Ordovician conodonts of the Katkoyeh formation in the Kuh-e-Bonorg section, Kalmard Horst (west of Tabas)

Abstract

Abstract In this research, Ordovician conodonts obtained from the sedimentary succession of the informal Katkoyeh formation is studied in a stratigraphic section at the south of Kuh-e-Bonorg, eastern flank of Kalmard Anticline in the west of Tabas (Central Iran). The formation which is partly equivalent to the succession of the Shirgesht Formation in Tabas Block is unconformably overlaid the Kalmard Formation (attributed to the Neoproterozoic) and disconformably overlain by the Carboniferous Gachal formation. The Katkoyeh formation with 214.8 meter thickness is divided into three lithostratigraphic members including the Lower Sandstone (mostly quartzarenite to litharenite), Middle Shale (red to green shales interbedded with some dolomitic and calcareous sandstones, in laminated wavy stromatolite structure at the base) and Upper Carbonate (limestones, dolostones and marls). The section was sampled bed by bed for conodont studies so that 80 samples have been selected, however only seven of them were productive. Within the selected samples, six genera and 14 species of coniform and ramiform conodont elements are determined, a few of them are reporting for the first time from Iran. The obtained conodonts demonstrate the faunal assemblages equal to the proteus, elegans-evae and diprion Zones which confirm the age of Early Ordovician (late Tremadocian–Floian) for the studied section. The whole conodont material display a faunal affinity with the Baltoscandic and Argentina paleobiogeographic provinces. Keywords: Conodont, Floian, Katkoyeh formation, Ordovician, Tremadocian. Introduction In spite of the moderately -exposed outcrops of the Ordovician successions throughout the country, most of the stratigraphical studies in Iran are focused on Alborz Range (e.g.; Gansser and Huber 1962, Muller 1973, Ghavidel-Syooki 2006, Ghobadi Pour et al. 2007, 2011; Jahangir et al. 2016) and the appropriate data from the other regions are restricted to a few published reports (e.g.; Ruttner et al. 1968, Aghanabati 1977, Hamedi 1995, Ghaderi et al. 2008). Especially our knowledge is very little about the conodont contents of the Ordovician rocks in Iran. The Ordovician sedimentary succession in the north of Tabas in central Iran have been introduced as the Shirgesht Formation by Ruttner et al. (1968). The rock sequences of this formation composed of brown, cream, and green to red limestones, shales and partly sandstones with hardly more than 800 m in thickness (Ghobadi Pour et al. 2006). Lithostratigraphically, the Ordovician sedimentary interval in adjacent Kalmard Block (Aghanabati 2010) is different, includes more siliciclastic rocks somewhat carbonate beds in upper part. These succession has been considered as the Katkoyeh formation by Hamedi and Wright (1992). The less known Katkoyeh formation as an informal stratigraphic rock unit in central Iran has been regarded to Arenigian to Ashgillian in the type area in Kerman region, but the age of the formation in the Kalmard type region is dissimilar in different studies. In the current research we have looked for the succession Katkoyeh formation in a section at the south of Kuh-e-Bonorg, eastern flank of the Kalmard anticline, from the age dating view, based on the conodont contents. Material & Methods In the Kuh-e-Bonorg section, 80 rock samples were collected bed by bed for conodont content while only seven of them were productive. For this purpose, sandy limestones, dolomitic limestones and pure limestones with about 3–5 kg were dissolved by 10% formic acid and 20% acetic acid following the standard procedure for conodont extraction (e.g., Jeppsson & Anehus, 1995). Residue washed on appropriate clean 18 and 230 mesh sieve-stack (0.85 to 0.063 mm). The extracted conodont collections stored in the Ferdowsi University of Mashhad, Iran under the collection name with the FUM#MC prefix. Discussion of Results & Conclusions The Katkoyeh formation in the south of Kuh-e-Bonorg section is unconformably overlaid the Neoproterozoic rocks attributed to the Kalmard Formation and disconformably overlain by the Carboniferous Gachal formation. The Katkoyeh formation of 214.8 meter thickness in the studied area is divided into three members such as the Lower Sandstone (mostly quartzarenite to litharenite), Middle Shale (red to green shale interbedded with some dolomitic and calcareous sandstone, in laminated wavy stromatolite structure at the base) and Upper Carbonate (limestones, dolostones and marls). Among the seven fossiliferous beds, we obtained the index conodonts of Lower Ordovician which belong to six genera and 14 species of coniform and ramiform elements. The assemblage is correlative with the same collections from the Baltoscandic and Argentina paleobiogeographic provinces (e.g.; Mannik and Virra 2012; Voldman ‌‌‌‌‌‌et al. 2017). The distribution of taxa in the stratigraphic succession is variable, in abundance of elements as well as in the number of taxa. The oldest conodonts in the section, Drepanodus arcuatus and Drepanoistodus costatus, come from the dolostones and dolomitic limestones of uppermost Tremadocian-early Floian proteus Zone (equivalent to Acodus apex Zone in NW Argentina Province) the sample MC208B. The next fossiliferous bed, MC232, comprises Acodus triangularis, Acodus cf. deltatus, Drepanoistodus cf. nowlani, Drepanoistodus cf. bassiovalis, Drepanoistodus bassiovalis and Gothodus costalatus which confirm Floian elegans - evae Zones (equivalent to Gothodus vetus–Gothodus andinus Zones in NW Argentina Province). The last productive beds, MC263–MC265, contain Trapezognathus diprion as the index taxon for upper evae Zone (equivalent to Trapezognathus diprion Zone in NW Argentina Province). This species confirm the age of late Floian for the uppermost part of the Katkoyeh formation in the Kuh-e-Bonorg section.

Keywords

• conodont
• floian
• katkoyeh formation
• ordovician
• tremadocian