On the occurrence of Neoplanorbulinella Matsumaru, 1976 (Foraminifera) from the late Oligocene of Central Iran (Qom Formation):Palaeobiogeographic implications

Mehdi Sarfi , Mohsen Yazdi-Moghadam

a School of Earth Sciences, Damghan University, Damghan 3671641167, Iran

b National Iranian Oil Company Exploration Directorate, Sheikh Bahaie Square, Tehran 1994814695, Iran

Abstract The shallow marine carbonates of the upper Oligocene Qom Formation yielded several occurrences of the foraminiferan genus Neoplanorbulinella Matsumaru.Neoplanorbulinella saipanensis Matsumaru has so far been recorded from the late Eocene—early Miocene of Japan and late Oligocene of NE Italy whereas N.malatyaensis Gedik is only reported from the Oligocene from its type locality in the Malatya Basin,eastern Turkey.The new records reported here from the Qom Formation indicate that both species occur in the Dobaradar section, ca.10 km south of the city of Qom in North—Central Iran.The palaeogeographic distribution of these two species therefore has to be extended as far as the palaeolongitude of current Central Iran.The accompanying larger benthic foraminifers, including Miogypsinoides complanatus (Schlumberger),M.formosensis Yabe and Hanzawa, Spiroclypeus margaritatus (Schlumberger), Operculina complanata(Defrance), and Risananeiza pustulosa Boukhary et al., indicate the upper Chattian SBZ 23 Zone.The coexistence of the N.saipanensis and N.malatyaensis points to suitable palaeobiogeographic conditions of Central Iran to host Western and Eastern Tethyan taxa.

Keywords Larger foraminifers, Planorbulinids, Biostratigraphy, Qom Formation, Central Iran

1.Introduction

Fig.1 A)Simplified geological map of Iran(after Yazdi-Moghadam et al.,2021b)showing the main tectonic subdivisions;B)Road map of the studied section,in the vicinity of the city of Qom;C)Geological map of the study area.Simplified from the geological map of the city of Qom,scale 1: 250,000 by Emami (1991).CEIM: Central East Iran Microplate; MZT: Main Zagros Thrust Fault; SSZ: Sanandaj-Sirjan Zone; UDMA:Uromia Dokhtar Magmatic Arc; ZFTB: Zagros Fold and Thrust Belt.

The upper Oligocene Qom Formation of Central Iran represents a thick and predominantly carbonate unit of shallow-water facies rich in smaller and larger benthic foraminifers (LBFs) (e.g., Furrer and Soder,1955; Bozorgnia, 1966; Rahaghi, 1973; Yazdi-Moghadam et al., 2018a, 2018b, 2021a).These include nummulitids, miogypsinids, planorbulinids,miliolids, rhapydioninids, soritids, and others.Having taxa with restricted palaeogeographic distributions,LBFs have become an increasingly important tool for studying palaeobiogeography of the Tethys realm.The LBFs composition and their areal distribution are not homogenous within the Paleogene and Neogene.Previous studies have suggested that several LBF species could live only at one of eastern or western palaeobiogeographic realms of the Tethys Ocean(e.g.,Adams,1984;Drooger,1993;Renema,2007; ¨Ozcan and Less, 2009).In fact, several taxa of LBFs are endemic forms.The endemic faunas being one of the main tools for palaeobiogeography of the faunal provinces, are used for reconstructing the palaeogeographic positions, and delimitation of climatic belts (e.g.,Hottinger et al.,1989).The genus Neoplanorbulinella is one of the taxa that the detailed distribution pattern of its different species in the Middle East is not clear.Therefore, this study reports several occurrences of Neoplanorbulinella based on new data from Central Iran and briefly discusses the palaeogeographic distribution of its different species.

2.Geological and stratigraphic setting

The present Iranian plateau comprises of different tectonic units,including the Alborz and Kopet Dagh in the northern part of the country, the Zagros Fold and Thrust Belt and the Sanandaj-Sirjan Zone in the southern part,separated by Central Iran in the middle part(Fig.1A).Each of these units is characterized by a long and distinctive depositional history (St¨ocklin and Setudehnia, 1971; Setudehnia, 1972; Berberian and King, 1981; Motiei, 1993).Central Iran was attached to the Zagros prior to the Permian rifting which formed a single continental unit shaping the current Iranian plateau.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 Ocean, the northward migration of the Arabian Plate continued up to the continental collision with the Eurasian/Iranian Plate probably in the late early Miocene (e.g., Su and Zhou, 2020).The occurrence of the Oligo-Miocene marine deposits in Central Iran, Uromia Dokhtar Magmatic Arc (UDMA), and Sanandaj-Sirjan Zone (SSZ) is the result of the subduction of Neo-Tethys oceanic crust below the southern margin of Central Iran,which led to the opening a back-arc basin(Central Iran)and a fore-arc basin(SSZ)separated by a magmatic arc (UDMA).From the tectonic point of view,the Qom Formation of the studied section is located in the back-arc basin(Reuter et al.,2009).In fact, the Qom Formation can be considered as part of a large depositional system of shallowmarine platform carbonates and deep-marine marlstones that characterized large parts of the presentday Iran during the Oligo-Miocene times (e.g., Yazdi-Moghadam et al., 2021a).At the type locality, the marine strata of the Qom Formation are bounded by two non-marine continental and clastic units respectively (the Lower Red and Upper Red formations)(Furrer and Soder, 1955; Gansser, 1955; Abaie et al.,1964; Bozorgnia, 1965).The Lower Red Formation(Oligocene), the Qom Formation (lower Oligocene—middle Miocene) and the Upper Red Formation (lower Miocene—lower Pliocene?) constitute the main stratigraphic units present in Central Iran.Furrer and Soder (1955) divided the Qom Formation into six members:basal limestones(a-Member),sandy marlstones (b-Member), alternating marlstones and limestones(c-Member),evaporites(d-Member),green marlstones (e-Member), and top limestones (f-Member).Soder (1959) further subdivided the c-Member into four subunits as c?-to c?-submembers.The present study is based on specimens from the upper Oligocene b-Member and c?-submember of the Qom Formation collected from one outcropping section (Dobaradar section)in Central Iran.Coordinates of the base of the section are 34°34′1.6″N, 50°52′41″E (Fig.1B).

Fig.2 Outcrop aspects of platform carbonates of the a-Member to c?-submember of the Qom Formation in the Dobaradar section,south of the city of Qom and its lower contact with the Lower Red Formation in the studied section.The electrical poles for scale are about 4 m high.LRF: Lower Red Formation.

Fig.3 Lithostratigraphic log of the Dobaradar section (b-Member and c?-submember of the Qom Formation) showing the distribution of larger benthic foraminifers.LRF: Lower Red Formation.

The Dobaradar section is located at Kuh-e Dobaradar, 10 km to the south of the city of Qom (Fig.1B and C).The Qom Formation in this area overlies the red, continental conglomerates and siltstones of the Lower Red Formation and underlies the conglomerates of the Upper Red Formation.Both the lower and upper contacts of the Qom Formation in this area are parallelly unconformable.The total thickness of the Qom Formation in this section is about 1275 m being divided into a-to f-members.The neoplanorbulinid foraminifers are present in the b-Member and c?-submember.These two units have an overall thickness of 354 m composed mainly of marine marlstones, thin-bedded sandstones and conglomerates,and medium-to thin-bedded sandy and bioclastic limestones containing larger benthic foraminifers(LBFs) and calcareous red algae (Figs.2 and 3).The LBF assemblage consists of Miogypsinoides complanatus, M.formosensis, Spiroclypeus margaritatus,and Operculina complanata.Amphistegina bohdanowiczi, gypsinids, Carpenteria sp., and Stomatorbina concentrica, are subordinate.Bryozoans are associated.Coralline algae are mainly represented by mastophoroids (Lithoporella melobesioides Foslie),neogoniolithoids(Spongites sp., Neogoniolithon sp.),and melobesioids.Peyssonneliales (Polystrata alba(Pfender) Denizot) and geniculate corallines rarely occur.Neoplanorbulinid foraminifers occur in the upper 45 m of the b-Member and lower 86 m of the c?-submember (Fig.3).

3.Material and methods

Fig.4 Neoplanorbulinella saipanensis Matsumaru(1976)from the upper Oligocene Qom Formation,the Dobaradar section,south of the city of Qom A—D)Axial section;E)Equatorial section;F)Oblique axial section.Scale bars represent 200 μm.A and C:DB 10212;B:DB 10216;D:DB 10186; E: DB 10196; F: DB 10190.

Fig.5 Neoplanorbulinella malatyaensis Gedik,2017 from the upper Oligocene Qom Formation, the Dobaradar section,south of the city of Qom.Scale bars represent 200 μm.A—D) axial section.A, C and D: DB 10144; B: DB 10180.

This study is based on one outcropping section of the Qom Formation(Dobaradar section)located in the south of the city of Qom.Our material includes specimens from random cuts of 295 thin sections belonging to 128 cemented carbonate rock samples of the Qom Formation (b-Member and c?-submember).Multiple thin sections were prepared from the collected samples to obtain oriented sections of the specimens.Thin sections were prepared with conventional methods,the rock samples were cut into thin slices and then ground down via abrasive powders to a thickness of almost 0.03 mm.Obtained petrographic thin sections were studied under a light microscope and photographed via a mounted camera under polarized light.All samples and thin sections are housed in the collection of National Iranian Oil Company Exploration Directorate (NIOCEXP) under the acronyms DB(Dobaradar section).

Fig.6 Neoplanorbulinella malatyaensis Gedik,2017 from the upper Oligocene Qom Formation, the Dobaradar section,south of the city of Qom.Scale bars represent 200 μm.A, B) Slightly oblique axial section; C, D) Oblique section.A and C: DB 10144; B and D: DB 10145.

4.Systematic palaeontology

Class Foraminifera d’Orbigny, 1826

Family Planorbulinidae Schwager, 1877

Genus Neoplanorbulinella Matsumaru, 1976

Type species: Neoplanorbulinella saipanensis Matsumaru, 1976

Neoplanorbulinella saipanensisMatsumaru,1976: Fig.4.

1976 Neoplanorbulinella saipanensis Matsumaru:Matsumaru (1976), p.201—202, pl.6, figs.1—12.

Fig.7 Hyaline larger benthic foraminifers of the Dobaradar section (b-Member and c?-submember).A—C) Miogypsinoides complanatus(Schlumberger);D,E)Miogypsinoides formosensis Yabe and Hanzawa;F)Operculina complanata(Defrance);G,H)Spiroclypeus margaritatus(Schlumberger);I,J)Risananeiza pustulosa Boukhary et al.A:DB 10171;B:10162;C:DB 10189;D:DB 10226;E:DB 10211;F:DB 10169;G:DB 10173; H: DB 10196; I and J: DB 10160.

1996 Neoplanorbulinella saipanensis Matsumaru:Matsumaru (1996), p.152, 154, pl.54, figs.8—12, pl.55, figs.1,4—7.

2007 Neoplanorbulinella saipanensis Matsumaru:Bassi et al.(2007), p.854, pl.3, fig.9.

Only megalospheric forms(A-forms)were observed.The attached test is discoidal with a flat to low conical dorsal part (convexo-concave to conico-concave)(Fig.4).Apical angle varies from 132°to 180°.Diameter of the test is between 532 μm and 988 μm and the maximum height is around 415 μm.The globular to subglobular protoconch (47 μm—76 μm) slightly embraces the deuteroconch (40 μm—65 μm).The proloculus is followed by trochospirally arranged equatorial chambers which became annular in the late ontogenic stages of the growth.The only one principal auxiliary chamber is highly developed (Fig.4E).Two to three rows of irregularly arranged lateral chambers are present in the ventral side of the test.Both the equatorial and lateral chambers are marked by an irregular size increase from the center of the test toward the periphery.The organic membrane gives a brownish color to the hyaline perforate wall of the test.

Fig.8 Paleogeographic map showing the distribution of Neoplanorbulinella saipanensis Matsumaru(1976)and N.malatyaensis Gedik(2017)plotted on an Oligocene paleomap (from Scotese, 2016).1: Japan (Matsumaru, 1976); 2 and 3: Central Iran (this study); 4: central Turkey(Gedik, 2017); 5: northern Italy (Bassi et al., 2007).

Neoplanorbulinella malatyaensisGedik (2017):Figs.5 and 6.

2017 Neoplanorbulinella malatyaensis Gedik:Gedik (2017), p.280,fig.6E—H.

Test is large in size with a diameter reaching 1.78 mm and a maximum height of 0.57 mm (Figs.5 and 6).Regarding test morphology, the studied specimens of N.malatyaensis are lenticular, low conical to discoidal,corresponding to the convexo-concave,and conico-concave forms.A few specimens show the tendency of having reflexed chamber margins(Fig.6B,D).Apical angle varies from 116°to 150°.The globular protoconch(68—75 μm)is followed by the subglobular deuteroconch (60—70 μm) and then by the arcuate equatorial chambers.The wide umbilical cavity(600—920 μm) is characterized by the presence of several underdeveloped and regularly arranged lateral chambers.

5.Stratigraphy

The shallow marine deposits of the b-Member and c?-submember of the Qom Formation at Dobaradar section are characterized by the presence of larger benthic foraminifers, including Miogypsinoides complanatus, M.formosensis, Spiroclypeus margaritatus,Operculina complanata,and Risananeiza pustulosa.In order to identify the zonal assignment for the Qom Formation in the studied section, the important biostratigraphic markers in the assemblage mentioned above are M.complanatus (Fig.7A—C) and M.formosensis (Fig.7D and E).These miogypsinid species are well known from the late Chattian of the Tethys realm (e.g., Drooger, 1952, 1993; ¨Ozcan et al., 2010;Hakyemez et al.,2016;Less et al.,2018;Schiavinotto and Benedetti, 2021).O.complanata (Fig.7F) ranges from the Rupelian to Tortonian ((Shallow Benthic Zones)SBZ 21—26)during which no clear evolutionary change could be observed (Cahuzac and Poignant,1997).S.margaritatus (Fig.7G and H) is a LBF known to have a Tethys-wide expansion during the late Chattian(SBZ 23)(e.g., ¨Ozcan et al.,2010;Ferr‵andez-Ca～nadell and Bover-Arnal,2017;Less et al.,2018).So far,R.pustulosa(Fig.7I and J)is only known from the late Chattian (SBZ 23) of the Middle East and Europe(Boukhary et al., 2008; Sirel and Gedik, 2011;Ferr‵andez-Ca～nadell and Bover-Arnal, 2017).Summing up, the LBF assemblages of the b-Member and c?-submember of the Qom Formation in the Dobaradar section correspond to SBZ 23 of Cahuzac and Poignant(1997) and indicate a late Chattian age.

6.Palaeobiogeographic implications

The genus Neoplanorbulinella and its type species N.saipanensis were first described by Matsumaru(1976) from the lower Miocene of Saipan and Mariana islands, south of Japan in the Western Pacific.Later,the author (Matsumaru, 1996) reported the species from the Sekimon Limestone and Minamizaki Limestone,south of Tokyo.According to Matsumaru(1996),the Sekimon Limestone is late Eocene whereas the Minamizaki Limestone is early Miocene in age.This species is also reported by Bassi et al.(2007)from the late Oligocene of Venetian area, NE Italy.Hence, the range of this taxon can be considered as late Eocene—early Miocene and the occurrence of N.saipanensis in the late Oligocene of Iran overlaps its overall range.Its occurrence from NE Italy is the only record of the species in the Western Tethys and represents its westernmost record.Recently,Gedik(2017)described two new species of the genus Neoplanorbulinella from the lower Oligocene(Rupelian)of the Malatya Basin(eastern Taurid)in Turkey,namely N.malatyaensis and N.matsumarui.The occurrence of N.malatyaensis Gedik from the upper Chattian of Central Iran represents its distribution eastwards to Central Iran and extends the range of this taxon to the upper Oligocene (Fig.8).So far, N.malatyaensis and N.saipanensis have never been reported from the same area.The co-existence of these two species in Central Iran implies the importance of the Oligocene—Miocene Qom Formation as a link between the Mediterranean Tethys to the west and the Indo-Pacific to the southeast.

7.Conclusions

The shallow-marine platform carbonates of the upper Oligocene Qom Formation of Central Iran yielded several occurrences of the genus Neoplanorbulinella, including N.saipanensis and N.malatyaensis.So far, the former species is reported from the Eastern and Western Tethys while the latter is only known from the eastern Turkey in the Western Tethys.The co-existence of N.malatyaensis and N.saipanensis in the upper Chattian shallow marine carbonates of the Qom Formation in Central Iran points to the specific palaeobiogeographic character of this area as a part of Tethyan Seaway between the Eastern and Western Tethys Oceans that could host both the widely distributed and endemic larger benthic foraminifers during the Paleogene.This find also implies that the palaeoecological conditions in this area were suitable for meroplankton larvas of the Neoplanorbulinella species to arrive and settle via oceanic water circulations.

Authors'contributions

The conceptualization, writing and editorial aspects, and interpretation of the data and samples within the study were all provided by both authors through their mutual collaboration.

Funding

The present study was funded by Damghan University (Grant No.653/19).

Conflicts of interest

The authors declare that they have no competing interests.

Acknowledgments

We are grateful to NIOC Exploration Directorate for permission of publication.The authors also thank Damghan University to support this study.Finally, we would like to express our gratitude to the two anonymous reviewers and the journal's editor for their valuable insights.