نانواستراتیگرافی نهشته‌های برش آویچ (حاشیه شرقی بلوک لوت)

در این پژوهش زیست‌ چینه ‌نگاری نهشته‌های منسوب به الیگومیوسن برش چینه شناسی آویچ واقع در حاشیه شرقی بلوک لوت برمبنای نانوفسیل‌های آهکی بررسی شده است. این نهشته‌ها حدود 550 متر ضخامت دارند و عمدتاً شامل واحدهای ماسه‌سنگی، آرژیلیتی و مارنی با میان لایه‌های ماسه‌سنگی می‌باشند. بر اساس مطالعات انجام‌شده، 45 گونه نانوفسیلی متعلق به 18 جنس شناسایی شد. بر مبنای نانوفسیل‌های آهکی شناسایی شده، زیست‌زون‌های nn2 و nn3 از زیست‌پهنه‌بندی martini (1971) برای نهشته‌های روی بخش ماسه‌سنگی و زیست‌زون‌های nn7 تا nn9 برای بالاترین بخش از نهشته‌های مورد مطالعه تعیین گردید. زیست‌زون‌های تعیین شده نشانگر سن آکیتانین تورتونین برای توالی مورد بررسی است. همچنین وجود زیست‌زون‌های np18 و np19 در نمونه‌های متعلق به بالاترین ردیف‌های گذر زیرین توالی یاد شده در برش آویچ، نشانگر سن ائوسن پسین (پریابونین) برای این نهشته‌ها است. نبود زیست‌زون‌های np20 تا nn1، می‌تواند ناشی از نبود نانوفسیل‌های آهکی در ماسه‌سنگ‌های منطقه و یا قطع شدگی توالی ناشی از گسل خوردگی بین نهشته‌های ائوسن و میوسن باشد.

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

IntroductionCalcareous 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 MethodsIn 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 OligoMiocene, 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 crosspolarized light (XPL) or (PPL). All calcareous nannofossil specimens were identified by using the taxonomic schemes of Perch Nielsen (1985) and Bown Young (1998). DiscussionCalcareous 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 AquitanianTortonian for the studied deposits attributed to OligoMiocene in the Avich section. In this section, the absence of NP20NN1 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. ConclusionThe nannofossil assemblages of the OligoMiocene 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. ReferencesBown, 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, 1628.Hadavi, F., Zohravi, M., 2010. Introduction to calcareous nannofossils from the Koronk section in west Birjand (S.E. Iran). 13th 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): 177184.Martini, E., 1971. Die Typen und Typoide des NaturMuseums Senckenberg, 46: Fossiles Nannoplankton. Senckenbergiana lethaea, 52: 115127PerchNielsen, K., 1985. Mesozoic Calcareous Nannofossils. In: Bolli, H.M., Saunders, J.B., PerchNielsen, K., (eds.), Plankton Stratigraphy. Cambridge University Press, 329426.