The Age dating of the Bidestan Member (Nayband Formation) at northwest of Ferdows based on foraminifers

The studied section, called Hassan-Abad section in this paper, is located near the village of Hasan-Abad about 13 km northwest of the city of Ferdows, northeast Iran. The studied section of Nayband Formation with altitude of 1300 meter is composed of a sequence of reefal carbonates and shales. The reefal carbonates contains rich macro-fauna, such as corals, sponges, gastropods, bivalves and micro-fauna including foraminifers, algae, and microproblematica. The thickness of Late Triassic Nayband Formation at the Hassan-Abad section is about 175 m. Considerable part of the section (about 100 m) consists of reef carbonates with micritic matrix that is gray in color. By approaching to the upper part, carbonate rocks decreases while intercalations of green shales are gradually appeared. Well-sorted red sandstones with iron oxides are present in the upper most parts of the measured section. The boundary of the Nayband Formation with Early Jurassic sandstones of Ab-Haji Formation is continuous.
Corals, hypercalcified sponges (including sphinctozoans, inozoans, chaetetids, spongiomorphs and stromatomorphs), and algae (Solenoporcea, dasycladaceans), are the most important reef organisms. Based on abundant occurrence of Heterastridium coglobatum Reuss, a possibly index fossil of Bidestan Member (Senowbari-Daryan, 1996), the studied section (Bidestan Member and Howz-e-Khan Member) is dated as being of Upper Norian and Rhaetian in age. Separation of the Howz-e-Khan Member, as the youngest part of the Nayband Formation, was not possible in this section therefore, this part of Nayband Formation doesn’t exactly same as one of the member of type locality, such as Bidstan or Howz-e-Khan, so that it has informal name which is Ferdows Member (Aghanabati, 2009). It can be indicative of different conditions for deposition of Upper Triassic reefs limestone in east of central Iran in Lute Block. Late Triassic reefs in the Tethys province and outside of the Tethys show the widest distribution during Norian- Rhaetian stages and are known from numerous localities in Mediterranean region including Alps, Caucasia, Southeast Asia and northwest America (Flügel, 1982). Although three different group of foraminifera are generally not abundant in the reefs of the Nayband Formation in this study, however comparison of foraminifera association of the Hassan Abad section with the associations known from Panthalassan and Tethys, according to Chablais et al. (2011), Iran can be considered in Provinces III (Eastern Laurasia).
The foraminifera divided to three Agglutinate, Hyaline and porcelaneouse groups. The porcelaneouse foraminifera including: Altinerina, Cucurbita, Decapoalina, Galeanella, Miliolechina, Miliolipora, Ophthalmidium, Orthotrinacria, Paraophthalmidium, Planiinvoluta, agglutinate forms: Agglutisolena, Endotebidae, Glomospira, Palaeolituonella, Tetrataxis, Trochammina and Hyaline taxa: Aulotortus, Austrocolomia, Duostomina, Coronipora, Trocholina.
In the Hassan Abad section, the porcelaneouse genera including Decapoalina, Galeanella, Miliolipora and Ophthalmidium are the most abundant the same as the Northern Calcareous Alps and south of Tethys. These genera are also abundant in all reefal facies of Upper Triassic deposits. Between Agglutinate foraminifera, Trochammina is abundant. Distribution of foraminifera in this area indicated that the Alp, SW Asia, Australia, Turkey and from other localities of Nayband Formation in Iran show most similarity to the Northern Calcareous Alps (Zaninetti, 1976; Schäfer and Senowbari-Daryan, 1981). Correlation of the Late Triassic reefs distribution indicates that northern margin of Lute Block, where Hassan Abad section (Ferdows member) deposited, had been situated in the lower latitudes and in the northern hemisphere during depositional time. In addition, similarities of the scleractinian corals of Hassan-Abad section (Amirhassankhani et al., 2010; Shepherd et al. 2012; Amirhassankhani & Ariayi, 2008) with scleractinian corals of the mentioned area proves a biogeographically connection and an appropriate condition for growing of Triassic corals. The aunal Association of the reefs in Hassan Abad supports their depositional environment in a tropical region. In this study, 28 species of foraminifera from families of Ammodiscidae, Ataxophragmidae, Cornuspiridae, Cuneolinidae, Duostominidae, Endothyridae, Hauerindae, Ichthyolariidae, Involutindae, Miliolechinidae, Milioliporidae, Nubecularidae, Ophthalmidiidae, Tetrataxadae, Trochammnindae and Verneuilnadae were identified and 11 species were introduced for first time from Iran. The abundance of porcelaneouse genera is more similar to the Northern Calcareous Alps and south of Tethys which indicate same condition during Late Triassic. The foraminifer’s assemblages prove Norian-Rhaetian age in carbonate deposits in Lute Block. The scleractinian corals and Comparison of foraminifers of the Hassan-Abad section with known taxa from Alps, SW Asia, Australia, Turkey, Oman and Iran as well as its precise correlation to trend of Late Triassic reefs distribution indicates that the Lute Block was situated in the low latitudes and in the northern hemisphere during the depositional time.