زیست‌ چینه‌ نگاری سازند بغمشاه بر مبنای نانوفسیل‌های آهکی در شمال طبس

در این پژوهش سازند بغمشاه در برش چینه‌ شناسی خروان با هدف مطالعه نانوفسیل‌ های آهکی مورد نمونه برداری قرار گرفته است. سازند بغمشاه با سنگ شناسی غالب شیل و مارن، در شمال طبس از رخنمون گسترده‌ ای برخوردار است و در برش مورد مطالعه 239 متر ستبرا دارد. مطالعات دیرینه‌شناسی انجام شده در این پژوهش به شناسایی 46 گونه متعلق به 26 جنس نانوفسیل آهکی با حفظ شدگی متوسط تا خوب انجامیده است. نظر به اهمیت حضور گونه‌ های شاخص و مجموعه گونه‌ های همراه، زیست‌زون‌ های cc1 تاcc5 از زون بندی (1977) sissingh تعیین گردید. بر پایه زیست‌زون ‌های مشخص شده، سن سازند بغمشاه در برش مورد مطالعه بریازین پیشین تا هوتروین پسین پیشنهاد می‌شود.

Biostratigraphy of Baghamshah Formation based on calcareous nannofossils in the North of Tabas

IntroductionThis research has studied calcareous nannofossils of the Baghamshah Formation in Tabas Block (eastern Iran). Calcareous nannofossils are one of the most important fossil groups for biostratigraphical analysis. The earliest paleontological studies on the Baghamshah Formation have focused on ammonites (e.g., SeyedEmami et al. 1988, 1991, 1997, 2001, 2002; Wilmsen et al., 2009). Kallanxhi et al. (2016) have also considered this formation’s ammonites and nannofossil contents. The current study is the first investigation of calcareous nannofossils of the Baghamshah Formation in the Kharvan section and aims to discuss the possibility of using the standard nannofossil zones for the Baghamshah Formation.Materials and MethodsThe Kharvan section with 239 meters thickness is located 40 km northeast of Tabas near the Kharvan village. The formation has lithologically consisted of green and gray shales and marls. Generally, 75 samples from the Baghamshah Formation have been taken. Samples were prepared via the smear slide method (Bown Young, 1998). All nannofossils were photographed in both crosspolarized light (XPL) and (PPL) and identified using PerchNielsen (1985) and Bown Young (1998).Result and Discussion46 species belonging to 26 genera were identified in the studied section. Amongst them, 38 species belong to 21 genera of nannoliths and coccoliths, and 8 species from 5 genera belong to Didemnid ascidian spicules. Didemnid ascidian spicules are organisms with soft bodies and rigid spicules, which carbonaterich seawaters and coral reefs are suitable environments for their colonization (Varol, 2006). The most dominant genera from the Baghamshah Formation are Watznaueria and Nannoconus. Among the nannofossil species, Nannoconus steinmannii, Retecapsa angustiforata, Calcicalathina oblongata, Tubodiscus verenae, Lithraphidites bollii, Cruciellipsis cuvillieri, and Nannoconus abundans are index species, used for biozonation. The Biostratigraphic studies in the Baghamshah Formation is led to the identification of calcareous nannofossils biozones CC1 to CC5 of Sissingh (1977) in the Kharvan section. Determined biozones represent the age of the early Berriasian late Hauterivian for the investigated strata in the Kharvan sectionConclusionThe nannofossil assemblages of the Baghamshah Formation in the Kharvan section have average to pretty good preservation. In this section, the presence of CC1 at the lower part of the Baghamshah Formation and the presence of CC5 at the upper part of the formation shows that the age of the Baghamshah Formation is Early Berriasian Late Hauterivian in the Kharvan section. ReferencesBown, P.R., Young, J.R., 1998. Introduction. In: Bown, P.R., (ed.), Calcareous Nannofossil Biostratigraphy. British Micropalaeontological Society Publications Series, Chapman and Kluwer Academic, London, 115.Kallanxhi M.E., Falahatgar, M., Javidan M., Sarfi M., Parvizi T., 2016. Calcareous nannofossils and ammonitesbased biostratigraphy of the Baghamshah Formation (Central Iran Basin, Iran). Studia UBB Geologia, 60 (1): 2942.PerchNielsen, K., 1985. Mesozoic calcareous nannofossils. In: Bolli, H.M., Saunders, J.B., PerchNielsen, K., (eds.), Plankton Stratigraphy. Cambridge University Press, 329–426.SeyedEmami, K., 1988. Jurassic and Cretaceous ammonite fauna of Iran and their paleobiogeographic significance. In: Wiedmann, J., Kullman, J., (eds.), Cephalopodes: present and past, Schweizerbartsche Verlagsbuchhandlung, Stuttgart, 599606.SeyedEmami, K., Fürsich, F.T., Schairer, G., 2001. Lithostratigraphy, ammonite faunas and palaeoenvironments of Middle Jurassic strata in North and Central Iran. Newsletters on Stratigraphy 38: 163184.SeyedEmami, K., Schairer, G., Aghanabati, S.A., 1997. Ammonitenaus der Baghamshah Formation (Callov, Mittlerer Jura), NW Tabas (Zentraliran). Mitteilungen der Bayerischen Staatssammlung für Paläontologie und historischen Geologie, 37: 24–40.SeyedEmami, K., Schairer, G., Aghanabati, S.A., Fazl, M., 1991. Ammoniten aus dem Bathon von Zentraliran (Tabas–Nayband region). Münchener Geowissenschaftliche Abhandlungen, A19: 65–100.SeyedEmami, K., Schairer, G., Fürsich, F.T., Wilmsen, M., Majidifard, M.R., 2002. Reineckeidae (Ammonoidea) from the Callovian (Middle Jurassic) of the Shotori Range (East Central Iran). Neues Jahrbuch für Geologie und Paläontologie, Monatshefte, 3: 184192.Sissingh, W., 1977. Biostratigraphy of Cretaceous calcareous nannoplankton. Geologie en Mijnbouw, 56: 37–65.Varol, O., 2006. Didemnid ascidian spicules from the Arabian Peninsula. Journal of Nannoplankton Research, 28 (1): 3555.Wilmsen, M., Fürsich, F.T., SeyedEmami, K., Majidifard, M.R., 2009. An overview of the stratigraphy and facies development of the Jurassic System on the Tabas Block, eastcentral Iran. In: Brunet, M.F., Wilmsen, M., Granath J.W., (eds.) South Caspian to central Iran basins. Geological Society, London, Special Publications, 312: 323–343.