Abstract

The Oligocene–Miocene Bluff Formation on Grand Cayman is formed of hard, white, finely crystalline dolostone in which the precursor textures and fossil microstructures are commonly preserved. The dolostones have a high porosity (up to 25%) because of leaching of skeletal material, which was originally aragonite, and dissolution of the bedrock during the various phases of exposure and karst development.A major disconformity divides the Bluff Formation into the Cayman Member (Oligocene) and Pedro Castle Member (Middle Miocene). Cavities in the Cayman Member are commonly filled or partly filled with caymanite, dolomitized skeletal grainstone, terra rossa, and flowstone. Available evidence suggests that the caymanite and skeletal grainstone were emplaced prior to deposition of the Pedro Castle Member in Middle Miocene times, whereas the emplacement of the terra rossa and flowstone postdates dolomitization of the Bluff Formation.Petrographic and geochemical data suggest that there was only one phase of dolomitization that was mediated by normal seawater. There is no signifiant difference among 87Sr/86Sr ratios of the dolostones of the Cayman and the Pedro Castle members. The average 87Sr/86Sr ratio of 0.70905 for these dolostones is significantly lower than the average 87Sr/86Sr ratio of 0.70917 obtained from modern seawater around Grand Cayman. The 87Sr/86Sr ratios for the dolostones of the Bluff Formation, ranging from 0.70900 to 0.70914, suggest the dolomitization occurred 2–5 Ma ago. The underlying cause of the pervasive dolomitization is uncertain. Although it appears that "normal" seawater was responsible for that dolomitization, there is little evidence pointing to why it occurred 2–5 Ma ago.

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