An integrated biostratigraphy using orbitolinids, calcareous nannofossils, and dinoflagellate cysts was conducted on the Lower Cretaceous Sarcheshmeh Formation in two stratigraphic sections from the western Koppeh-Dagh Basin (NE Iran). The analyzed successions consist mainly of deep-marine shales and marlstones with intercalations of thin-bedded orbitolinid-bearing limestones. These strata yielded common to abundant and fairly well to well-preserved calcareous nannofossil assemblages. The aforementioned assemblages led to the identification of parts of the NC6 and NC7a biozones that can be correlated with parts of the CC7a and CC7b zones. The organic-walled dinoflagellate cysts also show well-preserved and relatively diverse assemblages in the studied sections that corresponded to the Odontochitina operculata Zone. Additionally, within the limestone intercalations of the Sarcheshmeh Formation, two index species of orbitolinids, Praeorbitolina cormyi Schroeder, and P. wienandsi Schroeder, were recorded, indicating the presence of the Praeorbitolina cormyi Zone. According to the literature, these recognized biozones are compatible with the ammonite zones of Deshayesites forbesi, Deshayesites deshayesi, and Dufrenoyia furcata, as well as the Globigerinelloides blowi and Leupoldina cabri planktonic foraminiferal zones. By the compilation of all acquired data, an age of late early Aptian (late Bedoulian) is deduced for the Sarcheshmeh Formation in the studied sections.
