Applications and limitations of benthonic foraminifera and calcareous algae in biozonation of the Tirgan Formation: examples from Taher-Abad and Eshlir stratigraphic sections in the Eastern Kopet-Dagh

Introduction The Tirgan Formation, as a cliff forming carbonate stratigraphic unit, consists of medium to thick bedded gray limestone with some thin intercalations of shales, considered as a probable hydrocarbons reservoir rock in the Kopet-Dagh basin (Robert et al. , 2014). Throughout the basin, this formation overlaid the red siliciclastic succession of the Shurijeh Formation and overlain by the gray shales of the Sarcheshmeh Formation. Relative age of the Tirgan Formation, especially on the western Kopet-Dagh sections, have been discussed in a lot of publications (e. g. , Kalantari, 1969; Rivandi et al. , 2010; Khodadadi & Hadavi, 2012; Carevic et al. , 2013; Bucur et al. , 2013; Taherpour et al. , 2013). However the provided results are not practical for many of the sections particularly in the eastern parts of the basin. The outcomes of different biostratigraphical tools such as orbitolinids and calcareous nannofossils are dissimilar and it is not possible to find them in all paleoenvironments, especially in the shallower settings. Embryonic chambers of orbitolinids as the most important characters of their taxonomy (Schroeder et al. , 2010) are absent in most of specimens, not possible to find them even in the serial sections. The previously introduced biozones based on the foraminifera are problematic and the name of some biozone maker fossils had been chosen in a wrong way (for example Dictyoconus arabicus in Rivandi et al. , 2010). In addition, pelagic macrofauna like ammonites as the fundamental biostratigraphic tool for dating of the Cretaceous marine environments are invisible in the eastern outcrops of the Tirgan Formation. Due to these limitations, we try to use a new tool for biostratigraphy and provide a suitable explanation for relative age of the sequences at least in the TaherAbad and Eshlir anticline sections, east of Kopeh-Dagh. These two stratigraphic sections are respectively located at the distance of 75 and 95 kilometers northeast of Mashhad City. Totally, more than 150 thin sections belong to 100 limestone samples have been prepared. Furthermore, 25 shale samples for palynological investigations were taken and studied.

Results and Discussion Micropaleontological studies on the Tirgan Formation is led to identification of 17 genera and 21 species of the benthonic foraminifera and 16 genera and 20 species of the calcareous algae in the Taher-Abad section. They are as follow: Andersenolina alpina, Balkhania balkhanica, Charentia cuvillieri, Comaliamma sp. , Dictyoconus sp. , Haplophragmoides joukowskyi, Haplophragmoides sp. , Istriloculina eliptica, Istriloculina sp. , Lenticulina sp. , Mayncina bulgarica, Melathrokerion valserinensis, Nautiloculina oolithica, Neotrocholina aptiensis, Orbitolinids, Palorbitolina lenticularis, Quinqueloculina sp. , Rumanoloculina robusta, Rumanoloculina cf. pseudominima, Rumano-loculina sp. , Torinosuella peneropliformis, Vercorsella scarsellai, Acicularia sp. , Arabicodium sp. , Boueina sp. , Boueina hochstetteri, Cayeuxia sp. , Carpathoporella sp. , Coptocompylodon sp. , Deloffrella sp. , Deloffrella quercifoliipora, Girvanella sp. , Halimedaceae, Holosporella sp. , Kopetdagaria sphaerica, Marinella lugeoni, Montiella? elitzae, Neomeris cretacea, Permocaculus sp. , Russoella radoicicae, Terquemella sp. . In addition, 17 genera and 19 species of benthonic foraminifera and 12 genera and 16 species of calcareous algae are recognized in the Eshlir section. They comprise Andersenolina alpina, Balkhania balkhanica, Charentia cuvillieri, Dictyoconus sp. , Haplophragmoides joukowskyi, Istriloculina eliptica, Istriloculina sp. , Lenticulina sp. , Mayncina bulgarica, Melathrokerion valserinensis, Nautiloculina oolithica, Neotrocholina aptiensis, Novalesia cf. producta, Orbitolinids, Palorbitolina lenticularis, Quinqueloculina sp. , Rumanoloculina robusta, Torinosuella peneropliformis, Vercosella arenata, Vercorsella scarsellai, Acicularia sp. , Boueina sp. , Boueina hochstetteri, Coptocampylodon sp. , Deloffrella sp. , Deloffrella quercifoliipora, Halimedaceae, Holosporella sp. , Kopetdagaria sphaerica, Marinella lugeoni, Montiella? elitzae, Neomeris cretacea, Permocaculus sp. , Russoella radoicicae, Terquemella sp. The mentioned microfossils characterized three biozones in the sections: Palorbitolina lenticularis Range zone, Kopetdagaria sphaerica local zone and Balkhania balkhanica local zone. Palorbitolina lenticularis Range zone contains the first to last occurrences of the nominal species, demonstrates the Late Barremian – Early Aptain ages (Bachman & Hirsch, 2006) for the Taher-Abad section. This biozone is equivalent to two shorter local zones of Balkhania balkhanica and Kopetdagaria sphaerica. The first zone includes the entire existence of the Balkhania balkhanica, has a regional biostratigraphic value for the Late Barremian. The thickness of Balkhania balkhanica zone in the Taher-Abad and Eshlir sections is respectively 20. 65 and 37 meters, includes the carbonate units of the lower part of the Tirgan Formation. The second zone comprises the whole range of Kopetdagaria sphaerica in the studied sections, confirms the Late Barremian to Earlier Aptian (Bedoulian). Because of the incompleteness of Palorbitolina lenticularis Range zone in the Eshlir section, it is not possible to simply use of that. Instead, the later defined shorter Balkhania balkhanica and Kopetdagaria sphaerica biozones are appropriately useful in the Eshlir section. This biozone has a thickness of 43. 35 and 39. 65 meters in Taher-Abad and Eshlir, occupies a major upper part of the carbonate rocks of these two sections.

Conclusion In the absence of proper orbitolinid fauna as the index benthonic fossils of the Early Cretaceous (Late Barremian – Early Aptain), the Balkhania balkhanica and Kopetdagaria sphaerica biozones have a worthy performance in the age dating of the Tirgan Formation, particularly in the eastern Kopet-Dagh. These ages have been examined by palynological contents of the shaly intervals. Some species such as Cribroperidinium cf. tenuiceras, Muderongia staurota, Odontochitina operculata and Pseudoceratium pelliferum which formerly reported from the Late Barremian – Early Aptian strata of Australia and Egypt (e. g. , Oosting et al. , 2006; Deaf et al. , 2015) confirm the proposed age by foraminifera and calcareous algae