Petrography and geochemistry of the Shotori dolomites in Kalmard section, Central Iran

Carbonate sequences of the Shotori Formations (Middle Triassic) with a thickness of 308 m, were deposited in the Kalmard region of the Tabas city in Central Iran basin. The lower contact of the formation gradually and conformably overlies the Sorkhshale Formation and upper contact is faulted. The Shotori Formation is mainly composed of thick to medium bedded fine-coarsely crystalline dolomites with a thickness of 250 m with interbeds of thin bedded limestone and sandstone. The Shotori Formation is mainly composed of fine-coarsely crystalline dolomite. Based on petrographic (size and fabric), and elemental studies (Ca, Mg, Na, Sr, Fe, Mn), five dolomite types were recognized. Variation in dolomite types is mainly related to early to late diagenetic processes, changing the composition of dolomitizing fluids. Geochemical studies also indicate that medium to coarse grain dolomites formed in meteoric diagenesis under reducing conditions. Mechanism of dolomitization for dolomite type 1 is sabkha model, for dolomite types 2 and 3 is mixing zone and is burial model for dolomite types 4 and 5.