Biostratigraphy of Devonian deposits of Bahram Formation in North Tar section (Varcamar valley - Southwest Natanz) based on conodont fauna

The Iranian Plate is regarded as a marginal fragment of Gondwana, which has been separated from Gondwanan-Arabian plate during the Late Paleozoic or Early Mesozoic (Early Triassic) and has been connected with the Eurasian Turan plate at the end of Middle Triassic time (Berberian and King, 1981; Soffel and Förster, 1984; Weddige, 1984a; Scotese, 2001). Tectonically, Iran is subdivided into several structural zones that have been affected with numerous folds, faults and recurrences, so that respectively each zone has different geological characteristics with exclusive sedimentological features and fossil contents (Stocklin, 1968; Wendt et al., 2005). The studied section is located at the vicinity of Tar village (Tarq area), about 45 km southwest of Natanz city, 110 km northeast of Isfahan. The area structurally belongs to the Sanandaj-Sirjan Metamorphic belt at the contact area between Uromiyeh-Dokhtar volcanic belt and Central Iran Microplate. The project conducted the biostratigraphy of the Middle - Upper Devonian deposits based on conodont fauna to establish the precise age of the Bahram Formation in the studied profile. Bahram Formation in the studied section is composed of 3 separate units with different lithological characters and different fossil contents. The first clastic unit is an alternation of dolomites, sandstones and minor shale interbeds with rare fossils, disconformable overlaid Padeha Formation. The second and Third units are mainly carbonate with several fossiliferous horizons. Zahedi (1973), Adhamian (2003), Ghobadipour et al. (2013) and Bahrami et al. (2015) studied the Devonian deposits in the central Iran area, and they all reported pre-Permian erosional disconformity (Hercynian phase). Wendt et al. (2002; 2005) and Berberian and King (1981) also reported the same phase from the other localities where Paleozoic rocks are exposed in Iran. Sharland et al. (2001) and Ruban et al. (2007) believed Per-Permian erosional corresponds to the deformation and uplifts of contemporaneous to the Neothetian rifting in the Middle Permian time. Forthy –six samples collected for systematically conodont studies and processed with conventional acetic/formic acid. Besides, a few samples, that were prolific, let us to stablish the conodont zonation for the studied interval. Based on revealed data out of conodont studies, 26 conodont species belong to 4 genera were identified: Icriodus. obliquimarginatus, I. subterminus, I. brevis, I. expansus, I.excavatus, I. eslaensis, I. alternatus alternatus, I. sp. nov, I. aff. difficilis, Polygnathus. varcus, P. cf. parawebbi, P. ensensis, P. linguiformis linguiformis, P. linguiformis linguiformis γ1a, P. linguiformis linguiformis γ1b, P. linguiformis linguiformis γ2, P. linguiformis linguiformis γ4, P. pseodufoliatus, P. alatus, P. angustidiscus, P. webbi, P. politus, Polygnathus xylus, P. aequalis, Ancyrodella sp., and Bipennatus bipennatus. Three conodont zones were discriminated as follow: expansus Zone, subterminus Zone, Upper falsiovalis (Frasnian)?. The conodont biostratigraphy proves the Late Givetian to Early Frasnian? age for the deposits of the Bahram Formation in North Tar section. Icriodids and Polygnathids dominated between the studied conodont fauna yielded Icrodid-Polygnathid biofacies to the studied interval, although in the upper expansus zone and Lower Frasnian? interval of the section indicates more or less the Polygnathid-Icriodid biofacies, the privilege of the deeper condition than the subterminus part of the section. The CAI (color alteration index) of the conodonts were close to 4/5-5 shows high temperature after burial process that indicator of no gas and oil potential in the area.  This article is extracted from a research project No. 910712 entitled "Biostratigraphy of Devonian rocks (Bahram Formation) in the North-tar section (Southwest Natanz) based on conodonts". The Project financially and logistically supported by the Vice chancellor for Research and Technology office at University of Isfahan which deeply appreciated. The author also thanks to Dr. Vachik Hairapetian (Islamic Azad University Khorasgan Branch) for his valuable guidance in determination of the vertebrate micro-remains.