GEOLOGY OF THE CAYMAN ISLANDS

GEOLOGY OF THE CAYMAN ISLANDS

Brian Jones

Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada, T6G 2E3

Grand Cayman, Cayman Brac, and Little Cayman are exposed portions of the Cayman Ridge which stretches westward from the south of Cuba towards Honduras. These islands are located in a tectonically active zone with the Cayman Trench to the south and the Cayman spreading centre to the southwest of Grand Cayman. Although accurate data are lacking, it is estimated that each islands is capped by ~ 1300 m of limestones and dolostones. Exposures on Grand Cayman and Little Cayman are limited because the most of the terrain is less than 20 m above sea level. Cayman Brac offers better exposures with cliffs up to 50 m high.

The Bluff Group, which forms the core of each island, is overlain and onlapped by the Ironshore Formation. The Bluff Group is formed of the Brac Formation (Lower Oligocene), the Cayman Formation (Middle-Upper Miocene), and the Pedro Castle Formation (Lower Pliocene). The Brac Formation, exposed only at the east end of Cayman Brac, is formed of limestones and dolostone whereas the overlying Cayman Formation is formed almost entirely of finely crystalline dolostones. The uppermost Pedro Castle Formation is formed of limestones and dolostones. The original carbonate sediments, deposited on shallow banks in water no more than 30 m deep, contained numerous corals, bivalves, gastropods, algae, and foraminifera. No reefs are known from this succession. Boundaries between these formations are unconformities that represent periods of subaerial exposure and extensive karst development. The Brac Unconformity, which separates the Brac Formation from the Cayman Formation, has at least 25 m of relief on it. The Cayman Unconformity, which defines the upper boundary of the Cayman Formation, has at least 40 m of relief and many associated karst features. Diagenesis in the Bluff Group has been extensive. Porosity is high with caves and fossil moldic porosity dominating.

The Ironshore Formation is formed entirely of limestone with aragonite still being common. Superbly preserved corals, bivalves, gastropods, and algae are common and reefs are well developed in some parts of the succession. The Ironshore Formation is divided into units A, B, C, D, E, and F that each developed in response to specific Pleistocene highstands. Unconformities between these units are subtle and commonly difficult to recognize.

The sedimentary succession of the Cayman Islands provides an ideal example of how carbonate successions development is controlled by successive depositional-erosional cycles.

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