GOLDEN GROVE DOLOMITE, BARBADOS - ORIGIN FROM MODIFIED SEAWATER

The largest dolomite occurrence on Barbados, part of a roadcut at Gold en Grove, is about 40 m long and 0.5-2.5 m thick. Dolomite at Golden G rove constitutes about 10-40 vol % of the host limestone, an algal-for aminiferal facies of Late Pleistocene age, with local patches consisti ng of up to 100 vol % dolomite. Almost all dolomite is a replacement o f matrix and predominantly Mg-calcite skeletal grains. An average of l ess than 0.1 vol % of the rock is fibrous to bladed, radial and radiax ial dolomite cements with calcite banding. The partially dolomitized a lgal-foraminiferal facies is unconformably overlain by scleractinian c oral reef limestones. Replacive dolomitization appears to have been ve ry localized at the stratigraphic level of Golden Grove. The petrograp hic data suggest that replacive dolomitization took place after deposi tion of the algal-foraminiferal facies and prior to deposition of the overlying coral reef facies. delta(18)O (+1.1 to +4.4 parts per thousa nd PDB) values of the replacive dolomites suggest seawater with variab le temperature and/or slight degrees of evaporation as the principal a gent of dolomitization. In addition, variable and highly negative delt a(13)C values of the replacive dolomites (-23.5 to -8.9 parts per thou sand PDB) reflect mixing of normal marine carbon with a carbon reservo ir strongly depleted in C-13. The main source of this C-13-depleted ca rbonate probably was oxidized methane, with a possible but very minor contribution of carbonate derived from bacterial sulfate reduction. Me thane may have been supplied by bacterial methanogenesis relatively cl ose 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 me thane (via fractures?) and/or its localized oxidation somehow induced dolomitization at Golden Grove. Furthermore, the delta(13)C, delta(18) O and Sr-87/Sr-86 ratios (0.70913-0.70919) of replacive dolomites, of Golden Grove calcites (delta(13)C = -14.0 to -3.9 parts per thousand P DB; delta(18)O = -5.0 to -2.2 parts per thousand PDB), and of groundwa ters from the present coastal mixing zone and inland carbonate aquifer s (Sr-87/Sr-86 = 0.70911-0.70924; delta(13)C = -12.8 to -5.9 parts per thousand PDB; delta(18)O = -5.0 to -1.2 parts per thousand SMOW) effe ctively rule out that replacive dolomitization at Golden Grove was cau sed in a typical coastal freshwater-seawater mixing zone or that dolom itization was influenced by meteoric waters to any significant degree. The small amounts of banded dolomite cements, however, may have forme d in a typical coastal mixing zone.