The palynology of the Barut Formation (Early Neoproterozoic, Tonian) from the Alborz Mountains, northern Iran

Precambrian organic-walled microfossils provide significant evidence for early biological evolution and stratigraphic correlation. This study presents the analysis of 30 surface samples collected from the Barut Formation at the Bashm locality, situated 7 km north of Shahmirzad in the Alborz Mountains of northern Iran. Based on the recovered microfossil assemblage, this study reassigns the age of the Barut Formation from the broadly designated late Proterozoic to the early Neoproterozoic (Tonian), providing new constraints on the Precambrian stratigraphy of northern Iran. The investigation documents 47 carbonaceous microfossil taxa belonging to 25 genera, all recorded for the first time from the Barut Formation. The assemblage comprises 29 taxa from 14 genera interpreted as probable cyanobacteria, including Cephalonyx shahmirzadensis sp. nov., Eomycetopsis alborzensis sp. nov., Obruchevella parva, Proterocladus spp., and Siphonophycus spp. Eleven eukaryotic acritarch taxa from eight genera were also identified, including Dictyotidium iranense sp. nov., Germinosphaera spp., Jacutianema solubila, and Trachyhystrichosphaera aimika. Seven additional taxa assigned to Leiosphaeridia, Arctacellularia, and Navifusa exhibit uncertain affinities. The assemblage includes one new genus and seven new species. Several taxa serve as reliable Tonian index fossils, and, together with the absence of large acanthomorphic acritarchs, support a Tonian age (~1000–720 Ma) for the Barut Formation. This age reassignment indicates a ~720–538.8 Ma depositional hiatus before the early Cambrian Zaigun Formation and places the Barut Formation within the earliest phase of Neoproterozoic evolution associated with the initial breakup of Rodinia supercontinent.