PENECONTEMPORANEOUS DOLOMITIZATION IN SABKHA-FAISHAKH, QATAR - EVIDENCE FROM CHANGES IN THE CHEMISTRY OF THE INTERSTITIAL BRINES

Studies in Sabkha Faishakh on the west coast of Qatar in the early 196 0s showed that much of the aragonite sabkha sediment has been converte d to cryptocrystalline and microcrystalline dolomite in the last few t housand years under the influence of its interstitial, lagoon-derived brines whose Mg/Ca ratio was boosted by the precipitation of gypsum (I lling et al. 1965). Because of the comparatively simple hydrological s ystem in the part of the sabkha studied, we have been able to show fro m chemical analyses of water samples collected along a 2-km line of pi ts across the sabkha that, as dolomitization proceeds, the brines lose magnesium and sulfate ions in a nearly one-to-one ratio in conformity with the reaction 2CaCO(3) + Mg++ + SO4-- + 2H(2)O-->CaMg(CO3)(2) + C aSO4.2H(2)O The radical losses were determined by assuming that the ch loride content of the brines changes solely as a function of evaporati on. The correlation coefficient of magnesium vs. sulfate losses in 17 water samples analyzed is 0.98, and the slope of the line is 0.89. Cha nges in the bulk mineralogical composition of the sediment confirm the production of approximately equivalent amounts of dolomite and gypsum , as put forward by Klement nearly a hundred years ago.