ABSTRACT

Dolomite occurs in the Golden Grove reef terrace of late Pleistocene age in southeastern Barbados, where fore-reef lithologies have been partially to completely dolomitized. Interpretation of the possible modes of formation of this dolomite has been a subject of disagreement for over a decade. Among the points of contention are the timing of deposition, dolomitization and dolomite cement precipitation, and the relationship of replacement dolomite with dolomite cement. Data from this study indicate that dolomitization occurred during a single relative sea-level fall following reef development at marine oxygen isotope stage 7.3, at approximately 216 ka. Replacement dolomitization affected both micritic matrix and skeletal grains. During or after replacement dolomitization, dolomite and calcite cements filled primary and secondary pores. The cement paragenesis comprises early dolomite cement, complexly banded cements of alternating calcite and dolomite, late dolomite cement of near-stoichiometric composition, and a final phase of blocky calcite cement. Replacement dolomitization and cement precipitation represent a continuum of processes resulting from progressive modification of diagenetic fluids.

Stable carbon and oxygen isotope and strontium elemental compositions of replacive dolomite, dolomite cements, and calcite cements indicate that dolomitization occurred in a mixing zone between marine and meteoric groundwaters occupying the reef terrace shortly after deposition. Dolomite oxygen isotope values range from +0.18 to +3.85o/oo PDB, and become progressively more depleted throughout the paragenesis, indicating a greater influence of meteoric water through time. Remarkably depleted carbon isotope compositions, for both replacement dolomite and dolomite cement (-9.53 to -23.04o/oo PDB), originated from oxidation of thermogenic methane advected from the underlying accretionary complex. Methane was oxidized in marine waters that subsequently mixed with meteoric waters, as evidenced by a progressive enrichment in carbon isotope composition throughout the paragenesis. Dolomite strontium contents are substantially elevated above those expected for marine dolomitization, ranging from 640 to 1650 ppm, and increasing throughout the paragenesis. Meteoric water provided the excess strontium, as aragonite-to-calcite transformation in the exposed reef terrace increased the fluid Sr/Ca ratio. The continuum of geochemical compositions between the replacement dolomite and dolomite cement argues for their genetic association. These new data provide additional support to the mixing-zone interpretation for Golden Grove, and that the model of mixing-zone dolomitization remains viable.