EVAPORITIC MIXED-WATER DOLOMITIZATION ON ST-CROIX, USVI

Dolomite exists in only a small area of Pliocene carbonates on St, Cro ix, This area consists of reef, lagoonal, and platform facies that und erlie and rim the shoreline of Krause Lagoon, an embayment that existe d on the southern coastline of St, Croix prior to industrial developme nt in the 1960s, Since dolomite is not found in similar facies elsewhe re on St, Croix, this geographic distribution suggests that the format ion of the dolomite was related to hydrologic conditions in Krause Lag oon, The dolomite is calcium rich (57-60 mol % Ca) and exists both as euhedral rhombs 2-75 mu m in diameter and as a replace ment mineral in dolomitized bioclasts, Bioclasts in the dolomitic strata often show g ood preservation of microstructure, particularly in large benthic fora ms and coralline algae. The mean Sr-87/Sr-86 isotopic composition of t he dolomite is 0.70887 +/- 0.00002 (2 sigma, n = 3) which corresponds to the Sr-87/Sr-86 ratio of Late Miocene seawater, However, because th e dolomite resides in Pliocene strata, it is difficult to invoke unmod ified seawater as an agent of dolomitization, Dolomitization therefore requires a source of nonradiogenic strontium, Modern St, Croix ground water has Sr-87/Sr-86 compositions between 0.7076 and 0.7085 (n = 4), well below the ratio of both modern seawater and the dolomites, Mixing calculations show that modern St, Croix groundwater could be a signif icant source of nonradiogenic strontium in a dolomite formed from a tw o-component groundwater-seawater mix. On the basis of strontium-concen tration modeling, the groundwater component responsible for the St, Cr oix dolomites may have ranged from 40% to 80% of the dolomitizing flui ds. Stable isotopic values for the dolomite range from delta(18)O of 0.7 parts per thousand to +3.8 parts per thousand, and delta(13)C of 0.6 parts per thousand to +2.4 parts per thousand (PDB), with increasi ng delta(18)O and delta(13)C values from the margins to the center of the lagoon, The maximum delta(18)O values in these dolomites are too h igh to have formed from unaltered groundwater or seawater, even accoun ting for ice volume effects. Therefore the isotopically heaviest dolom ite must have precipitated from fluids enriched in O-18, probably as a result of evaporation. Dolomitization from fluids produced from a mix ture of evaporated seawater and St. Croix groundwater are consistent w ith the geochemistry and geologic distribution of the dolomite, Calcul ations show that such a scenario is possible, and may be fairly common , despite the relative complexity of the model, Documented block fault ing of the Krause Lagoon area may have provided a stable hydrologic re gime for a long enough time for dolomite to form, despite island uplif t during the late Tertiary, Other models of dolomitization can be show n to be less likely or untenable on the basis of chemical, lithologic, or hydrologic criteria.