Evaporitic mixed-water dolomitization on St. Croix, U.S.V.I.
- Univ. of New Orleans, LA (United States). Dept. of Geology and Geophysics
- Louisiana State Univ., Baton Rouge, LA (United States). Dept. of Geology and Geophysics
Dolomite exists in only a small area of Pliocene carbonates on St. Croix. This area consists of reef, lagoonal, and platform facies that underlie and rim the shoreline of Krause Lagoon, an embayment that existed on the southern coastline of St. Croix prior to industrial development in the 1960s. Since dolomite is not found in similar facies elsewhere on st. Croix, this geographic distribution suggest that the formation of the dolomite was related to hydrologic conditions in Krause Lagoon. The dolomite is calcium-rich (57--60 mol % Ca) and exists both as euhedral rhombs 2--75 {micro}m in diameter and as a replacement mineral in dolomitized bioclasts. Stable isotopic values for the dolomite range from {delta}{sup 18}O of +0.7{per_thousand} to +3.8{per_thousand}, and {delta}{sup 13}C of +0.6{per_thousand} to +2.4{per_thousand} (PDB), with increasing {delta}{sup 18}O and {delta}{sup 13}C values from the margins to the center of the lagoon. The maximum {delta}{sup 13}C values from the margins to the center of the lagoon. The maximum {delta}{sup 18}O values in these dolomites are too high to have formed from unaltered groundwater or seawater, even accounting for ice-volume effects. Therefore the isotopically heaviest dolomite must have precipitated from fluid enriched in {sup 18}O, probably as result of evaporation. Dolomitization from fluids produced from a mixture of evaporated seawater and St. Croix groundwater are consistent with the geochemistry and geologic distribution of the dolomite. Calculations show that such a scenario is possible, and may be fairly common, despite the relative complexity of the model. Documented block faulting of the Krause Lagoon area may have provided a stable hydrologic regime for a long enough time for dolomite to form, despite island uplift during the later Tertiary. Other models of dolomitization can be shown to be less likely or untenable on the basis of chemical, lithologic, or hydrologic criteria.
- OSTI ID:
- 118795
- Journal Information:
- Journal of Sedimentary Research, Section A: Sedimentary Petrology and Processes, Journal Name: Journal of Sedimentary Research, Section A: Sedimentary Petrology and Processes Journal Issue: 4 Vol. 65; ISSN JSRAEA; ISSN 1073-130X
- Country of Publication:
- United States
- Language:
- English
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