Depositional model, diagenesis and geochemistry of the Sefid- Kuh Formation (Late Scythian) in Aghe darband Tectonic Window, NE. of Iran

Shallow marine carbonate sedimentation prevailed during the Early Triassic in the north-east of Iran (Sefid Kuh Formation; Agh Darband tectonic window). This formation with the age of Late Schythian overlies conformably the Qara Gheithan Formation and is unconformably covered by the Nazarkardeh Formation in the type section. To evaluate the depositional model, original carbonate mineralogy and diagenetic processes in Kale Faqir (250m), Kale Angur (90m), Kale Ghalk (70m) and Kale Anabeh (30m), the Sefid Kuh Formation was investigated by petrographic and geochemical analyses. Based on sequence stratigraphy in this formation, four 3rd sequence in Kale Faqir detected. Three sequence in Kale Angur, two sequence in Kale Ghalk and one sequence in Kale Anabeh detected. Petrographic studies led to the recognition of 12 microfacies that were deposited in four facies belts: tidal flat, lagoon, and shoal in inner ramp and shallow open marine in mid to outer ramp environment. Of these microfacies, eight belong to inner ramp, one belong to mid ramp and three are located in the outer ramp setting. The wide distribution of tidal flat sediments along with the oolitic shoal facies and lack of a large shoal reef and calciturbidite indicate a homoclinal-ramp system. Mud mounds are one of the most sedimentary structure in the shallow-marine carbonates of the Sefid Kuh Formation and contained biotubation, worm tube and ostacod debries. Abundant aragonite skeletal and non-skeletal components, aragonite cements, oomold and presence of dolomites in Sefid-Kuh carbonates indicate original aragonite mineralogy. Cementation, dolomitization, dedolomitization, micritization, bioturbation, hematitization, compaction, dissolution, vein filling, phosphatitization, pyritization and stylolitization are diagenetic processes in the Sefid Kuh Formation, occurring in marine to meteoric and burial diagenetic environments. Cementation and dolomitization are the main diagenetic processes that affected the original texture. Values of major (Ca and Mg) and minor (Sr, Na, Fe and Mn) elements (such as high Sr value) shoes that the Sefid Kuh carbonates were deposited in a shallow warm-water subtropical environment and aragonite was the original carbonate mineralogy. Variations of Sr/Ca suggest that diagenetic alteration must have occurred in an open diagenetic system, with high water/rock interaction which dolomitization, dissolution and cementation support this conclusion.