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Abstract

Protodolomite is found in the sediments beneath hammocks of the tidal flats of west-central Andros Island, Bahamas. The protodolomite is disordered, contains 38-44 mole percent MgCO3, occurs as 1 μm equant rhombs, and fills pores between and grows around aragonite needles.

The tidal flats are composed of Holocene lime sediments which overlie Pleistocene limestones, forming a wedge 0-4 m thick. The tidal flat wedge has prograded westwards several km into a slightly hypersaline shallow water bank which resembles an epeiric sea in aerial dimensions. Elevated ridges (hammocks), are common features on the tidal flats. They are underlain by fresh groundwater lenses which are surrounded by groundwater of more normal marine sahnity.

Our analyses of several chemical parameters of these groundwaters indicate that all waters are supersaturated with respect to dolomite and that there is no special potential for precipitation of dolomite in the freshwater or adjacent mixing zones. Mg+2 loss from the system by precipitation, if it occurs, is in quantities too small to detect.

We took sediment cores through hammocks and other environments, and determined the mineralogy of the sediments by X-ray diffraction. Though protodolomite is more common in sub-hammock sediments, its distribution is not reasonably explained solely in terms of mixing zone precipitation.

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