ISSN 0869-5938, Stratigraphy and Geological Correlation, 2011, Vol. 19, No. 2, pp. 188–204. © Pleiades Publishing, Ltd., 2011. 188 1 INTRODUCTION The Kopet Dagh (Koppeh Dagh) sedimentary basin is situated in the northeastern Iran and southern Turkmenistan. The Iranian part of the Kopet Dagh Basin is geographically located between 54°00′ to 61°14′E and 36°00′ to 38°16′N (Fig. 1). The Kopet Dagh Basin was formed as an intracon- tinental basin in the northeastern Iran after the closure of Hercynian Ocean that followed Early Kimmerian orogeny (Berberian and King, 1981). From the Juras- sic through Eocene, relatively continuous sedimentary succession forms five major transgressive–regressive sequences in the eastern Kopet Dagh (Afshar-Harb, 1979, 1983). A fault-controlled subsidence of the Kopet Dagh Basin started in the late mid-Jurassic (Afshar-Harb, 1979; Seyed-Emami and Alavi-Naini, 1990; Seyed-Emami et al., 1994, 1996) and termi- nated in the Oligocene causing the deposition of up to 1 The article is published in the original. 10-km-thick sedimentary succession (Berberian and King, 1981) (Fig. 2). The Cretaceous deposits are divided into nine for- mations in the Kopet Dagh Mts. (Fig. 3). A single megasequence in the Kopet Dagh Basin embraces the whole Lower Cretaceous succession which is made up of conglomerates and sandstones of the Shurijeh Fm. in the lower part and of dark grey shales and siltstones of the Sanganeh Fm. in the upper part (Fig. 3). Detailed geological studies were conducted by geologists of the National Iranian Oil Company (NIOC) during the 1960s and 1970s. The most impor- tant publications were made by Afshar-Harb (1969, 1979) on general geology and petroleum geology, Kal- antari (1969) on the Jurassic and Cretaceous foramin- ifers, and by Hubber (1976) and Afshar-Harb (1982, 1983) as to geological mapping. Despite recent lithostratigraphic, biostratigraphic and paleontological studies of the Sarcheshmeh and Sanganeh Fms. in the Kopet Dagh Basin (Dehghan, 2002; Hadavi and Shokri, 2001, 2006; Hadavi and 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 1 S. N. Raisossadat a and M. H. Shokri b a Geology Department, Faculty of Sciences, Birjand University, Birjand, Iran b Exploration Directorate, National Iranian Oil Company, Tehran, Iran e-mail: snaser_rais@yahoo.co.uk e-mail: snraeisosadat@birjand.ac.ir e-mail: shokri502002@yahoo.com Received June 18, 2009; in final form, February 18, 2010 Abstract—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, Calcio- solenia, 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, Cru- cicribrum, Cyclagellosphaera, Diazomatolithus, Discorhabdus, Eiffellithus, Lithraphidites, Nannoconus, Pre- discosphaera, 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 nan- nofossil assemblages are typical for the Lower Cretaceous of the Tethyan realm and indicate warm surface water conditions. Keywords: Lower Cretaceous, Barremian, Aptian, nannofossils, ammonites, Kopet Dagh, Iran. DOI: 10.1134/S0869593811020109