Biostratigraphy of Oligo-Miocene marine deposits of Central Iran basin based on calcareous nannoplanktons

Abstract

Background and Objectives: The Oligo-Miocene marine deposits (Qom Formation) have been widely spread in the Central Iran Zone. According to economic value of these marine deposits, this Formation studied by several researchers. While there are many differences opinion among researchers. For this reason, to solve the existing ambiguities, it is necessary to study with new tools and techniques. Therefore, the present study is the first study aimed at biostratigraphy based on calcareous nannoplankton that has been performed on all succession exposed from the Oligo-Miocene marine deposits (members a to f of the Qom Formation). Methods: The studied deposits are located in the middle part of the Central Iran Zone and more precisely in the Navab anticline (northeast of Kashan city), which has a thickness of 417 meters. Due to the location and expansion of all members in this section, it has been a very suitable area for biostratigraphic assessments comparable to world standard models. In this study, a total of 370 microscopic slides prepared from rock samples were studied to the determination of the calcareous nannoplankton and accurately separate world standard biological zones. Findings: The present study led to the identification of 72 species belonging to 18 genera of this valuable group of the planktonic micro-organisms. According to the identified index species, standard nannoplankton biozones of the NP25 and NN1-NN4 were separated from this surface section of the Qom Formation. In addition, the equivalent biozones such as CNO5-6, CNM1- CNM6 as well as zones and subzones CP19b, CN1a, b, c, CN2- CN3, which belonging to the newest biozonation patterns, were also separated in this study. Conclusion: Therefore, based on the geological time value of the separated biozones, the age of late Chattian to late Burdigalian-early Langhian was proposed for the Qom Formation in the Navab anticline section. In this study, for the first time, the important chronostratigraphic boundaries such as the Paleogene- Neogene system boundary (or Chattian-Aquitanian stage boundary) in the upper parts of sub-member "c1", the Aquitanian- Burdigalian stage boundary at the base of member "e" and Burdigalian- Langhian in the upper part of member "f", were disaggregated based on biostratigraphic criteria.