We describe the biostratigraphy and palaeoecology of the Dalichai Formation based on calcareous nannofossils from the Guydagh section in the western Alborz Mountains, north-west Iran. The nannofossils are moderately to well-preserved, and 25 species belonging to 8 genera have been distinguished, representing the Early Berriasian to Late Hauterivian ages accord ing to the CC1–CC4b biozones of Sissingh (1977). The re corded nannofossils are gen er ally Tethyan and cos mo pol i tan, while the record of Nannoconus inornatus, a Bo real species, at the top of the section reflects the water mass connection between the Bo real and Tethyan do mains during the Late Hauterivian. The records of Nannoconus spp., Watznaueria spp., and Rhagodiscus asper show that the sedimentary basin of the Dalichai Formation was located at low latitudes of the Tethyan realm with warm surface waters. Palaeoecologically, the findings represent a marine transgression and increasing depth, and the palaeoenvironmental conditions change from low-high mesotrophic (shown by the dominance of W. britannica) to oligotrophic (from the dominance of nannoconids), r- to K-strategy of life (from the dominance of nannoconids), and became increasingly stable (shown by the reduction of W. britannica) towards the top of the section

Keywords