UPPER PLIENSBACHIAN AND LOWER TOARCIAN PALEOGEOGRAPHY AND SEDIMENTARY ENVIRONMENTS OF NORTHEAST ENGLAND

A core was recently drilled through the Lower Jurassic of North Yorkshire, U.K., approximately 10 km south of Whitby and 7 km west of Robin Hood’s Bay. This core allows the study of a fresh lateral equivalent of the upper Pliensbachian and lower Toarcian succession exposed along the Yorkshire coast. We studied the uppermost Staithes Sandstone Formation to lowermost Alum Shale Member interval of the core and sampled it approximately every 20–50 cm. We analyzed the samples for organic carbon isotopes, total organic carbon (TOC), and elemental geochemistry (major, trace, and rare earth elements). Using a combination of litho- and carbon isotope stratigraphy, we correlated the core with the classic exposure along the Staithes–Port Mulgrave coast, and our correlation shows that there are no significant lateral changes in stratigraphy between the two localities. The chief difference between the two successions is that the ironstones of the Cleveland Ironstone Formation are not laterally continuous. Our organic carbon isotope stratigraphy reveals a c. 6‰ negative excursion in the lower part of the Whitby Mudstone Formation, which we correlate to the Toarcian oceanic anoxic event (T-OAE) in the Dactylioceras tenuicostatum and Harpoceras exaratum zones of the lower Toarcian, and a smaller c. 2‰ excursion at the top of the Cleveland Ironstone Formation, which we correlate to the Sulphur Band and the Pliensbachian–Toarcian boundary. TOC values are generally <2% before, 5–20% during, and c. 3% after the T-OAE. Trends in our elemental geochemistry results show patterns of variation that we interpret as being related to climate and relative sea level. The supply of siliciclastic sediment during the deposition of the Staithes Sandstone and Cleveland Ironstone Formations was cyclic and may be related to transgressive–regressive cycles. Our data suggest that siliciclastic sediment supply was shutoff during the T-OAE, and we interpret this as a consequence of relative sea-level rise, which promoted trapping of sediment in nearshore environments. We therefore propose that sediments deposited on the northwest European continental shelf during the T-OAE are palimpsest rather than newly supplied detrital, and we suggest that some organic matter may be recycled too from, for example, Carboniferous sources.