Ostracod biostratigraphy of Abtalkh Formation in Babafaraji section (East of Kopeh Dagh)

The Kopeh‍ Dagh sedimentary basin is located in the North and North East of Iran (Stöcklin & Setudehnia, 1991) and contains the most complete Cretaceous sediments (Darvishzadeh, 2010). Abtalkh Formation is has a maximum thickness in the East of Kopeh Dagh basin (Stöcklin & Setudehnia, 1991). The lithology of Abtalkh Formation mainly consists of light gray, blue to green shale, and siltstone. It conformably overlies the Abderaz Formation and the upper boundary is conformable with Neyzar Formation (Khosro-Tehrani, 2007). In this research, we aimed to present biostratigraphy of ostracods from Abtalkh Formation in the Babafaraji section.

The thickness of the Abtalkh Formation is 840 m in the Babafaraji section. In this study from Abtalkh Formation, 80 samples were collected every 10 meters. The samples were prepared to identify the Ostracods and Nannofossils. The samples were taken from a depth of approximately 50 cm and have been prepared as below.For sample preparation, we placed 500 grams of a sample in a beaker and soaked it with a solution of water and hydrogen peroxide for 24 hours then the remaining sediment washed and then dried by using the sieves (30 and 60 mesh) which respectively are stacked from coarse to fine. Then all ostracods removed from each sample by brush and put on a slide. All slides were studied under a Zeiss ZH55 reflective microscope and photographed with a scanning electron microscope. Samples examined for calcareous nannofossils content were prepared using traditional smear-slide techniques. Besides ostracods from the Babafaraji section, calcareous nannofossils were also investigated, and the biozonation of ostracods has been discussed alongside the stratigraphic distribution of calcareous nannofossils. The ostracods of Abtalkh Formation in the Babafaraji section have good preservation. In this research, 31 genera and 88 species of ostracods have been identified representing a well-diversified fauna. Generally, ostracods reached their maximum diversity and abundance in the lower part of the formation and are considerably reduced towards the upper part, especially near the boundary of Abtalkh and Neyzar formations. Ostracods often are living in lakes, deep seas and oceans and are strongly influenced by environmental parameters due to sensitivity to environmental conditions more than other species. (Riha, 1995). Therefore, ostracods are useful to palaeoecology studies, reconstruction of environmental conditions, and also in determining the biological horizons and age determination of marine and continental strata on local and regional scale.

According to the stratigraphic distribution of identified species, four interval biozones are as follow:Cytherelloidea sp.1 biozone with a thickness of 210 m is equivalent to Quadrum sissinghii Zone (CC21) and the age of this zone is early Late Campanian.Alatacythere sp.2 biozone with a thickness of 273 m is equivalent to Quadrum trifidum Zone (CC22) and indicating the uppermost of Late Campanian age.Limburgina sp. biozone with a thickness of 227.5 m is equivalent to Tranolithus phacelosus Zone (CC23) and is very end of Late Campanian to Early Maastrichtian in age.Cythereis sp.12 biozone with a thickness of 73.5 m is equivalent to Reinhardtites levis zone (CC24) and indicating Early Maastrichtian age.According to the studied ostracods and corresponding with nannofossil zones, the age of Abtalkh Formation in Babafaraji section was identified upper Late Campanian to Early Maastrichtian.