Biostratigraphy of Abtalkh Formation based on Calcareous nannofossils in eastern Kopet-Dagh basin, Padeha stratigraphic section (south-east of Mashhad)

The Kopet-Dagh basin extended in the north and north east of Iran and consists of marine strata from Middle Jurassic to Paleogene time. Abtalkh Formation is one of the thickest lithostratigraphic units of Cretaceous in the Kopet-Dagh basin and is composed of light gray، green to bluish marl and silty marl. The Abtalkh Formation conformably overlies the marls and chalky limestones of the Abderaz Formation and is conformably overlain by the Nayzar Formation (sandstone، silty marl and limestone). In this study، the Padeha stratigraphical section in the eastern Kopet-Dagh basin was selected and studied in high resolution for its calcareous nannofossil biochronology. Padeha section contains very abundant، rich and moderately preserved calcareous nannofossil assemblages of Tethyan realm. A total of 268 samples in Padeha stratigraphical section were collected from the Abderaz، Abtalkh and Nayzar formations. The thickness of measured Abtalkh Formation in Padeha section was 974. 5 (m). One hundred and fourteen nannoplanktonic species belonging to forty nine genera and seventeen families were identified and the biostratigraphic study allowed the recognition of the calcareous nannofossil biozones of Sissingh (1977)، emended by Perch-Nielsen (1985) and Burnett (1998). From bottom to top، identified biozones in the Padeha section including a part of CC19، CC20، CC21، CC22، probably a part of CC23a and a part of UC14dTP، UC15bTP، UC15cTP، UC15dTP and probably a part of UC16. The results of this study indicate that the age of Abtalkh Formation in the Padeha section spans from late Early Campanian to Latest Campanian.