GEOCHEMISTRY OF THE WHITBIAN (UPPER LIAS) SEDIMENTS OF THE YORKSHIRE COAST

Abstract

Summary The Whitbian sediments exposed on the Yorkshire coast consist of grey pyritic shales with bands of calcareous and sideritic concretions, and are divided in upward succession into: the Grey Shales, the Jet Rock Series, the Alum Shale Series and the Peak Shales. Two slightly different successions are separated by a transcurrent fault (the Peak Fault). The paper describes the mineralogical composition and major and minor element geochemistry of detrital and non-detrital constituents of sediments from all parts of the Whitbian succession except the Peak Shales. The detritus was severely weathered and was probably derived mainly from Palaeozoic clastic sediments. The abundance of organic matter, especially in the Jet Rock Series, indicates anaerobic conditions of deposition, and it is suggested that precipitation of calcium carbonate depended partly on the reducing conditions. Post-depositional changes involving both detrital and non-detrital components of the sediments are discussed, and two main diagenetic phases are recognized. In the earlier phase some of the calcareous nodules developed and pyrites was formed by bacterial sulphate reduction. The later phase was characterized by development of sideritic nodules, though some calcareous concretions also formed. The origin of jet in the Jet Rock Series is discussed. Geochemical correlation of beds on either side of the Peak Fault supports correlations based on lithological and palaeontological similarities.