Nannostratigraphy of the Avij section deposits (Eastern margin of Lut Block)

Document Type : مقالات پژوهشی

Authors

• Marziyeh Notghi Moghaddam ¹
• Farah Jalili ²
• Fatemeh Hadavi ³

¹ Assistant Professor, Department of Geology, Payame Noor University, Tehran, I.R of Iran

² Ph.D. in Stratigraphy and Paleontology, Department of Geology, University of Birjand, Iran

³ Professor, Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Iran

10.22067/sed.facies.v13i1.74386

Abstract

Introduction
Calcareous nannofossils are one of the most important groups used for oceanic sections biostratigraphy. This research has studied calcareous nannofossils of the deposits attributed to Oligo- Miocene in Lut Block (eastern Iran). Previous nannofossils studies on Paleogene and Neogene deposits of east Iran were done by Hadavi & Zohravi (2010); Jalili et al. (2017). The present research is another investigation of calcareous nannofossils in the Avich section. The studied section with 550 meters thickness is located in the southwest of Qayen city and the south of the Avich village. The lithology of the deposits consists of sandstones, argillitic limestone, and green marl with interbedded sandstone. This study aims to determine the calcareous nannofossil assemblages and discuss the possibility of applying the standard zonation and age determination.
 
Materials and Methods
In this study, 20 samples from the upper part of the deposits attributed to Eocene, 100 samples from the lower part of the deposits attributed to Oligo-Miocene, and 100 samples from the upper part of these deposits from the Avich section have been studied. Samples were prepared as a smear slide method (Bown & Young, 1998). The examination of nannofloras was performed at 2500x magnification. All images were taken in either cross-polarized light (XPL) or (PPL). All calcareous nannofossil specimens were identified by using the taxonomic schemes of Perch- Nielsen (1985) and Bown & Young (1998).
 
Discussion
Calcareous nannoplanktons are widespread in the present oceans, from coastal areas to open ocean settings. They are suitable for biostratigraphical studies since they are abundant, planktonic, rapidly evolving, and largely cosmopolitan. Regarding calcareous nannofossils, 45 species belonging to 18 genera have been recognized at the studied interval in the Avich section. In the present samples, structures of the central area in many species such as Helicosphaera and Chiasmolithus; indicate the calcareous nannofossils in this section have medium to relatively low preservation. The most important identified species are Chiasmolithus oamaruensis, Isthmolithus recurvus, Discoaster druggii, Triquetrorhabdulus carinatus, Discoaster kugleri, Catinaster coalitus, and Discoaster hamatus. Some species belonging to Discoaster, Catinaster, Triquetrorhabdulus and Isthmolithus are existed from the lower part or upper part of the studied section. Other taxa such as Hughesiu, Coccolithus, Tetralithoides, Sphenolithus, and Helicosphaera are present in most samples.
In the present study, Martini's standard calcareous nannofossil zonation (1971) was applied. In this zonation, the NP abbreviation represents Paleogene nannoplanktons (Nannoplankton Paleogene), and NN abbreviation means Neogene nannoplanktons (Nannoplankton Neogene). In this study, according to the first and last occurrences of marker species of nannofossils, two calcareous nannofossil biozones NP18 (Chiasmolithus oamaruensis Zone) and NP19 (Isthmolithus recurves Zone) in the upper part of Eocene deposits, NN2 (Discoaster druggii Zone) and NN3 (Sphenolithus belemnos Zone) biozones in the lower part of Miocene deposits, and NN7 (Discoaster kugleri Zone) and NN9 (Discoaster hamatus Zone) in the upper part of Miocene deposits were recognized.
Calcareous nannoplankton zones, especially the presence of NP19 at the uppermost part of Eocene deposits and the presence of NN2 at the lowermost portion of Miocene deposits, indicate that the investigated boundary has the age of Priabonian – Aquitanian in this area. Other determined biozones represent the age of the Aquitanian-Tortonian for the studied deposits attributed to Oligo-Miocene in the Avich section. In this section, the absence of NP20-NN1 biozones may be due to the facies changes and the lack of calcareous nannofossils in sandstone rocks of the succession or the tectonic activities between the Eocene and Miocene deposits.
 
Conclusion
The nannofossil assemblages of the Oligo-Miocene deposits of the Avich section are relatively good in diversity with medium to relatively low preservation. Index calcareous nannofossils allowed us to infer the biozonation and record bioevents in these deposits. In this section, the presence of NN2 at the lower part of Miocene deposits and the presence of NN9 at the upper part of the studied deposits show that the investigated deposits are Aquitanian - Tortonian in age.
 
References
Bown, P.R., & Young, J.R., 1998. Techniques. In: Bown, P.R., (ed.), Calcareous Nannofossil Biostratigraphy. British Micropalaeontological Society Publications Series, Chapman and Kluwer Academic, London, 16-28.
Hadavi, F., Zohravi, M., 2010. Introduction to calcareous nannofossils from the Koronk section in west Birjand (S.E. Iran). 13^th International Nannoplankton Association Conference (Abstracts and Programs), Yamagata, Japan, (Poster), P. 59.  
Jalili, F., Hadavi, F., Rahimi, B., Khazaei, A.R., 2017. Investigation of middle and late Miocene flyschoid of east Afzalabad (North Birjand) based on calcareous nannofossils. Scientific Quarterly Journal Geosciences, 26 (102): 177-184.
Martini, E., 1971. Die Typen und Typoide des Natur-Museums Senckenberg, 46: Fossiles Nannoplankton. Senckenbergiana lethaea, 52: 115-127
Perch-Nielsen, K., 1985. Mesozoic Calcareous Nannofossils. In: Bolli, H.M., Saunders, J.B., & Perch-Nielsen, K., (eds.), Plankton Stratigraphy. Cambridge University Press, 329-426.

Keywords

• Calcareous nannofossils
• Lut Block
• Avich
• Aquitanian
• Tortonian
• Priabonian

Main Subjects

• Systematic Paleontology & Biostratigraphy

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