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

摘要

Norian and Rhaetian (Upper Triassic) sediments from the\udMercia Mudstone Group of the East Midlands are described. Four\udfacies are recognized; (1) sheet flood, (2) red-bed saline\udmudflat, (3) lacustrine and (4) marginal. Sedimentation occurred\udon an extensive peneplain which had areas of minor relief.\udInterstitial sulphate precipitation from hypersaline brines formed\udwidespread evaporitic gypsum and anhydrite horizons. Dolomitization\udof carbonate material and possibly extensive interstitial halite\udgrowth also occurred. Displacive sulphate masses in Fauld Mine are\udrecognized to be diapiric structures associated with an identifiable\udsource bed. Fault movement triggered flowage of water-saturated,\udsource bed gypsum.\udA new approach to correlating fine-grained, unfossiliferous\udred-beds utilizes variations in clay mineralogy and δ18o dolomite.\udThe Trent Formation is subdivided by this method into the Fauld\udMember and overlying Hawton Member. An isotopic study of the Trent\udFormation sulphates (S, Sr) and dolomites (C, 0) indicates that\udduring deposition of the Fauld Member, grabens and adjacent areas\udhad a marine brine regime with minor continental input (up to 20%\udsulphate derived from continental sulphides). The morst area was\udstrongly influenced by continental brines which derived sulphate\udfrom the exposed Carboniferous Hathern Anhydrite Series. The\udHawton member brine regime was predominantly continental with\udperiodic marine influxes.\udTwo distinct clay suites are recognized. The first is\udcharacterized by a predominance of detrital illite and chlorite\ud( = Hawton Member); the second is characterized by the presence of\udeither Mg-rich smectite/chlorite and/or palygorskite and/or\udsepiolite with minor illite and chlorite (=Fauld Member).\udSepiolite and palygorskite were neoformed, whilst smectite/chlorite\udwas transformed from degraded detrital illite. Both processes\udoccurred in an alkaline brine regime rich in Mg[superscript 2+] and amorphous\udsilica. The distribution of Mg-rich clays reflects availability\udof [superscript 2+], Al[superscript 3+] and Si within the brines.