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 a poor fauna, and have occasional intraclasts and bird's eye structure. They are interpreted as supratidal. In places they are capped by a thin 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 delta O 18 value of the dolomite is +2.1 per thousand relative to PDB and it is enriched in O 18 by nearly equal 3 per thousand relative to presently forming sediment and nearly equal 5 per thousand to associated limestone. The mean delta C 13 is + 1.8 per thousand roughly the same as presently forming sediment but nearly equal 8 per thousand heavier than associated calcite. The mean Sr value for limestone is 564 ppm, for dolomite 170 ppm. Stratal dolomites are lower in O 18 and mol % MgCO 3 than are the massive dolomites, and crystal structures are more poorly ordered. This may indicate that the two dolomite types formed in different fluids (i.e., that different processes operated) or that they reflect differing susceptibility to dolomitization. Our present state of knowledge does not allow an unequivocal answer as to the nature of the fluid(s) responsible for dolomitizing the San Salvador rocks.

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