An Integrated Biostratigraphy and Chemostratigraphy for the Dawn of the Jurassic (the Hettangian Stage in the Cheshire Basin, UK)

Abstract Highly resolved Earth System reconstructions of the Earliest Jurassic are hampered by complexities of existing reference sections. Here, an integrated stratigraphy of two core records is constructed, representing the expanded Hettangian sequence of the Cheshire Basin, UK. The combined record is based on ammonite and calcareous nannofossil biostratigraphy, organic carbon isotope ratios, and bulk rock chemical properties of legacy core material (Wilkesley) and Prees 2C, cored in 2020. Combination of ammonite records of these cores allows for substantial reductions in the uncertainties of ammonite zonal and subzonal boundaries. The new organic carbon isotope chemostratigraphy for the Hettangian of the Cheshire Basin improves temporal resolution compared to other published records, in particular for the Tilmanni and Planorbis zones. The limited occurrence of organic matter enrichment intervals associated with negative organic carbon isotope spikes in the Cheshire Basin allows for the identification of minor drifts throughout the Hettangian and into the Sinemurian which are likely of global significance. Organic carbon and macrofossil carbonate carbon isotope ratios largely follow the same trends in the Hettangian, but exhibit contrasting changes in the early Liasicus Zone, commensurate with a decrease in the isotopic difference of both records of ca. 2‰.