Improving Earliest Jurassic Integrated Stratigraphy Using Multiple Core Records

The Triassic-Jurassic transition is notable for one of the largest losses in biodiversity throughout the Phanerozoic, accompanied by major palaeoenvironmental changes and tectonic rearrangements. Despite this importance, the construction of a robust integrated stratigraphic scheme with fine biostratigraphic subdivision for the latest Rhaetian to Sinemurian stages has proven difficult, in turn holding back the construction of a detailed, accurate and precise Hettangian timescale. Sedimentary strata of the Cheshire Basin (United Kingdom) accessed through coring efforts in 1959-60 (Wilkesley core) and 2020-21 (Prees 2C core), provide a new opportunity to evaluate stratigraphic completeness and global significance of chemostratigraphic signals in present reference sections. Both core records yielded sufficient biostratigraphic constraints to establish that all Hettangian ammonite chronozones are represented in the rock record, while this was only partly feasible for subchronozones. Chemostratigraphic data for the Prees 2C core were determined at 20 cm resolution (TOC and δ13C of organic matter), and 25 cm resolution for Ca content and Rb/Zr ratios. Equivalent chemostratigraphic data were generated for the Wilkesley core at 30 cm intervals, the spacing of residual sample material retained from the original core. Ammonite occurrences from Wilkesley were spliced into the Prees 2C record based on these chemostratigraphic data, establishing that the Prees 2C record is complete at the ammonite subchronozone level for the entire Hettangian stage, even though some subchronozone boundaries remain poorly constrained. The Prees 2C δ13Corg record, with thus improved biostratigraphic control, has some advantages over reference sections from St Audrie’s Bay and East Quantoxhead (United Kingdom), because the Cheshire Basin is characterised by less extensive stratigraphic gaps in the lower Hettangian, and was less prone to phases of organic-rich mud deposition. Such intervals were likely only of regional or local significance, but associated with negative spikes in δ13Corg, masking global trends in δ13C. While intervals of TOC enrichment above c. 1.5 wt% TOC relate to negative spikes in δ13Corg also in the Cheshire Basin, this impacts only c. 10 % of the samples from the Planorbis to Angulata chronozones, leaving > 560 measurements for the construction of an additional Hettangian reference δ13Corg record. Drifts in δ13Corg following the main carbon isotope excursion of the Planorbis Chronozone are uncovered that likely have global correlation potential. The Cheshire Basin record shows a broad positive interval peaking around the Liasicus-Angulata boundary, a negative peak in the middle Angulata Chronozone, and a continuous rise until the lower Rotiforme Subchronozone of the Sinemurian, Bucklandi Chronozone. These features may be of importance to improve the reliability of correlations between European records with highly resolved floating time scales based on cyclostratigraphy, and sequences with ash beds for which absolute ages can be determined with great precision.