The Trent, Glen Parva and Blue Anchor Formations (Upper Triassic) of the East Midlands and their Sulphate Deposits

Norian and Rhaetian (Upper Triassic) sediments from the Mercia Mudstone Group of the East Midlands are described. Four facies are recognized; (1) sheet flood, (2) red-bed saline mudflat, (3) lacustrine and (4) marginal. Sedimentation occurred on an extensive peneplain which had areas of minor relief. Interstitial sulphate precipitation from hypersaline brines formed widespread evaporitic gypsum and anhydrite horizons. Dolomitization of carbonate material and possibly extensive interstitial halite growth also occurred. Displacive sulphate masses in Fauld Mine are recognized to be diapiric structures associated with an identifiable source bed. Fault movement triggered flowage of water-saturated, source bed gypsum. A new approach to correlating fine-grained, unfossiliferous red-beds utilizes variations in clay mineralogy and δ18o dolomite. The Trent Formation is subdivided by this method into the Fauld Member and overlying Hawton Member. An isotopic study of the Trent Formation sulphates (S, Sr) and dolomites (C, 0) indicates that during deposition of the Fauld Member, grabens and adjacent areas had a marine brine regime with minor continental input (up to 20% sulphate derived from continental sulphides). The morst area was strongly influenced by continental brines which derived sulphate from the exposed Carboniferous Hathern Anhydrite Series. The Hawton member brine regime was predominantly continental with periodic marine influxes. Two distinct clay suites are recognized. The first is characterized by a predominance of detrital illite and chlorite ( = Hawton Member); the second is characterized by the presence of either Mg-rich smectite/chlorite and/or palygorskite and/or sepiolite with minor illite and chlorite (=Fauld Member). Sepiolite and palygorskite were neoformed, whilst smectite/chlorite was transformed from degraded detrital illite. Both processes occurred in an alkaline brine regime rich in Mg[superscript 2+] and amorphous silica. The distribution of Mg-rich clays reflects availability of [superscript 2+], Al[superscript 3+] and Si within the brines.