About This Item
Share This Item
Recent dolomite in the process of formation has been discovered on exposed mud flats of the Bahama Islands and in smaller areas of the lower Florida Keys. On western Andros Island, up to 80 per cent dolomite occurs at or near the surface in an area covering hundreds of square miles. In both the Bahamas and the Florida Keys (Sugarloaf Key), the site of formation is inches above mean high tide level.
The dolomite crystals, <3 microns, occur in pelleted muds and skeletal and oolitic sands that are associated with laminations, stromatolites, mud cracks, and burrows. The concentration of dolomite increases as the soft sediments are progressively lithified. Partly dolomitized gastropod shells and pellets show that the dolomite is a penecontemporaneous replacement of calcium carbonate. Radiocarbon age determinations by Martin and Deffeyes (personal commun.) prove that the exposed dolomite is contemporary.
The dolomite occurs where alternate flooding and drying together with upward capillary movement provide a continual supply of magnesium from sea water. Surface evaporation intensified by a dark algal mat increases the concentration of dissolved salts, but in this humid climate no evaporites are preserved during dolomitization. With subsidence this unique zone periodically supplied with marine sediment and soaked in concentrated sea water can persist to form significant thicknesses of dolomite. Already in only 5,000 years up to 5 feet of Recent dolomitic sediment has formed in parts of the Bahamas.
This Recent dolomite may be called "primary" but actually it originates by replacement of calcium carbonate. Its occurrence in sediments with mud cracks, stromatolites, burrow mottling, and boudinage-like structures indicates that it may explain many ancient dolomites with similar structures.
End_of_Article - Last_Page 547------------