Abstract

Dolomite in the Early Ordovician Cool Creek Formation of the Arbuckle Group, Slick Hills, SW Oklahoma, occurs as massive bodies in sabkha-like sequences. Petrographically, the dolomite displays euhedral to anhedral (and xenotopic), fine- to coarse-crystalline textures. Chemically, it is characterized by nearly stoichiometric composition (Ca (sub 1.01) Mg (sub 0.99) [CO 3 ] 2 ) and low delta 18 O values (-7.6 to -9.9 per thousand , PDB), and has a large range of 87 Sr/ 86 Sr ratios (0.70843 to 0.70894) relative to associated limestone (0.70879 to 0.70897) and coeval Early Ordovician seawater. The 87 Sr/ 86 Sr ratios of most (65%) dolomite samples fall in the 87 Sr/ 86 Sr range of coeval seawater. The distribution of Cool Creek dolomite in sabkha-like sequences, coupled with dominant 87 Sr/ 86 Sr ratios similar to coeval seawater, suggests that the dolomite probably formed initially in a marine environment during early diagenesis. The low delta 18 O values of the dolomite, however, indicate considerable burial modification. Correlations between texture and geochemistry, as well as among various parameters (Sr, Fe, delta 18 O, and 87 Sr/ 86 Sr), demonstrate that dolomite samples with increased modification are characterized by coarser textures, lower Sr and higher Fe concentrations, and lower delta 18 O values and 87 Sr/ 86 Sr ratios. These correlations, along with geochemical comparison with associated limestone, not only suggest that the dolomite was modified by fluids depleted in 18 O, but also indicate that the lower 87 Sr/ 86 Sr ratios of many (35%) dolomite samples relative to Early Ordovician seawater resulted from diagenetic modification. On this basis, we conclude that 1) modification of the dolomite by basin-derived brines can be ruled out, 2) modification by younger seawater, seawater-meteoric mixtures, and fluids derived from early compaction of younger shales is possible, but 3) the dolomite seems most likely to have undergone long-lasting modification by meteoric water which gained low 87 Sr/ 86 Sr ratios by infiltrating younger carbonates, probably during emergence of the carbonate platform coincident with regional Paleozoic unconformities.

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