Scanning electron microscopy delineates several diagenetic products currently forming beneath the prograding Holocene wedge of carbonate sediments along southwest Andros Island. Various diagenetic reactions are associated with distinct chemical pore-water environments in the affected sediments.

Pore fluids of near normal marine concentrations occur in sediments over widespread areas of the tidal flats and, in these, small amounts of non-lithifying aragonite have been precipitated at isolated locations. Meteoric water infiltrates and is stored beneath topographic high areas (hammocks), while a halo of brackish groundwater surrounds the hammocks. Non-lithifying protodolomite cement occurs in sediments saturated with brackish water beneath the hammocks and adjacent areas. An unusual Mg-calcite (7 to 8 mole % MgCO₃) occurs as a beachrock cement where brackish hammock-water mixes with more normal-marine (bank) water. Sediments saturated with fresh water beneath hammocks are being calcitized. Groundwater near the surface of low positive topographic areas with a high exp sure index, such as flanks of hammocks, modern beach ridges, and tidal-creek levees, become hypersaline due to evaporation of capillary waters. The well-known protodolomitic crusts form in these environments. Most carbonate precipitation beneath the tidal flats is nonlithifying; some nodule formation, nodular beachrock, and, of course, the crusts of varying mineralogy are exceptions.

End_of_Article - Last_Page 788------------