Accepted Manuscript Microfacies models and sequence stratigraphic architecture of the Oligocene– Miocene Qom Formation, south of Qom City, Iran Mahnaz Amirshahkarami, Mahnaz Karavan PII:
S1674-9871(14)00126-1
DOI:
10.1016/j.gsf.2014.08.004
Reference:
GSF 321
To appear in:
Geoscience Frontiers
Received Date: 15 March 2014 Revised Date:
11 August 2014
Accepted Date: 18 August 2014
Please cite this article as: Amirshahkarami, M., Karavan, M., Microfacies models and sequence stratigraphic architecture of the Oligocene–Miocene Qom Formation, south of Qom City, Iran, Geoscience Frontiers (2014), doi: 10.1016/j.gsf.2014.08.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
Microfacies models and sequence stratigraphic architecture of the Oligocene–Miocene Qom Formation, south of Qom City, Iran Mahnaz Amirshahkaramia,∗, Mahnaz Karavanb Geology Department, Faculty of Science, Payame Noor University, Esfahan, IRAN.
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a
Fax: +98-0312-4222511
Geology Department, Faculty of Science, Payame Noor University, Esfahan, IRAN.
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b
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Abstract
The Oligocene–Miocene Qom Formation has different depositional models in the Central Iran, Sanandaj–Sirjan and Urumiehh–Dokhtar magmatic arc provinces in Iran. The Kahak section of the Qom Formation in the Urumiehh–Dokhtar magmatic arc has been studied, in order to determinate its microfacies, depositional model and sequence stratigraphy. The textural analysis and faunal assemblages reveal ten microfacies. These microfacies are indicative of five
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depositional settings of open marine, patch reef, lagoon, tidal flat and beach of the inner and middle ramp. On the basis of the vertical succession architecture of depositional system tracts, four third-order sequences have been recognized in the Oligocene–Miocene Kahak succession of Qom Formation. Based on the correlation charts, the transgression of the Qom Sea started from the southeast and continued gradually towards the north. This resulted in widespread
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northward development of the lagoon paleoenvironment in the Aquitanian-Burdigalian stages. Also, the sequence stratigraphic model of the Oligocene–Miocene Qom Formation has an
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architecture similar to those that have developed from Oligocene–Miocene global sea level changes.
Keywords: Rupelian-Chattian; Aquitanian-Burdigalian; paleoenvironment; Sedimentary model; sequence stratigraphy.
1. Introduction
∗
Corresponding author : E-mail:
[email protected], Fax: +98-0312-4222511
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ACCEPTED MANUSCRIPT The Oligocene–Miocene Qom Formation includes marine marlstones and limestones with gypsum and siliciclastic rocks and is an important gas reservoir in Central Iran (Fig. 1). There is no particular section which has been specified as type section for the Qom Formation but generally its type area has been accepted to be the Qom plain in Central Iran (Aghanabati, 2011).
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The first reports of the Qom Formation were published by Loftus (1855) and Abich (1878) in the Lake Rezayeh (Uromieh) region and by Tietze (1875) in Central Iran. Furrer and Soder (1955) subdivided the Oligocene–Miocene marine strata of the Qom Formation in the type locality of the formation near the town of Qom, into six members.
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These rock units are a – basal limestone, b – sandy marl, c – marl and limestone alternation, d – evaporites, e – green marls and f – top limestone. Bozorgnia (1965)
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expanded the subdivision into ten units using lithological and palaeontological characteristics. He left the Rupelian strata unnamed and correlated it with the lower part of the Lower Asmari Formation in the Zagros Basin in south of Iran (Fig. 1). Generally, the Qom Formation is unconformably underlain by red and green-gray shale and siltstones of the Oligocene Lower Red Formation and unconformably overlain by the Miocene Upper Red Formation. In the Tanbour and Bujan sections in the Sanandaj–
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Sirjan province, the Qom Formation is unconformably underlain by
Paleozoic
metamorphic rocks and unconformably overlain by the Quaternary sediments (Anjumshoa, 2013; Anjomshoa and Amirshahkarami, 2014). Biostratigraphic data on larger benthic foraminifera of the Qom Formation were
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established and dated as Oligocene–Miocene by Rahaghi (1973, 1976, 1980). A biostratigraphic revision of the Qom Formation was made by Naimi and
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Amirshahkarami (2011) and Yazdi-Moghaddam (2011). Paleoecology and biostratigraphy of Qom Formation have been studied by VaziriMoghaddam and Torabi (2004), Daneshian and Ramezani Dana (2007), Reuter et al., 2009, Mohammadi et al. (2011), Anjomshoa (2013), Anjomshoa and Amirshahkarami (2013), Mohammadi et al., (2013a,b) and Karavan et al. (2014) . Karavan et al. (2014) have studied the sequence stratigraphy of the Qom Formation in the Bijegan section. Qom
Formation
with
variable
lithostratigraphic,
biozonal
and
microfacies
characteristics is widespread in the Sanandaj–Sirjan, Urumiehh–Dokhtar magmatic arc and Central Iran provinces. Most of the researches on the Qom Formation involve the
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ACCEPTED MANUSCRIPT characterization of biostratigraphy and microfacies. Also, there is no exact correlation among the Qom Formation successions. The Qom sequence stratigraphy explains the depositional model and transgression of the Tethyan Seaway in Central Iran. The Kahak is one of the most important sections of the Qom Formation in the eastern margins of the Urumieh–Dokhtar magmatic arc in Iran.
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Qom Formation contains numerous benthic foraminifera species which provided useful data for reconstruction of the depositional paleoenvironments (Anjomshoa and Amirshahkarami, 2013).
Biostratigraphy of Qom Formation has been studied by Naimi and Amirshahkarami
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(2011) in Kahak area (Table 1). The assemblage biozones of the study area are explained by the late Oligocene–early Miocene biostratigraphy (Laursen et al., 2009).
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The current paper has three purposes: (1) The explanation of the depositional settings of the Qom Formation in the Kahak section using the microfacies analysis (2) The sequence stratigraphy and microfacies correlation of Qom Formation in the study section with Oligocene-Miocene Asmari succession in the Zagros Basin (3) Study of the sedimentary basin transgression of Qom Formation in the Tethyan Seaway in Iran.
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2. Geological and Geographical position of the study area The microplate of Central Iran originated during final collision of the African/Arabian Plate with the Iranian Plate, the process of which has already started in the Mesozoic (Coleman-Sadd, 1982).
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An important effect of the collision of these plates was the closure of the Tethyan Seaway during the Miocene (Harzhauser and Piller 2007, Reuter et al., 2009).
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The termination of migration of marine biota and exchange of tropical waters between the eastern Mediterranean and the western Indo-Pacific Tethys has been called the Terminal Tethyan Event (TTE) by Harzhauser et al. (2007). Accordance to Adams et al. (1983) the timing of TTE is of Aquitanian age, while Harzhauser et al. (2002) proposed a Burdigalian age. Amirshahkarami (2013b) discussed a disconnection seaway between the shallow marine limestone of the Oligocene–Miocene Asmari Formation in Zagros Basin (southwest Iran) and the western Indo-West Pacific region in the Aquitanian and Burdigalian times.
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ACCEPTED MANUSCRIPT Another effect of the plate collision was the formation of a fore-arc basin (Sanandaj– Sirjan Basin) and a back-arc basin (Qom Basin) on the Iranian Plate at the north-eastern margin of the Tethyan Seaway (Fig. 2a). These basins are separated by a volcanic arc system which developed during Eocene times (Stöcklin and Setudehina, 1991). According to Mohammadi et al. (2013) ddeposition of the Qom Formation (Rupelian–
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Burdigalian in age) took place in three NW–SE-trending basins: Sanandaj–Sirjan (forearc basin), Urumiehh–Dokhtar magmatic arc (intra-arc basin) and Central Iran (back-arc basin) (Fig. 2a). Also, transgression of the Tethyan Seaway in the Iranian Plate started from southeast and continued northwestward gradually.
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Within this study one section was investigated in the eastern margin of Urumiehh– Dokhtar magmatic arc (Intra-arc basin). The study section is located at the Kahak
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village, about 30 km south of Qom in central Iran (Figs. 1–2). The Kahak section of the Qom Formation (studied in detail at its mid-point at 34°31'13.3"N and 50°56'29.5"E) is unconformably underlain by the Oligocene Lower Red Formation and unconformably overlain by the Miocene Upper Red Formation.
3. Material and Methods
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A complete section of the Qom Formation was measured and sampled in Kahak area, south of Qom City. The thickness of the Kahak section is 220 m. In the section studied, the lithology of the Qom Formation is mainly characterized by limestone, sandy limestone, sandstone and coral limestone. The microfacies characteristics were
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described from thin sections in 75 rocky samples. The reorganization of carbonates microfacies is basis on the nomenclature of Dunham (1962) and Embry and Klovan
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(1971). Analyses have allowed the interpretation of carbonate marine environments, depositional system tracts, sea level changes and sequence stratigraphic architectures of Qom Formation in the study area.
4. Microfacies analyses Based on the textural, allochemical and orthochemical characteristics, ten microfacies were identified from Qom Formation in the study area (Figs. 3–5).
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ACCEPTED MANUSCRIPT 4.1. Microfacies A - Bioclastic corallinaceaen Lepidocyclinidae packstone – grainstone This microfacies has a grain–supported texture in a micritic matrix (packstone) or in a sparry calcite cement (grainstone). The major allochems are perforated larger benthic
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foraminifera such as Amphistegina, Rotalia, Nummulites, Operculina, Eulepidina and Nephrolepidina. Other skeletal grains include rhodolite, bryozoans, corallinaceaen algae, bivalves and echinids. This microfacies occurs in Rupelian-Chattian lower layers of Kahak succession (Fig. 7).
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Interpretation
The depositional setting of the microfacies A is the lower photic zone of open marine
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environment with medium to high energy. Large and flat Nummulitidae, Lepidocyclinidae, bryozoans, and echinids are the typical open marine 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 Tanbour sections of Qom Formation byAnjumshoa and Amirshahkarami (2014) and Karavan et al. (2014). A similar microfacies is present
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in the Rupelian-Chattian lower beds of Asmari Formation in Zagros Basin in southwest of Iran (Vaziri-Moghaddam et al., 2010; Amirshahkarami, 2013a).
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4.2. Microfacies B - Bioclastic bryozoan packstone-floatstone The characteristic of the microfacies B is grain-supported texture in a micritic matrix
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with abundant bioclasts (Figure 3b). The texture is different from that of microfacies A and includes packstone-floatstone with coarse-grained bioclasts of bryozoans which are larger than 2 mm in size. Other bioclasts are corallinaceaen algae, larger benthic foraminifera (Lepidocyclinidae, Nummulitidae), small benthic foraminifera and fragments of mollusca. This microfacies occurs in Rupelian-Chattian lower layers of Kahak succession (Fig. 7).
Interpretation
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4.3. Microfacies C - Coral boundstone This microfacies is characterized by colonial corals and has been identified in the Burdigalian stage. Macroscopically, it is a discontinuous massive limestone containing
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autochthonous Scleractinian colonial corals (Figs. 3c and 4). Interpretation
The occurrence of in-situ organisms such as colonial corals suggests a reef environment
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(Wilson, 1975; Flügel, 2004). The discontinuous coral boundstone layers interbedded with lagoon microfacies indicate a patch reef depositional environment (Fig. 6). 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 C indicates oligotrophic condition.
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4.4. Microfacies D - Bioclastic miliolid coral floatstone The texture of this microfacies is a micritic matrix with more than 10% coarse grains larger than 2 mm (Fig. 3d). The texture includes floatstone with coarse-grained bioclasts
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of colonial corals. Other bioclasts are miliolids, mollusca, ostracods and small benthic foraminifera. The microfacies D has been identified in the Burdigalian stage.
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Interpretation
This microfacies is characterized by abundant coarse–grained fragments of colonial coral with imperforate foraminifera (miliolids). This microfacies is distinguished from the reef facies by the absence of in-situ boundstone fabrics (Wilson, 1975; Flügel, 2004; Vaziri-Moghaddam et al., 2010). Most fossils are well preserved and indicated a low energy environment with circulation in the back reef setting (Wilson, 1975; Flügel, 1982). The co-occurrence of normal marine skeletal (corals) and restricted biota (imperforate foraminifera) suggest a semi-restricted lagoon. A similar microfacies occurs in Asmari Formation in Zagros Basin (Vaziri-Moghaddam et al., 2010)
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ACCEPTED MANUSCRIPT 4.5. Microfacies E - Bioclastic miogipsinoidae miliolid corallinaceaen packstonegrainstone This microfacies is Oligocene–Miocene in age and is a packstone-grainstone with corallinaceaen algae, perforate and imperforate benthic foraminifera, micritized
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bioclasts (Figs. 3e and 7). In this microfacies, perforate foraminifera are Miogypsinidae and Rotalia and imperforate foraminifera are miliolids and Dendritina. Other bioclasts are small benthic foraminifera, molluscs, echinids and ostracods.
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Interpretation
Perforate foraminifera (e.g. Miogypsinidae, Rotalia) and imperforate foraminifera (e.g. miliolids and Dendritina) are found together in the semi-restricted lagoon.
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Miogypsinoids lived in shallow waters with normal salinity (Geel, 2000) and recent Rotalia lives in a shallow water photic zone (Romero et al., 2002). The imperforate foraminifera such as miliolid, and Denditina occur in a restricted lagoon (Hallock and Glenn, 1986; Geel, 2000; Romero et al., 2002). The mixed open marine bioclasts and protected environment bioclasts indicate semi-restricted lagoon (Vaziri-Moghaddam et
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4.6. Microfacies F - Bioclastic peloid corallinaceaen imperforate foraminifera
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packstone-grainstone
The depositional grain-supported texture of this microfacies is represented by
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packestone–grainstone (Figs. 3f and 5a). This microfacies is composed of imperforate foraminifera
(Borelis,
Meandropsina,
Peneroplis,
Dendritina
and
miliolids)
corallinaceaen algae, peloids and is Burdigalian in age (Fig. 7). Other bioclasts include small benthic foraminifera, Ostracodes, and micritic skeletal components. The packestone–grainstone textures are fine–medium grained, poorly–moderately sorted, subangular and well rounded. Interpretation In the microfacies F, the lagoon depositional setting is suggested by poor marine biota and abundant restricted biota (such as miliolids, peneroplids and alveolinids). The
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ACCEPTED MANUSCRIPT imperforate foraminifera indicate the restricted conditions (Hallock and Glenn, 1986; Geel, 2000; Romero et al., 2002). A similar microfacies has also been reported from Tanbour and Bijegan successions of Qom Formation (Anjumshoa and Amirshahkarami, 2014; Karavan et al., 2014). A similar microfacies has been identified in Asmari
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Formation in Zagros Basin (Amirshahkarami, 2013a)
4.7. Microfacies G - Sandy bioclastic miliolid wackestone–packstone
This microfacies is characterized by a wackestone–packstone texture with low diversity
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of imperforate foraminifera such as miliolids, Borelis and Dendritina of the Burdigalian stage (Figs. 5b, 7). Fine–medium grained detrital quartz is recognized in this
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microfacies. The wackestone–packstone is poorly–medium sorted and grains are angular to subangular. Secondary bioclasts are mainly fragments of corallinaceaen algae, mollusks and micritic skeletal grains. Textularia, peloids, ostracods, echinids and small benthic foraminifera are found in minor amounts. Interpretation
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The imperforate foraminifera such as miliolids and quartz grains in micritic texture may indicate deposition in a lagoon with low energy (Geel, 2000; Romero et al., 2002; Vaziri-Moghaddam and Torabi, 2004). A similar microfacies has also been reported in
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Bijegan section of Qom Formation (Karavan et al., 2014).
4.8. Microfacies H - Lithoclastic perforate foraminifera corallinaceaen
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packstone- floatstone
This microfacies is characterized by a lithoclastic packestone–floatstone texture with coarse corallinaceaen algae fragments, rhodolith and perforate foraminifera such as miogipsinoidae, Lepidocyclinidae and Amphistegina (Fig. 5c,d). The medium–coarse grained and angular–subangular lithoclasts have originated from extrabasinal igneous rocks. The matrix consists of fine-grained angular quartz and igneous rocks fragments which are embedded in a micritic matrix. The age of the microfacies representing basal section of the stratigraphic succession is early Rupelian (Fig. 7).
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ACCEPTED MANUSCRIPT Interpretation In the microfacies H, the extrabasinal sediment supply into the carbonate environment may indicate deposition in a tidal zone. Mixed carbonate-siliciclastic can be deposited in near-coast environments (Flügel, 2004). The microfacies characteristics, mixed carbonate-extrabasinal siliciclastic sediments with bioclasts (e.g. corallinaceaen algae
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fragments and perforate foraminifera) are common in near coastal inner shelf and tidal flat environments. The sedimentary features of the facies may indicate the intra-arc
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depositional setting within the Urumieh–Dokhtar magmatic arc (Fig. 2).
4.9. Microfacies I - Sandy mudstone
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This microfacies is a poorly sorted lime mudstone with 15 to 20% detrital grains of subangular quartz of 0.1 to 0.2 mm size (Fig. 5e). This facies, with alternating sandy limestone and carbonates, is Rupelian in age (Figure 7).
Interpretation
In the tidal zone and coastal environment, the clastic sediments may originate from the
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bedload of estuaries (Flügel, 2004). The fenestrate fabric and lack of fauna indicate the tidal flat depositional setting (Alsharhan and Kendall, 2002; Rasser et al., 2005; VaziriMoghaddam et al., 2010; Amirshahkarami, 2013a). A similar microfacies has also been
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reported in the Bijegan succession of Qom Formation (Karavan et al., 2014).
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4.10. Microfacies J - Sandstone This microfacies is submature subarkosic sandstone (Fig. 5f). The sandstone is mainly composed of 65 to 90% subangular to angular detrital quartz, 5 to 25% feldspars (chiefly plagioclase) and 7% lithic grains. The grains are bounded with clayey matrix and sparry calcite cement. The hematite and glauconite minerals are rarely present. The age of microfacies is late Rupelian (Fig. 7).
Interpretation
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ACCEPTED MANUSCRIPT The sandstone beds of this facies alternate with the marine carbonate layers of the Qom Formation. The detrital quartz and feldspar minerals may suggest a coastal environment. A similar microfacies has also been reported from the Bijegan succession of Qom
5. Sedimentary model
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Formation (Karavan et al., 2014).
Microfacies analyses from Kahak section of the Qom Formation show open marine, patch
reef,
lagoon,
semi-restricted
lagoon,
tidal
flat
and
costal
environments.Palaeoenvironmental reconstruction based on the textures and microfacies
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suggests a homoclinal carbonate ramp for Kahak section of Qom Formation (Fig. 6).
In classical facies models, a carbonate ramp is separated into inner ramp, middle ramp
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and outer ramp (Burchette and Wright, 1992).
The larger benthic foraminifera are effective tools in the recognition of the Cenozoic palaeoenvironments. Distribution of foraminiferal assemblages depends on intrabasinal conditions. The light gradient of environment is an important factor in the distribution of species and it is effective on symbioses and on nutrient availability too (Hottinger, 1983). The size, degree of flatness and wall of the large foraminifer shells may give
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indications of the paleoenvironmental conditions (Hallock and Gleen, 1986; Geel, 2000).
Perforate foraminifera with symbiotic algae (e.g. Lepidocyclinidae and Nummlitidae in microfacies A and B) are major foraminifera in euphotic shallow water of open marine
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environment (Hallock and Gleen, 1986; Geel, 2000). In a lagoon environment, imperforate foraminifera (e.g. miliolids, Dendritina,
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Peneroplis and Borelis in microfacies F and G) are abundant (Hallock and Gleen, 1986; Geel, 2000). In the Kahak section of Qom Formation, inner ramp includes the lagoon, tidal flat and shoreline or beach environments and middle ramp includes shallow water environments of open marine and patch reef. The outer ramp environment has not been recognized in the studied section of Qom Formation. The high energy coastal environment is characterized by detrital and mixed siliciclasticcarbonate sediments (microfacies I and J). Sandy mudstone (microfacies H) indicates a tidal flat environment.
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ACCEPTED MANUSCRIPT The most common microfacies of the inner ramp are wackestone, packestone, grainstone with imperforate foraminifera (microfacies E, F and G). The imperforate foraminifera such as Dendritina, Meandropsina, Peneroplis, Borelis and miliolids (microfacies F and G) indicate a lagoon environment (Geel, 2000; Romero et al., 2002). A semi-restricted lagoon is recognized by coexistence of restricted marine fauna such as
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imperforate foraminifera and open marine fauna such as perforate foraminifera (microfacies E).
The middle ramp includes proximal and distal parts. The proximal part of middle ramp is characterized by patch reef and reef-derived bioclasts such as colonial corals in a
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floatstone texture (microfacies C and D).
The distal part of middle ramp includes perforate foraminifera such as Lepidocyclinidae
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and Nummulites (microfacies A, B). The perforated larger benthic foraminifera such as Lepidocyclinidae and Nummulitidae indicate symbiotic bearing strategy (Hottinger, 1983; Leutenegger, 1984; Hallock and Glenn, 1986; Hottinger, 1997). Generally, in Kahak section of Oligocene–Miocene Qom Formation, sedimentary environments correspond to inner–middle ramp, and hence the inner ramp is more
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widespread than the middle ramp in the Aquitanian–Burdigalian.
6. Sequence stratigraphic interpretation
Sequence stratigraphy is considered by many authors as one of the latest conceptual revolutions in the broad field of sedimentary geology and emphasizes facies
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relationships and stratal architecture within a chronological framework (Miall, 1995; Catuneanu et al., 2009). A sequence stratigraphic framework includes genetic units that
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resulting from the interplay of accommodation and sedimentation (Catuneanu et al., 2009). The sequence stratigraphic genetic units are lowstand system tract (LST), transgressive system tract (TST), highstand systems tract (HST) and maximum flooding surface (MFS) which are bounded by sequence stratigraphic surfaces (SB) (Miall, 1995; Emery and Myers, 1996; Nicols, 1999; Catuneanu et al., 2009). Sequence stratigraphic analyses interpreted four third-order depositional sequences in the Kahak section of Qom Formation (Fig. 7):
6.1. Sequence 1
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ACCEPTED MANUSCRIPT The sequence 1 is 17 m thick and Rupelian in age. This sequence can be divided into LST, TST and HST. The tidal flat sandstone beds of the microfacies J on the inner ramp is the basal part of sequence 1 which has been deposited unconformably on the conglomerate beds of Oligocene Lower Red Formation (Figs. 6–7). These sandstones have been deposited possibly during the period of falling sea level as a LST.
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The overlying open marine carbonate rocks of the microfacies A and B are interpreted as transgressive deposits during the period of sea level rise as a TST (Fig. 7).
In the sequence 1, the TST is characterized by packstone, grainstone and floatstone textures with perforate benthic foraminifera (Lepidocyclinidae, Nummulitidae,
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Miogypsinidae), bryozoans and corralinacean algea. Large and flat Lepidocyclinidae of the open marine fauna show a rising sea levels which reaches the maximum, equivalent
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to the MFS.
The packstone–floatstone of the microfacies H with lithoclasts of igneous rocks and perforate benthic foraminifera overlies the MFS. These sediments were mostly deposited in a near-shore environment and are interpreted as the deposition of the HST (Fig. 7). The upper part of the HST is characterized by sandy mudstone indicating
6.2. Sequence 2
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possibly a tidal zone and a type 2 of sequence boundary (SB2).
The sequence 2 is 45 m thick and Chattian in age. This sequence can be divided into TST and HST. The open marine carbonate rocks of the microfacies B are interpreted as
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transgressive deposits formed during a period of sea level rise as a TST (Fig. 7). The microfacies B is packstone–floatstone and includes bryozoans larger than 2 mm in
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size (Fig. 3b). Other bioclasts are corallinaceaen algae, larger benthic foraminifera (Lepidocyclinidae, Nummulitidae), small benthic foraminifera and fragments of mollusca.
Bryozoans, Nummulitidae and large and flat Lepidocyclinidae of the open marine fauna show an increase and rising sea level changes reach a maximum, equivalent to the MFS. The packstone–grainstone of the microfacies E with Miogypsinidae, Rotalia, miliolids, Dendritina overlies the MFS. These sediments were mostly deposited in a semirestricted lagoon and are interpreted as deposition of the HST (Fig. 7). The upper part of the HST is characterized by Microfacies H with lithoclastic perforate foraminifera
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ACCEPTED MANUSCRIPT corallinaceaen packstone- floatstone (Fig. 5c,d) indicating near-coast inner shelf and tidal flat environments and a type 2 sequence boundary (SB2).
6.3. Sequence 3 The sequence 3 is approximately 80 m thick and late Chattian to early Burdigalian in
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age. The thickness of sequence 2 is approximately 80 m and its facies associations can be divided into TST and HST (Fig. 7). The open marine microfacies A in the basal part of the sequence 3 is interpreted as TST. This depositional system includes open marine fauna such as perforate benthic foraminifera (Fig. 3a). The upper bed of this package
is
a
packstone–grainstone
with
densely
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depositional
packed,
flat
Lepidocyclinidae and Nummulitidae and indicates the MFS.
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The restricted marine sediments of the back reef and lagoon environments (Microfacies D, E, F and G) overlying the MFS indicate the HST. The type 2 sequence boundary (SB2) between sequence 3 and sequence 4 is characterized by sandstones of the microfacies J.
6.4. Sequence 4
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The sequence 4 is about 62 m thick and Burdigalian in age. This sequence can be divided into TST and HST (Figure 7). The basal part of the sequence 3 is interpreted as the TST. This depositional system includes carbonate deposits of the microfacies C, D, F deposited in the patch-reef, back-reef, semi-restricted lagoon and lagoon
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paleoenvironments. The upper bed of this depositional package is coral boundstone and indicates the MFS. According to Neumann and Macintyre (1985) reef accretion has
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several strategies including keep-up reefs, catch-up reefs and give-up reefs. The reef growths, including the shallow water and deep water reefs, are illustrated by sea level changes in the marine environments (James and Bourque, 1992). In the shallow water, the TST reef grew in a keep-up strategy. In the keep-up reefs the rock record should illustrate homogeneous vertical sequences which have uniform microfacies. Therefore, the patch reef identified in the sequence 3 show a shallow water TST reef with a keepup strategy (Fig. 7). The HST has an aggradational stacking pattern which has been deposited in the back reef, open lagoon and lagoon paleoenvironments of the microfacies D, E, F, G. This
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ACCEPTED MANUSCRIPT sequence is overlain by the Miocene Upper Red Formation. There is an unconformity (SB1: type 1 of sequence boundary) between Qom Formation and Upper Red Formation. As a consequence, based on the vertical succession of the microfacies and sedimentary paleoenvironments, four third-order sequences have been recognized in Oligocene–
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Miocene succession of Qom Formation in the Kahak section. There is a MFS in the early Rupelian sediments of the Qom Formation, and it could be a correlation level.
7. Correlation of sedimentary sequences
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The biostratigraphic and paleoenvironmental correlation of the Kahak section of Qom Formation with Bijegan section documented by Karavan et al. (2014) provided the
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following consequences (Fig. 8).
(1) In the Kahak section, Naimi and Amirshahkarami (2011) have identified four biozone assemblages of foraminifera in the Rupelian, Chattian, Aquitanian and Burdigalian stages and in the Bijegan section two biozone assemblages of foraminifera have been identified in the Rupelian and Chattian stages. In other words, the Miocene sediments of Qom Formation have been deposited towards the northeast of the intra-arc
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basin into the Qom basin (Figs. 2A, 8). According to the studies based on the larger benthic foraminifers’ biostratigraphy by Anjumshoa (2013) andAnjomshoa and Amirshahkarami (2014), Qom Formation is Rupelian– Chattian in age in northeast Sirjan in the Sanandaj–Sirjan fore arc basin (Fig. 2A). Therefore, in accordance with
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biostratigraphic correlation, deposition of Qom Formation started earlier in the southeastern parts.
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(2) A very-low-gradient homoclinal carbonate ramp model, including inner ramp and middle ramp, has been suggested for Qom Formation in both of the Kahak and Bijegan sections (Fig. 8). The coral reef with coral boundstone facies has been identified in the Bijegan section in the Rupelian and Chattian stages but a patch reef has been recognized in the Kahak section in the Burdigalian stage. In the Kahak section, a widespread lagoon paleoenvironment has been identified in the Aquitanian and Burdigalian stages (Figs. 7– 8). Also sandstone layers with a beach facies has been deposited in the Kahak section in the Aquitanian stage.
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ACCEPTED MANUSCRIPT (3) Due to the vertical succession of microfacies and sedimentary paleoenvironmental features, four third-order sequences have been recognized in the Kahak section of the Oligocene–Miocene succession of Qom Formation. In Urumieh–Dokhtar intra arc basin, depositions of Qom Formation in the north (Kahak section) have more complete sedimentary sequences (sq1–sq4) in the Rupelian–Burdigalian stages (Fig. 8).
started from the southeast and continued northwest gradually.
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Therefore, transgression of the Tethyan Seaway on the Urumieh–Dokhtar intra arc basin
(4) The Rupelian sediments of Qom Formation are thicker in Bijegan than in the Kahak section. Rupelian sediments of the Bijegan section include three third-order depositional
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sequences (sq1–sq3) and Kahak section includes only one third-order depositional sequence. Anyway, in both of the sections, a MFS has been identified in the early
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Rupelian.
(5) There are some similarities between Oligocene–Miocene Qom Formation fauna in Central Iran, Urumieh–Dokhtar and Sanandaj–Sirjan Province of Iran with Oligocene– Miocene Asmari Formation fauna in Zagros Basin in southwest Iran (Anjumshoa and Amirshahkarami, 2014). Also, sedimentary paleoenvironment of the Asmari Formation is a homoclinal ramp (Amirshahkarami et al., 2007; Vaziri-Moghaddam et al., 2010;
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Amirshahkarami, 2013a).
The faunal dissimilarity between the shallow marine limestone of the Asmari Formation and Indo-West Pacific suggests a disconnected seaway in the Aquitanian and Burdigalian (Amirshahkarami, 2013b). In accordance with correlation diagrams in
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Figure 8, a MFS has been identified in the early Rupelian in both the Qom as well as the Asmari Formation.
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The middle ramp with open marine paleoenvironment has been recognized in the
lower part of both the Asmari Formation and the Qom Formation in the RupelianChattian stages (Fig. 8). The lagoonal paleoenvironment is widespread in both Asmari Formation and Qom Formation in the Aquitanian-Burdigalian stages. As a consequence, various numbers of the third-order sequences have been recognized in the different sections of Qom Formation. However, there is a MFS in the early Rupelian sediments of Qom Formation so that it can be a correlation level. The deposition of Qom Formation started earlier in the southeast. Therefore, transgression of
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ACCEPTED MANUSCRIPT the Tethyan Seaway on Iran started from the southeast and continued northwest gradually. According to the correlation of eustatic curves in figure 9, the sequence stratigraphic model of the Oligocene–Miocene Kahak section of Qom Formation has similar architecture with third-order cycle 4.5 of supercycle (second-order cycle) TA4, third-
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order cycle 1.3 of supercycle (second-order cycle) TB1 and third-order cycle 2.2 of supercycle TB2 from Haq et al., (1987). The third-order cycles from Qom Formation in Kahak section (sq1 to sq4) transform to another by sequence boundaries second type (SB2) rather than by subaerial unconformities corresponding to successive sea-level
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lows in sequence boundaries first type (SB1). According to Carter (1998) the global sealevel model comprises an assembly of local relative sea-level events which are widely
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recognizable within their parent sedimentary basin; and the sequence stratigraphic model is robust, and its application is therefore an insightful way to approach the interpretation of sedimentary rocks.
8. Conclusions
The Rupelian–Burdigalian Kahak section of Qom Formation in the Urumiehh–Dokhtar
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magmatic arc in Iran has been deposited in an open marine depositional setting of patch reef, lagoon, tidal flat and coastal environments of the inner and middle ramp. Also, on the basis of the sequence stratigraphic architecture, four third-order sequences have been recognized in the Oligocene–Miocene Kahak section of Qom Formation
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which has similar architecture with global sea level model. Transgression of Qom Sea started from the southeast and continued northwest
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gradually. Based on the chronostratigraphic sequence correlation, there is a similarity in upward shallowing succession of Qom Formation with the Oligocene–Miocene Asmari Formation from the Zagros Basin in southwest of Iran too.
Acknowledgements The authors thank the Vazvan-Esfahan Payame Noor University for the laboratory and the financial facilities provided. We appreciate the comments of the reviewers. Also we thank Prof. Hossein Vaziri Moghaddam from Esfahan University and Editor of Geoscience Frontiers journal for helpful comments and editing of manuscript.
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Figure 1. General map of Iran showing the eight geologic provinces (adapted from Heydari et al., 2003).
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ACCEPTED MANUSCRIPT Table 1. Biozonation of Qom Formation in Kahak section (Naimi and Amirshahkarami, 2011).
Figure 2. Location and geological setting of the study area: (A) general map of Iran
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showing the basins in which deposition of the Oligocene–Miocene Qom Formation has taken place: QB or Qom Basin (Back-arc basin); UDMA or Urumieh–Dokhtar Magmatic arc (Intra-arc basin); SSB or Sanandaj-Sirjan Basin (Fore-arc basin) (adapted
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and revised from National Iranian Oil company (NIOC) Geological map 1969, 1975, 1977a,b, 1978; Heydari et al. 2003; Harzhauser and Piller, 2007; Reuter et al., 2009).
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(B) Location of the study area at the Urumieh-Dokhtar Magmatic arc.
Figure 3. Microfacies type of Qom Formation in Kahak section: (a) Microfacies A: Bioclastic
corallinaceaen
Nummulitidae
Lepidocyclinidae
packstone-grainstone
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(Sample No. K002). (b) Microfacies B: Bioclastic bryozoans corallinaceaen packstonefloatstone (Sample No. K009). (c) Microfacies C: Coral boundstone (Sample No.
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K046). (d) Microfacies D: Bioclastic miliolid coral floatstone (Sample No. K044). (e) Microfacies E: Bioclastic Miogypsinoidae miliolid corallinaceaen wackestone-
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packstone (Sample No. K053). (f) Microfacies F: Bioclastic corallinaceaen imperforate foraminifera wackestone-packstone; (Sample No. K037).
Figure 4. Outcrop photographs of a patch reef of Qom Formation in the Urumieh– Dokhtar intra basin arc, Iran.
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No. K034). (c-d) Microfacies H: Lithoclastic perforate foraminifera corallinaceaen packstone-floatstone under polarized light (Sample No. K004). (e) Microfacies I: Sandy mudstone (Sample No. K007). (f) Microfacies J: Sandstone (subarkose) under polarized
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light (Sample No. K042).
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Figure 6. Qom Formation depositional model in Kahak section. Interpretation adapted from Hottinger (1997), Pomar (2001) and Rasser and Nebelsick (2003). FWWB: Fair weather wave base; SWB: Storm wave base; A to J: Microfacies defined in Figs. 3, 5.
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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.
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6).
Figure 8. Chronostratigraphic scheme for Qom Formation in Kahak section (study area) and Bijegan section (Karavan et al., 2014) in Urumieh–Dokhtar intra arc basin of Iran with Asmari Formation in Chaman-Bolbol section (Amirshahkarami et al., 2007; Amirshahkarami and Taheri, 2010) in Zagros Basin in southwest of Iran. Correlation of
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Red Formation are in Central Iran).
Figure 9. Chronostratigraphic correlation of stratigraphic sequences. A: Sea level changes of Qom Formation in Kahak section, Urumieh–Dokhtar intra arc basin, Iran
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(for symbols see Fig. 7). B: Global sea level model for Rupelian to Burdigalian stages
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(adapted from Hag et al., 1987).
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Table 1. Larger benthic foraminifera biozonation in the Kahak section of the Qom Formation (Naimi and Amirshahkarami, 2011).
Assemblage biozone
Aquitanian
III
Pseudolituonella reicheli, Peneroplis sp., 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.
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Burdigalian
Biozone no. IV
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Borelis melo, Borelis curdica
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The Rupelian-Burdigalian Kahak section of the Qom Formation in the Urumiehh– Dokhtar magmatic arc in Iran has been deposited in open marine, patch reef, lagoon, tidal flat and coastal environments. The microfacies analyses have been interpreted as indicative of the inner and middle
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Sequence stratigraphic architecture revealed four third-order sequences having
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