Cave-fills in Miocene-Pliocene strata on Cayman Brac, British West Indies: Implications for the geological evolution of an isolated oceanic island.

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  • An 8-m-high wall in a quarry on the west end of Cayman Brac exposes the upper part of the Cayman Formation (Miocene), the lower part of the overlying Pedro Castle Formation (Pliocene), and the Cayman Unconformity, which is a karstic unconformity that separates these formations. The modern-day karst surface caps the Pedro Castle Formation. This exposure also includes cross-sections through two filled caves—the “Lower Cave” (> 8 m long, up to 2.5 m high) and “Upper Cave” (> 23 m long, up to 2 m high)—that are housed in the Cayman Formation and Pedro Castle Formation, respectively. The Lower Cave is filled with caymanite, which is formed of laminated, varicolored dolomitized mudstones, and grainstones that contain scattered marine fossils (e.g., foraminifera, red algae). This cave, connected to the Cayman Unconformity by a small-diameter tunnel, evolved as part of the karst system that developed during the Messinian lowstand (7.3–5.3 Ma). The cave was filled and dolomitized prior to deposition of the Pedro Castle Formation. The Upper Cave is filled with a wide spectrum of lithotypes, including dolostones, calcareous mudstones, terra rossa, gastropod coquina, coated grains, and speleothems. U/Th dating indicates that some of the flowstones are > 500,000 years old whereas others are only ~ 21,000 years old. Dolostones and mudstones in the basal part of the Upper Cave contain marine fossils (foraminifera, red algae) whereas the younger deposits are devoid of such fossils. The Upper Cave and its deposits developed after the sediments of the Pedro Castle Formation had been deposited and lithified. Development of the cave filling deposits, which includes a clear transition from marine to non-marine influences, was controlled by eustatic sea-level changes and/or westward tectonic tilting of Cayman Brac that occurred after the Pedro Castle Formation became exposed, probably during the Late Pliocene.

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    Attribution-NonCommercial-NoDerivatives 4.0 International