New Constraints on Dolomitization Models and Neogene Tectono-eustatic Events in the SE Bahamas From Exposed Messinian and Upper Pliocene Dolostones on Mayaguana Island

For the first time, massive dolostone units of Messinian and Late Pliocene age have been observed on the surface of the Bahamas archipelago, providing new constraints on dolomitization models and the Late Cenozoic sea-level and tectonic history in this area. In the Bahamas, massive dolostone units have so far only been identified from cores at depths ranging from 9 m in Mayaguana to 44 m on San Salvador. We report here on an extensive, one km-long sea-cliff exposure near Little Bay, on the north coast of Mayaguana, displaying two vertically stacked dolostone units separated by a karstic surface, and reaching up to 3 m above sea level. The lower crypto-crystalline dolostone is very hard, greenish-brown in colour (white when broken) and displays well-preserved cross bedding and ripple laminations, suggesting a tide- influenced depositional setting. The upper coarse-crystalline mimetic dolostone is also very hard, brown in colour, poorly stratified and contains numerous dm-sized fossils (corals, red algae, mollusks) indicating a high-energy peri-reefal depositional environment. The lower crypto-crystalline dolostone yielded an average 87Sr/ 86Sr ratio of 0.708988, and δ18O and δ13C values of +3.12 and +1.94 per mil, respectively, whereas the upper coarse-crystalline mimetic dolostone gave a 87Sr/ 86Sr ratio of 0.709083, and δ18O and δ13C values of +1.46 and -1.10 per mil, respectively. The oldest possible age of dolomitization, and thus the youngest possible age of deposition, can be constrained between 5.4 and 5.6 Ma (Messinian) for the lower unit, and between 2.2 and 2.3 Ma (Late Pliocene) for the upper one. Preliminary stable-isotope data suggest that the dolomitizing fluid could have been seawater in both cases. The negative δ13C value measured from the upper unit is likely related to a late phase of freshwater diagenesis. These rocks represent the oldest stratigraphic units observed so far at the surface of the Bahamas archipelago. If confirmed, the Messinian age of the lower crypto-crystalline dolostone would signify a yet undetected phase of dolomitization in the Caribbean region. Finally, further processing of field data and expected new geochemical results will certainly provide new constraints on dolomitization models and the Late Cenozoic eustatic and tectonic history in the southeastern Bahamas.