Early Ordovician Cool Creek Dolomite, Middle Arbuckle Group, Slick Hills, Sw Oklahoma, U.S.A.: Origin and Modification

Abstract

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 (Ca1.01Mg0.99[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.