Petrography and geochemistry of dolomites of Shahbazan formation and investigation of its possible boundary with Asmari formation from the elemental geochemical point of view (Northeastern Kohdasht, South Lorestan)

The Shahbazan Formation is composed of dolomitic, dolomitic limestone and marl sequences of middle to late Eocene age. The studied area is located in the south of Lorestan and 20 km northeast of Kohdasht city. In this section, the Shahbazan formation with the dominant lithology of limestone and dolomite is 121 meters thick, its lower boundary is discontinuous and sloping on the Keshkan debris formation, and its upper boundary is discontinuous with the carbonates of the Asmari formation. The facade is covered continuously. Field and petrography studies were carried out on 90 sedimentary thin sections and geochemical studies on 30 samples from Shahbazan dolomite formation and 10 samples from Asmari carbonate formation by (EDS) and (EPMA) methods. Based on elemental analysis, two groups of dolomites were identified. Primary dolomites (dolomicrites) were observed in sizes less than 10 microns and secondary dolomites (delomicrosparites and delvasparaites) were observed in sizes larger than 10 microns. Using elemental analysis, it was found that from the primary dolomites to the secondary dolomites, the amount of Fe, Mg and Mn elements increases, while the amount of Sr, Ca and Na elements decreases. In the possible boundary between Shahbazan and Asmari formations, the amount of Sr, Ca, Na, Sr/Mn and Ca/Mg increases, and the amount of Mn, Mg elements and the ratio of Mn/Ca decrease. Geochemical evidence showed that all studied dolomites are non-stoichiometric (Ca/Mg ≥2) and the high amount of Na element with an average of 0.6% by weight indicates this issue.