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Calibration, analysis and interpretation of depositional cycles in the Early Toarcian of Yorkshire, UK

Abstract The Toarcian of Yorskhire, UK, has been analysed and interpreted in order to determine whether shale-nodule couplets or cycles represent a regular cyclicity, and, if so, what the cyclicity could represent. This was done by calculating rates of deposition by analysing shale laminae and calculating durations of hiatus from rates of growth of nodules which are believed to be formed during hiatuses. This has enabled a new ammonite geochronology to be calculated, giving average zonal durations of 700 ka and subzonal durations close to 300 ka. The cyclicity is not regular enough to represent Milankovitch cycles and a tectonic origin related to pulsed North Sea thermal doming is suggested as an alternative mechanism.