Halokinetic-related Dolomitization in the Middle Eocene Carbonates, Saudi Arabia

Salt movement influenced the eastern part of the Arabian platform during the Eocene time. The halokinesis was not only important for creating hydrocarbon trap but also controlling dolomitization via thermal convection of fluid and provision of pressure gradient. Previous works on mid-Eocene dolomites of the Arabian platform proposed various models as the mechanisms for the genesis of the dolomite. However, most did not consider the impact of the halokinesis in building their predictive mechanisms. To contribute to this knowledge gap, integrated mineralogical, petrographic, chemical, and stable isotope analyses were employed on Mid-late Eocene samples taken from shallow cores. Core descriptions revealed two dolomite types: an upper light grey, coarse, indurated dolomite (Type I) and a bottom dark grey, more sucrosic but less indurated dolomite (Type II). Petrographic results showed pervasive, fabric non-selective and non-mimetic dolomites with dominant euhedral-subhedral (idiotopic-hypidiotopic mosaic) fabric in Type II dolomite and dominant subhedral in Type I. Geochemical analyses indicated that Type I dolomite is near-stoichiometric, well-ordered, and low in Sr, while Type II is non-stoichiometric, poorly ordered, and high in Sr content. Both dolomite types exhibit lower Fe (av.5238ppm) and Sr (av.823ppm) values compared to the associated limestones but comparable Mn (av.261ppm=dolomite and av.273ppm=limestone). Both the dolomites and the adjacent limestones have tight clusters of 13C value ranges of between 0.89 and 1.83 but a slightly wider range of 18O values of between -2.59 and 0.95 for the dolomites and -3.34 to +0.36 for the limestone. The dolomite isotopic and paleotemperature (av.32.8C) values are comparable to the Eocene marine water values with very little variation. Thus, contrary to previous interpretations involving meteoric mixing or evaporation of the seawater, we propose that the dolomite originated from seawater, especially since there is an absence of evaporites throughout the core, but slightly modified by salt diapir-driven convection. The absence of deep burial features, co-enrichment of Sr and Fe-Mn elements, and low paleotemperatures of the studied dolomites suggest early dolomitization at near-surface and sub-oxic condition. Hence, our work provides a new understanding of dolomitization in shallow carbonate platforms that are underlain by deep-seated salt diapirs.