Benthic Foraminiferal and Calcareous Algae Assemblages in the Tirgan Formation ( Urgonien Facies Type ) in South Flank of Ghorogh Syncline ( North of Chenaran ) , NE Iran

In this research, the Tirgan Formation in the south flank of Ghorogh Syncline (north of Chenaran), northeastern Iran with the thickness of 251 was studied. This section consists of limestone, marly limestone and shale with impressions of echinoderms toxasteridae family, many of foraminifera from orbitolinidae family and also lots of calcareous algae that create facies variation along with other non-skeletal allochems. Micropaleontological investigation has been carried out on 90 thin-sections. These criteria reveal that an equivalent of the Urgonian facies type (in France-Swiss area) and open marine, low depth sedimentary basin are extended in the Kopet-Dagh basin. In this article, we tried to introduce some of the important foraminiferal and calcareous algal assemblages of this area.


Introduction
The Tirgan Formation is a Lower Cretaceous succession in the Kopet Dagh basin in northeast of Iran which mainly consists of micritic limestones, thick-bedded bio-micritic limestones, shaly limestones, marly limestones and bio-sparite limestones [1].The study area is located in south flank of Ghorogh Syncline (north of Chenaran) in the Kopet-Dagh basin with coordinates: 36˚50'38"N and 59˚02'38"E (Figure 1).The Tirgan Formation in study area with a thickness of 251 m, consists of limestone, marly limestone and shale (Figure 2).Detailed micropaleontological investigations have been carried out on one section of the Tirgan Formation supported by the analysis of 90 thin-sections.In this section, the benthic foraminiferal assemblages constitute the largest proportion of the total microfaunal content in terms of abundance.

Geological Setting
The For the eastern part of Kopet Dagh, thickness of 50 m or less is indicated but in the type area of the Tirgan Valley the thickness is about 700 m [3].The Tirgan Formation overlies the Shurijeh Formation and underlies by the Sarcheshmeh Formation; the contacts with both are conformable and a transitional inter-fingering between the Tirgan and Shurijeh formations have been observed in the southeastern Kopet Dagh.The lithology of the Tirgan Formation includes oolithic, partly fossiliferous (e.g.orbitolinid limestones), partly marly limestones, and marls allowing further differentiation of the formation.The age of Tirgan Formation was essentially Neocomian, inplaces extending as high as Aptian.Barremian deposits of the Tirgan Formation may reach up to 300 m thickness and consist of alternating calcareous sand and oolitic sandy limestone containing bryozoans, calcareous algae and both benthonic and planktonic foraminifera [4].The Aptian part of the Tirgan Formation may reach up to 1100 m in thickness and is mainly represented by massive limestones; its foraminiferal fauna includes beside others orbitolinids such as Palorbitolina lenticularis.

Discussion
Chenaran area is a part of Kopet Dagh sedimentary basin located in the Khorasan Razavi.In this area, some outcrops of the Mozdooran, Shurijeh, Tirgan and Sarcheshmeh formations are present with the Jurassic and Cretaceous age.This area is located about 25 km north of Chenaran.In this section, the thickness of the Tirgan Formation is 251 m in south flank of Ghorogh Syncline.For micropaleontological studies, 66 samples were taken and 90 thin-sections were prepared.Several references such as [5]- [11] used to identify benthic foraminifera and calcareous algae.
The following benthic foraminifera have been identified (Figure 3 and Figure
According to the mentioned biozones, the Late Barremian-Early Aptian age is suggested for the formation.
Overall, the importance of this research is that it will lead to determine the age of the Tirgan Formation more precisely and also it conveniently enables us to compare this formation with other sections of the basin on the future studies.

Conclusions
The main results of the present research as follow: 1) Tirgan Formation in the study area consists mainly of medium-to thickbedded biogenic oolithical limestones with intercalations of shales.
3) The existence of Urgonian facies in the NE Iran is accurately certified according to the micro-and macrofauna in the studied area.
4) Based on the identified benthic foraminifera as well as dasycladalean algae, the Late Barremian-Early Aptian age can be ascribed for the Tirgan Formation in the studied area.

Figure 2 .
Figure 2. Stratigraphic section of the Tirgan Formation in south flank of Ghorogh Syncline.