Diagenesis and Geochemistry of the Ziyarat Formation in the Type Section, Tochal, (Alborz)

The Ziyarat Formation is an Upper Paleocene to Middle Eocene carbonate sequences، located in North of Tochal Village (south-east of Tehran)، near Emam Zadeh Abdollah. The Ziyarat Formation with the total thickness of 212. 5 m conformably overlies the Fajan conglomerate and is overlain by greenish tufaceous siltstone of the Karj Formation. Petrographic studies led to the recognition of 11 microfacies. Different facies and evidences such as the absence of reefal facies، calciturbidite deposits، and widespread tidal flat deposits indicate that the Ziyarat Formation was deposited in a homoclinal carbonate ramp environment. Micritization، cementation، compaction، neomorphism، dissolution، dolomitization، fracturing and silicification are the major diagenetic processes in the Ziyarat Formation، occurring in meteoric and burial diagenetic environments. Major and minor elements and oxygen and carbon isotope values indicate that Ziyarat carbonates were deposited in a shallow warm water tropical environment and original carbonate mineralogy was aragonite. Bivariate plots of Mn versus Sr/Ca and δ18O illustrate that Ziyarat limestones were affected by open system diagenesis with high water/rock interaction. Early burial diagenetic temperature calculation based on heaviest oxygen isotope values of micrite and δw of Eocene seawater of 0. 85 SMOW، shows that temperature was around 39˚C. Cathodluminescence studies of carbonate cements illustrated dull luminescence، because these carbonates afftected by both meteoric and burial diagenesis، and thus typical yellow to orange luminescence and zonations، evidence of meteoric diagenesis، are absent. This statement is confirmed by isotropic evidences.