مقایسه زیست‌زون‌‌های آمونیتی با نانوفسیلی در سازند آیتامیر، شرق حوضه رسوبی کپه‌داغ (برش امیرآباد)

در این مطالعه سازند آیتامیر در برش امیرآباد در شرق حوضه کپه‌داغ بر اساس نانوپلانکتون‌های آهکی مورد مطالعه قرار گرفت و نتایج حاصله با یافته‌های آمونیتی مطابقت داده شد. ضخامت سازند آیتامیر در این برش 668 متر اندازه‌گیری شده است. رخدادهای زیستی نانوفسیلی در قالب دو زیست‌پهنه‌بندی cc و nc استفاده شده و زیست‌زون‌های cc8 تا cc10 و nc9 تا nc13 تعیین شده است. بر اساس نانوفسیل‌های آهکی سن آلبین سنومانین پسین برای سازند آیتامیر در نظر گرفته شده است. زیست‌زون‌های آمونیتی تعیین شده در برش امیرآباد سن آلبین پیشین سنومانین میانی را برای سازند آیتامیر (تا حدود 20 متر انتهایی سازند آیتامیر) تعیین کرده است که منطبق بر نتایج نانوفسیلی است.

Ammonite and Nannofossil Biozonation of the Aitamir Formation in Eastern Kopet-Dagh Basin (Amirabad section)

IntroductionThe KopetDagh Basin is located in the northeast of Iran, extended over Iran, Turkmenistan, and Afghanistan with a westnorthwest to eastsoutheast trend. The Iranian part of the basin is located between 61°14ˊ and 54°00ˊE and 38°16ˊ and 36°00ˊ N. Cretaceous successions are thick and well exposed in the northeast of Iran. The Cretaceous Ammonites are studied by SeyedEmami (1980), SeyedEmami Aryai (1981), SeyedEmami et al. (1984), Immel et al. (1997), Raisossadat (2004, 2006), Mosavinia et al. (2007, 2014), and Mosavinia Wilmsen (2011). Calcareous nannoplankton biostratigraphy of Aitamir formation is studied by Susani (2005) and Hadavi Musazadeh (2005).Materials and MethodsThe studied section, the Amirabad section, is located at a distance of 65 km to the northeast of Mashhad city, on the MashhadKalat road (coordinates: N36°34ˊ59˝ and E60°09ˊ24˝; altitude: 720 m). Aitamir Formation is measured 668 m in this section and comprised siltstones, fossiliferous (bivalve and ammonite) sandstones, glauconitic sandstones, dark gray to greenishgray silty shales, and dark gray shales. It conformably overlies the Sanganeh Formation and is overlain by the Abderaz Formation. A total of 139 samples were collected from the section at intervals 35 m to study the calcareous nannofossils. In addition, 40 m part of the topmost of the underlying Sanganeh Formation as well as the lowermost of the overlying Abderaz Formation were also measured and sampled. For nannofossil biostratigraphic investigations, all samples were processed using the gravity settling technique (Bown Young, 1998). The prepared slides were observed under a Nikon Optiphot II Pol light microscope with a magnification ×1000. Bibliographic references for the calcareous nannofossils are given in PerchNielsen (1985), Varol (1992), and Burnett (1998). Ammonite samples were collected across the section and named and photographed at the paleontology laboratory of the PayameNoor University of Mashhad. Discussion and ConclusionNannofossil bioevents have been utilized to biostratigraphically classify the exposed sedimentary succession based on the CC biozonation scheme of Sissingh (1977, modified by PerchNielsen, 1985) and the NC biozonation scheme of Roth (1978, modified by Bralower et al., 1995). The biozones CC8 to CC10 and NC9 to NC13 were determined, and accordingly, the Aitamir Formation is attributed to Albian to late Cenomanian time interval. The lithology of sandstones and siltstones at the lower part of the Aitamir Formation influenced the preservation of nannofossil taxa. In this part, the Ammonite biozones have covered the gap of nannofossil biozones. The upper part of Leymeriella tardefurcata, Hoplites dentatus, Euhoplites loricatus, Euhoplites lautus Mortoniceras (M.) inflatum, Stoliczkaia dispar, Mantelliceras mantelli, Mantelliceras dixoni, Cunningtoniceras inerme and Acanthoceras rhotomagense biozones were determined at the Aitamir Formation except for the upper 20 meters, which contains no ammonite sample. Based on Ammonite biozones, the age of Late Early Albian to Middle Cenomanian is attributed to the Aitamir Formation.ReferencesBown, P.R., Young, J.R., 1998. Techniques. In: Bown, P.R., (ed.), Calcareous Nannofossil Biostratigraphy. British Micropalaeontological Society Publications Series, Chapman and Hall/Kluwer Academic Publishers, 1628.Burnett, J.A., 1998. Upper Cretaceous. In: Bown, P.R., (ed.), Calcareous nannofossil biostratigraphy. British Micropalaeontological Society Publications Series, Chapman and Hall/Kluwer Academic Publishers, 132199.Hadavi, F., Musazadeh, H., 2005. Characters of stratigraphic system of the Aitamir formation based on Calacareous nannofossils in Shurab section Kopetdagh region. 24th Symposium of Geosciences, Geological Survey of Iran, Tehran, Iran.Immel, H., SeyedEmami, K., AfsharHarb, A., 1997. KreideAmmoniten aus dem iranischen teil des KoppehDagh (NEIran). Zitteliana, 21: 159190.Mosavinia, A., Wilmsen, M., 2011. Cenomanian Acanthoceratoidea (CretaceousAmmonoidea) from the Aitamir Formation (Koppeh Dagh, NE Iran): taxonomyand stratigraphic implications. Acta Geologica Polonica, 61: 175192.Mosavinia, A., Lehmann, J., Wilmsen, M., 2014. Late Albian ammonites from the Aitamir Formation (Koppeh Dagh, NE Iran). Cretaceous Research, 50: 7288.Mosavinia, A., Wilmsen, M., Aryai, A., Shahida, M., Lehmann, J., 2007. Mortoniceratinae (Ammonitina) from the Upper Albian (cretaceous) of the Atamir Formation, Koppeh Dagh Mountains, NE Iran. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen, 246 (1): 8395.PerchNielsen, K., 1985. Mesozoic Calcareous Nannofossils. In: Bolli, H.M., Saunders, J.B., Perch Nielsen, K., (eds.), Plankton Stratigraphy. Cambridge University Press, Cambridge, 329426.Raisossadat, S.N., 2004. The ammonite family Deshayesitidae in the Kopet Dagh Basin, northeast Iran. Cretaceous Research, 25: 115136.Raisossadat, S.N., 2006. The ammonite family Parahoplitidae in the Sanganeh Formation of the Kopet Dagh Basin, northeastern Iran.Cretaceous Research, 27: 907922.Roth, P.H., 1978. Cretaceous nannoplankton biostratigraphy and oceanography of the northwestern Atlantic Ocean. In: Benson, W.E., Sheridan, R.E., (eds.), Initial Reports of the Deep Sea Drilling Project 44. U.S. Government Printing Office, Washington, 731760.SeyedEmami, K., Aryai, A.A., 1981. Ammoniten aus dem unteren Cenoman von Nordostiran (Koppeh Dagh). Mitteilungen der Bayerischen Staatssammlung für Paläontologie und Historische Geologie, 21: 2339.SeyedEmami, K., 1980. Parahoplitidae (Ammonoidea) aus dem Nordost und Zentral Iran. Neues Jahrbuch für Geologie und Paläeontologie, Monatshefte 1980, 719737.SeyedEmami, K., Forster, R., Mojtahedi, A., 1984. Ammoniten aus dem mittleren Cenoman von NordostIran (KoppehDagh). Neues Jahrbuch für Geologie und Paläontologie Monatshefte, 159172.Sissingh, W., 1977. Biostratigraphy of Cretaceous calcareous nannoplankton. Geologie en Mijnbouw, 56: 3765.Susani, A., 2005. Biostratigraphy of lower part of Aitamir Formation based on calcareous nannofossils in Shurab section. M.Sc. thesis, Ferdowsi University of Mashhad.Varol, O., 1992. Taxonomic revision of the Polycyclolithaceae and its contribution to Cretaceous biostratigraphy. Newsletters on Stratigraphy, 27: 93127.