Glauconite within Albian-Cenomanian Aitamir Formation, Kopet-Dagh Basin, northeastern Iran: origin and implications of cretaceous seawater

An integrated petrographical, geochemical, and mineralogical investigation reveals the wide variation in the mineral chemical composition of glauconites within the Albian-Cenomanian Aitamir Formation. Bounded between a black shale (Sanganeh Formation) below and chalky limestone (Abderaz Formation) above the Aitamir Formation represents an inner shelf deposit. Glauconite may occur either as pellets or as altered zones within feldspar and mica. Poor sorting and deeply penetrative crack indicate the authigenic nature of glauconite pellets. The contents of Fe2O3, MgO, Al2O3, and SiO2 of glauconites vary widely while the K2O exhibits a limited spread indicating the evolved nature of the glauconite. The glauconite matures at a fixed and high value of K2O with variable Fe2O3 (total) supporting the replacement origin. The X-ray diffraction study reveals a negligible interstratification within glauconites. The micro-textural study attests to the highly evolved nature of the glauconite. The shallow marine glauconite within the Albian-Cenomanian Aitamir Formation formed under sub-oxic, shallow depositional setting during the middle Cretaceous global warming phase within the Tethyan domain.