The Gachsaran Formation from the Fars Group has a vast extent in Saudi Arabia, United Arab Emirates and Qatar (Dam Formation). It extends from the Zagros Folded Belt Zone in the south of Iran to the north of Iraq, and even to northeast Syria (Fatha Formation). In order to eliminate the ambiguities of the proposed age and cover the regional information misplacements in cases where biostratigraphy studies can be effective, surface outcrops of these deposits were investigated in three stratigraphic sections near the border of Iran and Iraq, in the area behind the mountains of the Lurestan region (Posht-e-Kuh arc). According to similar results obtained in two other sections, the age of the Gachsaran Formation in the studied region is considered to be Early Miocene (Late Burdigalian). Comparison of the results of this study with biostratigraphic studies from other parts of the Middle East, which includes 16 study areas, shows that the Gachsaran Formation in Iran and its equivalent formations (Fatha and Dam) in neighboring countries have an age range of early Miocene (Aquitanian–Burdigalian) and even Burdigalian. This age has been determined by comparing the biological contents of the Gachsaran Formation with valid official biozones, as well as the presence of the index fossil Borelis melo curdica, to cover the information gap in the area. Isotopic studies of these sections in the region show that the maximum age of these sediments is early Miocene (Burdigalian), and this confirms the results of biostratigraphic studies.

In the present study, the geochemical characteristics of sequences of the Gachsaran Formation, located in the west of Kermanshah province, Iran - Iraq border zones, were studied. In order to determine the concentration of the elements, the XRF and ICP-mass techniques were employed, and the XRD technique was used to identify the mineralogical composition and finally, the evaluation of the level of pollution caused by these elements were carried out using statistical and pollution index software.The results illuminated that the concentrations of CaO, MgO, TiO2, and concentrations of two elements, i.e., Cd and Sb were higher than their mean values in the earth’s crust. Based on the CF pollution index, the elements of Cd and Sb with the values of 1.52 and 2 show the moderate contamination. Enrichment factor (EF) revealed moderate contamination for Cs (2.46), Ga (3.86), Rb (2) and Ti (2.35). This index showed the high pollution and anthropogenic origin for Ti (8), Cd (10.41), U (11.26) and Sb (13.43). The results of the correlation between the elements indicated the presence of positive and significant correlation between Cs, Hf, La, Nb, Nd, Rb, Sc, Sm, Ta, Tb, Th, Tl, W, Y, Yb, and Zr. There was no positive and significant correlation between U and none of the elements. Three elements of Sb, U, and Cd showed a negative correlation with most of the studied elements. According to the results of cluster analysis, three separate groups were obtained so that each of Ti and Fe was classified as separate groups and Fe showed the highest difference in comparison with other elements. Based on the results of the Principal Component Analysis (PCA), the highest effect was related to the elements of Cs, Hf, La, Nb, Rb, Sc, Sm, Tb, Th, Tl, W, Y, Yb, Zr, Fe in the first Component, Sb, Cd in second component and U in the third component.