Biostratigraphic studies of the lower cretaceous (Upper Barremian-lower Aptian) Sarcheshmeh and Sanganeh Formations in the Kopet Dagh basin, NE Iran: An integration of calcareous nannofossil and ammonite stratigraphies

摘要

Lower Cretaceous sediments of the northwestern part of the Kopet Dagh sedimentary basin have been sampled with the purpose to study stratigraphic distribution of calcareous nannofossils. A total of 87 samples from the 1900-m-thick marly limestones, shales and siltstones of the Sarcheshmeh and Sanganeh Formations (late Barremian-early Aptian) displayed diverse nannofossil assemblages. Representative species of the following genera were recorded from the Sarcheshmeh Fm.: Braarudosphaera, Calcicalathina, Calciosolenia, Chiastozygus, Conusphaera, Cretarhabdus, Cyclagelosphaera, Eprolithus, Haqius, Hayesites, Lithraphidites, Manivitella, Micrantholithus, Nannoconus, Radiolithus, Retecapsa, Rhagodiscus, Rucinolithus, Watznaueria, and Zeugrhabdotus. In the Sanganeh Formation, Biscutum, Broinsonia, Cribrosphaerella, Crucicribrum, Cyclagellosphaera, Diazomatolithus, Discorhabdus, Eiffellithus, Lithraphidites, Nannoconus, Prediscosphaera, Rhagodiscus, Tranolithus, and Watznaueria were found. The identified nannofossil assemblages enabled the recognition of NC5-NC7A zones in the studied part of the section. Paleoecologically, these nannofossil assemblages are typical for the Lower Cretaceous of the Tethyan realm and indicate warm surface water conditions.