ISLA DE MONA, PUERTO RICO AND CURACAO, NETHERLAND ANTILLES AS A MODEL FOR BAHAMIAN DOLOMITIZATION

Mixing zone dolomitization has been discredited thermodynamically and kinetically. However, many islands with young dolostones, such as Curacao and Isla de Mona, have been identified as a mixing zone origin. Petrographic analyses of rock samples from Isla de Mona and Curacao have yielded results that point to a microbial origin of dolostone on these islands.

On both islands, uplift has brought the dolomitized limestone to their current subaearial location. On Curacao, microscopy of the matrix porosity of the dolomitized Seroe Domi Formation has shown the presence of cyanophytes and other microbes, whose organic membranes appear to be acting as nucleation sites for organic dolomite precipitation. On Isla de Mona there are two carbonate units: the Mona Dolomite and the Lirio Limestone. The Lirio Limestone lies stratigraphically on top of the Mona Dolomite with extensive flank margin cave development separating the two units. The dolostone gradually grades into calcitic dolomite to high magnesium limestone to pure limestone of the Lirio Limestone. The size and position of the flank margin caves at the boundary suggests that sea level was relatively constant during their formation. Uplift eventually brought the flank margin caves to their current position; however, the sparse distribution of flank margin caves below the contact suggests that uplift was relatively constant.

The origin of the microbial communities identified is likely primary deposition within matrix porosity. Once a freshwater lens develops on an island, fine particulate organic matter and organic acids collect at the density interface along the halophreatic zone. The microbial community creates a geochemical environment that promotes dolomite precipitation. The mechanism presented here is that dolomitization occurs at the base of the freshwater lens, with seawater as the magnesium source, and promoted by microbial metabolism of organics collecting at the density interface using sulfate from seawater as an electron acceptor. This explains the large amount of dolostone found on Isla de Mona, representing the stable position of the dolomitization zone with slow uplift of the island through that zone. Quaternary glacioeustasy, which would not allow enough time for dolomitization, may be why dolomite is so rare in the shallow Bahamian platform.