The biostratigraphy and palaeobiogeography of Cambrian and Ordovician acritarchs and chitinozoa from the Simeh-Kuh, NW Damghan City, the Alborz Mountains, northern Iran

Abstract

A study in Simeh-Kuh, northern Iran, focuses on Ordovician strata, documenting acritarchs and chitinozoa, and has helped to understand ancient environments. The aim is chronostratigraphical calibration, using palaeontological data including brachiopods, trilobites, and graptolites. Ninety-three surface samples were collected and analysed, yielding a rich collection of 117 morphotypes, including 59 acritarch species and 58 chitinozoan species. Eight acritarch assemblage biozones (A–H) and 14 chitinozoan biozones (1–14) were established. Biozones A–F indicate Late Cambrian to Middle Ordovician ages in the Lashkarak Formation, while biozones G–H indicate the Late Ordovician in the Ghelli Formation. The Eremochitina brevis-Linochitina pissotensis biozones suggest late Floian to late Darriwilian ages, and the Belonechitina robusta-Armoricochitina nigerica biozones support a Late Ordovician age. These findings place the Alborz Mountains in the North Gondwana Domain during the Ordovician. Palynomorphs with a thermal alteration index of 4.0–4.5 indicate overmature organic matter in Lower Palaeozoic deposits, potentially linked to the Ordovician metamorphic event in the southern Caspian Sea. Additionally, four new species were identified: Cyathochitina sp. A, Tanuchitina hosseini-nezhadi sp. nov., Dactylofusa kazzazii sp. nov., and Inflatarium alborzensis sp. nov.

Highlights

• A palynological study at Simeh-Kuh in the Alborz Mountains, northern Iran has uncovered a richly preserved assortment of Cambrian–Ordovician acritarchs and chitinozoa.

• Fifty-eight chitinozoan species, spanning 22 genera, were identified from the Lashkarak and Ghelli formations at Simeh-Kuh. This facilitated the delineation of 14 distinct chitinozoan biozones.

• Fifty-nine acritarch species across 33 genera were identified from the Late Cambrian to Late Ordovician strata, contributing to recognising eight distinct assemblage biozones.

• The Lashkarak Formation has yielded four distinctive acritarch genera: Arbusculidium, Coryphidium, Striatotheca, and Arkonia. These genera hold notable significance as they are characteristic elements of the Peri-Gondwanan palaeoprovince.

• The identification of chitinozoans in the Lashkarak and Ghelli formations has facilitated the establishment of 14 chitinozoan biozones. Notably, these biozones are considered classical elements of the Northern Gondwana Domain/Peri-Gondwana palaeoprovince.

• The Belonechitina robusta, Tanuchitina fistulosa, Acanthochitina barbata, and Armoricochitina nigerica biozones were recognised in the Ordovician deposits at Simeh-Kuh. This discovery marks the region’s first confirmation of Sandbian–Katian strata.

• A significant hiatus exists between the Ghelli and Geirud formations, spanning the Late Ordovician (latest Katian–Hirnantian), Silurian, and Early–Middle Devonian periods.

• Four new species have been established: Cyathochitina sp. A, Tanuchitina hosseini-nezhadi sp. nov., Dactylofusa kazzazii sp. nov., and Inflatarium alborzensis sp. nov.

Keywords:

• Marine microflora and microfauna
• biostratigraphy
• Cambrian–Ordovician
• Gondwana/Peri-Gondwana
• Alborz Mountains (northern Iran)
• palaeobiogeography

Acknowledgements

The authors express sincere gratitude to Dr Jim Riding, the chief editor of Palynology, and three anonymous reviewers for their valuable comments and suggestions that significantly contributed to the improvement of this manuscript. Special thanks also to Dr Mahmoud Hosseini-Nezhad and Dr Roohollah Memarzadeh for their indispensable logistical support and field assistance. Further, the first author acknowledges the unwavering encouragement from his wife, Fahimeh Ghaemi-Nejad, and daughters, Hedieh Ghavidel-Syooki and Mona Ghavidel-Syooki, throughout the finalisation of this contribution. It is important to note that this research received no specific grant from public, commercial, or not-for-profit funding agencies.

Disclosure statement

This manuscript has not been published or presented elsewhere in part or in its entirety and is not under consideration by any other journal. The authors have read and understood the journal’s policies, and believe neither the manuscript nor the study violates any of these. There are no conflicts of interest to declare.

Additional information

Notes on contributors

Mohammad Ghavidel-Syooki

MOHAMMAD GHAVIDEL-SYOOKI, PhD received his BSc in geology in 1974 and an MSc in palynology in 1977 from the University of Tehran. He received his PhD in geology (palynology) from the Michigan State University (MSU) in 1988. He returned to Iran in 1989 and began working at the Exploration Directorate of the National Iranian Oil Company (NIOC) as a senior geologist, teaching palynology at different universities across his country. He was the manager of geological and geochemical research at the NIOC from 1999 to 2007. He is now a professor of petroleum geology at Tehran University and of palaeontology at the universities of Tehran, Amirkabir, Shahid Beheshti, Isfahan, and Shiraz. He has supervised over 52 MSc and PhD dissertations. He was recognised as Iran’s memorable figure of NIOC in 2006, the best geologist, and the father of palynology in Iran the same year. Currently, his research is focused on Lower Palaeozoic acritarchs, chitinozoa and cryptospores/spores in the Alborz Mountains, northern Iran. He has participated in many international geological conferences from 1984 until now.

Mohammad Hossein Piri-Kangarshahi

MOHAMMAD HOSSEIN PIRI-KANGARSHAHI received his BSc in geology in 2012 and his MSc in palynology in 2017. His thesis focused on the Central Iranian basin’s Ordovician–Silurian boundary (acritarchs, chitinozoans, and cryptospores). His study led to a regional palynological scheme with those recorded in the Gondwana landmass and the effects of the Upper Ordovician glaciation on palynomorph assemblages. Since 2013, he has been a research assistant to Professor Mohammad Ghavidel-Syooki at the Institute of Petroleum Engineering, School of Chemical Engineering, College of Engineering, University of Tehran. He formulated a new method for preparing SEM images to save time and improve the palynomorph entities’ extraction and quality. Currently, he is a PhD student at the Tehran University.