Paleoenvironment, paleoclimate, and stratigraphy controls on the mineralogy of the Devonian-Carboniferous Ora Formation, Iraq

The siliciclastic-carbonate succession of the Devonian-Carboniferous Ora Formation documents the shift from the mixture (fluvial-marine) clastics of the Kaista Formation to the marine carbonate rocks of the Harur Formation in Iraq with dominated reduced paleoredox conditions and created a conducive environment for the hydrocarbon accumulation. X-ray diffraction analysis supported by scanning electron microscopy used to determine the various mineralogical components across the Ora Formation. To establish a correlation between the variation in mineralogical composition and the paleoenvironmental, paleoclimatic, and stratigraphic settings, various analyses for total organic carbon (TOC), total sulfur (S), and insoluble residue (IR) were employed. The study revealed that clay minerals of mixed layers of illite-smectite, illite, chlorite and kaolinite, framework silicates of quartz and alkali feldspars, carbonates (calcite, dolomite and siderite) while the others are represented by anatase, pyrite, goethite and jarosite. Stratigraphic variation of these minerals is used to determine the paleoenvironmental and paleoclimatic conditions prevailing during deposition. Specifically, the results demonstrated that the interval containing the highest values of TOC, S, and IR was the most volatile in terms of mineralogical content. The minerals such as pyrite, palygorskite, and barite contain traits indicative of paleoenvironmental conditions, including limited alkaline water circulation in shallow water deposition associated with high content of organic matter. The occurrence of illite from precursor kaolinite delineates the transgression systems tracts in hot and humid paleoclimatic conditions, inversely, illite and palygorskite signify hot and dry environments in the high stand systems tracts. Based on V vs Al₂O₃ and P₂O₅ vs Al₂O₃ diagrams the paleoenvironment is mainly shallow marine deltaic settings, while the paleo-oxygenation conditions refer to oxic to dysoxic conditions based on V/Cr and Cu/Zn ratios.