In order to study of calcareous nannofossils biostratigraphy, the upper part of the Pabdeh Formation and the lower parts of the Asmari Formation were selected in one well in Parsi oil field in southwest of Iran. Investigation of calcareous nanofossils in this section resulted in detection of 16 genera and 39 species. According to the first occurrence of index species and distribution of fossil assemblages, four biozones including Helicosphaera reticulate Zone (NP22/ Rupelian), Sphenolithus predistentus Zone (NP23/Rupelian), Sphenolithus distentus Zone (NP24/ Rupelian- Chatian) and Sphenolithus ciperoensis Zone (NP25/ Chatian), which is equivalent to zones CP16c - CP19b, was recognized in the upper part of the Pabdeh Formation. As a result of this study, and based on the identified calcareous nannofossil biozones, the age of the uppermost part of the Pabdeh Formation is Rupelian to Chatian. In the basal anhydrite portion of the Asmari Formation, no biozone of calcareous nanofossils was determined. In this study, the dominant conditions of the sedimentary environment, were determined by the presence of calcareous nannofossils index species, so that the presence of these species confirms the warm climate and basin location at low latitudes.

Keywords: Biozone, Biostratigraphy, Zagros, Calcareous nannofossils

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