Abstract

In order to compare the validity of petrographic and isotopic techniques in determining the depositional and diagenetic history of limestones, both techniques were used on samples from a Pleistocene outcrop on Barbados, where depositional and disgenetic models are relatively simple and have well-documented recent analogues. The outcrop consists of a reasonably homogeneous grainstone cut by a discontinuity surface. The carbonate immediately above and below the discontinuity forms well-cemented layers, but the rest of the grainstone is highly permeable. Only the lower well-cemented layer contains isopachous, bladed cement. Carbon of this layer is isotopically light, and oxygen is slightly heavy. In addition, delta 18 O distinctly changes across the discontinuity. Based on the cement morphology, the lower cemented layer formed as a submarine hardground; however, the isotopic analyses indicate evaporative precipitation of calcite at a subaerial exposure at or just above the submarine hardground following a regression. This regression and following transgression formed the discontinuity. The distinct offset of delta 18 O values across the discontinuity indicates that the limestone above and below the discontinuity underwent mineralogical stabilization in solutions of different isotopic compositions. Diagenesis of the unit below the discontinuity preceded deposition of the unit above. The upper cemented layer resulted from cementation in a perched phreatic environment resting on the discontinuity. We consider these results to be generally applicable to other limestones. Stratigraphic variation in delta 18 O is especially useful in identifying discontinuity surfaces and in mapping individual depositional/diagenetic packages within otherwise similar limestones. Integration of petrographic and isotopic data is necessary to determine the origin of discontinuity surfaces.

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