Foraminiferal biostratigraphy of the lower Miocene ...

Geobios 51 (2018) 231–246

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Original article

Foraminiferal biostratigraphy of the lower Miocene Hamzian and Arashtanab sections (NW Iran), northern margin of the Tethyan Seaway§ Mohsen Yazdi-Moghadam a,*, Abbas Sadeghi a, Mohammad Hossein Adabi a, Alireza Tahmasbi b a b

Department of Geology, Faculty of Earth Sciences, University of Shahid Beheshti, Tehran, Iran National Iranian Oil Company Exploration Directorate, Sheikh Bahayi Square, 1994814695, Tehran, Iran

A R T I C L E I N F O

A B S T R A C T

Article history: Received 24 June 2017 Accepted 17 April 2018 Available online 21 April 2018

Lower Miocene strata exposed in the Khoy-Bostan Abad area (NW Iran) document marine platform environments. Paleontological and lithological characteristics of these deposits were investigated to develop detailed bio- and lithostratigraphic frameworks. The shallow marine units, composed of bioclastic carbonates in Bostan Abad and mixed siliciclastic carbonates in Khoy area, were deposited under different local tectonic regimes. Relatively diverse assemblages of benthic foraminifera from these shallow marine units were studied to establish a high resolution biostratigraphy in the context of the standard shallow benthic zonation of Western Tethys. The co-occurrence of Miogypsina intermedia Drooger and Borelis melo curdica (Reichel) characterizes Zone SBZ25, indicating a Burdigalian age as also indicated by planktonic foraminifera. C 2018 Elsevier Masson SAS. All rights reserved.

Keywords: Benthic foraminifera Planktonic foraminifera Burdigalian Qom Formation Biostratigraphy

1. Introduction During the Oligo-Miocene, the Mediterranean region and the Middle East (including Iran) were characterized by deposition of shallow-marine deposits often rich in larger benthic and planktonic foraminifera. Although the Oligo-Miocene shallow and deep marine rocks of southern Iran have been formally named (Asmari, Gachsaran, Mishan and Pabdeh formations), in Central Iran they are generally mapped as the Qom Formation (e.g., Furrer and Soder, 1955; James and Wynd, 1965; Adams and Bourgeois, 1967; Sto¨cklin and Setudehnia, 1971; Setudehnia, 1972; Motiei, 1993). At its type locality south of Qom city, the Qom Fm. is about 1300 m thick and consists of bioclastic limestones, marlstones, marly limestones, and evaporites. Furrer and Soder (1955) were the first to subdivide the Qom Fm. into six members (a–f) including, from base to top: Member a: basal limestones; b: sandy marlstones; c: alternating marlstones and limestones; d: evaporites; e: green

§

Corresponding editor: Fre´de´ric Quille´ve´re´. * Corresponding author. E-mail address: [email protected] (M. Yazdi-Moghadam).

https://doi.org/10.1016/j.geobios.2018.04.008 C 2018 Elsevier Masson SAS. All rights reserved. 0016-6995/

marlstones; and f: top limestones. Member c was later subdivided into four sub-members (c1–c4) by Soder (1959). Bozorgnia (1966) recognized two sedimentary cycles in the type locality of the Qom Fm. He expanded the lithostratigraphic subdivisions of the formation into ten units by defining the Rupelian Nummulitesbearing beds as ‘Unnamed Member’. The lithological units of the Qom Fm. change rapidly over short distances. Therefore, the application of the above-mentioned terminology for the measured sections far from the type locality is unfeasible and similarly named units may have different lithologies and not be timeequivalent. Wynd (1965), Adams and Bourgeois (1967) and Van Buchem et al. (2010) established local biozonal schemes for the OligoMiocene Asmari Formation of southern Iran mainly on the basis of larger foraminifera. These local schemes have been widely used in southern and central Iran (Seyrafian and Hamedani, 1998; Seyrafian, 2000; Vaziri-Moghaddam et al., 2006; Amirshahkarami et al., 2007; Sadeghi et al., 2011; Rahmani et al., 2012; Daneshian and Dana, 2007). Adams (1970) zoned the Asmari Fm. using the TLetter Stages of the Indo-West Pacific (IWP) region. This scheme, in addition to those of Wynd (1965) and Adams and Bourgeois (1967), was later followed by Mossadegh et al. (2009) for the

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M. Yazdi-Moghadam et al. / Geobios 51 (2018) 231–246

Asmari Fm. in the ‘Kohgiluyeh va Bouyer Ahmad Province’, southwestern Iran. As suggested by these authors, the stage boundaries based on these biozonal schemes remain uncertain. Reuter et al. (2009) and Yazdi-Moghadam (2011) correlated the larger foraminiferal assemblages of the Qom Fm. in Central Iran with the standard shallow benthic zonation (SBZ) of Europe by Cahuzac and Poignant (1997). They recognized Zones SBZ21 and SBZ22b from northwest (Uromieh) and south central Iran (Abadeh). One of the main issues in Oligo-Miocene larger foraminiferal biozonation is extending the zonal scheme of Western Tethys eastwards to the western Indo-Pacific region. Due to its position in the Tethyan Seaway during the OligoMiocene, the Qom Fm. plays an important role as a connection between the Mediterranean Tethys to the northwest and the

Indo-Pacific to the southeast. Actually, this formation is an important link between the European-Mediterranean and IndoPacific regions that acts as a ‘‘bridge’’ to correlate the faunal assemblages of these two palaeogeographic bioprovinces. The main purpose of this study is to provide a bio- and lithostratigraphic framework for the Khoy-Bostan Abad area. Our biostratigraphic analysis is mainly based upon the comparison of the recognized larger foraminifera assemblages from the study area with the well-known assemblages from the Western Tethys and circum-Mediterranean areas. This sets the lower Miocene strata of the Qom Fm. in a global biochronological framework. The biostratigraphic results are also compared with the T-Letter Stages of the Eastern Tethys and local biozonations of the Zagros Mountains, southern Iran.

Fig. 1. A. Simplified geological map of Iran showing the main tectonic subdivisions and location of the studied sections (modified after Agard et al., 2011). B. Road map showing the position of the studied sections. The Hamzian (A) and Arashtanab (B) sections are marked by asterisks. MZT: Main Zagros Thrust; SSZ: Sanandaj-Sirjan Zone; ZFTB: Zagros Fold Thrust Belt; CEIM: Central East Iran Microplate; UDMA: Uromia Dokhtar Magmatic Arc.


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Fig. 2. Geological maps of the study area and locations of the studied sections. A. Simplified from the geological map of Khoy, scale 1:100,000 by Emami (2012). B. Simplified from the geological map of Bostan Abad, scale 1:100,000 by Emami (1997).

2. Geological setting The Iranian plateau is considered as a tectonically active region within the Alpine-Himalayan orogenic belt. It extends over a number of continental fragments that have been welded together along suture zones of oceanic character (Nadimi, 2007; Azizi and Moinevaziri, 2009). The fragments are bounded by major faults; each fragment differs in its sedimentary sequence, nature, age of magmatism and metamorphism and structural character together with intensity of deformation. Considering these differences, the

Iranian plateau can be divided into different segments (Falcon, 1967; Sto¨cklin, 1968; Dewey et al., 1973; Jackson and McKenzie, 1984; S¸engo¨r, 1984; Byrne et al., 1992; McCall, 2002; Allen et al., 2003; Blanc et al., 2003; Walker and Jackson, 2004; Alavi, 2004, 2007). Based on structural trends, Sto¨cklin and Nabavi (1973) proposed a division into eight segments including the Zagros folding, Zagros thrusting, Sanandaj-Sirjan, Uromia-Dokhtar Magmatic Arc, Central Iran, Alborz, Kopet Dagh and eastern Iran (Fig. 1(A)). The Central Iranian Basin exhibits complicated structural features that are the result of long structural history

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Fig. 3. Lithostratigraphic log of the Arashtanab section (top Cretaceous strata to basal part of the Upper Red Formation). Up. Cret.: Upper Cretaceous; URF: Upper Red Formation.

from Palaeozoic time up to the Present (Letouzey and Rudkiewicz, 2005). The Central Iran segment was attached to the Zagros segment prior to Permian rifting and formed a single continental unit. These two segments were then separated by rifting which led to the opening of the Neo-Tethys Ocean. Following the Late Cretaceous closure of the Neo-Tethys, the northward migration of the Arabian plate continued up to the continental collision with the Eurasian/Iranian plate at the Oligocene-Miocene transition. The Eocene to early Miocene Central Iran volcanic arc is a remnant of this subduction (Berberian and King, 1981; Bina et al., 1986; Stampfli and Borel, 2002; Agard et al., 2005; Agard et al., 2011). Eocene volcanism was widespread in central and northern Iran, and appears to represent both volcanic arc and back arc volcanism (Vincent et al., 2005). The Central Iran segment is thus considered as a back arc basin resulting from the collision of the Arabian and Iranian plates. In fact, the occurrence of the Oligo-Miocene marine deposits in the

Central Iran Basin is the result of the subduction of Neo-Tethys oceanic crust below the southern margin of Central Iran, which led to opening a back-arc and a fore-arc basin separated by a magmatic arc. The onset of extensive sedimentation in the Qom Basin may indicate both back-arc and fore-arc depositions (Schuster and Wielandt, 1999). The Qom Fm. is isochronous with the Asmari Fm., the main fractured hydrocarbon reservoir in southern Iran (Bozorgnia, 1966; Sepehr and Cosgrove, 2004). An Eocene sequence, consisting mainly of arc volcanics and volcanics together with subordinate marine carbonates and evaporites, unconformably overlies Cretaceous and Jurassic sedimentary and meta-sedimentary rocks (Berberian and King, 1981; Bina et al., 1986). The Eocene succession, which starts with basal conglomerates and coarse clastics, is followed by conspicuous predominantly calco-alkaline volcanic series, which dominate the Eocene stratigraphy (Sto¨cklin, 1968). The volcanics are interbedded with limestones (sometimes


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Fig. 4. Outcrop aspects of the Arashtanab section. A. Lower part of the Qom Formation (Unit I) overlying the Upper Cretaceous strata. B. Thick to massive limestone beds of Unit I (the standing person is about 1.75 m tall). C. Contact between the Qom Formation (Unit II) and the Upper Red Formation.

nummulitic) and evaporites, indicating that the volcanism occurred in shallow waters close to the sea level (e.g., Morley et al., 2009). The Eocene succession was deformed, uplifted and eroded prior to deposition of the Oligo-Miocene sedimentary rocks of the Central Iran Basin (Huber, 1952; Gansser, 1955). The Lower Red Fm. (Oligocene), Qom Fm. (lower Oligocenemiddle Miocene) and the Upper Red Fm. (lower Miocene-lower Pliocene?) constitute the main stratigraphic units present in the Central Iran Basin (Furrer and Soder, 1955; Gansser, 1955; Abaie et al., 1964). In many areas of Central Iran, the continental clastic deposits of the Lower Red Fm. overlie the Eocene volcanics (Sto¨cklin and Setudehnia, 1971; Berberian and King, 1981) and usually underlie the laterally extensive marine succession of the Qom Fm. (Bozorgnia, 1965; Rahaghi, 1973, 1976, 1980; Okhravi and Amini, 1998; Daneshian and Dana, 2007; Guoqiang et al., 2007; Reuter et al., 2009). The Qom Fm. itself is overlain by a nonmarine clastic-dominated unit (Upper Red Fm.) composed of marlstones, siltstones, sandstones and conglomerates. In this work, two sections were investigated in the UromiaDokhtar Magmatic Arc and Sanandaj-Sirjan Zone, an area extending from northwest Bostan Abad to the north of Khoy, NW Iran (Figs. 1–2). The study area is situated in the UromiaDokhtar Magmatic Arc and Sanandaj-Sirjan Zone (Fig. 1(A)). Compared to the type locality (Qom area) where all the members are present (i.e., members a to f), the measured sections cover only the upper part of the Qom Fm. (Member f; Figs. 3–8).

3. Material and methods The present study is based on two outcrop sections located in northwest Iran: Hamzian (388460 34.900 N, 448540 29.800 E) and Arashtanab (378560 2300 N, 468450 100 E) (Figs. 1, 2). A total of 178 samples were studied from both sections, covering the whole marine strata of the Qom Fm. with a maximum spacing interval of 3 m. As larger foraminifera occur in cemented hard rocks and matrix-free specimens were not available, multiple thin sections were cut from the limestone samples in order to obtain oriented sections of the specimens. The generic classification of foraminifera follows Loeblich and Tappan (1987), updated with Hottinger et al. (1991) for the rotaliids. The larger foraminiferal biozonal scheme by Cahuzac and Poignant (1997) for the Oligocene and Miocene of the Western Tethys is applied to the studied sections. The zonation has been compared with the T-Letter Stages of the Indo-West Pacific (Renema, 2007) and the local biozonal schemes already established for the Zagros Mountains by Wynd (1965), Adams and Bourgeois (1967) and Van Buchem et al. (2010) (Fig. 9). The biozonal scheme for the planktonic foraminifera follows Wade et al. (2011). All the samples and thin sections presented in this paper are deposited in the collection of the National Iranian Oil Company Exploration Directorate (NIOCEXP), Tehran, Iran and labelled as ARB 1573ARB 1603 and ARB 2170-ARB 2318.


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Fig. 5. Log of the Arashtanab section showing the distribution of benthic and planktonic foraminifera together with non-foraminiferal macro-skeletal components.

4. Systematic paleontology Order Foraminiferida Eichwald, 1830 Family Miogypsinidae Vaughan, 1928 Genus Miogypsina Sacco, 1893 Miogypsina intermedia, Drooger 1952 Fig. 10 1952. Miogypsina (Miogypsina) intermedia n. sp. – Drooger, p. 35–36, pl. 2, figs. 30–34; pl. 3, 4a, b. 1975. Miogypsina intermedia Drooger – de Mulder, pl. 3, fig. 4. 1991. Miogypsina intermedia Drooger – Wildenborg, p. 114, pl. 2, fig. 3; pl. 5, figs. 4–8. Measurements: see Table 1. Remarks: According to Drooger (1952), M. intermedia is defined by mean V-values between 45 and 70. More than 50% of the specimens possess a second principal auxiliary chamber (PAC II). All our specimens exhibit second principal auxiliary

chambers and have V-values ranging from 46.2 to 59.5 (Table 1), making them matching Drooger’s (1952) criteria. Family Alveolinidae Ehrenberg, 1839 Genus Borelis de Montfort, 1808 Borelis melo curdica (Reichel, 1937) Fig. 11 1937. Neoalveolina melo curdica n. sp. – Reichel, p. 108, pl. 10, figs. 4–7. 1966. Borelis melo curdica (Reichel) – Reiss and Gvirtzman, pl. 1, fig. 8; pl. 2, fig. 1. Description: Spherical to slightly nautiloid Borelis with a diameter ranging from 0.95 to 1.01 mm. Globular proloculus is 0.5–0.6 mm wide. Chamberlets become large and small alternately, resulting in the development of intercalary chamberlets separated by Y-shaped septula, which are well developed in nearly all whorls.


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Fig. 6. Lithostratigraphic log of the Hamzian section (top Lower Red Formation to basal part of the Pliocene conglomerates). LRF: Lower Red Formation.

5. Results 5.1. Arashtanab section The Arashtanab section starts with a disconformable contact between the light gray marine marlstones and argillaceous limestones of the underlying Upper Cretaceous strata and the overlying carbonates of the Qom Fm. (Fig. 4(A)). In turn, the Qom Fm. is composed of two lithotypes in this area: shallow marine carbonates and deep marine marlstones. This permits a further lithostratigraphic subdivision into two units (Fig. 3).

The lower unit (Unit I), with a thickness of 30 m, is mainly composed of massive to thick-bedded shallow marine limestones. Considering the lithostratigraphic subdivisions by Furrer and Soder (1955) (members a to f), this unit corresponds to Member f (Figs. 3, 4). The significant larger foraminifera of this unit, shown in Fig. 5, are Miogypsina intermedia Drooger (Fig. 10(B, H, M, N)), Operculina complanata (Defrance) (Fig. 12(C, D), Amphistegina gr. radiata (Fichtel et Moll) (Fig. 12(I, J) and Borelis melo curdica (Reichel) (Fig. 11(A, C)). Planktonic foraminifera occur rarely, including the genera Globigerina and Globigerinoides. An associated assemblage of

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Fig. 7. Outcrop aspects of the Hamzian section. A. General overview of the Qom Formation. B–F. Close-up views showing different features of the terrigenous and carbonate strata.

smaller benthic foraminifera is dominated by Spiroloculina excavata d’Orbigny (Fig. 13(J)), Textularia sp., Heterolepa sp. (Fig. 13(E, F)), Lobatula lobatula (Walker et Jacob) (Fig. 13(G)) and Elphidium crispum (Linnaeus). Encrusting foraminifers such as Discogypsina sp., Miniacina sp., Carpenteria utricularis (Carter) (Fig. 13(A, B)) and Haddonia heissigi Hagn coexist. In addition, bivalves (Fig. 14(D)), barnacles (Fig. 14(E)), Ditrupa sp., Kuphus shells (Fig. 14(A)), corals, bryozoans, echinoderms and coralline algae are present in this unit. The shallow marine limestones of the Unit I are followed by 60 m-thick gray marine marlstones of Unit II (Figs. 3, 5). As shown in Fig. 5, this unit is marked by the occurrence of planktonic foraminifera including Catapsydrax dissimilis (Cushman et Bermu´dez) (Fig. 15(K, L)), Catapsydrax sp. (Fig. 15(M, N)), Dentoglobigerina baroemoenensis (LeRoy) (Fig. 15(I, J)), Globigerina sp. (Fig. 15(O, P)), Globigerinella obesa (Bolli) (Fig. 15(F–H)), Globigerinoides bisphericus Todd (Fig. 15(D, E)), Globigerinoides trilobus (Reuss) (Fig. 15(A-C)) and Tenuitella clemenciae (Bermu´dez) (Fig. 15(Q-T)). Finally, the red continental deposits of the Upper Red Fm. which are composed mainly of sandstones, conglomerates and siltstones unconformably cover the marine succession of the Qom Fm.

5.2. Hamzian section The lower contact between the Qom Fm. and the underlying strata is not exposed at Hamzian due to alluvial cover (Fig. 7(A)). The marine succession of the Qom Fm., 570 m thick, is a carbonatesiliciclastic sequence consisting of massive, thick or occasionally thin-bedded shallow-marine limestones interbedded frequently with thick to massive conglomerates and sandstones (Figs. 6, 7(B– F)). The influx of clastics may indicate frequent sea level drops during deposition. Limestones are cream to gray in color. Polymictic conglomerates contain particles varying in size from gravel to pebble. The sandstones and conglomerates contain the same fauna and flora as the limestones. Among the fauna and flora in Fig. 8, larger foraminifera are the main constituents of the carbonate beds, including Miogypsina intermedia (Fig. 10(A, C–G, I–L)), Nephrolepidina sp. (Fig. 12(A, B, E)), Neorotalia sp. (Fig. 12(F, G)), Amphistegina gr. radiata, Operculina complanata and Borelis melo curdica (Fig. 11(B, D)). Encrusting foraminifers are represented by Carpenteria utricularis, Discogypsina sp. (Fig. 13(K)), Miniacina sp. (Fig. 13(L)) and Haddonia heissigi (Fig. 13(M)). Smaller benthic foraminifera also occur, including Lobatula lobatula (Fig. 13(C, D)), Elphidium crispum (Fig. 12(K, L)), Eponides sp. (Fig. 12(H)), Textularia sp. (Fig. 13(N, O))


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Fig. 8. Log of the Hamzian section showing the distribution of benthic foraminifera together with non-foraminiferal macro-skeletal components.

and Miliolidae (Fig. 13(H, I)). Planktonic foraminifera including indeterminable representatives of the genera Globigerina and Globigerinoides occur rarely. The associated fauna and flora include bryozoans, bivalves, Kuphus shells, echinoderms, corals (Fig. 14(C)), coralline algae and Ditrupa sp. (Fig. 14(B)). Finally, the Qom succession is unconformably overlain by Pleistocene conglomerates.

6. Discussion Unit I at Arashtanab and the entire outcropping succession of the Qom Fm. in the Hamzian section are characterized by shallow-marine carbonates and mixed shallow-marine carbonates and siliciclastics, respectively. These strata yield both

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Fig. 9. Correlation of the Shallow Benthic Zone 25 (Cahuzac and Poignant, 1997) with corresponding zones from Indo-West Pacific and southern Iran.

Fig. 10. Miogypsina intermedia Drooger, a miogypsinid representative of the studied sections. A, G. ARB 2271. B, H. ARB 1578. C, I. ARB 2252. D, J. ARB 2175. E, K. ARB 2251. F, L. ARB 2281. M. ARB 1582. N. ARB 1573. G-L are embryonic-nepionic drawings of A–F microphotographs, respectively. Scale bars: 250 mm (A–L), 500 mm (M, N).


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Table 1 Numerical parameters of Miogypsina intermedia Drooger from the Hamzian and Arashtanab sections. Sample

DI

DII

a

b

g

V = (200a/b)

Hamzian section ARB 2174 ARB 2251 ARB 2252 ARB 2271 ARB 2281 Mean Standard deviation

165 156 229 175 165 178.0 29.29

223 192 275 168 232 218.0 40.76

74 70 60 77 78 71.8 7.51

267 271 260 283 262 268.6 9.12

20 35 18 27 38 27.6 8.84

55.4 51.7 46.2 54.4 59.5 53.43 4.96

Arashtanab section ARB 1574 ARB 1578 ARB 1579 Mean Standard deviation

159 205 166 176.66 24.78

216 303 217 245.33 49.94

61 68 58 62.33 5.13

258 257 235 250 13

38 27 35 33.33 5.68

47.28 52.91 49.36 49.85 2.84

DI: inner cross diameter of protoconch, in mm; DII: inner cross diameter of deuteroconch, in mm; a: arc length of the circumference of the protoconch underlying the shorter spiral; b: arc length of the circumference of the protoconch underlying both protoconchal spirals; g: angle between the apical frontal line through center of protoconch and line connecting the centers of the embryonic chambers (after Amato and Drooger, 1969).

Fig. 11. Borelis melo curdica (Reichel). A, C. ARB 1575. B, D. ARB 2248. f: foramen; chl: chamberlet; ichl: intercalary chamberlet; prp: preseptal passage; s: septum; sl: septulum. Scale bar: 200 mm.

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Fig. 12. Hyaline larger benthic foraminifera of the studied sections. A, B, E. Nephrolepidina sp., ARB 2172. C, D. Operculina complanata (Defrance). C. ARB 1573. D. ARB 1577. F, G. Neorotalia sp. F. ARB 225. G. ARB 2176. H. Eponides sp., ARB 2318. I, J. Amphistegina gr. radiata (Fichtel et Moll). I. ARB 1579. J. ARB 1573. K, L. Elphidium crispum (Linnaeus). K. ARB 2304; L. ARB 2305. Scale bars: 250 mm.

smaller and larger benthic foraminifera. As larger benthic foraminifera usually have short stratigraphic ranges, they are excellent biostratigraphic markers (Pignatti and Papazzoni, 2017). Among the larger benthic foraminifera present in the studied sections, Miogypsina intermedia and Borelis melo curdica are the key taxa for zonal assignment and dating. Miogypsina intermedia was widely distributed throughout the Tethys Ocean and is known to be restricted to the late early Miocene (Burdigalian; Wildenborg, 1991; Drooger, 1993; Cahuzac and ¨ zcan et al., 2009). As a zonal marker in Europe Poignant, 1997; O and the circum-Mediterranean area, the range of M. intermedia is restricted to Zone SBZ25. The species occurs in Unit I of the

Arashtanab section and also throughout the Qom Fm. in the Hamzian section. Borelis melo curdica has a total range from the early Burdigalian to the late Langhian in the Indo-Pacific, whereas it first occurred during the late early Burdigalian and ranges up into the Langhian in the Mediterranean area (Adams, 1970; Cahuzac and Poignant, 1997; Jones et al., 2006). So far, the species have not been recorded in strata younger than the Burdigalian either in southern or central Iran (Vaziri-Moghaddam et al., 2006; Daneshian and Dana, 2007; Hakimzadeh and Seyrafian, 2008; Dill et al., 2010; Rahmani et al., 2012; Mohammadi et al., 2015). The Burdigalian stage has also been confirmed based on strontium isotope calibrations in southern


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Fig. 13. Smaller benthic foraminifera of the studied sections. A, B. Carpenteria utricularis (Carter). A. ARB 1572. B. ARB 1576. C, D, G. Lobatula lobatula (Walker et Jacob). C. ARB 2227. D. ARB 2178. G. ARB 1575. E, F. Heterolepa sp. E. ARB 1572. F. ARB 1580. H, I. Miliolidae. H. ARB 1575. I. ARB 1572. J. Spiroloculina excavata d’Orbigny, ARB 2248. K. Discogypsina sp., ARB 2305. L. Miniacina sp., ARB 2201. M. Haddonia heissigi Hagn, ARB 2196. N, O. Textularia sp., ARB 2210. Scale bars: 500 mm.

Iran (Ehrenberg et al., 2007; Van Buchem et al., 2010). Recently, Sirel and Gedik (2011) and Gedik (2014, 2015) recorded the species from the Burdigalian of the Malatya basin, eastern Turkey (Eastern Mediterranean area), within Zone SBZ25. Summing up, the larger foraminiferal assemblages of the Qom Fm. in the Arashtanab (Unit I) and Hamzian sections can be referred to Zone SBZ25 of Cahuzac and Poignant (1997), which correlates with the Burdigalian stage. The assemblages also correlate to the Tf1 Letter Stage of the IWP region (see Renema, 2007 for letter classification). In terms of local biozonal schemes already established for the Asmari Fm. (Zagros Mountains, SW Iran), the above-recorded foraminiferal association corresponds to the Borelis melo curdica Zone 61 of Wynd (1965), Borelis melo curdica-Meandropsina iranica Zone 1 of Adams and Bourgeois (1967) and Borelis melo curdica-Borelis melo melo Zone of Van

Buchem et al. (2010) for the ‘Upper Asmari’ Fm. (see also Fig. 9 for comparison). The lower 15 m of Unit II at Arashtanab is characterized by the occurrence of planktonic foraminifera, including Catapsydrax dissimilis, Catapsydrax sp., Dentoglobigerina baroemoenensis, Globigerinoides trilobus, Globigerinella obesa and Tenuitella clemenciae. The top of this interval coincides with the last occurrence of C. dissimilis, allowing the correlation of this assemblage with Zone M3 (= Globigerinatella sp.-Catapsydrax dissimilis Zone) of Wade et al. (2011). The upper 45 m of Unit II yields Dentoglobigerina baroemoenensis, Globigerinoides trilobus, G. bisphericus, Globigerinella obesa and Tenuitella clemenciae. This assemblage corresponds to Subzone M4a (= Dentoglobigerina venezuelana) of Wade et al. (2011). Based on these occurrences of Zone M3 and Subzone M4a, we conclude that Unit II in the Arashtanab section deposited during the Burdigalian.

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Fig. 14. Macro-skeletal components of the studied sections. A. Kuphus shell, ARB 1573. B. Ditrupa sp., ARB 2173. C. coral, ARB 2241. D. Bivalve shell, ARB 1573. E. barnacle shell, ARB 1573. Scale bar: 1 mm.

7. Conclusions This study records for the first time the larger foraminifera of the Qom Formation from the Khoy-Bostan Abad area, NW Iran. Based on the occurrence of larger benthic foraminiferal index taxa including Miogypsina intermedia and Borelis melo curdica, the shallow-marine successions of the Qom Fm. correlate with Zone SBZ25 and deposited during the Burdigalian. Our biostratigraphic data also provide a precise chronology for the marine transgression of the Qom Fm. in the study area, starting during the late early Miocene (Burdigalian). The investigated strata were previously broadly referred to the Oligo-Miocene in the geological maps of Bostan Abad and Khoy, scale 1:100,000 (Emami, 1997, 2012). Zone SBZ25 is recorded for the first time in Central Iran. It demonstrates that the standard shallow benthic zonal schemes can be applied to the Oligo-Miocene shallow-marine deposits of Central Iran as a global biochronological framework. The results clearly show that while the shallow-marine carbonate sedimentation took place during the Burdigalian in both Khoy and Bostan

Abad areas, it was followed by a sea level rise in the southeast (Arashtanab) that resulted in the deposition of deep marine marlstones rich in planktonic foraminifera. At the same time, a major drop in sea level occurred in the northwest (Hamzian), resulting in platform emersion. These observations emphasise the important role of synsedimentary local tectonics in the deposition of the Oligo-Miocene marine deposits in Central Iran. The shallowmarine platform successions of the Qom Fm. in the Arashtanab and Hamzian sections can be considered as time equivalents of the ‘Upper Asmari’ Fm. of southern Iran. Acknowledgements We thank the Shahid Beheshti University and National Iranian Oil Company (NIOC) for supporting this study. We acknowledge constructive comments by two anonymous reviewers that remarkably improved an earlier version of this article. We extend our sincere gratitude for English improvement to Johannes Pignatti (Rome) and Bruno Granier (Brest). The authors are also grateful to


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Fig. 15. Planktonic foraminifera of the Arashtanab section (Unit II). A–C. Globigerinoides trilobus (Reuss), ARB 1588. D, E. Globigerinoides bisphericus Todd, ARB 1603. F– H. Globigerinella obesa (Bolli). F, G. ARB 1602. H. ARB 1587. I, J. Dentoglobigerina baroemoenensis (LeRoy), ARB 1603. K, L. Catapsydrax dissimilis (Cushman et Bermu´dez), ARB 1584. M, N. Catapsydrax sp., ARB 1588. O, P. Globigerina sp., ARB 1584. Q–T. Tenuitella clemenciae (Bermu´dez). Q, R. ARB 1585; S, T: ARB 1593. Scale bars: 100 mm.

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