The biostratigraphy and palaeogeography of acritarchs, chitinozoans, and cryptospores from the Upper Ordovician Ghelli Formation, Khoshyeilagh region, southern Caspian Sea, Alborz Mountains, northern Iran

In the Khoshyeilagh region, 70 km north of Shahroud city, northern Iran, a siliciclastic succession of Lower Palaeozoic strata, including the Lalun, Mila, Lashkarak, and Ghelli formations occurs alongside igneous rocks. These strata contain various organic-walled microfossils, spanning acritarchs, chitinozoans, cryptospores, scolecodonts, and graptolite remains. This study assesses the biostratigraphical significance of marine microfauna (chitinozoans), microflora (acritarchs), and terrestrial cryptospores in this part of the Alborz Mountains. Eighty-eight surface samples were collected from the above-mentioned formations, with six samples barren due to red sediments or igneous rocks. The remaining samples yielded 118 palynomorph taxa: 55 species of acritarchs (28 genera), 48 species of chitinozoa (18 genera), and 15 cryptospore species (10 genera). Scolecodonts and graptolite remains were observed but not studied in detail. No palynomorphs were found in the Lalun, Mila, and Lashkarak formations, while the Ghelli Formation contained well-preserved, abundant organic-walled microfossils. These findings suggest that the Alborz Mountains were part of the Peri-Gondwana palaeo-province during the Late Ordovician. The co-occurrence of acritarchs, chitinozoans, and cryptospores in the Ghelli Formation indicates their potential for global chronostratigraphical correlation. The palynological assemblages suggest a shallow inner shelf environment during the Late Ordovician in the Khoshyeilagh region. Two hiatuses were identified: between the Lalun and Lashkarak formations and between the Ghelli and Soltan Meidan formations. This study also introduces 15 new morphotypes, including ten new species of acritarchs and five new species of chitinozoa, detailed in the systematic sections.