution geochemical data show that an ~−5‰ δ13Corg excursion occurs similar to other lower Toarcian sections in the world, δ13Corg shifts ‘A’ to ‘C’ are missing in the hiatus, but that δ13Corg shift ‘D’ is present.Similar to the Cleveland Basin, the C. exaratum Subzone of the East Midlands Shelf succession is dominated by three epifaunal bivalve species, but their ranges differ significantly between the successions. An increase in the faunal diversity occurs within the upper C. exaratum Subzone at Holwell and other UK sections. The biotic data indicate that conditions on the East Midlands Shelf were more hospitable than in the Cleveland Basin. The bivalve populations on the shelf may have provided a source of new recruits for the basins.