The Triassic–Jurassic boundary beds of eastern and northeastern England: facies, environments and carbon isotopes

Upper Triassic to lowermost Jurassic strata in eastern and northeastern England are poorly exposed with the result that environmental changes during the associated end-Triassic mass extinction (ETME) interval have not been fully evaluated in the region. Here, we report on two records: an offshore, mudrock-dominated section at Bantycock Quarry in Nottinghamshire, and a sandstone-rich succession from the Dove's Nest borehole from the Cleveland Basin, North Yorkshire. The latter provides an expanded record and has a complete organic carbon isotope stratigraphy comparable to that seen elsewhere in the British Isles. The Westbury Formation at Dove's Nest is unusually dominated by bioturbated sandstones, whilst more typical offshore, dark-grey mudstones are seen at Bantycock Quarry. A sequence boundary marks the transition to pale-grey mudstones of the Cotham Member, and the loss of marine fauna in the basal centimetres records the level of the first phase of the ETME. These losses coincide with the development of freshwater conditions. The return of marine conditions higher in the Cotham Member is marked by flooding and a major negative carbon isotope excursion. Contrary to previous reports, the Langport Member is well developed in the Cleveland Basin where it is manifest as a dark-grey mudstone with thin storm-generated sandstones. At Bantycock Quarry, erosion has removed most of the Langport Member, although thin limestone conglomerate lenses at the base of the Blue Lias have likely derived their clasts from erosion of the White Lias (a unit characteristic of more southerly outcrops in England). The major sequence boundary at the base of the Blue Lias at Bantycock is attributed to local tectonic uplift, because there is no evidence for base-level fall in the more northern Cleveland Basin. The second phase of the ETME occurs at the top of the Langport Member, and as with the first phase this is associated with a negative carbon isotope excursion. This level is overlain by anoxic facies indicating oxygen deficiency played a role in the second extinction phase.