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Journal of the Geological Society

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

¹ School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK
² Isotope Geology Unit, Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 OQU, UK

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. ¹⁸O_dol values = 0%o are the heaviest reported from the Great Estuarine Group and probably formed by evaporative fractionation from meteoric water. ¹³C_dol values –2%o 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%o respectively. It is suggested that the minerals were not co-precipitates from one solution. Strontium is preferentially concentrated in the 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 ages are 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 ratios and thereby promoting dolomitization. During humid wet periods Sr-poor CaCO₃ precipitated with lighter ¹⁸O and ¹³C values.

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