Abstract

The largest dolomite occurrence on Barbados, part of a roadcut at Golden Grove, is about 40 m long and 0.5-2.5 m thick. Dolomite at Golden Grove constitutes about 10-40 vol % of the host limestone, an algalforaminiferal facies of Late Pleistocene age, with local patches consisting of up to 100 vol % dolomite. Almost all dolomite is a replacement of matrix and predominantly Mg-calcite skeletal grains. An average of less than 0.1 vol % of the rock is fibrous to bladed, radial and radiaxial dolomite cements with calcite banding. The partially dolomitized algal-foraminiferal facies is unconformably overlain by scleractinian coral reef limestones. Replacive dolomitization appears to have been very localized at the stratigraphic level of Golden Grove. The petrographic data suggest that replacive dolomitization took place after deposition of the algal-foraminiferal facies and prior to deposition of the overlying coral reef facies. delta 18 O (+1.1 to +4.4 per thousand PDB) values of the replacive dolomites suggest seawater with variable temperature and/or slight degrees of evaporation as the principal agent of dolomitization. In addition, variable and highly negative delta 13 C values of the replacive dolomites (-23.5 to -8.9 per thousand PDB) reflect mixing of normal marine carbon with a carbon reservoir strongly depleted in 13 C. The main source of this 13 C-depleted carbonate probably was oxidized methane, with a possible but very minor contribution of carbonate derived from bacterial Gsulfate reduction. Methane may have been supplied by bacterial methanogenesis relatively close to the site of dolomitization or by upward seepage of thermogenic methane derived from underlying Tertiary hydrocarbon reservoir(s). It is possible, albeit speculative, that the localized availability of methane (via fractures?) and/or its localized oxidation somehow induced dolomitization at Golden Grove. Furthermore, the delta 13 C, delta 18 O, and 87 Sr/ 86 Sr ratios (0.70913-0.70919) of replacive dolomites, of Golden Grove calcites (delta 13 C = -14.0 to -3.9 per thousand PDB; delta 18 O = -5.0 to -2.2 per thousand PDB), and of groundwaters from the present coastal mixing zone and inland carbonate aquifers ( 87 Sr/ 86 Sr = 0.70911-0.70924; delta 13 C = -123 to -5.9 per thousand PDB; delta 18 O = -5.0 to -1.2 per thousand SMOW) effectively role out that replacive dolomitization at Golden Grove was caused in a typical coastal freshwater-seawater mixing zone or that dolomitization was influenced by meteoric waters to any significant degree. The small amounts of banded dolomite cements, however, may have formed in a typical coastal mixing zone.

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