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, 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. Eremochitina brevis-Linochitina pissotensis biozones suggest late Floian to late Darriwilian ages, and 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.