Ammonites Biostratigraphy of the Sarcheshmeh Formation (Aptian); Hosein Abad section, northeast of the Kopet Dagh Basin

To determine the relative age of the Sarcheshmeh Formation in the northeast of the Kopet Dagh Basin, a stratigraphic section (Hosein Abad) with a thickness of 90 meters is selected. Lithologically, Sarcheshmeh Formation is mainly composed of gray to green marls with muddy limestone beds. In this research, based on ammonites, 10 species attributed to 4 genera of ammonites were identified. Accordingly, three biological zones, Deshayesites consobrinus, Deshayesites deshayesi, and Dufrenoyia mackesoni, are proposed confirming the Early’s relative age Aptian (Early Cretaceous) for Sarcheshmeh Formation in Hosein Abad section. The identified biological zones were also compared with some previous studies in the Kopet Dagh, Caucasus, and Mediterranean basins and it was found that the Sarcheshmeh Formation in the study area (northeast of the Kopet Dagh) shows well correlation to biological zones in the late Early Aptian. While, some studies on this Formation have reported biological zones related to the early Early Aptian.