Stratigraphy of the Worston Shale Group (Dinantian), Craven Basin, north-west England

Abstract

SUMMARY The Worston Shale Group of Earp et al. (1961) is made up of four formations: the Clitheroe Limestone, Hodder Mudstone, Hodderense Limestone and Pendleside Limestone. Within these formations fifteen members are defined: the Thornton Limestone, Coplow Limestone, Peach Quarry Limestone, Bellman Limestone, Limekiln Wood Limestone. Phynis Mudstone, Whitemore Limestone, Hetton Beck Limestone, Leagram Mudstone, Buckbanks Sandstone, Embsay Limestone, Rain Gill Limestone, Chaigley Limestone, Twiston Sandstone and Rad Brook Mudstone. A particular bed within the Hodder Mudstone Formation, the Dunbarella Bed, is also recognised. Four unconformities are recognised. The most widespread is at the base of the Hodder Mudstone Formation; the others, which are more restricted, occur at the base of the Embsay Limestone Member (and equivalents), at the base of the Twiston Sandstone Member and at the base of the Pendleside Limestone Formation. Diagnostic Visean foraminifera and conodonts first enter within the basal part of the Hodder Mudstone Formation, suggesting that the Tournaisian-Visean boundary lies much higher than previously supposed. All the Dinantian stage boundaries in the Arundian to Asbian interval have been re-correlated. The Craven Basin is thought to have been underlain by an asymmetric graben within the pre-Carboniferous basement and the depositional history of the basin to have resulted from a combination of both syndepositional tectonics and eustatic processes. Consequently, the basinal sediments portray a transition from carbonate shelf to slope (Gawthorpe 1987), involving a change during deposition of the Clitheroe Limestone and Hodder Mudstone formations from shallow-water carbonates, generated within the basin, to deep-water hemi-pelagic sediments with limestone turbidites fed from the surrounding carbonate shelves and platforms. Intermittent supply into the basin of terrigenous sand and reworked faunas is related to the emergence and erosion of the surrounding areas.