Dolomitization of Lower Cretaceous Peritidal Carbonates By Modified Seawater: Constraints From Clumped Isotopic Paleothermometry, Elemental Chemistry, and Strontium Isotopes

This study investigates one of the few examples of dolomitization of Lower Cretaceous shallow-water limestonesfrom the southern Tethys carbonate platform from outcrops on the Haushi-Huqf High in central-east Oman. Mud-dominatedperitidal carbonates rich in microbial mats are replaced by fine crystalline dolomite along at least 60 km in the lower 10 m of theJurf Formation. Two, meter-thick beds in the overlying Qishn Formation are also dolomitized over lateral distances of one to two kilometers. The stratabound geometry and petrographic relations of the dolomite with all other diagenetic phases indicate that thedolomite precipitated early. The presence of only rare anhydrite relicts suggests that seawater was below gypsum saturation during most of the dolomitization, supporting the hypothesis that Cretaceous slightly evaporated water (d18O fluid values range from 2.8% to 3.5%SMOW) can affect pervasive dolomitization. Despite being composed of peritidal facies with features suggesting high salinity, most of the carbonate succession was not dolomitized, suggesting that the presence of microbial mats exerted a major control on the distribution of dolomite, and that salinity is only a minor control on dolomitization. Clumped-isotope results indicate that the early formed dolomite re-equilibrated with fluids at 44±3uC in a shallow-burial setting. The elevated iron content (on average 6011 ppm) and 87Sr/86Sr (on average 0.70782) with respect to Cretaceous seawater suggests that the burial fluids interacted with Permian clay and feldspars of the Gharif Formation that underlies the Cretaceous carbonates. The vertical gradients of radiogenic Sr and the occurrence of small volumes of coarse crystalline dolomite in stylolites and fractures suggests that burial fluids were driven vertically and laterally by differential compaction, probably during maximum burial in the Late Cretaceous