Stratigraphy of the Lower Oligocene Nummulitic Limestones, North of Sonqor (Nw Iran)
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Rivista Italiana di Paleontologia e Stratigrafia (Research in Paleontology and Stratigraphy) vol. 124(2): 407-419. July 2018 STRATIGRAPHY OF THE LOWER OLIGOCENE NUMMULITIC LIMESTONES, NORTH OF SONQOR (NW IRAN) MOHSEN YAZDI-MOGHADAM1*, ABBAS SADEGHI1, MOHAMMAD HOSSEIN ADABI1 & ALIREZA TAHMASBI2 1 *Corresponding author, Department of Geology, Faculty of Earth Sciences, Shahid Beheshti University, 1983963113 Evin, Tehran, Iran. E-mail: [email protected] 2National Iranian Oil Company Exploration Directorate, Sheikh Bahayi Square, 1994814695 Tehran, Iran. To cite this article: Yazdi-Moghadam M., Sadeghi A., Adabi M.H. & Tahmasbi A. (2018) - Stratigraphy of the lower Oligocene nummulitic limestones, north of Sonqor (Nw Iran) Riv. It. Paleontol. Strat., 124(2): 407-416. Keywords: Qom Formation; biostratigraphy; Rupelian; larger foraminifera. Abstract. The lower Oligocene hyaline and porcellaneous larger foraminifera of a carbonate platform setting, north of Sonqor, were studied for high-resolution biostratigraphy in the context of European standard zonation (Shallow Benthic Zones). According to the geological map of Kermanshah, these beds were previously ascribed to the Miocene. The identified larger foraminifera include Nummulites fichteli Michelotti, Nummulites vascus Joly & Leymerie, Operculina complanata (Defrance), Asterigerina rotula (Kaufmann), Planorbulina bronnimanni Bignot & Decrouez, Discogypsi- na discus (Goës), Gypsina mastelensis Bursch, Halkyardia maxima Cimerman, Stomatorbina concentrica (Parker & Jones), Praerhapydionina delicata Henson, Penarchaias glynnjonesi (Henson), Austrotrillina aff. paucialveolata Grimsdale, and Haddonia heissigi Hagn, associated with the coralline alga Subterraniphyllum thomasii Elliott. The foraminiferal association characte- rises the SBZ 21 Zone (early Rupelian). INTRODUCTION Buchem et al. 2010; Sadeghi et al. 2011; Rahmani et al. 2012) whereas there are only a few published The Oligocene-Miocene deposits of the As- stratigraphic information for the Qom Formation mari and Qom formations are predominantly com- (e.g., Rahaghi 1973; Reuter et al. 2009; Hadavi et posed of shallow water marine carbonates which al. 2010; Yazdi-Moghadam 2011; Mohammadi et are often rich in hyaline and porcellaneous larger al. 2013, 2015; Yazdi-Moghadam et al. 2018). The foraminifera. These two formations constitute the lower Oligocene Nummulites-bearing carbonates main reservoirs of southern and central Iran (e.g., of the Qom Formation cropping out at north of James & Wynd 1965; Stöcklin & Setudehnia 1991). Sonqor, NW Iran are the target of this study for The Asmari Formation is widely distributed in the stratigraphy (Fig. 1). The proposed biozonation Zagros Mountains, and the Qom Formation also schemes for shallow water marine strata of the crops out in many localities in Central Iran, Sanan- Mediterranean, the Middle East, and the Indo-West daj-Sirjan Zone, and Uromia-Dokhtar Magmatic Pacific (IWP) regions are mainly based on larger Arc where it forms a series of carbonate and mixed foraminifera (e.g., Adams 1970; Schaub 1981; Ad- siliciclastic-carbonate deposits (e.g., Stöcklin & ams 1984; Drooger & Laagland 1986; Cahuzac & Setudehnia 1991; Agard et al. 2005). Several studies Poignant 1997; Boudagher-Fadel & Banner 1999; dealing with litho- and biostratigraphy of the As- Renema 2007; Özcan et al. 2010; Less et al. 2011). mari Formation have been carried out and are still The widespread distribution of lower Oligocene in progress (e.g., Seyrafian & Hamedani 1998; Sey- Nummulites-bearing facies in the Tethys Ocean from rafian 2000; Vaziri-Moghaddam et al. 2006; Amir- Europe and the circum-Mediterranean area through shahkarami et al. 2007; Ehrenberg et al. 2007; Van the Middle East and western Indo-Pacific allows a biostratigraphic correlation for these deposits to be Received: December 04, 2017; accepted: June 04, 2018 carried out throughout the Tethys. In Iran, however, 408 Yazdi-Moghadam M., Sadeghi A., Adabi M.H. & Tahmasbi A. except for Yazdi-Moghadam (2011) who correlated on the definition of Oligocene-Miocene stages at the lower Oligocene foraminiferal association of that time and must be updated now. Recently, the south Uromia (NW Iran) with the Shallow Ben- Oligocene-Miocene deposits of Central Iran have thic Zone 21, no attempt has been made to ascribe been the target of several stratigraphic studies (e.g., the larger foraminiferal association of this age to a Reuter et al. 2009; Yazdi-Moghadam 2011; Moham- standard biozonation scheme. Therefore, this study madi et al. 2013; Amirshahkarami & Karavan 2015; aims to set the Qom Formation beds in a global Mohammadi et al. 2015; Daneshian & Ghanbari time framework through establishing the global SB 2017; Daneshian & Ramezani Dana 2017). Howev- system of biochronozones as proposed by Pignatti er, there is still no agreement on applying a unified & Papazzoni (2017). standard biozonation scheme for larger foraminif- eral associations, and most of the previous studies are based on local zonations. STRATIGRAPHICAL AND PALEONTOLOGICAL BACKGROUND GEOLOGICAL SETTING The first studies on the Oligocene-Miocene marine strata of Central Iran were focused on the Iran is composed of different structural units general stratigraphy of these deposits (e.g., Loftus including Zagros fold and thrust belt, Sanandaj- 1855; Tietze 1875; Von Abich 1878). The earlier Sirjan Zone (SSZ), Uromia-Dokhtar Magmatic Arc workers used local informal names for these beds, (UDMA), Central Iran, Alborz, Kopet Dagh, and e.g., marine formation (Gansser 1955). The name eastern Iran (e.g., Stöcklin & Nabavi 1973) (Fig. of the Qom Formation was first informally intro- 1A). The Oligocene-Miocene marine deposits of duced by Dozy (1945). Through studying several Central Iran are generally referred to as the Qom sections at the type locality, south of the Qom city, Formation in Iranian literature. Lithologically, the Furrer & Soder (1955) divided the formation into Qom Formation consists mainly of bioclastic lime- six members by letters (a-f) including: (a-Member) stones, marine marlstones, evaporites and silici- basal limestones, (b-Member) sandy marlstones, (c- clastics varying in thickness in different areas and Member) alternating marlstones and limestones, exceeding ̴ 1 km at its type locality near the city (d-Member) evaporites, (e-Member) green marl- of Qom (Bozorgnia 1965; Stöcklin & Setudehnia stones, and (f-Member) top limestones. Then Soder 1991; Reuter et al. 2009). The main area of Cen- (1959) further subdivided the c-Member into four tral Iran, north of the Uromia-Dokhtar Magmatic sub-members as c1, c2, c3, and c4. The Qom For- Arc, is characterised by flat-lying topography with mation as a formal name (including its members) occasional low hills (Morley et al. 2009). As a part was accepted by the National Iranian Stratigraphic of the Alpine-Himalayan orogenic belt, Central Committee (NISC) in 1975 (Stöcklin & Setudehnia Iran exhibits complex structural features that are 1991) and it is used in the Iranian literature since the result of long structural history from Palaeo- that time. Bozorgnia (1965), introduced the shallow zoic time up to the present (Letouzey & Rudkie- water marine Nummulites-bearing carbonates of the wicz 2005; Zanchi et al. 2009, 2016; Zanchetta et Qom Formation elsewhere outside the type locality al. 2013, 2018)). The opening of Neotethys Ocean in the central part of the basin. He considered these led to the separation of Central Iran from Zagros lower Oligocene strata, completely unknown at the during the late Palaeozoic. Following the subduc- type locality, as a part of the Qom Formation and tion of the Neotethys which started during the correlated them with their time equivalent “Lower Late Triassic, the northward migration of Arabian Asmari” of southern Iran. Bozorgnia (1965) called plate continued up to the continental collision with these deposits as “Unnamed Member”. Works of the Eurasian/Iranian plate at Oligocene/Miocene Rahaghi (1973, 1976, 1980, 1984) on larger foramin- (Berberian & King 1981). The extensive Eocene ifera of Central Iran were the first major contribu- volcanism in Central Iran is a record of these sub- tions to report and describe the LF associations of duction processes (Berberian & King 1981; Bina the Qom Formation. However, his interpretations et al. 1986; Stampfli & Borel 2002; Agard et al. for dating the foraminiferal assemblages were based 2005). In Central Iran, a thick series of Eocene Stratigraphy of the lower Oligocene nummulitic limestones, north of Sonqor (Nw Iran) 409 Fig. 1 - A) Simplified geological map of Iran (modified after Agard et al. 2011) showing the main tectonic subdivi- sions and location of the studied section. B) Geo- logical map of the study area. Simplified from the geological map of Kerman- shah, scale 1:250,000 by Braud & Aghanabati (1978). CEIM: Central East Iran Microplate, MZT: Main Za- gros Thrust, SSZ: Sanandaj- Sirjan Zone, UDMA: Uro- mia Dokhtar Magmatic Arc, ZFTB: Zagros-Fold-Thrust Belt. volcanic and volcaniclastic deposits with subordi- begins with a basal conglomerate, followed by vol- nate marine carbonates and evaporites unconform- canic rocks with a calc-alkaline composition which ably underlies the Oligocene-Miocene succession are interbedded with nummulitic and evaporite of the Qom Formation (Berberian & King 1981; beds (Stöcklin 1968). The SSZ is characterised by Bina et al. 1986; Agard et al. 2005). The Eocene moderately metamorphosed rocks of Jurassic age, series in turn unconformably