Biostratigraphy of the Abderaz Formation in Sheikh stratigraphic section (north east of Bojnurd) based on planktonic foraminifera

Introduction The Kopet Dagh Sedimentary Basin is located at some parts of Turkmenistan, Afghanistan and NE of Iran (Berberian and King, 1981). This sedimentary basin formed after the closure of Paleotethys Ocean owing to the Middle Triassic Orogenic event and mainly consists of Jurassic to Tertiary deposits (Afshar-Harb, 1994). The Abderaz Formation is various in thickness and consists mainly of brown shales and chalky limestone in its type section which is located in 75 km of east of Mashhad city at Mozduran area (Afshar-Harb, 1994). It unconformably overlies the Aitamir Formation and conformably covers by the Abtalkh Formation (Afshar-Harb, 1994). There are several investigations on the Abderaz Formation biostratigraphy and stratigraphical properties such as Kalantari (1969), Vahidinia (1973), Foroughi (2004), Bakhshandeh et al. (2007, 2008), Shafiee et al. (2011, 2012), Vahidinia et al. (2014). To study the planktonic foraminifera of the Upper Cretaceous deposits in the west of the Kopet Dagh Basin, the Sheikh stratigraphic section in south flank of Sheikh syncline at north east of Bojnurd was selected and investigated.
Materials and methods In order to biostratigraphic analysis of the Abderaz Formation, one stratigraphic section in south flank of Sheikh syncline were studied. The thickness of the Abderaz Formation in the studied section is 252 m and a total of 235 thin sections were prepared and analyzed. The identification of species and classification of genera follows that of Postuma (1971), Wonders (1980), Loeblich and Tappan (1988), Bolli et al. (1989), Robaszynski et al. (1984), Robaszynski and Caron (1979) and Premoli Silva and Verga (2004).
Lithostratigraphy The Kalat Formation in the Sheikh section is 252 m in thickness and it conformably overlies the sandy limestones of the Aitamir Formation and conformably covers by argillaceous limestones of the Abtalkh Formation. Lithostratigraphically, the Abderaz Formation are divided into three parts. Generally, these parts are mainly composed of thick bedded to massive limestone, chalky limestone, argillaceous limestone and marly limestone.
Biostratigraphy The biostratigraphic analysis of the Abderaz Formation in Sheikh section led to recognition of 44 species belonging to 14 genera of planktonic foraminifera, 4 species belonging to 12 genera of benthic foraminifera and 4 species belonging to 2 genera of oligosteginids and then 7 biozones are identified based on biozonation scheme of Premoli Silva and Verga (2004). These biozones are briefly described below: 1: Helvetoglobotruncana helvetica Total range Zone: This biozone is characterized by first appearance to last appearance of Helvetoglobotruncana helvetica. The foraminifera content of this biozone include Helvetoglobotruncana helvetica, Marginotruncana coronata, Marginotruncana pseudolinneiana, Marginotruncana marginata, Marginotruncana schneegansi, Marginotruncana renzi, Whiteinella archaeocretacea, Whiteinella baltica, Whiteinella aprica, Whiteinella praehelvetica, Whiteinella paradubia, Praeglobotruncana cf. stephani, Archaeoglobigerina cretacea, Archaeoglobigerina blowi, Dicarinella algeriana and Dicarinella imbricata.
Age: Early to Middle Turonian
2: Dicarinella primitiva-Marginotruncana sigali Partial range Zone: This biozone is marked between the last appearance of Helvetoglobotruncana helvetica and the first appearance of Dicarinella concavata. The associating foraminifera in this biozone include Dicarinella primitiva, Marginotruncana sigali, Marginotruncana marginata, Marginotruncana pseudolinneiana, Whiteinella baltica, Whiteinella paradubia, Whiteinella inornata, Dicarinella algeriana, Archaeoglobigerina cretacea and Archaeoglobigerina blowi.
Age: Middle to Late Turonian
3: Dicarinella concavata Interval Zone: This biozone is characterized by first appearance of Dicarinella concavata and the last appearance of Dicarinella asymetrica.The foraminifera content of this biozone consist of Dicarinella concavata, Dicarinella primitiva, Marginotruncana marginata, Marginotruncana pseudolinneiana, Marginotruncana schneegansi, Marginotruncana sigali, Marginotruncana sinuosa, Marginotruncana paraconcavata, Whiteinella baltica and Archaeoglobigerina cretacea.
Age: Late Turonian-Lowermost Santonian
4: Dicarinella asymetrica Total range Zone: This biozone is characterized by total range of Dicarinella asymetrica. The foraminifera content of this biozone include Dicarinella asymetrica, Dicarinella concavata, Marginotruncana pseudolinneiana, Marginotruncana sigali, Marginotruncana sinuosa, Marginotruncana paraconcavata, Contusotruncana fornicata, Globotruncanita elevata, Globotruncana bulloides, Globotruncana lapparenti and Globotruncana linneiana.
Age: Early Santonian-Lowermost Campanian
5: Globotruncanita elevata Partial range Zone: This biozone is marked between the last appearance of Dicarinella asymetrica and the first appearance of Globotruncana ventricosa. The associating foraminifera in this biozone include Globotruncanita elevata, Globotruncanita stuartiformis, Contusotruncana fornicata, Globotruncana bulloides, Globotruncana lapparenti, Globotruncana hilli, Globotruncana linneiana, Macroglobigerinelloides bollii, Macroglobigerinelloides prairiehilensis, Macroglobigerinelloides messinae and Archaeoglobigerina cretacea.
Age: Early Campanian
6: Globotruncana ventricosa Interval Zone: This biozone is marked between the first appearance of Globotruncana ventricosa and the first appearance of Radotruncana calcarata.The foraminifera content of this biozone include Globotruncana ventricosa, Contusotruncana patelliformis, Contusotruncana plummerae, Contusotruncana fornicata, Globotruncanita elevata, Globotruncana stuarti, Globotruncana lapparenti, Globotruncana bulloides, Globotruncana hilli, Globotruncana linneiana and Globotruncana mariei.
Age: Middle to Late Campanian
7: Radotruncana calcarata Total range Zone: This biozone is characterized by total range of Radotruncana calcarata. The foraminifera content of this biozone include Radotruncana calcarata, Globotruncana ventricosa, Contusotruncana patelliformis, Contusotruncana fornicata, Globotruncanita stuartiformis, Globotruncanita elevata, Globotruncana stuarti, Globotruncana lapparenti, Globotruncana bulloides, Globotruncana linneiana and Globotruncana mariei.
Age: Late Campanian
Finally, according to the recognized planktonic foraminifera and indicated biozones, the Early Turonian to Late Campanian age for deposits of the Abderaz Formation in the Sheikh stratigraphic section at north east of Bojnurd is proposed