Abstract

A 55 m thick Pleistocene dolomite section was sampled in 33 borings in the Arabian Gulf, offshore of Al Jubayl, Saudi Arabia. The regional setting, suite of sedimentary structures, abundance of evaporites (crystals, nodules, and chickenwire horizons), and geochemistry of the dolomite are all consistent with accumulation as part of a series of vertically stacked sabkha depositional complexes. Analyses of the ordering, stoichiometry, and overall stable-isotope composition indicate that after dolomitization the sediment underwent diagenesis in contact with meteoric waters. Stable-isotope analyses of 385 samples have demonstrated that within this sequence there are four correlatable horizons that display marked excursions to low delta 13 C values; some shifts exceed 6 per mil between vertically adjacent samples. The delta 18 O values show a similar, but less pronounced, shift to low values at the same stratigraphic horizons. The samples with low delta 13 C and delta 18 O values have a greater admixture of fine-grained siliciclastics, are light gray (i.e., reducing conditions), and have higher concentrations of Fe, Mn, and Zn than other parts of the deposit. These diagenetic attributes, especially the marked change in delta 13 C values, may be due to either diagenesis due to the descent of meteoric water beneath a soil horizon or diagenesis associated with bacterially mediated sulfate-reduction processes; sulfate reduction is the favored hypothesis. It is envisioned that after the initial dolomitization diagenesis associated with bacterially mediated sulfate reduction of laterally extensive microbial mats within the top parts of sabkha sequences produced the correlatable horizons of low delta 13 C and delta 18 O values.

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