oa Early dolomitization of a Lower Cretaceous shallow water carbonate platform: was microbial activity a major controlling factor?

Early dolomitization of carbonate rocks has long been problematic as precipitation experiments at room temperature have shown that dolomite will not be precipitated chemically under Earth surface conditions. To explain the widespread occurrence of early dolomite in the geological record, bacterial mediation of dolomitization has been proposed. The strongest evidence comes from modern hypersaline environments where dolomitization was shown to take place under anoxic conditions.

In this study, part of the Qatar Carbonates and Carbon Storage Research Center (QCCSRC), we investigate whether microbial mediation of early dolomitization took place on the Barremian-Berriasian carbonate platform of the Jurf Formation of Southern Oman. The sediments are characterized by a finely crystalline, poorly ordered dolomite present in various proportions. The amount of dolomite seems to be controlled by facies with tidal flats and algal/microbial facies being totally dolomitized and restricted lagoonal facies being only partially dolomitized. Results indicate that the dolomite is penecontemporaneous to sedimentation and that the dolomitizing fluids are low temperature seawater formed in an evaporative lagoon. Thus, reflux mechanisms played a role in the process of dolomitization.

One of the major questions of our study is what controls the heterogeneity of the distribution of dolomite on this platform. We hypothesize that bacteria present in tidal flats and algal/microbial mats facies played an important role in controlling dolomitization. Our goal is to highlight the role of biomineralization in controlling the early diagenesis of the Jurf Formation. To test this we will use a multi-proxy approach using mineralogy, trace elements, stable and clumped isotopes, and SEM analysis, in an integrated field, petrographic and geochemical study of a Barremian-Berriasian carbonate platform exposed in the Haushi-Huqf area. A bed-by-bed sampling was done for two 15-meter thick outcrop sections separated by one kilometer. Initial results highlight the importance of considering the presence of microbial activity to build more accurate dolomitization models and in assessing the lateral heterogeneity of dolomite bodies.