Abstract

Finely crystalline Cenozoic island dolostones, like those found in the Cayman Formation on Grand Cayman, are commonly assumed to be petrographically and compositionally homogeneous. Backscatter electron images (BSEI), however, show that the constituent dolomite crystals (< 100 μm long and commonly < 20 μm long) are commonly zoned with respect to their mol % CaCO3 content. Moreover, such images allow (1) depiction of growth patterns in the constituent crystals, irrespective of their origin, (2) recognition of replacive dolomite as opposed to dolomite cement, and (3) delineation of "stratigraphic packages" in the dolomite cements that reflect different episodes of cementation. Integration of this information forms the basis for paragenetic interpretations of the dolostones.

On Grand Cayman, the Miocene Cayman Formation can be divided into friable, high-porosity dolostones and hard, low-porosity dolostones. Backscatter electron images show that the hard dolostones are characterized by complex arrays of zoned dolomite cements that have occluded most of the pores. Caymanite, an internal sediment, has occluded many of the larger cavities. In contrast, the high-porosity dolostones contain little cement and no internal sediments. Precipitation of the cements and the deposition of internal sediments were related to the passage of large volumes of water through some of the dolostones. Thus, the hard, low-porosity dolostones are found in the "cap rock" of the formation, in coastal locations, and in areas close to solution-widened fractures. Conversely, the friable, high-porosity dolostones form the lower "porous unit" of the formation in the interior of the island, where the passage of water was more restricted.

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