Polygenetic dolomite in subtidal sediments of northern Kuwait Bay, Kuwait

Abstract

The origin of dolomite has been enigmatic. It is a common constituent of carbonate hydrocarbon reservoirs, it is abundant in Phanerozoic sedimentary carbonate rocks, but dolomite is rare in Quaternary and Recent rocks. Many different models have been suggested to explain its origin. Authigenic dolomite has been found to be forming in the Persian Gulf, but rather in small quantities. Additionally, mixing of authigenic dolomite with allochthonous types has been difficult to characterize. While it is clear that dolomite forms under a wide range of geochemical conditions, this study focuses on the occurrence of dolomite in modern sediments in a restricted embayment (Kuwait Bay), whose sediments derive from multiple sources. In this study, recently developed quantitative mineralogical tools (QEMSCAN analyses) were used in conjunction with traditional approaches (XRD, SEM, whole-rock geochemistry, and isotope geochemistry) to characterize polygenetic dolomite types occurring in subtidal sediments in Kuwait Bay. This study is the first to employ these methods to characterize the sediment mineralogy and geochemistry. Dolomite occurs as both authigenic and detrital phases, and analytical techniques have allowed characterization of these different types of dolomite. Three distinct phases of dolomite are present in the sediments: stoichiometric dolomite, near-stoichiometric dolomite, and calcium-rich, poorly ordered dolomite (protodolomite). The data suggest that stoichiometric and near-stoichiometric dolomite are transported (allochthonous/detrital), while the protodolomite is most likely an in situ authigenic precipitate. Allochthonous/detrital dolomite occurs within composite grains that show evidence for transportation. They are typically in the 50 to 150 μm size fraction. The mineralogic composition and characteristics of the composite grains suggests two likely sources: eolian and fluvial. These detrital dolomites are extrabasinally sourced. However, isolated rhombohedra of pristine dolomite crystals are present in the sediments. These dolomite rhombs are typically less than 10 μm in diameter, and are calcium rich. They make up about 10% of the total dolomite in the sediments. Their origin is consistent with authigenic microbially mediated dolomite precipitation in organic-rich sediments in hypersaline waters.