Geoscience Frontiers 6 (2015) 593e604

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China University of Geosciences (Beijing)

Geoscience Frontiers

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Research paper

Microfacies models and sequence stratigraphic architecture of theOligoceneeMiocene Qom Formation, south of Qom City, Iran

Mahnaz Amirshahkarami*, Mahnaz KaravanGeology Department, Faculty of Science, Payame Noor University, Esfahan, Iran

a r t i c l e i n f o

Article history:Received 15 March 2014Received in revised form11 August 2014Accepted 18 August 2014Available online 30 September 2014

Keywords:RupelianeChattianAquitanianeBurdigalianPaleoenvironmentSedimentary modelSequence stratigraphy

* Corresponding author. Fax: þ98 (0) 312 4222511.E-mail address: m_amirshahkarami@yahoo.com (M. APeer-review under responsibility of China University

http://dx.doi.org/10.1016/j.gsf.2014.08.0041674-9871/� 2015, China University of Geosciences (

a b s t r a c t

The OligoceneeMiocene Qom Formation has different depositional models in the Central Iran,SanandajeSirjan and Urumieh-Dokhtar magmatic arc provinces in Iran. The Kahak section of the QomFormation in the Urumieh-Dokhtar magmatic arc has been studied, in order to determinate itsmicrofacies, depositional model and sequence stratigraphy. The textural analysis and faunal assem-blages reveal ten microfacies. These microfacies are indicative of five depositional settings of openmarine, patch reef, lagoon, tidal flat and beach of the inner and middle ramp. On the basis of thevertical succession architecture of depositional system tracts, four third-order sequences have beenrecognized in the OligoceneeMiocene Kahak succession of Qom Formation. Based on the correlationcharts, the transgression of the Qom Sea started from the southeast and continued gradually towardsthe north. This resulted in widespread northward development of the lagoon paleoenvironment in theAquitanian-Burdigalian stages. Also, the sequence stratigraphic model of the OligoceneeMiocene QomFormation has an architecture similar to those that have developed from OligoceneeMiocene globalsea level changes.

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1. Introduction

The OligoceneeMiocene Qom Formation includes marinemarlstones and limestones with gypsum and siliciclastic rocks andis an important gas reservoir in Central Iran (Fig. 1). There is noparticular section which has been specified as type section for theQom Formation but generally its type area has been accepted to bethe Qom plain in Central Iran (Aghanabati, 2011).

The first reports of the Qom Formationwere published by Loftus(1855) and Von Abich (1878) in the Lake Rezayeh (Uromieh) regionand by Tietze (1875) in Central Iran. Furrer and Soder (1955) sub-divided the OligoceneeMiocene marine strata of the Qom Forma-tion in the type locality of the formation near the town of Qom,into six members. These rock units are basal limestone, sandy marl,marl and limestone alternation, evaporites, green marls and toplimestone. Bozorgnia (1965) expanded the subdivision into tenunits using lithological and paleontological characteristics. He leftthe Rupelian strata unnamed and correlated it with the lower part

mirshahkarami).of Geosciences (Beijing).

Beijing) and Peking University. Pro

of the Lower Asmari Formation in the Zagros Basin in south of Iran(Fig. 1).

Generally, the Qom Formation is unconformably underlain byred and green-gray shale and siltstones of the Oligocene Lower RedFormation and unconformably overlain by the Miocene Upper RedFormation. In the Tanbour and Bujan sections in the San-andajeSirjan Province, the Qom Formation is unconformably un-derlain by Paleozoic metamorphic rocks and unconformablyoverlain by the Quaternary sediments (Anjomshoa, 2013;Anjomshoa and Amirshahkarami, 2014).

Biostratigraphic data on larger benthic foraminifera of the QomFormation were established and dated as OligoceneeMiocene byRahaghi (1973, 1976, 1980). A biostratigraphic revision of the QomFormation was made by Naimi and Amirshahkarami (2011) andYazdi-Moghaddam (2011).

Paleoecology and biostratigraphy of Qom Formation have beenstudied by Vaziri-Moghaddam and Torabi (2004), Daneshian andRamezani Dana (2007), Reuter et al. (2009), Mohammadi et al.(2011), Anjomshoa (2013), Anjomshoa and Amirshahkarami(2014), Mohammadi et al. (2013, 2015) and Karavan et al. (2015).Karavan et al. (2015) have studied the sequence stratigraphy of theQom Formation in the Bijegan section. Qom Formation with vari-able lithostratigraphic, biozonal and microfacies characteristics is

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Figure 1. General map of Iran showing the eight geologic provinces (adapted from Heidari et al., 2003).

Table 1Larger benthic foraminifera biozonation in the Kahak section of the Qom Formation(Naimi and Amirshahkarami, 2011).

Stage Biozone no. Assemblage biozone

Burdigalian IV Borelis melo, Borelis curdicaAquitanian III Pseudolituonella reicheli, Peneroplis sp.,0

Dendritina ranji, Triloculina trigonula,Rotalia sp., Pyrgo sp.

Chattian II Eulepidina sp., Eulepidina dilitata,Nephrolepidina sp., Amphistegina sp.,Operculina sp., Bozorginella qumiensis,Miogypsina sp., Pseudolituonella reicheli,Rotalia sp. Triloculina trigonula,Triloculina tricarinata

Rupelian I Nummulites fichteli, Nummulites vasc,Eulepidina dilitata, Nephrolepidina sp.,Neprolepidina tournoueri, Eulepidina sp.,Pseudolituonella reicheli, Miogypsina irregularis,Amphistegina sp., Operculina sp.

M. Amirshahkarami, M. Karavan / Geoscience Frontiers 6 (2015) 593e604594

widespread in the SanandajeSirjan, Urumieh-Dokhtar magmaticarc and Central Iran provinces. Most of the researches on the QomFormation involve the characterization of biostratigraphy andmicrofacies. Also, there is no exact correlation among the QomFormation successions. The Qom sequence stratigraphy explainsthe depositional model and transgression of the Tethyan Seaway inCentral Iran.

The Kahak is one of the most important sections of the QomFormation in the eastern margins of the Urumieh-Dokhtarmagmatic arc in Iran.

Qom Formation contains numerous benthic foraminifera spe-cies which provided useful data for reconstruction of the deposi-tional paleoenvironments (Anjomshoa and Amirshahkarami, 2014).

Biostratigraphy of Qom Formation has been studied by Naimiand Amirshahkarami (2011) in Kahak area (Table 1). The assem-blage biozones of the study area are explained by the late Oligo-ceneeearly Miocene biostratigraphy (Laursen et al., 2009).

The current paper has three purposes: (1) the explanation of thedepositional settings of the Qom Formation in the Kahak sectionusing the microfacies analysis; (2) the sequence stratigraphy andmicrofacies correlation of Qom Formation in the study sectionwithOligoceneeMiocene Asmari succession in the Zagros Basin; (3)study of the sedimentary basin transgression of Qom Formation inthe Tethyan Seaway in Iran.

2. Geological and geographical position of the study area

Themicroplate of Central Iran originated during final collision ofthe African/Arabian Plate with the Iranian Plate, the process ofwhich has already started in the Mesozoic (Coleman-Sadd, 1982).

An important effect of the collision of these plates was theclosure of the Tethyan Seaway during theMiocene (Harzhauser andPiller, 2007; Reuter et al., 2009).

The termination of migration of marine biota and exchange oftropical waters between the eastern Mediterranean and the

western Indo-Pacific Tethys has been called the Terminal TethyanEvent (TTE) by Harzhauser et al. (2007). Accordance to Adams et al.(1983) the timing of TTE is of Aquitanian age, while Harzhauseret al. (2002) proposed a Burdigalian age. Amirshahkarami (2013b)discussed a disconnection seaway between the shallow marinelimestone of the OligoceneeMiocene Asmari Formation in ZagrosBasin (Southwest Iran) and the western Indo-West Pacific region inthe Aquitanian and Burdigalian times.

Another effect of the plate collision was the formation of a fore-arc basin (SanandajeSirjan Basin) and a back-arc basin (Qom Basin)on the Iranian Plate at the northeastern margin of the TethyanSeaway (Fig. 2A). These basins are separated by a volcanic arc sys-tem which developed during Eocene times (Stöcklin andSetudehina, 1991).

According to Mohammadi et al. (2013, 2015) deposition of theQom Formation (RupelianeBurdigalian in age) took place in threeNWeSE-trending basins: SanandajeSirjan (fore-arc basin),

Figure 2. Location and geological setting of the study area. (A) General map of Iran showing the basins in which deposition of the OligoceneeMiocene Qom Formation has takenplace: QB e Qom Basin (Back-arc basin); UDMA e Urumieh-Dokhtar Magmatic arc (Intra-arc basin); SSB e Sanandaj-Sirjan Basin (Fore-arc basin) (adapted and revised fromNational Iranian Oil company (NIOC) Geological map 1969, 1975, 1977a,b, 1978; Heidari et al., 2003; Harzhauser and Piller, 2007; Reuter et al., 2009). (B) Location of the study area atthe Urumieh-Dokhtar Magmatic arc.

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Urumieh-Dokhtar magmatic arc (intra-arc basin) and Central Iran(back-arc basin) (Fig. 2A). Also, transgression of the Tethyan Seawayin the Iranian Plate started from southeast and continued north-westward gradually.

Within this study one section was investigated in the easternmargin of Urumieh-Dokhtar magmatic arc (Intra-arc basin). Thestudy section is located at the Kahak village, about 30 km south ofQom in central Iran (Figs. 1 and 2). The Kahak section of the QomFormation (studied in detail at its mid-point at 34�31013.300N and50�56029.500E) is unconformably underlain by the Oligocene LowerRed Formation and unconformably overlain by the Miocene UpperRed Formation.

3. Material and methods

A complete section of the Qom Formation was measured andsampled in Kahak area, south of Qom City. The thickness of theKahak section is 220 m. In the section studied, the lithology of theQom Formation is mainly characterized by limestone, sandy lime-stone, sandstone and coral limestone. The microfacies character-istics were described from thin sections in 75 rocky samples. Thereorganization of carbonates microfacies is basis on the nomen-clature of Dunham (1962) and Embry and Klovan (1971). Analyseshave allowed the interpretation of carbonate marine environments,depositional system tracts, sea level changes and sequence strati-graphic architectures of Qom Formation in the study area.

4. Microfacies analyses

Based on the textural, allochemical and orthochemical charac-teristics, ten microfacies were identified from Qom Formation inthe study area (Figs. 3e5).

4.1. Microfacies A - bioclastic corallinaceaen Lepidocyclinidaepackstone e grainstone

This microfacies has a grainesupported texture in a micriticmatrix (packstone) or in a sparry calcite cement (grainstone). Themajor allochems are perforated larger benthic foraminifera suchas Amphistegina, Rotalia, Nummulites, Operculina, Eulepidina andNephrolepidina. Other skeletal grains include rhodolite, bryozoans,

corallinaceaen algae, bivalves and echinids. This microfacies occursin RupelianeChattian lower layers of Kahak succession (Fig. 7).

The depositional setting of the microfacies A is the lower photiczone of open marine environment with medium to high energy.Large and flat Nummulitidae, Lepidocyclinidae, bryozoans, andechinids are the typical openmarine skeletal fauna found (Hottinger,1983, 1997; Leutenegger, 1984; Reiss and Hottinger, 1984;Hohenegger, 1996; Hohenegger et al., 1999; Romero et al., 2002).These fauna have also been reported from Bijegan and Tanboursections of Qom Formation by Anjomshoa and Amirshahkarami(2014) and Karavan et al. (2015). A similar microfacies is present inthe RupelianeChattian lower beds of Asmari Formation in ZagrosBasin in southwest of Iran (Vaziri-Moghaddam et al., 2010;Amirshahkarami, 2013a).

4.2. Microfacies B - bioclastic bryozoan packstone-floatstone

The characteristic of the microfacies B is grain-supportedtexture in a micritic matrix with abundant bioclasts (Fig. 3b). Thetexture is different from that of microfacies A and includespackstone-floatstone with coarse-grained bioclasts of bryozoanswhich are larger than 2 mm in size. Other bioclasts are cor-allinaceaen algae, larger benthic foraminifera (Lepidocyclinidae,Nummulitidae), small benthic foraminifera and fragments of mol-lusca. This microfacies occurs in RupelianeChattian lower layers ofKahak succession (Fig. 7).

This microfacies has been deposited in an open marine envi-ronment. The bioclastic bryozoan corallinaceaen packstonemicrofacies indicates the lack of an effective barrier (Wilson, 1975;Flügel, 2004) to the marine environment.

4.3. Microfacies C - coral boundstone

This microfacies is characterized by colonial corals and has beenidentified in the Burdigalian stage. Macroscopically, it is a discon-tinuous massive limestone containing autochthonous Scleractiniancolonial corals (Figs. 3c and 4).

The occurrence of in-situ organisms such as colonial coralssuggests a reef environment (Wilson, 1975; Flügel, 2004). Thediscontinuous coral boundstone layers interbedded with lagoonmicrofacies indicate a patch reef depositional environment (Fig. 6).

Figure 3. Microfacies type of Qom Formation in Kahak section. (a) Microfacies A: Bioclastic corallinaceaen Nummulitidae Lepidocyclinidae packstone-grainstone (Sample No.K002). (b) Microfacies B: Bioclastic bryozoans corallinaceaen packstone-floatstone (Sample No. K009). (c) Microfacies C: Coral boundstone (Sample No. K046). (d) Microfacies D:Bioclastic miliolid coral floatstone (Sample No. K044). (e) Microfacies E: Bioclastic Miogypsinoidae miliolid corallinaceaen wackestone-packstone (Sample No. K053). (f) MicrofaciesF: Bioclastic corallinaceaen imperforate foraminifera wackestone-packstone (Sample No. K037).

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Coral reef communities are adapted to oligotrophic environments.The coral reef suffers losses because of high nutrient concentration(Hallock and Schlager, 1986; Flügel, 2004). Therefore microfacies Cindicates oligotrophic condition.

4.4. Microfacies D - bioclastic miliolid coral floatstone

The texture of this microfacies is a micritic matrix with morethan 10% coarse grains larger than 2 mm (Fig. 3d). The texture in-cludes floatstone with coarse-grained bioclasts of colonial corals.Other bioclasts are miliolids, mollusca, ostracods and small benthicforaminifera. The microfacies D has been identified in the Burdi-galian stage.

This microfacies is characterized by abundant coarseegrainedfragments of colonial coral with imperforate foraminifera

(miliolids). This microfacies is distinguished from the reef facies bythe absence of in-situ boundstone fabrics (Wilson, 1975; Flügel,2004; Vaziri-Moghaddam et al., 2010). Most fossils are well pre-served and indicated a low energy environment with circulation inthe back reef setting (Wilson, 1975; Flügel, 2004). The co-occurrence of normal marine skeletal (corals) and restricted biota(imperforate foraminifera) suggest a semi-restricted lagoon. Asimilar microfacies occurs in Asmari Formation in Zagros Basin(Vaziri-Moghaddam et al., 2010).

4.5. Microfacies E - bioclastic miogipsinoidae miliolidcorallinaceaen packstone-grainstone

This microfacies is OligoceneeMiocene in age and is apackstone-grainstone with corallinaceaen algae, perforate and

Figure 4. Outcrop photographs of a patch reef of Qom Formation in the Urumieh-Dokhtar intra basin arc, Iran.

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imperforate benthic foraminifera, micritized bioclasts (Figs. 3e and7). In this microfacies, perforate foraminifera are Miogypsinidaeand Rotalia and imperforate foraminifera are miliolids and Den-dritina. Other bioclasts are small benthic foraminifera, molluscs,echinids and ostracods.

Perforate foraminifera (e.g. Miogypsinidae, Rotalia) and imper-forate foraminifera (e.g.miliolids andDendritina) are found togetherin the semi-restricted lagoon.Miogypsinoids lived in shallowwaterswithnormal salinity (Geel, 2000) and recentRotalia lives in a shallowwater photic zone (Romero et al., 2002). The imperforate forami-nifera such as miliolid, and Denditina occur in a restricted lagoon(Hallock and Glenn, 1986; Geel, 2000; Romero et al., 2002). Themixed open marine bioclasts and protected environment bioclastsindicate semi-restricted lagoon (Vaziri-Moghaddam et al., 2010).

4.6. Microfacies F - bioclastic peloid corallinaceaen imperforateforaminifera packstone-grainstone

The depositional grain-supported texture of this microfacies isrepresented by packestoneegrainstone (Figs. 3f and 5a). Thismicrofacies is composed of imperforate foraminifera (Borelis,Meandropsina, Peneroplis, Dendritina and miliolids) corallinaceaenalgae, peloids and is Burdigalian in age (Fig. 7). Other bioclastsinclude small benthic foraminifera, ostracodes, andmicritic skeletalcomponents. Thepackestoneegrainstone textures arefineemediumgrained, poorlyemoderately sorted, subangular and well rounded.

In the microfacies F, the lagoon depositional setting is suggestedby poor marine biota and abundant restricted biota (such as mil-iolids, peneroplids and alveolinids). The imperforate foraminiferaindicate the restricted conditions (Hallock and Glenn, 1986; Geel,2000; Romero et al., 2002). A similar microfacies has also beenreported from Tanbour and Bijegan successions of Qom Formation(Anjomshoa and Amirshahkarami, 2014; Karavan et al., 2015). Asimilar microfacies has been identified in Asmari Formation inZagros Basin (Amirshahkarami, 2013a).

4.7. Microfacies G - sandy bioclastic miliolidwackestoneepackstone

This microfacies is characterized by a wackestoneepackstonetexture with low diversity of imperforate foraminifera such asmiliolids, Borelis and Dendritina of the Burdigalian stage (Figs. 5band 7). Fineemedium grained detrital quartz is recognized in thismicrofacies. The wackestoneepackstone is poorlyemedium sortedand grains are angular to subangular. Secondary bioclasts aremainly fragments of corallinaceaen algae, mollusks and micriticskeletal grains. Textularia, peloids, ostracods, echinids and smallbenthic foraminifera are found in minor amounts.

The imperforate foraminifera such as miliolids and quartz grainsin micritic texture may indicate deposition in a lagoon with lowenergy (Geel, 2000; Romero et al., 2002; Vaziri-Moghaddam andTorabi, 2004). A similar microfacies has also been reported inBijegan section of Qom Formation (Karavan et al., 2015).

4.8. Microfacies H - lithoclastic perforate foraminiferacorallinaceaen packstone-floatstone

This microfacies is characterized by a lithoclastic packestoneefloatstone texture with coarse corallinaceaen algae fragments,rhodolith and perforate foraminifera such as miogipsinoidae, Lep-idocyclinidae and Amphistegina (Fig. 5c, d). The mediumecoarsegrained and angularesubangular lithoclasts have originated fromextrabasinal igneous rocks. The matrix consists of fine-grainedangular quartz and igneous rocks fragments which are embeddedin a micritic matrix. The age of the microfacies representing basalsection of the stratigraphic succession is early Rupelian (Fig. 7).

In the microfacies H, the extrabasinal sediment supply into thecarbonate environment may indicate deposition in a tidal zone.Mixed carbonate-siliciclastic can be deposited in near-coast envi-ronments (Flügel, 2004). The microfacies characteristics, mixedcarbonate-extrabasinal siliciclastic sediments with bioclasts (e.g.corallinaceaen algae fragments and perforate foraminifera) arecommon in near coastal inner shelf and tidal flat environments. Thesedimentary features of the facies may indicate the intra-arc depo-sitional setting within the Urumieh-Dokhtar magmatic arc (Fig. 2).

4.9. Microfacies I - Sandy mudstone

This microfacies is a poorly sorted lime mudstone with 15e20%detrital grains of subangular quartz of 0.1e0.2 mm size (Fig. 5e).This facies, with alternating sandy limestone and carbonates, isRupelian in age (Fig. 7).

In the tidal zone and coastal environment, the clastic sedimentsmay originate from the bedload of estuaries (Flügel, 2004). Thefenestrate fabric and lack of fauna indicate the tidal flat deposi-tional setting (Alsharhan and Kendall, 2002; Rasser et al., 2005;Vaziri-Moghaddam et al., 2010; Amirshahkarami, 2013a). Asimilar microfacies has also been reported in the Bijegan successionof Qom Formation (Karavan et al., 2015).

4.10. Microfacies J - sandstone

Thismicrofacies is submature subarkosic sandstone (Fig. 5f). Thesandstone is mainly composed of 65e90% subangular to angulardetrital quartz, 5e25% feldspars (chiefly plagioclase) and 7% lithicgrains. The grains are bounded with clayey matrix and sparry

Figure 5. Microfacies type of Qom Formation in Kahak section. (a) Microfacies F: Bioclastic corallinaceaen imperforate foraminifera wackestone-packstone (Sample No. K061).(b) Microfacies G: Sandy Bioclastic miliolid wackestone-packstone (Sample No. K034). (ced) Microfacies H: Lithoclastic perforate foraminifera corallinaceaen packstone-floatstoneunder polarized light (Sample No. K004). (e) Microfacies I: Sandy mudstone (Sample No. K007). (f) Microfacies J: Sandstone (subarkose) under polarized light (Sample No. K042).

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calcite cement. The hematite and glauconite minerals are rarelypresent. The age of microfacies is late Rupelian (Fig. 7).

The sandstone beds of this facies alternate with the marinecarbonate layers of the Qom Formation. The detrital quartz andfeldspar minerals may suggest a coastal environment. A similarmicrofacies has also been reported from the Bijegan succession ofQom Formation (Karavan et al., 2015).

5. Sedimentary model

Microfacies analyses from Kahak section of the Qom Formationshow open marine, patch reef, lagoon, semi-restricted lagoon, tidalflat and costal environments. Palaeoenvironmental reconstructionbased on the textures and microfacies suggests a homoclinal car-bonate ramp for Kahak section of Qom Formation (Fig. 6).

In classical facies models, a carbonate ramp is separated intoinner ramp, middle ramp and outer ramp (Burchette and Wright,1992).

The larger benthic foraminifera are effective tools in the recog-nition of the Cenozoic palaeoenvironments. Distribution of fora-miniferal assemblages depends on intrabasinal conditions. The lightgradient of environment is an important factor in the distribution ofspecies and it is effective on symbioses and on nutrient availabilitytoo (Hottinger, 1983). The size, degree of flatness and wall of thelarge foraminifer shells may give indications of the paleoenvir-onmental conditions (Hallock and Glenn, 1986; Geel, 2000).

Perforate foraminifera with symbiotic algae (e.g. Lep-idocyclinidae and Nummlitidae in microfacies A and B) are majorforaminifera in euphotic shallow water of open marine environ-ment (Hallock and Glenn, 1986; Geel, 2000).

Figure 6. Qom Formation depositional model in Kahak section. Interpretation adapted from Hottinger (1997), Pomar (2001) and Rasser and Nebelsick (2003). FWWB: Fair weatherwave base; SWB: Storm wave base; A to J: Microfacies defined in Figs. 3 and 5

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In a lagoon environment, imperforate foraminifera (e.g. mil-iolids, Dendritina, Peneroplis and Borelis in microfacies F and G) areabundant (Hallock and Glenn, 1986; Geel, 2000). In the Kahaksection of Qom Formation, inner ramp includes the lagoon, tidal flatand shoreline or beach environments and middle ramp includesshallow water environments of open marine and patch reef. Theouter ramp environment has not been recognized in the studiedsection of Qom Formation.

The high energy coastal environment is characterized bydetrital and mixed siliciclastic-carbonate sediments (microfacies Iand J). Sandy mudstone (microfacies H) indicates a tidal flatenvironment.

The most common microfacies of the inner ramp are wacke-stone, packestone, grainstone with imperforate foraminifera(microfacies E, F and G). The imperforate foraminifera such asDendritina, Meandropsina, Peneroplis, Borelis and miliolids (microf-acies F and G) indicate a lagoon environment (Geel, 2000; Romeroet al., 2002).

A semi-restricted lagoon is recognized by coexistence ofrestricted marine fauna such as imperforate foraminifera and openmarine fauna such as perforate foraminifera (microfacies E).

The middle ramp includes proximal and distal parts. The prox-imal part of middle ramp is characterized by patch reef and reef-derived bioclasts such as colonial corals in a floatstone texture(microfacies C and D).

The distal part of middle ramp includes perforate foraminiferasuch as Lepidocyclinidae and Nummulites (microfacies A, B).The perforated larger benthic foraminifera such as Lep-idocyclinidae and Nummulitidae indicate symbiotic bearingstrategy (Hottinger,1983; Leutenegger, 1984; Hallock and Glenn,1986; Hottinger, 1997).

Generally, in Kahak section of OligoceneeMiocene Qom For-mation, sedimentary environments correspond to inneremiddleramp, and hence the inner ramp is more widespread than themiddle ramp in the AquitanianeBurdigalian.

6. Sequence stratigraphic interpretation

Sequence stratigraphy is considered by many authors as one ofthe latest conceptual revolutions in the broad field of sedimentarygeology and emphasizes facies relationships and stratal architec-ture within a chronological framework (Miall, 1995; Catuneanuet al., 2009). A sequence stratigraphic framework includes geneticunits that resulting from the interplay of accommodation andsedimentation (Catuneanu et al., 2009). The sequence stratigraphicgenetic units are lowstand system tract (LST), transgressive systemtract (TST), highstand systems tract (HST) and maximum floodingsurface (MFS) which are bounded by sequence stratigraphic sur-faces (SB) (Miall, 1995; Emery and Myers, 1996; Nicols, 1999;Catuneanu et al., 2009).

Sequence stratigraphic analyses interpreted four third-orderdepositional sequences in the Kahak section of Qom Formation(Fig. 7).

6.1. Sequence 1

The sequence 1 is 17 m thick and Rupelian in age. This sequencecan be divided into LST, TST and HST. The tidal flat sandstone bedsof themicrofacies J on the inner ramp is the basal part of sequence 1which has been deposited unconformably on the conglomeratebeds of Oligocene Lower Red Formation (Figs. 6 and 7). Thesesandstones have been deposited possibly during the period offalling sea level as an LST.

The overlying open marine carbonate rocks of the microfacies Aand B are interpreted as transgressive deposits during the period ofsea level rise as a TST (Fig. 7).

In the sequence 1, the TST is characterized by packstone,grainstone and floatstone textures with perforate benthic forami-nifera (Lepidocyclinidae, Nummulitidae, Miogypsinidae), bryo-zoans and corralinacean algea. Large and flat Lepidocyclinidae of

Figure 7. Qom Formation sequence stratigraphy in Kahak section, eastern margin of the Urumieh-Dokhtar intra arc basin, Iran (For biozones and symbols see Table 1 and Fig. 6).

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Figure 8. Chronostratigraphic scheme for Qom Formation in Kahak section (study area) and Bijegan section (Karavan et al., in press) in Urumieh-Dokhtar intra arc basin of IranwithAsmari Formation in Chaman-Bolbol section (Amirshahkarami et al., 2007; Amirshahkarami and Taheri, 2010) in Zagros Basin in southwest of Iran. Correlation of depositionalenvironments and biozones is shown (Pabde Formation and Gachsaran Formation are in Zagros Basin in Southwest Iran and Lower Red Formation and Upper Red Formation are inCentral Iran).

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the open marine fauna show a rising sea levels which reaches themaximum, equivalent to the MFS.

The packstoneefloatstone of the microfacies H with lithoclastsof igneous rocks and perforate benthic foraminifera overlies theMFS. These sediments were mostly deposited in a near-shoreenvironment and are interpreted as the deposition of the HST(Fig. 7). The upper part of the HST is characterized by sandymudstone indicating possibly a tidal zone and a type 2 of sequenceboundary (SB2).

6.2. Sequence 2

The sequence 2 is 45 m thick and Chattian in age. This sequencecan be divided into TST and HST. The open marine carbonate rocksof the microfacies B are interpreted as transgressive depositsformed during a period of sea level rise as a TST (Fig. 7).

The microfacies B is packstoneefloatstone and includesbryozoans larger than 2 mm in size (Fig. 3b). Other bioclasts arecorallinaceaen algae, larger benthic foraminifera (Lepidocyclinidae,

Nummulitidae), small benthic foraminifera and fragments ofmollusca.

Bryozoans, Nummulitidae and large and flat Lepidocyclinidae ofthe open marine fauna show an increase and rising sea levelchanges reach a maximum, equivalent to the MFS.

The packstoneegrainstone of the microfacies E with Miogypsi-nidae, Rotalia, miliolids, Dendritina overlies the MFS. These sedi-ments were mostly deposited in a semi-restricted lagoon and areinterpreted as deposition of the HST (Fig. 7). The upper part of theHST is characterized by microfacies H with lithoclastic perforateforaminifera corallinaceaen packstone-floatstone (Fig. 5c,d) indi-cating near-coast inner shelf and tidal flat environments and a type2 sequence boundary (SB2).

6.3. Sequence 3

The sequence 3 is approximately 80 m thick and late Chattian toearly Burdigalian in age. The thickness of sequence 2 is approxi-mately 80 m and its facies associations can be divided into TST and

Figure 9. Chronostratigraphic correlation of stratigraphic sequences. A: Sea level changes of Qom Formation in Kahak section, Urumieh-Dokhtar intra arc basin, Iran (for symbolssee Fig. 7). B: Global sea level model for Rupelian to Burdigalian stages (adapted from Haq et al., 1987).

M. Amirshahkarami, M. Karavan / Geoscience Frontiers 6 (2015) 593e604602

HST (Fig. 7). The open marine microfacies A in the basal part of thesequence 3 is interpreted as TST. This depositional system includesopen marine fauna such as perforate benthic foraminifera (Fig. 3a).The upper bed of this depositional package is a packstoneegrain-stone with densely packed, flat Lepidocyclinidae and Nummuliti-dae and indicates the MFS.

The restricted marine sediments of the back reef and lagoonenvironments (Microfacies D, E, F and G) overlying the MFS indi-cate the HST. The type 2 sequence boundary (SB2) betweensequence 3 and sequence 4 is characterized by sandstones of themicrofacies J.

6.4. Sequence 4

The sequence 4 is about 62 m thick and Burdigalian in age.This sequence can be divided into TST and HST (Fig. 7). The basalpart of the sequence 3 is interpreted as the TST. This depositionalsystem includes carbonate deposits of the microfacies C, D, Fdeposited in the patch-reef, back-reef, semi-restricted lagoon andlagoon paleoenvironments. The upper bed of this depositionalpackage is coral boundstone and indicates the MFS. According toNeumann and Macintyre (1985) reef accretion has several stra-tegies including keep-up reefs, catch-up reefs and give-up reefs.The reef growths, including the shallow water and deep waterreefs, are illustrated by sea level changes in the marine environ-ments (James and Bourque, 1992). In the shallow water, the TSTreef grew in a keep-up strategy. In the keep-up reefs the rockrecord should illustrate homogeneous vertical sequences whichhave uniform microfacies. Therefore, the patch reef identified inthe sequence 3 show a shallow water TST reef with a keep-upstrategy (Fig. 7).

The HST has an aggradational stacking pattern which has beendeposited in the back reef, open lagoon and lagoon paleoenviron-ments of the microfacies D, E, F, G. This sequence is overlain by the

Miocene Upper Red Formation. There is an unconformity (SB1: type1 of sequence boundary) between Qom Formation and Upper RedFormation.

As a consequence, based on the vertical succession of themicrofacies and sedimentary paleoenvironments, four third-ordersequences have been recognized in OligoceneeMiocene succes-sion of Qom Formation in the Kahak section. There is an MFS in theearly Rupelian sediments of the Qom Formation, and it could be acorrelation level.

7. Correlation of sedimentary sequences

The biostratigraphic and paleoenvironmental correlation of theKahak section of Qom Formation with Bijegan section documentedby Karavan et al. (2015) provided the following consequences(Fig. 8).

(1) In the Kahak section, Naimi and Amirshahkarami (2011) haveidentified four biozone assemblages of foraminifera in theRupelian, Chattian, Aquitanian andBurdigalian stages and in theBijegan section two biozone assemblages of foraminifera havebeen identified in the Rupelian and Chattian stages. In otherwords, the Miocene sediments of Qom Formation have beendeposited towards the northeast of the intra-arc basin into theQom basin (Figs. 2A and 8). According to the studies based onthe larger benthic foraminifers’ biostratigraphy by Anjomshoa(2013) and Anjomshoa and Amirshahkarami (2014), Qom For-mation is RupelianeChattian in age in northeast Sirjan in theSanandajeSirjan fore arc basin (Fig. 2A). Therefore, in accor-dance with biostratigraphic correlation, deposition of QomFormation started earlier in the southeastern parts.

(2) A very-low-gradient homoclinal carbonate ramp model,including inner ramp and middle ramp, has been suggested forQom Formation in both of the Kahak and Bijegan sections

M. Amirshahkarami, M. Karavan / Geoscience Frontiers 6 (2015) 593e604 603

(Fig. 8). The coral reef with coral boundstone facies has beenidentified in the Bijegan section in the Rupelian and Chattianstages but a patch reef has been recognized in the Kahak sec-tion in the Burdigalian stage. In the Kahak section, a wide-spread lagoon paleoenvironment has been identified in theAquitanian and Burdigalian stages (Figs. 7 and 8). Also sand-stone layers with a beach facies has been deposited in theKahak section in the Aquitanian stage.

(3) Due to the vertical succession of microfacies and sedimentarypaleoenvironmental features, four third-order sequences havebeen recognized in the Kahak section of the Oligoce-neeMiocene succession of Qom Formation. In Urumieh-Dokhtar intra arc basin, depositions of Qom Formation in thenorth (Kahak section) have more complete sedimentary se-quences (sq1esq4) in the RupelianeBurdigalian stages (Fig. 8).Therefore, transgression of the Tethyan Seaway on the Uru-mieh-Dokhtar intra arc basin started from the southeast andcontinued northwest gradually.

(4) The Rupelian sediments of Qom Formation are thicker inBijegan than in the Kahak section. Rupelian sediments of theBijegan section include three third-order depositional se-quences (sq1esq3) and Kahak section includes only one third-order depositional sequence. Anyway, in both of the sections,an MFS has been identified in the early Rupelian.

(5) There are some similarities between OligoceneeMiocene QomFormation fauna in Central Iran, Urumieh-Dokhtar and San-andajeSirjan Province of Iranwith OligoceneeMiocene AsmariFormation fauna in Zagros Basin in Southwest Iran (Anjomshoaand Amirshahkarami, 2014). Also, sedimentary paleoenviron-ment of the Asmari Formation is a homoclinal ramp(Amirshahkarami et al., 2007; Vaziri-Moghaddam et al., 2010;Amirshahkarami, 2013a).

The faunal dissimilarity between the shallow marine limestoneof the Asmari Formation and Indo-West Pacific suggests a discon-nected seaway in the Aquitanian and Burdigalian (Amirshahkarami,2013b). In accordance with correlation diagrams in Fig. 8, an MFShas been identified in the early Rupelian in both the Qom as well asthe Asmari Formation.

The middle rampwith open marine paleoenvironment has beenrecognized in the lower part of both the Asmari Formation and theQom Formation in the RupelianeChattian stages (Fig. 8). Thelagoonal paleoenvironment is widespread in both Asmari Forma-tion and Qom Formation in the AquitanianeBurdigalian stages.

As a consequence, various numbers of the third-order sequenceshave been recognized in the different sections of Qom Formation.However, there is an MFS in the early Rupelian sediments of QomFormation so that it can be a correlation level. The deposition ofQom Formation started earlier in the southeast. Therefore, trans-gression of the Tethyan Seaway on Iran started from the southeastand continued northwest gradually.

According to the correlation of eustatic curves in Fig. 9, thesequence stratigraphic model of the OligoceneeMiocene Kahaksection of Qom Formation has similar architecture with third-order cycle 4.5 of supercycle (second-order cycle) TA4, third-order cycle 1.3 of supercycle (second-order cycle) TB1 and third-order cycle 2.2 of supercycle TB2 from Haq et al., (1987). Thethird-order cycles from Qom Formation in Kahak section (sq1 tosq4) transform to another by sequence boundaries second type(SB2) rather than by subaerial unconformities corresponding tosuccessive sea-level lows in sequence boundaries first type (SB1).According to Carter (1998) the global sea-level model comprisesan assembly of local relative sea-level events which are widelyrecognizable within their parent sedimentary basin; and thesequence stratigraphic model is robust, and its application is

therefore an insightful way to approach the interpretation ofsedimentary rocks.

8. Conclusions

The RupelianeBurdigalian Kahak section of Qom Formation inthe Urumieh-Dokhtar magmatic arc in Iran has been deposited inan open marine depositional setting of patch reef, lagoon, tidal flatand coastal environments of the inner and middle ramp.

Also, on the basis of the sequence stratigraphic architecture, fourthird-order sequences have been recognized in the Oligoce-neeMiocene Kahak section of Qom Formation which has similararchitecture with global sea level model.

Transgression of Qom Sea started from the southeast andcontinued northwest gradually. Based on the chronostratigraphicsequence correlation, there is a similarity in upward shallowingsuccession of Qom Formation with the OligoceneeMiocene AsmariFormation from the Zagros Basin in southwest of Iran too.

Acknowledgments

The authors thank the Vazvan-Esfahan Payame Noor Universityfor the laboratory and the financial facilities provided. We appre-ciate the comments of the reviewers. Also we thank Prof. HosseinVaziri Moghaddam from Esfahan University and Editor of Geo-science Frontiers journal for helpful comments and editing ofmanuscript.

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