The geochemistry of early diagenetic dolostones from a low-salinity Jurassic lagoon

In the Middle Jurassic Great Estuarine Group of NW Scotland, argillaceous dolostones and dolomitic limestones occur interbedded with limestones and mudstones of low salinity, 'closed lagoon' palaeoenvironments. These microcrystalline dolostones are imperfectly ordered and calcium-rich, and have escaped pervasive dissolution-reprecipitation processes during burial diagenesis. Geochemical studies show that the dolostones retain a significant early diagenetic memory. d18Odol values≈0‰ are the heaviest reported from the Great Estuarine Group and probably formed by evaporative fractionation from meteoric water. d13Cdol values≈-2‰ may represent precipitation from a standing body of meteoric water partially equilibrated with atmospheric CO2. Calcite coexisting with the dolomite is lighter in both 13C and 18O by 0.8 and 3.7‰ respectively. It is suggested that the minerals were not co-precipitates from one solution. Strontium is preferentially concentrated inthe dolomite with 87Sr/86Sr values between 0.7095 and 0.7099. These values, which are slightly more radiogenic than Jurassic seawater, may originate from the continental weathering of ancient limestones, mixed with seawater derived from the compaction of underlying sediments. Sm-Nd model agesare consistent with weathering of Dalradian rocks in the drainage hinterland. Dolomite formation proceeded by dolomitization of a CaCO3 precursor in a warm seasonal climate. During dry, arid seasons the lagoon water evaporated concentrating 18O, raising Mg/Ca and Sr/Ca ratiosand thereby promoting dolomitization. During humid wet periods Sr-poor CaCO3 precipitated with lighter d18O and d13C values.