Mixing Zone Dolomite in Tidal-Flat Sediments of Central-West Andros Island, Bahamas: ABSTRACT

Abstract

The central-west coast of Andros Island is a complex carbonate facies mosaic, deposited at or near mean sea level. The shore has prograded intermittently during the past 3,000 years to produce a seaward-thickening wedge of sediments up to 25 km wide and about 4 m thick beneath the present shoreline. Old channel levees and beach ridges have been preserved during progradation as topographically high ridges which allow the development of freshwater lenses beneath. These lenses show considerable seasonal variation, both in geometry and pore-water chemistry. Mixing zones, extending laterally a few hundred meters around the lenses and down to Pleistocene bedrock, separate the fresh waters of the lenses (with high alkalinity and low ionic concentration), from adjacent saline and hypersaline waters (with low alkalinity and high ionic concentration). Fresh waters are generally undersaturated with aragonite, calcite, and dolomite, but calcite saturation may occur locally. The ends of aragonite needles, skeletal grains, and pellets are commonly corroded, although low-magnesian calcite crystals may locally enclose and replace aragonite by dissolution-reprecipitation. Water in the mixing zone, between 2,500 and 15,000 ppm Cl-, is undersaturated with respect to aragonite and low-magnesian calcite, but supersaturated with respect to calcium magnesium carbonates. X-ray diffraction (XRD) studies indicate small amounts of (< 6%) of protodolomite (38 to 44 mole % of MgCO3) distributed in patches across whichever sedimentary facies are intersected by the mixing zone. The mixing-zone origin of the protodolomite is however equivocal, as similar compositions occur more rarely with saline pore waters not associated with present or past mixing zones. SEM reveals 1µm euhedral rhombic protodolomite crystals between and engulfing aragonite needles. The needles may later dissolve. Chemical data from mixing zones indicate precipitation of a magnesium-rich phase frequently in excess of observed protodolomite concentrations; a huntite phase is indicated by considerations of stability, but is unsupported by XRD results. End_of_Article - Last_Page 457------------