Calcareous nannofossil biostratigraphy of Sarcheshmeh and Sanganeh formations at Qaleh Jegh and Yazdan Abad sections (West of Kopet-Dagh Basin)

Kopet Dagh Basin is located on the northeast of Iran, extending over Iran, Turkmenistan and Afghanistan with a west-northwest to east-southeast trend. The Iranian part of the basin is located between 61°14ˊ and 54°00ˊE and 38°15ˊ and 35°38ˊ N. It constitutes the second most important hydrocarbon province of Iran after Zagros Basin. Cretaceous sediments are well developed and extensively exposed in this basin. In order to study the Biostratigraphy of Sarcheshme and Sanganeh Formations, the calcareous nannofossil assemblages have been investigated in two stratigraphic sections. Shokri (2000), Hadavi and Shokri (2000), Hadavi and Bodaghi (2000 & 2009), Dehghan (2002), Mahanipour et al. (2011a,b & 2012) and Mahanipour and Kani (2015) studied the calcareous nannofossil biostratigraphy of Sarcheshme or Sanganeh Formation.
Material & The studied sections, the Qaleh Jegh section (located at a distance of 35 km to the northeast of Bojnord, on the Baba Boland mountain; N37°47ˊ57˝ and E57°16ˊ53˝) and Yazdan Abad section (located at north of Yazdan Abad village; N37°55ˊ34˝ and E57°35ˊ10˝). Sarcheshmeh Formation conformably overlies the Tirgan Formation and is in turn overlain by the Sanganeh Formation in both sections and Atamir Formation conformably overlies the Sanganeh Formation.
Sarcheshmeh Formation measures 2660 m in Qaleh Jegh section and comprises argillaceous limestone (with limestone intercalations) in lower part and shale (with limestone intercalation) in upper part. A fossiliferous limestone separates the Sarcheshmeh and Sanganeh. Sanganeh Formation measures 1400 m and consists of siltstones (with sandstone, shale or limestone intercalation) in lower part and shale containing chert nodules (with siltstone intercalation) in upper part.
At Yazadn Abad section the 655 m thick succession of Sarcheshmeh Formation consists of alternating argillaceous limestone and limestone in lower part and calcareous marl, marl and argillaceous limestone in upper part. A fossiliferous limestone separates Sarcheshmeh and Sanganeh formations. Sanganeh Formation is 485 m thick comprising siltstones with sandstone, marl (with argillaceous limestone or limestone intercalation).
A total of 1286 samples (1006 from Qaleh Jegh section and 280 from the Yazdan Abad section) were collected at about 4 m intervals. For the purpose of biostratigraphic investigations, all of the samples were processed using the gravity settling technique (Bown and Young, 1998) and the prepared slides were observed under a Leica DMLP Pol light microscope at ×1000 magnification.
Discussion of Results & Nannofossil bioevents have been utilized to biostratigraphically classify the exposed sedimentary succession based on schemes of Roth (1978; modified by Bralower et al., 1993) and Sissingh (1977; modified by Perch-Nielsen, 1985). Overall, 56 species from 28 genera of 14 families in Qaleh Jegh section, and 45 species from 26 genera of 13 families in Yazdan Abad section have been identified and their distribution recorded. Based on the index calcareous nannofossil taxa, the upper part of NC6, NC7A, NC7 (B&C) and lower part of NC8 nannofossil zones are delineated in Ghaleh Jegh and the upper part of NC5, NC6, NC7B and lower part of NC7A are recorded in Yazdan Abad. The distributions point to Early to Late Aptian time of deposition for the rocks of Sarcheshmeh Formation and Late Aptian to Early Albian for those of Sanganeh Formation in Ghaleh Jegh, while At Yazdan Abad, Sarcheshmeh Formation ranges from Late Barremian to Early Aptian and Sanganeh Formation dates Early to Late Aptian.