Middle–Late Devonian conodont fauna of the Bahram Formation in the Shahzadeh Mohammad section, North of Kerman

Abstract

Abstract Devonian deposits of the Bahram Formation with a thickness of 300 m were studied based on conodont fauna in Shahzadeh Mohammad section (33 km east of Zarand, 95 km north of Kerman city) in central Iran structural Zone. According to 31 identified species and subspecies and their stratigraphic development, nine conodont biozones belonging to the middle Givetian to the lower Famennian age were identified. The presence of Polygnathus ansatus determines the ansatus Zone and Icriodus arkonensis arkonensis, determines the age of middle Givetian for the base of the Bahram Formation, which is the oldest age determined for the base of the Bahram Formation. Based on conodont distributions and conodont biofacies, polygnathid-icrodid biofacies are the most abundant biofacies in the section, indicating the middle shelf environment for the studied deposits. Keywords: Devonian, Conodont, Bahram Formation, Givetian–Famennian, Biofacies. Introduction The Devonian, one of the most important periods in the Paleozoic, was a time of major sea-level changes and catastrophic events (Sandberg et al. 2002). Besides, it was a time of exceptionally high sea-level stand. It inferred widespread equable climates, which resulted in the most significant carbonate production and diversity of marine fauna in the Paleozoic (Gradstein et al. 2005). The Late Devonian was a time of many sea-level changes, catastrophic events, and two mass extinctions, knowledge of which is gained mainly through a high-resolution conodont biochronology (Sandberg and Ziegler 1996). The Devonian is the time for the radiation of fish and the appearance of the first ammonoids, insects, and amphibians. Vascular plants and forests became established; plants took over the land and became so plentiful that the first coal deposits were formed in tropical swamps. Besides, before the end of the period, land tetrapods appeared. Due to the greenhouse climate condition and sea-level highstand, global reef growth reached an acme in the Middle Devonian (Gradstein et al. 2005). Due to the relative rise of sea level, Middle and Late Devonian sediments were deposited in the northern, eastern, and central parts of Iran. In Central Iran, Middle and Late Devonian succession are exposed in the Kerman, Tabas, Yazd, and Isfahan regions. In Kerman, these deposits are limited to the northern parts of the region and are introduced and studied as Sibzar and Bahram formations. The Sibzar Formation, has few occurrences and only in two disjunct areas can be traced (Wendt et al. 2002). This formation is placed between the Padeha and the Bahram formations. The Bahram Formation consists of siliciclastic and carbonate strata. There is a complete profile from Lower to Upper Devonian deposits in the Shahzadeh Mohammad section, east of Zarand. The studied section is part of the Kerman-Tabas Block, which constitutes one of the individual structural units of the Central-East-Iranian Microcontinent. In this study, the biostratigraphy, biofacies, and sea-level changes of the Bahram Formation in the Shahzadeh Mohammad section have been investigated based on conodont fauna. Material & Methods In total, 300 m of the Middle–Upper Devonian successions (Bahram Formation) at the Shahzadeh Mohammad section has been measured. Forty samples, about 4–5 kg, were systematically collected and treated with conventional preparation methods. The samples were processed with diluted acetic/formic acid (20%). The residue was washed by sieving ((using sieve Mesh numbers 10, 30, 60, and 100). The conodonts were extracted from residues by hand picking. Discussion of Results & Conclusions Ancyrodella, Icriodus, Pelekysgnathus, Polygnathus are the present genera in this section. Conodont biofacies and relative sea-level changes of the Middle–Upper Devonian Shahzadeh Mohammad section have been investigated and compared with the global sea-level curve. The biofacies interpretation is based on the distribution of the above four genera in the conodont biofacies presented by Sandberg and Dreesen (1984). Since the dominant genera of this section are Icriodus and Polygnathus, frequency changes of these genera determine conodont biofacies and relative sea-level changes. Polygnathus indicates an increase and Icriodus indicates a decrease in the relative sea-level. The following biofacies were identified: icriodid, icriodid-polygnathid, polygnathid-icriodid, and polygnathid biofacies. Polygnathid-icriodid biofacies is the most abundant biofacies. Due to the presence of shallow-water species, the abundance of Icriodus and Polygnathus, the absence of the genus Palmatolepis, the middle shelf environment is determined for the Bahram Formation in the Shahzadeh Mohammad section. In general, all of the Devonian succession from the Kerman area have been deposited in a shallow-water environment, and generally indicates the shallow parts of the Paleotethys. Our examination has yielded new conodont data from the Bahram Formation in the Shahzadeh Mohammad section. The ansatus Zone to Palmatolepis glabra pectinata Zone (middle Givetian to lower Famennian) were assigned to the Bahram Formation within nine conodont zones (Zamani et al. 2020). Conodonts from the sample Sh7 (Polygnathus ansatus and Icriodus arkonensis arkonensis) indicate Givetian age for the basal part of the Bahram Formation; but Middle Frasnian age is reported in previous studies in this area. The first appearance of Ancyrodella rotundiloba pristina represents Givetian/Frasnian boundary at sample Sh12.

Keywords

• devonian
• conodont
• bahram formation
• givetian–famennian
• biofacies