عنوان مقاله [English]
The Kopet-Dagh Basin is located in the north and north east of Iran, within the border triangle of Iran, Turkmenistan and Afghanistan. Sediments of Cretaceous age are well exposed in this basin. Sarcheshmeh and Sanganeh formations belong to the Cretaceous age in this basin and have the highest thickness in the west of the basin. Calcareous nannofossil investigation at the Sheikh section (in the west of the Kopet-Dagh Basin) lead to the recognition of 84 species from 35 genera which are belonging to 16 families. According to the index calcareous nannofossils, the upper part of NC5 Zone (Watznaueria oblonga Zone), NC6 Zone (Chiastozygus litterarius Zone) and the lower part of NC7a biozone (Rhagodiscus angustus Zone) are recognized at the Sarcheshmeh Formation and NC7a biozone (Rhagodiscus angustus Zone) is recognized at the basal part of Sanganeh Formation. Based on the calcareous nannofossils data, the age of Sarcheshmeh Formation is Late Barremian-early Late Aptian and the age of Sanganeh Formation is early Late Aptian -?Late Aptian.