A stable isotope study of the Mercia Mudstones (Keuper Marl) and associated sulphate horizons in the English Midlands

Abstract

ABSTRACT Widespread evaporitic sulphate horizons occur in the frequently dolomitic Mercia Mudstones (Upper Triassic) of the English Midlands. The mudstones were deposited on an extensive peneplain which had areas of upstanding minor relief (Charnwood Massif, Derbyshire—Pennines, London Platform). Horst and graben structures formed during the early and middle Triassic, controlled late Triassic sedimentation rates and peneplain slope directions. A stable isotope study (S,C,O) of the sulphates and dolomites of the Trent Formation indicates that during the deposition of the lower Fauld Member grabens such as the Need wood Basin in Staffordshire and areas marginal to a graben such as Newark in Nottinghamshire had a marine brine regime with minor continental input. Using constraints applied by sulphate concentrations of modern waters in arid environments, the ratio of volume of marine input to volume of continental input lay between 3:1 and 4:1. The horst area (East Leake, Nottinghamshire) was strongly influenced by continental brines which derived sulphate from the exposed Carboniferous Hathern Anhydrite Series. A predominantly continental brine regime existed across the whole area during deposition of the overlying Hawton Member. Periodic marine influxes gave rise to thin sulphate horizons whilst continental run‐off was occasionally sufficient to form temporary lakes. The Blue Anchor Formation (Rhaetian) was deposited in a lacustrine environment with waters of mixed marine and continental origin. Salinities were lower than those of the preceding Trent Formation brines.