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Geological Society, London, Special Publications; 1987; v. 36; p. 173-190;
DOI: 10.1144/GSL.SP.1987.036.01.14
© 1987 Geological Society of London

Regional Studies and Burial Diagenesis

Diagenesis of carbonate cements in Permo-Triassic sandstones in the Wessex and East Yorkshire-Lincolnshire Basins, UK: a stable isotope study

A. H. Bath & A. E. Milodowski

Fluid Processes Research Group, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK

B. Spiro

Mineral Science and Isotope Geology Research Group, British Geological Survey, 64 Gray’ Inn Road, London WC1X 8NG, UK

The 13C/12C and 18O/16O ratios of carbonate cements in Permo-Triassic fluvio-deltaic sandstones differ between the Wessex and East Yorkshire-Lincolnshire Basins. The isotopic data indicate different conditions of deposition and early post-depositional carbonate precipitation which supports petrological and mineralogical studies of the same sandstones focussed on the development of reservoir properties. Isotopic compositions of carbonate cements vary across the Wessex Basin; they indicate early derivation of dissolved carbonate during recrystallization of detrital limestone. Within the basin, calcite cements show little variation in {delta}13C but considerable differences in {delta}18O between early calcrete calcites and isotopically-light later calcites; {delta}18O decreases to values close to isotopic equilibrium with present water while {delta}13C is not in isotopic equilibrium with bicarbonate in present-day brines. Two distinct generations of replacive dolomite, early non-ferroan and later ferroan, are found in the basin centre and have different {delta}18O but similar {delta}13C values. 34S/32S and 18O/16O ratios of anhydrite cements could indicate a remote Zechstein source of sulphate or a mixed marine sulphate-sulphide source. The Permo-Triassic sandstones of the East Yorkshire-Lincolnshire Basin, in contrast, contain dominantly dolomite cement. {delta}18O and {delta}13C values are relatively heavy and indicate early dolomitization of calcite over large areas. {delta}18O values of minor calcites indicate that dedolomitization is continuing in the present-day groundwater regime. The isotopic differences between carbonate cements in the two basins is tentatively attributed to the differing early post-depositional conditions and to the differences in carbonate chemistry between calcite dissolution/precipitation reactions and dolomitization reactions.