Biostratigraphy of the upper part of Aitamir Formation based on calcareours nannofossils, Cherli section (NW Kopet-Dagh)

Abstract

Introduction The Kopet-Dagh sedimentary basin is located in north-east Iran, southern Turkmenistan and Afghanistan and is composed of Mesozoic and Cenozoic sedimentary successions. Fifteen formations have been introduced in the Kopet-Dagh Basin. The Cretaceous formations include Shurijeh, Zard, Tirgan, Sarcheshmeh, Sanganeh, Aitamir, Abderaz, Abtalkh, Neyzar and Kalat (Afshar-Harb 1994). Aitamir Formation is one of the main rock units in the Kopet-Dagh basin. This paper presents the biostratigraphic data obtained based on nannofossils from the upper part of the Aitamir Formation. In order to study calcareous nannofossils a stratigraphic section was chosen in northwest Kopet-Dagh. The measured section is located about 35 km west of Maraveh Tappeh city and near Cherli village at 37 50' N latitude and 55 31' E longitude. Based on foraminifera assemblage, Kalantari (1969) suggested Cenomanian age for the Aitamir Formation. Thereafter, Seyed-Emami (1980), Seyed-Emami and Aryai (1981), Seyed-Emami et al. (1984), Immel et al. (1997), Mosavinia et al. (2007) and Mosavinia and Wilmsen (2011), based on ammonites, reported a late Albian-middle Cenomanian age for this formation. Moreover, Moradi Salimi (2012), and Raisossadat et al. (2013) in the center and west of Kopet-Dagh, Sharifi et al. (2012) and Garmabi and Mosavinia (2012) in the east of Kopet-Dagh discussed the ammonite contents and paleobiogeography of ammonites in the Aitamir Formation. The calcareous nanoplanktons of the formation have been studied by Susani (2005), Mousazadeh (2005), Moheghy et al. (2014) and Mousavi et al. (2016a, b). In the measured section, the formation conformably overlies the Sanganeh Formation and underlies the Abderaz Formation. The lower sandstone member is 711 m, consists of glauconitic coarse to fine grained sandstones with some intercalation of silty sandstones while the upper shale member is 359 m, composed of green to greenish grey, grey shales and marlstone that changes to limestone beds of Abderaz Formation at the top. A total of 100 samples were collected from the section with a thickness of 1070 m. The preservation and abundance of calcareous nannofossils in the lower sandstone member of the Aitamir Formation is very rare and poor but in the upper shale member it is good and the most abundant nannofossils belong to this member. Thirty-five (35) samples were collected from the shale member and used for calcareous nannofossil studies. For detailed calcareous nannofossil examination, simple smear slides were prepared using standard procedures. Smear slides were made from the fresh surface of raw sediment samples. Prepared slides were studied by using a polarized microscope and photographed by a digital camera. Corel draw and Photoshop softwares were used for drawing charts and figures. Discussion and Conclusion The standard Upper Cretaceous calcareous nannofossil biozonation Sissingh (1977) (the CC zones), which was modified by Perch-Nielsen (1985) and UC biozonation of Burnett (1998) were used. This scheme largely resulted from the examination of potential stage-boundary sections. Both biozonations were applied to the data on the range-charts. The biostratigraphic studies of the shale member of the Aitamir Formation resulted to the identification of 23 genera and 32 species of calcareous nannofossils are as follows; Amphizygus megalosa, Broinsonia enormis, Calcicalathina alta, Corollithion kennedy, Cyclagelosphaera margerelii, Eiffellithus turriseiffelii, Eprolithus floralis, Gartnerago segmentatum, Haqius circumradiatus, Icrorhabdulus belgicus, Lithraphidites carniolensis, Manivitella pemmatoidea, Marthasterites furcatus, Microrhabdulus decorates, Prediscophaera intercisa, Prediscosphaera columnata, Quadrum gartneri, Radiolithus planus, Retecapsa angustiforata, Retecapsa crenulata, Rhagodiscus angustus, Rhagodiscus splendens, Stoverius achylosus, Tranolithus orionatus, Tranolithus phacelosus, Watznaueria barnesiae, Watznaueria biporta, Watznaueria fossacincta, Zeugrhabdotus fibuliformis, Zeughrabdotus diplogrammus, Zeugrhabdotus embergeri, Zeugrhabdotus xenotus. The following biozones are suggested based on Sissingh (1977) and Burnett (1998). According to the first occurrence and last occurrence of index species and fossil assemblages, three biozones of Sissingh (1977) Eiffellithus turriseiffelii Zone (CC9), Microrhabdulus decorates Zone (CC10) and Quadrum gartneri Zone (CC11) equivalent to biozones UC0-UC7 of Burnett (1998) are suggested. The identified nannofossils zones, could confirm a Late Albian to Early Turonian age for the shale member at the Cherli section in the north western Kopet-Dagh. The determined age is nearly equal with the age obtained by foraminifera (Motamedalshariati et al. 2012). The age of the uppermost part of the Aitamir and the lowest part of Abderaz formations is the Early Turonian. Therefore, it appears that the contact between the Aitamir and Abderaz formations conform and sedimentation was continuous. In the boundary, lithology transitionally changes from shale in the uppermost part of the Aitamir Formation to limestone in the lowest part of the Abderaz Formation. However, more studies are required on this boundary in the western part of the basin.

Keywords

• Aitamir
• Albian
• Biostratigraphy
• Cenomanian
• Cretaceous
• Kopet-Dagh
• Calcareous nannofossils