Application of petrography, major and trace elements, carbon and oxygen isotope geochemistry to reconstruction of diagenesis of carbonate rocks of the Sanganeh Formation (Lower Cretaceous), East Kopet-Dagh Basin, NE Iran

Abstract

Postdepositional history of the Sanganeh Formation (Lower Cretaceous), in eastern Kopet-Dagh Basin in northeast of Iran was considered. The Sanganeh Formation mainly composed of shale, marl, siltstone, and interbeds of limestone. The study of limestone interbeds led to recognizing several diagenetic processes including micritization, cementation (including isopachous rim, syntaxial overgrowth, equant mosaic spary, blocky, and poikilotopic cements), neomorphism, compaction (including physical and chemical), dolomitization, pyritization, fracturing, and vein filling. Limestone samples were analyzed for major (Ca, Mg) and minor (Na, Sr, Fe, and Mn) elements as well as oxygen and carbon isotopic compositions (δ18O and δ13C). Many of diagenetic processes such as cementation, neomorphism, dolomitization, and pyritization happen in several diagenetic environments. In order to differentiate them, geochemical data (including element and isotopic) have been used. In addition, blocky, mosaic spar, and poikilotopic cements show different luminescence under the cathodoluminescence microscope that is accordant to their geochemical contents. Therefore, the luminescence was a guide for recognizing cements and neomorphed lime muds of different diagenetic environments of studied samples. The neomorphed lime muds and lime muds were differentiated by geochemical data. The marine lime muds and micrites includes mean δ18O 0.04 ‰, δ13C 1.95 ‰, Na mean 1491.21 ppm, Sr mean 5233.68 ppm, Fe mean 61.34 ppm, and Mn mean 35.96. The meteoric cements and includes mean δ18O −1.02 ‰, δ13C 8.85‰, Na mean 126.34 ppm, Sr mean 300.03 ppm, Fe mean 2883.88 ppm, and Mn mean 382.88. The burial cements and neomorphed lime mud samples contained mean δ18O −9 ‰, δ13C −6.33‰, Na mean 308.29 ppm, Sr mean 731.72 ppm, Fe mean 816.02 ppm, and Mn mean 37.28. Paragenetic sequence of the Sanganeh Formation carbonate rocks was interpreted and depicted using integration of petrographic, major and trace elements, and isotopic data in three phases including (i) eogenesis, (ii) mesogenesis, and (iii) telogenesis.