SUBSURFACE DOLOMITES, SAN SALVADOR, BAHAMAS'

ABSTRACT: The carbonates from a 168-meter well on the north coast of San Salvador, Bahamas consist mostly of stratal and massive dolomite below 37 meters. Stratal dolomites are thinly bedded (<15 cm) and laminated, contain apoor fauna, and have occasional intraclasts and bird's eye structure. They are interpreted assupratidal. In places they are capped by athin reddish, iron-rich crust, which may indicate soil forming conditions. Most of the section is massive dolomite, which formed as replacement of grain supported sediments or rocks of backreef or lagoonal facies. Multiple stages of dolomitization are indicated. The mean 60 l ~ value of the dolomite is +2.1% ~ relative to PDB and it is enriched in O 's by ~3%c rela-tive to presently forming sediment and-5%c to associated limestone. The mean 5C 13 is + 1.8%c roughly the same as presently forming sediment but-8%, heavier than associated calcite. The mean Sr value for limestone is 564 ppm, for dolomite 170 ppm. Stratal dolomites are lower in O TM and mol % MgCO3 than are the massive dolomites, and crystal structures are more poorly ordered. This ma_v indicate that the two dolomite types formed in different fluids (i.e., that different processes operated) or that they reflect differing susceptibility to dolomitiza-tion. Our present state of knowledge does not allow an unequivocal nswer as to the nature of the fluid(s) responsible for dolomitizing the San Salvador ocks