The upper Permian Cadeby Formation in the Boston Spa area, West Yorkshire, UK

Abstract

Undescribed carbonate exposures in the Boston Spa area have been studied in detail in order to give a comprehensive account of the stratigraphy and facies there of the upper Permian (Lopingian Series) Cadeby Formation of the Zechstein Group. The Cadeby Formation was deposited close to the western margin of the Zechstein Basin, in the Yorkshire sub-basin. All the strata in the study area are of shallow water origin, and are thought to have close analogues with modern carbonate sediments in the Arabian/Persian Gulf. The Cadeby Formation comprises the Wetherby Member overlain by the Sprotbrough Member. The contact between the members is here placed at the Hampole Discontinuity, which marks a subaerial erosion surface. The Wetherby Member includes bryozoan patch reefs and was deposited on a marginal carbonate shelf of the Zechstein Basin. The Hampole Beds, which are here placed at the base of the Sprotbrough Member, were deposited during a period of relatively low basin-wide sea-level. They include features that suggest deposition in intertidal and supratidal environments. Influxes of fine clays gave rise to thin mudstones within the Hampole Beds. Particularly around Wetherby, there are also several impersistent mudstones in the strata above the Hampole Beds. These are considered to have been deposited during regression; the associated dolomites have features typical of supratidal conditions, such as fenestrae and microbial laminae, indicative of local temporary emergence. Shallow subtidal dolomites of the Sprotbrough Member above the Hampole Beds include cross-stratification that indicates sandwaves. A lens of clastic material near the top of the formation at Wetherby is interpreted as a flash flood deposit, probably sourced from emergent areas to the west. The upper beds of the Sprotbrough Member show dedolomitization, brecciation and karstic weathering, precursors to the deposition of the terrestrial red clays of the overlying Edlington Formation. Minor post-Permian barite mineralization is found and its distribution is related to underlying structural features.