Peri-Gondwanan acritarchs and chitinozoans from the Lower–Middle Ordovician Lashkarak Formation in the Alborz Mountain Ranges, northern Iran: regional stratigraphical significance and palaeogeographical implications

Abstract

The Lashkarak Formation (Lower–Middle Ordovician) in the Gerdkuh locality, 10 km west of Damghan city, northern Iran, has been found to contain acritarchs and chitinozoans. This study aimed to understand these chitinozoan and acritarch successions as well as brachiopods in this part of the Alborz Mountains, in a novel manner. Ninety-seven surface samples from this succession were collected and analysed. Thirty samples yielded well-preserved palynomorph taxa such as acritarchs, chitinozoans, and scolecodonts, as well as graptolite remains. In total, 53 taxa were identified among the acritarchs (38 species belonging to 21 genera) and chitinozoans (15 species belonging to 10 genera). Although scolecodonts and graptolite remains were also observed, they were not studied in detail. The palynological analyses revealed the presence of several diagnostic acritarchs in the Lashkarak Formation, including Vulcanisphaera simplex, Arbusculidium filamentosum, Coryphidium bohemicum, Dactylofusa velifera, Striatotheca mutua, Arkonia virgata, and Orthosphaeridium ternatum. These acritarchs allowed the recognition of five acritarch assemblage zones. Similarly, seven chitinozoan biozones were recognised: Eremochitina brevis, Desmochitina ornensis, Belonechitina henryi, Cyathochitina protocalix, Cyathochitina calix, Siphonochitina formosa, and Laufeldochitina clavata. These findings confirm the inclusion of the Alborz Mountains in the peri-Gondwana palaeoprovince during the Early–Middle Ordovician. The co-occurrence of the acritarch and chitinozoan taxa with previously identified brachiopods allowed for the establishment of a more detailed Early–Middle Ordovician biozonation, demonstrating their potential usefulness for global chronostratigraphy. Based on palynological and brachiopod assemblages, a shallow marine inner shelf setting is suggested for the Early–Middle Ordovician at the Gerdkuh locality. Moreover, in this study, four new morphotype species were erected, consisting of one new chitinozoan (Cyathochitina gerdkuhensis sp. nov.) and three new acritarchs (Baltisphaeridium razii sp. nov., Navifusa alborzensis sp. nov., and Orthosphaeridium iranense sp. nov.). However, Othosphaeridium cf. ternatum was left in open nomenclature.

Highlights

• Palynomorphs from the Lower–Middle Ordovician (Lashkarak Formation) were recorded from the Alborz Mountains.

• 15 chitinozoan species (10 genera) were recorded from the Lashkarak Formation, S. Caspian Sea.

• 38 acritarch taxa (21 genera) were identified, indicating the N. Gondwanan Domain.

• The diagnostic acritarch taxa are Arbusculidium, Coryphidium, Striatotheca, and Arkonia.

• Chitinozoan biozones are E. brevis, D. ornensis, B. henryi, C. protocalix, C. calix, S. formosa, and L. clavata.

• Baltisphaeridium razii sp. nov., Navifusa alborzensis sp. nov., Cyathochitina gerdkuhensis sp. nov., and Orthosphaeridium iranense sp. nov. is recorded for the first time.

• A major hiatus corresponds to the Late Ordovician, Silurian, and Early–Middle Devonian.

Keywords:

• Early Ordovician (Tremadocian–Floian)
• Peri-Gondwana
• Lashkarak Formation
• chitinozoan and acritarch biostratigraphy
• Alborz Mountains
• Middle Ordovician (Dapingian–Darriwilan)
• Northern Iran

Acknowledgements

We gratefully acknowledge the chief editor of Palynology, Dr Jim Riding, and four anonymous reviewers for their helpful comments and suggestions regarding improvement of this manuscript. The authors also thank Dr Mahmoud Hosseini-Nejad and Dr Roohollah Memarzadeh for their logistical support and assistance in the field. The first author is grateful to his wife, Fahimeh Ghaemi-Nejad, and daughters, Hedieh Ghavidel-Syooki and Mona Ghavidel-Syooki, for their encouragement in finalising this contribution. This research did not receive any specific grant from public, commercial, or not-for-profit funding agencies.

Disclosure statement

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

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Additional information

Notes on contributors

Mohammad Ghavidel-Syooki

MOHAMMAD GHAVIDEL-SYOOKI, PhD

• I received my BSc degree in geology in 1974 and an MSc degree in palynology in 1977 from the University of Tehran. I received my PhD in geology (palynology) from Michigan State University (MSU) in 1988.

• I returned to Iran in 1989 and began to work at the National Iranian Oil Company (NIOC) as a senior geologist and teaching in different universities across the country.

• I was manager of geological and geochemical research of the NIOC from 1999 to 2007.

• I am now a professor of petroleum geology at Tehran University and palaeontology at the universities of Tehran, Amirkabir, Shahid Beheshti, Isfahan, Shiraz and have supervised over 52 MS and PhD dissertations.

• I was recognised as one of the top NIOC scientific people, the best scientific figure of the Iranian Oil Ministry in 2006, and the best geologist of Iran the same year.

• I have taken part in different international geological conferences.

Mohammad Hossein Piri-Kangarshahi

MOHAMMAD HOSSEIN PIRI-KANGERSHAHI graduated with a master’s degree in geology, stratigraphy, and palaeontology in 2017. His thesis focussed on various Ordovician–Silurian palynomorphs (acritarchs, chitinozoans, and cryptospores) in the Central Iranian Plate, and their systematics. This study led to a regional palynological scheme with correlations with those previously recorded in Gondwana and a greater understanding of the effects of the Late Ordovician glaciation on Gondwana palynomorph assemblages. Since 2013, he has been working as a research assistant to Professor Mohammad Ghavidel-Syooki at the Institute of Petroleum Engineering, University of Tehran, collaborating on Paleozoic studies of Iran in Alborz, Zagros, and Central Iran regions. After training in the palynology laboratory of the Museum Office of the Environmental Protection Organization of Iran and the Institute of Petroleum Engineering of the University of Tehran, he now specialises in the extraction of palynomorphs and microfossils. He has achieved new results in the preparation of SEM images to improve the time and quality of extraction and separation of palynomorphs.