PALEOENVIRONMENTAL AND PETROGRAPHIC ANALYSIS OF LATE NEOGENE CARBONATES FROM WEST END, GRAND BAHAMAS ISLAND, BAHAMAS

Thin section petrology of carbonate samples from a 47 m deep borehole near West End, Grand Bahamas Island revealed Late Neogene (Pliocene and Pleistocene) paleoenvironmental history and permitted analysis of porosity types and development in shallow aquifers. Thin sections were prepared from fragmented split-spoon samples collected at 1.5 m intervals. Before thin section preparation a suite of 1 to 4 cm sized representative fragments from each sampled interval were vacuum impregnated in blue epoxy. Thin sections from the lower portion of the borehole were stained for dolomite identification. The upper portion of the borehole section from about 6 to 15 m is dominated by ooid and peloid-rich grainstones and packstones, with abundant mollusk, foraminifera, and Halimeda bioclasts typical of shallow open platform environments. Micrite matrix and coral bioclasts are uncommon in this interval. Deeper portions of the borehole from about 20 to 40 m are dominated by facies with more abundant coral, red algal, mollusk, and foram bioclasts that are more typical of high energy reef facies or very shallow platform environments on the flanks of reefs. Interestingly, no dolomite or dolostone was detected in any stained thin sections from the West End borehole. In deeper boreholes we studied in the vicinity of Freeport, Grand Bahamas Island, thick continuous dolostones were encountered beginning at about 102 m and in several other published boreholes on Grand Bahamas Island thick sections of stratal dolostones were first encountered at depths ranging from about 20 to 35 m. Intraparticle and interparticle porosity in thin sections ranged from 13% to 31% of rock volume and although there was not a pronounced trend in total porosity in the borehole, vuggy and moldic porosity is more abundant in the upper 30 m.