New information on the Dyrham Formation (Lower Jurassic, Pliensbachian) of the East Midlands Shelf, UK

Abstract

Previously unpublished Lower Jurassic sections of the East Midlands Shelf, along with reassessment of museum collections and previously published successions, indicate that seafloor sedimentation was dominated by complex erosional and depositional processes driven by dynamic and complex shelfal seafloor energy regimes during the Davoei and Margaritatus Zones of the Pliensbachian. The possible influence of tectonically driven relative sea level fluctuations is uncertain. Distinct condensation events, marked by authigenic mineralisation and clast reworking, can be identified at the Davoei–Margaritatus zonal boundary at the base of the Stokesi Subzone (‘Lower Authigenic Mineral Bed’), and at the boundary between the Stokesi and Subnodosus Subzones (‘Upper Authigenic Mineral Bed’). These correspond to a recognised candidate sequence boundary and candidate maximum flooding surface respectively. The Stokesi Subzone is well defined in all the studied sections by the ammonites between the two marker beds, whilst the Subnodosus Subzone can only be proven at Middleton Cheney. Nutrient supply for mineralisation observed in the ‘Authigenic Mineral Beds’ was derived from the erosion of uppermost Davoei Zone and lowermost Stokesi Subzone sediments. The high concentrations of authigenic mineralisation ions in the Davoei sediments were generated by large amounts of organic material derived from continental run-off.