ABSTRACT: 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 (Aquitanianâ€“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.