Origin of Island Dolostone: Case Study of Cayman Formation (Miocene), Grand Cayman, British West Indies

Abstract

Grand Cayman is located on an isolated fault block that is part of the Cayman Ridge that defines the southern margin of the North American Plate. The exposed part of the Oligocene to Pleistocene carbonate succession that forms the island comprises the Bluff Group (Brac Formation, Cayman Formation, Pedro Castle Formation) and Ironshore Formation. The Cayman Formation (Miocene), which is up to ~140 m, is formed of dolostones and limestones. Analyses of numerous dolostone samples from numerous wells drilled on the eastern part of the island show that there are no obvious stratigraphic variations in the dolostones. In contrast, there are significant geographic variations in the petrography, dolomite stoichiometry, and stable isotope signatures of these dolostones. Thus, from a geographic perspective, the Cayman Formation can be divided into the concentrically arranged peripheral dolostone, transitional dolostone, interior dolostone, and interior (dolomitic) limestone zones. 87Sr/86Sr ratios from the dolostones indicate that they probably resulted from two major phases of dolomitization that occurred during the late Miocene to early Pliocene and late Pliocene to early Pleistocene. Dolomitization was mediated by seawater as it flowed from the coast inland. As it migrated inland the composition of the water progressively changed as it interacted with the host rock and mixed with meteoric water. These changes were responsible for the landward variations in the petrographic and geochemical signatures of the dolostones. This model for the origin of island dolostones stresses the geographic variability in the dolostones and dolomitizing fluids that are controlled by various intrinsic and extrinsic factors. Over the last 1 Ma, the Cayman Formation has experienced rapid and frequent changes in diagenetic environments because of the frequent and rapid glacio-eustatic changes in sea level. The diagenetic fabrics evident in the dolostones and limestones of the formation, however, do not record all of these diagenetic regimes. The Cayman Model for island dolomitization can be applied to many island dolostone succession found throughout world. The extent and distribution of the concentrically arranged zones vary from island to island because their development is controlled by many different intrinsic and extrinsic factors. The Cayman Model, highlights the complexity of the dolomitization processes, clearly illustrates that geographic and stratigraphic variations must be integrated into any proposed dolomitization model. This study suggests that dolomitization models should not be based on a single geographic location because the progressive lateral changes in the dolomitizing fluids and environmental conditions cannot be assessed.