Biostratigraphy, biofacies and conodont CAI (Conodont Alteration Index) of the Middle-Late Devonian deposits in Kuh-e-Bande-Abdol-Hossein (SE Anarak)

The Kuh-e-Bande-Abdol-Hossein section, which is located southeast of Anarak, was first mentioned by Reyer and Mohafez (1970) and later examined in more detail by Sharkovski et al. (1984) and Wendt et al. (2005). We re-examined this section because it is mainly composed of sedimentary rocks ranging from Ordovician to Permian age (Hairapetian et al. 2015; Lensch and Davoudzadeh 1982). Herein we focus on the Middle to Upper Devonian strata in order to establish the biostratigraphic framework of this section by means of conodonts. We also briefly discuss the conodont biofacies of the Kuh-e-Bande-Abdol-Hossein.

The measured profile is located approximately 32 km southeast of Anarak and 180 km northeast of Isfahan (E 53° 52′ 55″ and N 33° 10′ 90″ WGS coordinates). The entire section has a thickness of approximately 1200 m. In order to improve the biostratigraphy of the Kuh-e- Bande-Abdol-Hossein section, 78 conodont samples of roughly 2 to 3 kg each were taken from the 366m of carbonates and processed by conventional methods using 10% formic acid. Washed residues were sieved and separated into three fractions, and conodonts were handpicked utilizing a microscope. Depending on the depositional facies setting, the number of conodonts per sample is highly variable, e.g., in dolostones, no conodonts were found. In contrast, in shallow-water limestones, a good number of species occurred in separate beds. A total number of 1917 conodonts were obtained from the residues, which led to the identification of 41 species and subspecies within six genera.

Based on the revealed conodont data, six conodont zones were discriminated as follow: expansus zone, subterminus zone, Upper falsiovalis to transitans zones, transitans to lower rhenana zones,upper rhenana to linguiformis zones, triangularis to termini zones. Bahram Formation at the studied profile spans late Givetian (expansus zone) to Early Famennian (triangularis to termini zones. Biofacies interpretation and conodont frequency reveal the Icriodid-Polygnathis to Polygnathid-Icriodid biofacies due to the deepening of the depositional basin. CAI interpretation also shows the variation from CAI=1/5-2 to CAI=4-4/5. The studied interval is composed of an overall shallow-water, nearshore to open marine facies setting.

This study is undertaken at the Department of Geology, Faculty of Sciences, University of Isfahan. The financial supports by the Vice-Chancellor for Research and Technology, University of Isfahan, was highly appreciated.