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

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Abstract
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. δ 18 O dol values =≈ 0%o are the heaviest reported from the Great Estuarine Group and probably formed by evaporative fractionation from meteoric water. δ 13 C dol values ≈ −2%o may represent precipitation from a standing body of meteoric water partially equilibrated with atmospheric CO 2 . Calcite coexisting with the dolomite is lighter in both 13 C and 18 O by 0.8 and 3.7%o respectively. It is suggested that the minerals were not co-precipitates from one solution. Strontium is preferentially concentrated in the dolomite with 87 Sr/ 86 Sr 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 ages are consistent with weathering of Dalradian rocks in the drainage hinterland. Dolomite formation proceeded by dolomitization of a CaCO 3 precursor in a warm seasonal climate. During dry, arid seasons the lagoon water evaporated concentrating 18 O, raising Mg/Ca and Sr/Ca ratios and thereby promoting dolomitization. During humid wet periods Sr-poor CaCO 3 precipitated with lighter δ 18 O and δ 13 C values.