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

A coastal exposure at Wine Haven, Robin Hood's Bay (North Yorkshire, UK) fu lfils the criteria for definition as the Global Stratotype Section and Poin t (GSSP) for the base of the Pliensbachian Stage (Lower Jurassic). This mar ine sequence was deposited during a long-term transgression and is relative ly expanded stratigraphically. A rich fauna of ammonites above and below th e 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. aculeat um horizons of earliest Pliensbachian age. A suitable level for the boundar y is characterized by the faunal association of Bifericeras donovani Dommer gues & Meister and Apoderoceras sp. Strontium-isotope stratigraphy, based o n analysis of belemnites, yields a calcite Sr-87/Sr-86 ratio for the sugges ted boundary level of 0.707425+/-0.000021 (combined uncertainties based on line fit to stratigraphic dataset (+/-0.000004) and measurement of the stan dard (+/-0.000017)). Alternative uncertainties of similar to+/-0.000008 are associated with the most extreme interpretation of sedimentation-rate hist ory allowed by the strontium-isotope data (that is, abrupt changes in sedim entation rate at precisely the boundary level); however, sedimentological c onsiderations, and measured strontium-isotope values at the boundary, suppo rt condensation rather than hiatus. Belemnite oxygen-isotope data suggest a significant temperature drop(similar to5 degreesC) across the boundary at this locality.