Sulfur isotope (δ34S) stratigraphy of evaporites: Application to the UK Triassic

Evaporite-prone, dryland continental basin-fills, such as the UK Triassic, lack abundant fossil control on their chronostratigraphy and tend to rely on other techniques, such as heavy mineral correlation and magnetostratigraphy, for correlation and age dating. Here we report on the novel use of evaporite-based sulfur isotope (δ 34 S evap ) profiles for the Sherwood Sandstone and Mercia Mudstone groups, from the Larne, East Irish Sea, and Somerset basins. Through comparison with the established global composite δ 34 S evap age curve, our δ 34 S evap profiles assist in providing further localised chronostratigraphic constraint to evaporitic strata of the UK Triassic, beyond what is achievable with sparse biostratigraphy alone. We report δ 34 S evap values from the lowermost Sherwood Sandstone Group that appear to align well with those of Permian seawater, potentially in agreement with previous research that reported comparable δ 34 S evap values in the Sherwood Sandstone of the Cleveland and UK southern North Sea basins. Our δ 34 S evap data from the Larne Basin provide further chronostratigraphic calibration to the Sherwood Sandstone-Mercia Mudstone boundary, agreeing well with the age assignment of this lithostratigraphic boundary within the established stratigraphic framework for this region. In addition, we provide the first high-resolution δ 34 S evap profile from halites and evaporitic mudstones of the East Irish Sea Basin, providing additional chronostratigraphic constraint to a basin that is especially impoverished in fossil content. Our δ 34 S evap profiles from the Mercia Mudstone Group of the Somerset Basin and the Glenstaghey and Knocksoghey formations of the Larne Basin are largely offset from the Triassic seawater δ 34 S evap age curve, which we interpret as being dominated by a terrestrial δ 34 S sulfate signal. Thus, through comparison with the global composite δ 34 S evap curve, we apply our δ 34 S evap data to provide further insight into the degree of marine influence during evaporite deposition. This helps to reconcile the long-standing controversy over the marine vs continental origin of UK Triassic evaporites, paving the way for an enhanced understanding of the sedimentary dynamics of these basins.