Calcareous nannofossils are described from 4 sections of the Lower Cretaceous Dalichai Formation from the Eastern (Tepal, Talu and Lavan sections) and central Alborz Mountains (Yonjezar section), Iran. Forty-two species belonging to 18 genera have been identified from the 4 sections studied, including 13 species belonging to 8 genera of ascidian didemnids only from the Tepal section. These taxa indicate the CC1–CC5 biozones of Sissingh’s (1977) biozonation and an age of Berriasian to Early Barremian. The nannofossils recorded from the Dalichai For ma tion are gen er ally Tethyan and cos mo pol itan, while the presence of some Boreal taxa (Nannoconus abundans and N. borealis) may reflect connection between the Boreal and Tethyan realms in the Early Barremian. The sedimentary basin of the Dalichai Formation of the Tepal, Talu, and Yonjezar sections was located at low latitudes of the Tethyan realm with warm surface water. Also, trends of increasing depth and productivity, of a shift from r-selected to k-selected strategies and of a change from low-to-high mesotrophic to oligotrophic conditions, were deduced. A change from unstable to stable conditions towards the tops of sections is inferred for these localities. Laterally in the Dalichai Basin from the eastern to central Alborz, there was an increase in depth and productivity, and also a trend towards oligotrophic conditions and a dominance of low-stress conditions.

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