Recent Dolomitization of Pleistocene Limestones by Hypersaline Brines, Great Inagua Island, Bahamas: ABSTRACT

Abstract

Pleistocene carbonate rocks were sampled beneath a shallow saline lake on Great Inagua Island, Bahamas, to test the hypothesis of dolomitization by seepage refluxion. Commercial salt has been produced in part of the lake by solar evaporation of sea water for more than 100 years; hence, the writers reasoned that brines formed might have affected the Pleistocene carbonate bedrock. The upper few inches of rock beneath salt ponds consists of fine-grained, moldic dolomite; no dolomite was found in the Pleistocene limestone marginal to the lake or in rock underlying the lake in areas away from the salt ponds. Relict structures in the moldic dolomite show that it formed by replacement of partly cemented, aragonitic, oolitic limestone identical with the underlying and adjacent Pl istocene limestone. The dolomite crystals are cryptocrystalline (< 5 µ) in size and have a calcium-rich composition (about Ca60Mg40 to Ca56Mg44). The dolomite is enriched in O18; measured apparent fractionation values for ^dgrO18 between the dolomite and the aragonite and calcite of the parent oolitic limestone range from 3.8 to 4.6^pmil. This O18 enrichment should be expected from dolomitization resulting from interaction of carbonate material with hypersaline brines formed by solar evaporation. Radiocarbon dates of the dolomite range from 2,930 to 3,420 yr B.P. These dates, older than anticipated if the dolomitization is controlled by brines formed since salt production began, may be in error because o incorporation of old carbon atoms from the replaced Pleistocene limestone. End_of_Article - Last_Page 522------------