A potential global stratotype for the Sinemurian–Pliensbachian boundary (Lower Jurassic), Robin Hood's Bay, UK: ammonite faunas and isotope stratigraphy

Abstract

A coastal exposure at Wine Haven, Robin Hood's Bay (North Yorkshire, UK) fulfils the criteria for definition as the Global Stratotype Section and Point (GSSP) for the base of the Pliensbachian Stage (Lower Jurassic). This marine sequence was deposited during a long-term transgression and is relatively expanded stratigraphically. A rich fauna of ammonites above and below the boundary interval allows recognition of the Leptechioceras gr. meigeini, Paltechioceras aureolum and Paltechioceras tardecrescens horizons of latest Sinemurian age, and the Bifericeras donovani, and Apoderoceras gr. aculeatum horizons of earliest Pliensbachian age. A suitable level for the boundary is characterized by the faunal association of Bifericeras donovani Dommergues & Meister and Apoderoceras sp. Strontium-isotope stratigraphy, based on analysis of belemnites, yields a calcite 87Sr/86Sr ratio for the suggested boundary level of 0.707425±0.000021 (combined uncertainties based on line fit to stratigraphic dataset (±0.000004) and measurement of the standard (±0.000017)). Alternative uncertainties of ∼±0.000008 are associated with the most extreme interpretation of sedimentation-rate history allowed by the strontium-isotope data (that is, abrupt changes in sedimentation rate at precisely the boundary level); however, sedimentological considerations, and measured strontium-isotope values at the boundary, support condensation rather than hiatus. Belemnite oxygen-isotope data suggest a significant temperature drop (∼5 °C) across the boundary at this locality.