Biostratigraphy of the Abtalkh Formation at the Bahadorkhan section (Central Kopet-Dagh) based on planktonic foraminifera

Plankton organisms are particular importance due to the high-speed evolution, vast geographical distribution and abundance in deep marine sediments. These criteria caused to found these organisms great importance in biostratigraphy (Bolli & Krasheninnikov, 1977). The main aim of this study was to evaluate content of Planktonic Foraminifera and present biozonation model. The studied section is located along the main road from Ghoochan to Dargaz about 40 kilometers to Dargarz. After travelling for three kilometers on a sidetrack, we arrive at the section site located in the north-western mountains of Bahadorkhan village. The place is known as Hussein Kadkhoda. The Geographical coordinates of the studied section are 58ᵒ 35′ 01 east latitude and 37ᵒ 41′ 25″ north latitude. Abtalkh Formation is 407/5 meters thick at Bahadorkhan section. Bahadorkhan section consists of calcareous shale, lime marl, marl and a few dispersed chalky limestone beds. Unlike the type section, the upper and lower boundaries of Abtalkh Formation are conformable with Abderaz and Neyzar formations. This section is composed of 7 lithostratigraphy portions.
micropaleontological studies of the Bahadorkhan section led to identification of 40 species belong to17 genera of planktonic foraminifera. According to vertical dispersion range of index forms and biozone maker, this stratigraphy section is divided into 4 biozones: 1. Globotruncanita elevata Partial range Zone: this biozone was introduced by Postuma (1971), from extinction of Dicarinella asymetrica which was not recorded in the studied section, to the first occurrence of Globotruncana ventricosa. The mentioned interval is composed of grey marl and brown calcareous marl.
2. Globotruncana ventricosa Interval zone: this biozone was introduced by Dalbiez (1955), interval from the FO of Globotruncana ventricosa to the FO of Rodotruncana calcarata. Mentioned part is composed of calcareous shale and calcareous marl.
3. Rodotruncana calcarata Total range Zone: this biozone the first was introduced by Herm (1926). Mentioned part is composed of calcareous shale and calcareous marl. Interval from the FO of Rodotruncana calcarata to the LO of Radotruncana calcarata.
4. Globotruncanella havanensis Partial range Zone: this biozone the first was introduced by Lapparent (1918). From extinction of Rodotruncana calcarata to the first occurrence of Globotruncana aegyptica. The mentioned part is of composed marl. According to extinction of both species of Marginotruncana: Marginotruncana marginata and Marginotruncana coronata and also first occurrence of Globotruncanita stuartiformis this boundary can be found in sample. No 30.
Lithostratigraphic comparison of Abtalkh Formation at Bahadorkhan section with Mozduran and Hamam-Ghale sections respectively in east Kopet-Dagh and east of central Kopet-Dagh and Aitamir section in west Kopet-Dagh showed that the thickness decreases in the central and western Kopet-Dagh basin, so that the least thickness belongs to Bahadorkhan section in central Kopet-Dagh. Also biostratigraphy correlation of Abtalkh Formation at type locality and Hamam-ghale section in east and central basin show Campanian- Maastrichtian and Early Campanian to Middle Masstrichtian age whereas the Abtalkh Formation in the Bahadorkhan area deposited during the late Late Santonian to Late Campanian. Microstratigraphy study conducted on 100 shaly and marly samples that collected from Abtalkh Formation successions led to identification of 40 species belong to 17 genera of planktonic foraminifera.