Holocene carbonate sediments that compose small buildups in the Joulters Cays area of the Bahamas have been extensively altered by freshwater. Sediments, which include skeletal packtones and grainstones, and oolitic grainstones, were deposited in open shelf and ooid shoal environments. Evidence of the subsequent freshwater diagenesis includes mineralogic, petrographic, isotopic, and geochemical data. Mineralogically, the sediments are calcite with no high magnesium-calcite and only minor amounts of aragonite. Aragonitic grains such as mollusks and ooids have been dissolved resulting in skelmoldic and oomoldic textures. Such dissolution results in relatively high porosity despite extensive pore-filling cement. Values of ^dgrC¹³ are negative indicating exchange w th isotopically light meteoric water. Analyses by atomic absorption spectroscopy and electron microprobe reveal low Mg⁺⁺ and Sr⁺⁺ values which reflect the flushing of freshwater through the system.

There are two possible sources for the freshwater: (1) the Pleistocene high just seaward of South Joulters Cay which could serve as a barrier to open-water exchange with the ocean. Ponding and freshening (by rainfall) of seawater would result in freshwater phreatic alteration of underlying sediments; and (2) the freshwater lens associated with a previous, older Joulters Cays. A modern analog is the Florida coastal acquifer which extends seaward several tens of km and phreatically alters carbonate rocks which remain well below sea level. It is likely that an earlier Joulters Cays existed and that the sediments were altered by the freshwater lens associated with those islands.

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