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

A study in Simeh-Kuh, northern Iran, focuses on Ordovician strata, documenting acritarchs and chitinozoa, which is vital for understanding ancient environments. It aims to chronostratigraphically calibrate 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 fourteen chitinozoan biozones (1-14) were established. Biozones A-F indicate Late Cambrian to Middle Ordovician ages in the Lashkarak Formation, while biozones G-H in the Ghelli Formation indicate Late Ordovician. <i>Eremochitina brevis-Linochitina pissotensis</i> biozones suggest late Floian to late Darriwilian ages, and <i>Belonechitina robusta-Armoricochitina nigerica</i> 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: <i>Cyathochitina</i> sp. A, <i>Tanuchitina hosseini-nezhadi</i> sp. nov., <i>Dactylofusa kazzazii</i> sp. nov., and <i>Inflatarium alborzensis</i> sp. nov. The palynological analysis at Simeh-Kuh in the Alborz Mountains 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 fourteen 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 hosts four distinctive acritarch genera: Arbusculidium, Coryphidium, Striatotheca, and Arkonia. These genera hold notable significance as they are characteristic elements of the Peri-Gondwanan paleoprovin The identification of chitinozoans in the Lashkarak and Ghelli formations has facilitated the establishment of fourteen chitinozoan biozones. Notably, these biozones are considered classical elements of the Northern Gondwana Domain/Peri-Gondwana palaeo-province. <i>Belonechitina robusta, Tanuchitina fistulosa, Acanthochitina barbata,</i> and <i>Armoricochitina nigerica</i> biozones were recognized in the Ordovician deposits at Simeh-Kuh. This revelation 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: <i>Cyathochitina persianense</i> sp. nov., <i>Tanuchitina hosseini-nezhadi</i> sp. nov., <i>Dactylofusa kazzazii</i> sp. nov., and <i>Inflatarium alborzensis</i> sp. nov.