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Journal of the Geological Society

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Mixing-zone dolomites in the Gully Oolite, Lower Carboniferous, South Wales

School of Earth Sciences, University of Birmingham, PO Box 363, Birmingham B15 2TT, UK

The Gully Oolite at St Lythams, Glamorgan, is extensively dolomitized towards its base. The dolomite is non-ferroan and is both replacive and void-filling in nature. ¹⁸O varies from –6.06%0 to –2.96%0 (PDB) and ¹³C from +2.32%0 to 3.55%0 (PDB) with a marked covariation of ¹⁸O and ¹³C. Dolomitization is believed to have occurred within a mixing-zone environment at salinities up to 50% of that of seawater. The variation of dolomite isotopic composition versus depth in the sediment is explained in terms of successive upwards migration of the mixing-zone and associated multiple phases of dolomitization. The tops of these successive mixing-zones coincide with evidence of sediment emergence, and the presence of dolomite intraclasts implies that dolomitization occurred at shallow depths within the sediment. This suggests dolomitization occurred in the absence of an overlying meteoric-phreatic lens. The high degree of dolomitization at St Lythams as opposed to that occurring in modern mixing-zones is related to: (1) longer duration of the mixing-zone environment in the sediment; (2) the proximity of the mixing-zone to the sediment surface; (3) possible chemical differences between Carboniferous and modern seawater, particularly with respect to pco_2.

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