Investigation of facies, elemental and isotopic changes of carbonate deposits of Tirgan Formation (Barrmian-Aptian) in Kopeh Dagh sedimentary basin (Northeast of Iran)

The Kopeh Dagh sedimentary basin is an intercontinental basin formed following the closure of the Paleo-tethys ocean due to a collision between the two continents of Iran and Turan and in has arisen the Early Cimmerian orogenic phase. Deposition of shallow carbonates during the Lower Cretaceous (Barmin-Apsin) in the Kopet Dagh sedimentary basin has been introduced as the Tirgan Formation. This formation is located in three studied sections on the Shurijeh detrital siliceous formation and below the limestone-marni formation of Sarcheshmeh.

In this research, this formation uses petrographic studies and geochemical analyzes to evaluate the sedimentary model, primary carbonate mineralogy and diagenetic processes in Shirvan (197.7 m), Quchan (464.4 m) and Chenaran (127.8) sections. 720 thin sections were prepared for petrographic studies. To separate ferroan calcite and dolomite from nonferroan samples, the samples were stained with potassium ferrocyanide solution (Dickson, 1996). In order to name carbonate rocks, the method (Dunham, 1962) and detrital rocks, classification (Folk, 1980) were used. The method (Flugel, 2010) was also used to interpret the facies and present the sedimentary model. For geochemical studies, 41 samples were analyzed by ICP-OES method in Iran-Karaj Mineral Processing Research Center (Emidro Company) for elemental analysis and by ICP-MS method in Science Laboratory Center of Ottawa University of Canada for carbon isotope and oxygen.

Petrographic studies led to the identification of 18 microfacies that were deposited in four facies belts open marin, lagoon, Barrier and tidal flat. The frequency of tidal facies along with barrier facies and the absence of large reef barrier and turbidite sediments indicate the deposition of this formation in a ramp system with a carbonate slope. Diagenetic processes in this formation include: cementation, micritization, dissolution, compaction, dolomitization, fracture, stylolitification and silicification. The abundance of skeletal and non-skeletal components of calcite along with calcite cement, abundance and presence of dolomite with high content of stransiom refers to the initial mineralization of calcite in the carbonates of Tirgan Formation in time. Also, the values ​​of major (Mg, Ca) and minor elements (Sr, Na, Mn, Fe) and the isotopic range of oxygen 18 and carbon 13 of the limestones of Tirgan Formation, also indicate the composition of primary calcite mineralogy in these areas. This value also shows the impact of the burial diagenesis environment on these deposits in a semi-closed diagenesis system with a low water-to-rock ratio. The average diagenetic temperature of seawater at the time of sedimentation of carbonates of Tirgan Formation using the heaviest oxygen isotope in calcareous samples is estimated to be 18.21 ° C.

Comparison of the data obtained from the three studied sections and the information provided from the eastern and western regions, shows an increase in temperature and thus an increase in strontium to the west, which it is indicates a change in the mineralogical composition from calcite to aragonite from east to west basin