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_1.01Mg_0.99[CO₃]₂) and low ¹⁸O values (-7.6 to -9.9, PDB), and has a large range of ⁸⁷Sr/⁸⁶Sr ratios (0.70843 to 0.70894) relative to associated limestone (0.70879 to 0.70897) and coeval Early Ordovician seawater. The ⁸⁷Sr/⁸⁶Sr ratios of most (65%) dolomite samples fall in the ⁸⁷Sr/⁸⁶Sr range of coeval seawater.

The distribution of Cool Creek dolomite in sabkha-like sequences, coupled with dominant ⁸⁷Sr/⁸⁶Sr ratios similar to coeval seawater, suggests that the dolomite probably formed initially in a marine environment during early diagenesis. The low ¹⁸O values of the dolomite, however, indicate considerable burial modification. Correlations between texture and geochemistry, as well as among various parameters (Sr, Fe, ¹⁸O, and ⁸⁷Sr/⁸⁶Sr), demonstrate that dolomite samples with increased modification are characterized by coarser textures, lower Sr and higher Fe concentrations, and lower ¹⁸O values and ⁸⁷Sr/⁸⁶Sr ratios. These correlations, along with geochemical comparison with associated limestone, not only suggest that the dolomite was modified by fluids depleted in ¹⁸O, but also indicate that the lower ⁸⁷Sr/⁸⁶Sr 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 ^{Sr/86Sr ratios by infiltrating younger carbonates, probably during emergence of the carbonate platform coincident with regional Paleozoic unconformities.}