Conodont biostratigraphy and biofacies of Late Devonian-Early Carboniferous deposits at Chelcheli section, Northwest Shahrud (Eastern Alborz)

Abstract

Abstract Chelcheli section (397m thick) about 70 km northwest of Shahroud on the Shahroud-Gorgan road (Tuskistan route), about 5 km east of the Chahar-Bagh village is sampled and studied to investigate the DC transition and the conodont facies changes during the Global Hangenberg Crisis. The measurement of one hundred eighty two meters of the upper part of Khoshyilagh formation and 215 meters of the Lowermost Mobarak formation led to the discrimination of 11 rock units with different sedimentary features. In total, forty limestone samples, about 4-5 kgs were systematically collected and treated with conventional preparation methods. Three hundred forty conodont elements were obtained, which led to the identification of 47 species and subspecies belonging to 16 genera that allowed the separation of 15 conodont bio-zones from the early Famennian to the Late Tournasian. Keywords: Biostratigraphy; Late Devonian; Mississippian; Devonian – Carboniferous boundary, Hangenberg Crisis; Chelcheli; Eastern Alborz. Introduction In Iran, Devonian-Carboniferous sequences are found in limited areas, but their spread is more extensive in eastern and central Alborz and central Iran (Wendt et al. 2005). However, these deposits are located in different structural units and prevailing geological setting have caused different sedimentary facies and sequences (Ashuri 1990, 1997, 1998, 2001, 2002, 2004, 2006; Bahrami et al. 2011; Habibi et al. 2008; Sardar Abadi et al. 2015; Yazdi 1999, Yazdi and Turner 2000; Wendt et al. 2002, 2005). The Alborz range in northern Iran is an active fold-and-thrust belt (Alavi, 1996; Berberian, 1983) and is situated about 200 – 500 kms to the north of the Neo-Tethyan suture. The closure of the Palaeotethys between the Iran Plate as a part of Gondwana and the Turan Plate (Laurussia) occurred in the early Late Triassic and was accomplished during the Early/earliest Mid Jurassic (Golonka 2002). During the Palaeozoic, Iran was situated at the northern margin of Gondwana (Berberian and King 1981; Scotese 2001). In the mid-Palaeozoic, most of Iran was located about 20°–25° south of the palaeoequator. During the Mississippian, the Alborz Basin was positioned at a palaeolatitude of approximately 45–50°S (Muttoni et al. 2009; Torsvik & Cocks 2004, 2013; Vachard 1996). Most of the D/C sections from central and eastern Alborz Mountains have been deposited in a shallow-water, carbonate ramp setting (Königshof et al. 2020 in press). However, in contrast to other D/C sections in Iran (Bahrami et al. 2011; Habibi et al. 2008), the Chelcheli section exhibits “characteristic rock” types around the D/C boundary such as black shale and sandstone. In this study, we have sampled conodonts from the Khoshyeilagh Formation and the overlying Mobarak Formation of the Chelcheli section with a special focus on the D/C boundary. Material and Methods In total, forty limestone samples, about 4-5 kgs were systematically collected and treated with conventional preparation methods. The samples were processed with diluted acetic/formic acid (20%). The conodonts were extracted from residues by hand picking and heavy liquid technique at the University of Isfahan, I.R. Iran. Discussion of Results and Conclusions Discriminated conodont species: Alternognathus, Bispathodus, Branmehla, Clydagnathus, Gnathodus, Icriodus, Mehlina, Palmatolepis, Polygnathus, Neoplygnathus, Protognthodus, Pseudopolygnathus, Scaliognathus, Doliognathus, Siphonodella and Scaphignathus are amongst the cosmopolitan species, the index and indicator spacies in the study of DC boundary. The upper Palmatolepis triangularis Zone to Bispathodus ultimus Zone (early-late Famennian) were assigned to the uppermost portion of the Khoshyeilagh formation within 12 conodont zones. Protognathodus kockeli Zone to Scaliognathus anchoralis-Doliognathus latus zone was also assigned to the Tournasian part of Mobarak formation classified in 3 distinct conodont zones. The presence of tiny black shale horizons just bellow the DC boundary and its association with offshore conodont species such as bispatotides and palmatolpids indicates an increase in the sea level to the highest level into near the end of the latest Famennian. Subsequently 8-10 meters of quartzitic sandstone unit with cross-stratification and no conjunctival fauna due to lowering of sea level show the evidence of biological and sedimentological changes during the Hangenbeg Crisis in the Chelcheli section. Gradual and recurrent increase in polygnathids, protognathids, and Siphonodellis, which are subsequently replaced by offshore Gnathoids also indicates that water levels are re-emerging into the Upper Tournasian

Keywords

• biostratigraphy؛ late devonian؛ mississippian؛ devonian – carboniferous boundary
• hangenberg crisis؛ chelcheli؛ eastern alborz