Inorganic geochemistry and palaeoenvironments of the
Early Jurassic Cleveland Basin

Abstract

The uppermost Staithes Sandstone Formation to lowermost Alum Shale Member were studied in a core drilled recently approximately 10 km south of Whitby and 7 km west of Robin Hood’s Bay to investigate the evolution of Pliensbachian-Toarcian (lower Jurassic) depositional environments of the Cleveland Basin, Yorkshire, UK. Samples were analysed for organic carbon isotopes, total organic carbon (TOC), and elemental geochemistry (major and trace elements). The carbon isotope excursion in the Whitby Mudstone Formation correlates to the Toarcian oceanic anoxic event (T-OAE). The elemental geochemistry data were used to assess the palaeoredox state of the depositional environment, the role of dilution of organic matter by siliciclastic material, and the rates of organic productivity in the photic zone above the site of deposition. The data reveal that the supply of detrital material varied, possibly resulting from climate or relative sea-level variations, during the deposition of the Staithes Sandstone and Cleveland Ironstone Formations, but abruptly stopped at the onset of the deposition of the Grey Shale Member of the Whitby Mudstone Formation. It is proposed that siliciclastic starvation, resulting from a transgression, was responsible for the shutoff of detrital material supply and that from the Pliensbachian-Toarcian boundary deposition was predominantly of palimpsest sediment. The redox proxies suggest that the water column was well oxygenated throughout the deposition of the Staithes Sandstone and Cleveland Ironstone Formations, but became anoxic, and eventually euxinic, during the deposition of the upper half of the Grey Shale Member. It is suggested that the transgression responsible for the detrital shutoff also resulted in the stratification of the water column and that the TOC enrichment during and after the negative carbon isotope excursion may be a product of enhanced preservation.