ABSTRACT

On the Cayman Islands, the Tertiary Bluff Group (Brac Formation, Cayman Formation, Pedro Castle Formation) is onlapped and overlain by the Pleistocene Ironshore Formation. On Grand Cayman, the Brac Formation and Cayman Formation are formed of finely crystalline dolostones whereas the Pedro Castle Formation is formed of finely crystalline dolostones, dolomitized limestones, and limestones. No dolomite has been found in the Ironshore Formation. Dolostones in the Bluff Group, which retained their original depositional textures and lack evidence of any recrystallization, are formed of small (typically 5-15 µm long) interlocking, euhedral dolomite crystals. Dolomite cement is present in the Brac Formation and Cayman Formation but is very rare in the Pedro Castle Formation. Most of the dolomite crystals are characterized by oscillatory zoning with alternating zones of low-Ca calcian dolomite and high-Ca calcian dolomite. Grand Cayman is ideal for assessing the temporal evolution of Tertiary dolostones because the dolostones are young, have not been recrystallized, and are geographically isolated by the deep oceanic waters around the island.

Interpretation of 158 new ⁸⁷Sr/⁸⁶Sr ratios from the dolostones in the Bluff Group indicate that the succession underwent three time-transgressive phases of dolomitization during the Late Miocene, the Late Pliocene, and Pleistocene. Petrographically similar dolomite was produced during each phase of dolomitization that was mediated by the same type of fluid and the same general conditions. Dolomitization was part of a dynamic cycle of processes that followed major lowstands. Karst development during the lowstands preconditioned the limestones for dolomitization by increasing their porosity and permeability. Thus, vast quantities of the dolomitizing fluids could freely circulate through the strata during the subsequent transgression. Dolomitization ceased once a stable highstand had been attained.