Biostratigraphy, sequence stratigraphy, and ...

Biostratigraphy, sequence stratigraphy, and paleoecology of the Lower–Middle Miocene of Northern Bandar Abbas, Southeast Zagros basin in south of Iran Akbar Heidari, Asadollah Mahboubi, Reza Moussavi-Harami, Luis Gonzalez & Seyed Ali Moalemi Arabian Journal of Geosciences ISSN 1866-7511 Arab J Geosci DOI 10.1007/s12517-012-0803-3

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Author's personal copy Arab J Geosci DOI 10.1007/s12517-012-0803-3

ORIGINAL PAPER

Biostratigraphy, sequence stratigraphy, and paleoecology of the Lower–Middle Miocene of Northern Bandar Abbas, Southeast Zagros basin in south of Iran Akbar Heidari & Asadollah Mahboubi & Reza Moussavi-Harami & Luis Gonzalez & Seyed Ali Moalemi

Received: 24 August 2012 / Accepted: 11 December 2012 # Saudi Society for Geosciences 2013

Abstract The Guri Member is a limestone interval at the base of the calcareous marls of the Mishan Formation. It is the youngest hydrocarbon reservoir of the southeast part of the Zagros sedimentary basin. This Member overlaid siliciclastic rocks of Razak Formation and is overlain by green and gray marls of the Mishan Formation. In order to consider the paleoecology and paleoenvironments of the Lower–Middle Miocene (Guri Member), we have studied biostratigraphy and sequence stratigraphy of the Guri Member based on foraminifer and microfacies in two stratigraphic sections including Dorahi–Homag and Chahestan. A total of 33 genera and 56 species of benthic and planktonic foraminifera were identified in two studied stratigraphic sections. Benthic and planktonic foraminifera demonstrate Aquitanian to Langhian age (Early– Middle Miocene) for this Member at the study area. Studied interval has deposited in four facies association including supratidal, lagoon, coral reef, and open sea on a carbonate ramp. Carbonate rocks of the Guri Member have precipitated in two and three depositional sequences at Chahestan and Dorahi–Homag sections, respectively. Sedimentation of marine carbonates of the Guri Member on siliciclastic deposits reflects a major transgression of sea level at Lower to Middle Miocene that led to creating a new sea in the Zagros basin at A. Heidari (*) : A. Mahboubi : R. Moussavi-Harami Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran e-mail: [email protected] L. Gonzalez Department of Geology, University of Kansas, Lawrence, KS, USA S. A. Moalemi Reservoir Geology Division, Exploration and Production Department, Research Institute of Petroleum Industry, Tehran, Iran

that age. Increasing siliciclastic influx along with a sea level fall finally caused burying of the carbonate ramp. Except for the beginning of sedimentation of carbonate at the base of both stratigraphic sections (depositional sequence 1), most of the system tracts are not matched to global sea level curve that reflect local effects of the basin. Distribution of foraminifera suggests precipitation in tropical to subtropical in mesotrophic to oligotrophic and eutrophic to oligotrophic conditions. Based on large benthic foraminifera (porcelaneous large benthic foraminifera and hyaline larger benthic foraminifera), water temperature average was determined between 25 and 30 °C that was confirmed by analyzing oxygen and carbon stable isotopes. Finally, we have utilized achieved data to reconstruction and modeling of paleoecology, paleoenvironments, and sea level changes in the southeast part of the Zagros basin. Keywords Foraminifera . Sedimentary environment . Paleoecology . Guri Member . Bandar Abbas . Zagros basin

Introduction Unlike siliciclastic lithofacies, where deposition can result from purely physical process, carbonate sediment are often the products of complex interaction between physical, chemical, and biological processes (Beavington-Penney et al. 2006). In shallow platform settings, many of the different elements (habitats) are not simply related to water depth (Rankey 2004) and are mobile in the sense that one habitat can replace another over short time periods because of subtle environmental changes (Beavington-Penney et al. 2006). Bandar Abbas hinterland located in southeast part of Zagros sedimentary Basin in south of Iran and north of the Arabian plate. The eastern boundary of this hinterland is

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Minab Fault and its southern border crosses through Persian Gulf. Its northern border is according to the Razak fault (Barzegar 1981). Zagros sedimentary rocks and structures (anticlines and synclines) in Bandar Abbas area show a different orientation than other parts of this basin (Haynes and McQuilan 1974). Anticlines and synclines of this zone are mostly eastern–western that is different from NW–SE trend of other zones of Zagros Basin (Molinaro et al. 2004). The Guri Member has two different trends of thickness including northern–southern and eastern–western in the Zagros basin (Motiei 1993). The first thickness trend has a range from 100 m in the west and southwest to 1,000 m in the north of Bandar Abbas. The second trend has a range from 1,000 m in the north of this zone to 100 m in the south of Bandar Abbas (Movahed 1995). This Member extends from the south of Shiraz (Fars Province) to the Bandar Abbas area (Motiei 1993); some recent studies have reported the development of the Guri Member in the Dezful area in Khuzestan province (Homaiun Zadeh 2002). The Guri Member is a part of the calcareous marl Mishan Formation and is laid at the base of this formation. Its section type was introduced by James and Wynd (1965) in the Tange Guri area in SE Lar city and has a 111 m thickness. The Guri Member in the study area composed of medium to thick buff-colored limestone with a rough and rugged appearance with some interbeds of marl, shale, and sandstone in the study area. It contains abundant macro- and microfossils with some gray to green interbeds of marl. James and Wynd (1965) primarily introduced the Guri Member as Operculina Member because it has a large number of Operculina in most of the stratigraphic sections especially at the section type. Moreover, planktonic foraminifera, bivalves, gastropods, brachiopod, coral, and echinoderms from these Formations are known as Lower to Middle Miocene (Aghanabati 2004). The Guri limestone is the youngest hydrocarbon reservoir of the Zagros basin (Aghanabati 2004). This Member has not been studied enough in the Zagros basin; hence, examinations of foraminiferal content in this study are useful for determining the age, stratigraphy, geographic expansion, facies associations, and interpret paleoecology and paleoenvironments. Heidari et al. (2012) studied the Guri Member in the Bandar Abbas region and determined the Early Miocene age for this Member based on crab’s body fossils. It conformably has overlaid siliciclastic sediments of the Razak Formation (1 in Fig. 1 and 1 in Fig. 2) and underlain Mishan Formation marls (2 in Fig. 1). Because of economic aspects, most of Oligo-Miocene studies in Zagros basin have focused on Asmari Formation. Unfortunately, there are not enough studies on other equal successions such as the Guri Limestone. Therefore, this study is one of the scarce studies that considered the Guri Member (Lower– Middle Miocene) in the Zagros basin. The objectives of this

study are summarized as: (1) examination of biostratigraphy of Lower to Middle Miocene of the southeastern Zagros basin based on foraminiferal contents; (2) differentiate facies associations and interpret their depositional environments; (3) illustrate sea level changes based on sequence stratigraphy studies; and (4) reconstructing and modeling the paleogeography and paleoecology during Lower to Middle Miocene age in southeast of Zagros basin in south of Iran.

Method and materials Chahestan and Dorahi–Homag areas are located in the north of Bandar Abbas. Two stratigraphic sections were measured in the southeast Zagros basin in south of Iran (Fig. 3). The thickness of the Guri Member ranges from 171 m at C ha he s t a n to 47 8 m at D o r ah i– H o m a g s e c t i o n . Geographical coordinates of the Chahestan section is north latitude of 27°33′01″ and east longitude of 56°42′40″ in northeast Bandar Abbas and Dorahi–Homag is located at north latitude 28˚ 49′ 9″ and east longitude of 56°26′7″ (Fig. 3). A total of 252 samples from two stratigraphy sections including 22 marl and 57 rock samples from Chahestan section and 26 marl and 147 rock samples from Dorahi–Homag were collected. After washing the marl samples on 70 mesh (210 μm), 120 mesh (125 μm), and 230 mesh (63 μm) sieves, they were studied at a paleontology lab under stereomicroscope in the Paleontology Laboratory of the Ferdowsi University of Mashhad and University of Kansas. Proper recognized foraminifera were picked and isolated, then they were transferred to the scanning electron microscope lab of the Ferdowsi University of Mashhad and micron scale photos were taken. The foraminiferal contents were identified based on Loeblich and Tappan (1988) and Bolli et al. (1987). Rock samples were transferred to thin section lab and 204 thin sections were provided for consideration in the petrography lab. Thin sections were also stained by a solution of mixed alizarin red and potassium ferrycianide in order to differentiate calcite from dolomite (Dickson 1966). Textural classification was done according to Dunham (1962) and Embry and Klovan (1971). The sequence stratigraphy techniques and concepts developed by many workers were used in this study to delineate sequence boundaries (unconformities), parasequences, and system tracts based on marine flooding surfaces (e.g., Haq et al. 1987; Vail et al. 1991; Hardenbol et al. 1998; Cataneunu et al. 2010). Sea level changes were interpreted for the Lower to Middle Miocene interval based on facies variations and investigation with the sequence stratigraphic framework established in this study. In addition, 45 samples were selected from grains and micrite for consideration of stable isotopes of oxygen and carbon. The stable isotopes of the Guri Member from the Mishan Formation were analyzed at the University of


Fig. 1 Summarized geology map and situation of studied sections; 1:10,000,000; depicted by authors based on Fakhari (1994)

Kansas Stable Isotope Laboratory. Drilling was done by microscope-mounted drill assembly with a 0.3 μm tungsten carbide burr. In a stainless steel boat, 30–80 μg of pure carbonate powder were measured each. Analysis was done using a Kiel Carbonate Device III+ Finnigan MAT253 isotope ratio mass spectrometer (ThermoFinnigan, Germany). Isotope ratios are reported relative to the Vienna PeeDee Belemnite (VPDB) standard with analytical precision of better than ±0.1‰ δ C18 and δ O18 values. Phosphoric acid preparation performed according to Stanford University Stable Isotope Laboratory’s online manual. Standards NBS-18 carbonatite (NIST Ref. Mat. 8543), NBS-19 limestone (NIST Ref. Mat. 8544), and an internally calibrated calcite standard were analyzed at regular intervals. A standard calibration curve was generated for d13C and d18O and sample data is then calibrated and reported versus the VPDB scale.

Discussion The Adams and Bourgeois (1967) study was focused on Asmari Formation in the Lurestan and Khuzestan provinces in southwestern Iran. Their studies led to presenting a biostratigraphic base for Oligo-Miocene in Iran (e.g., Vaziri

1987; Vaziri et al. 2006; Daneshian and Dana Ramadan 2007; Ranjbaran et al. 2007; Amirshahkarami et al. 2010; Seyrafian et al. 2011). They have measured and studied 50 stratigraphic sections and determined three assemblage zones for the Asmari Formation including: (1) Borelis melo group–Meandropsina iranica assemblage zone (Burdigalian in age); (2) Miogypsinoides–Archaias–Valvulinid assemblage zone (Aquitanian in age) which they subdivided into two assemblage biozones: (2a) Elphidium sp.14– Miogypsina assemblage subzone that overlaid (2b) Archaias asmaricus–Archaias hensoni assemblage subzone; and (3) Eulepidina–Nephrolepidina–Nummulites assemblage zone (Oligocene). In this study, we also compared the Guri Member biozones with other studies such as Laursen et al. (2009), Wynd (1965), and Cahuzac and Poignant (1997; Table 1). The Laursen et al. (2009) assemblage zones for Miocene age of the Zagros basin (Asmari Formation) are: (1) Miogypsina–Elphidum sp. 14Peneroplis farsensis (Aquitanian in age) and (2) B. melo curdica–B. melo melo (Burdigalian in age; Table 1). The assemblage zones of Wynd (1965) for Miocene age of the Zagros basin (Asmari Formation) are (1) Austrotrilina howchini–Peneroplis evolutus (zone 59, Aquitanian in age); and (2) B. melo curdica (zone 61, Burdigalian in age; Table 1).

Author's personal copy Arab J Geosci Fig. 2 Field photos of the Guri Member at the Chahestan section. 1 The lower boundary with siliciclastic rocks of the Razak Formation is marked by dashed yellow line; 2 the conglomerate at the base of the Member that reflect a transgression of sea level and is sequence boundary of DS1; 3 the extra-large bivalves in the lower part of section, some of them are larger than 1 m; 4 the branch red corals in lower part of the section, close to lower boundary

The Cahuzac and Poignant (1997) assemblage zones for Miocene age of Zagros basin (Asmari Formation) are (1) A. howchini–Miogypsina–Miogypsinoides deharti (Aquitanian in age) and (2) B. melo group–Miogypsina (Burdigalian in age; Table 1).

Fig. 3 Field photos of the Guri Member at the Dorahi–Homag section. 1 The lower boundary of the Member with siliciclastic rocks of the Razak Formation, 2 the upper boundary of Member with Mishan Formation marls; 3 the branch corals in the lower part of the Member; 4 the lower part of section that contain crab beds

Biostratigraphy of Guri Member at Chahestan section Based on studies of thin sections and washed samples, a total of 18 spices and 12 genus of benthic foraminifera were recognized (Fig. 4; Plates 1, 2, and 3): Amphistegina sp.,

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A. asmaricus Asmari hensoni

MiogypsinoidesArchaias Valvinid Elphidium sp. 14-Miogypsina Aquitanian

Miogypsina-Elphidium sp. 14-P. farsensis

B. melo melo Indeterminate

A. howchini P. evolutus (zone 59)

B. melo curdica

M. iranica Elphidium sp. 14 Miogypsina A. hensoni A. howchini P. evolutus P. farsensis M. iranica

B. melo curdica B. melo group

Miogypsina A. howchini Miogypsina M. deharti

B. melo group–M. iranica B. melo cordica Burdigalian Miocene

B. melo curdica (zone 61)

Dorahi–Homag section Chahestan section Cahuzac and Poignant (1997) Adams and Bourgeois (1967), Vaziri (1987), Vaziri-Moghaddam et al. (2006), Daneshian Dana and Ramadan (2007), Ranjbaran et al. (2007), Amirshahkarami et al. (2010), Seyrafian et al. (2011) Wynd (1965) Laursen et al. (2009) Age Epoch

Table 1 Comparing biozones of Guri Member from Mishan Formation at Dorahi–Homag and Chahestan sections in the southeast Zagros Basin with Miocene part of bizonation studies if Asmari Formation (Laursen et al. 2009; Wynd 1965; Adams and Bourgeois 1967; Cahuzac and Poignant 1997)

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Archaias sp., Asterigerina sp., Bolivina sp., Elphidum sp., Lepidocyclina sp., Miogypsina sp., Miogypsinoides sp., Nonionella sp., Operculina sp., Peneroplis sp., Quinqueloculina sp., Ammonia beccarii, Archaias kirkokensis, A. hensoni, Archaias operculiniformis, A. howchini, Borelis pygmaea, B. melo curdica, Cibicides kullenbergi, Cibisides mediocris, Cibicides novozelandicus, Dentalina basiplanata, M. iranica, Miogypsinoides complanata, Miogypsina irregularis, P. evolutus, Peneroplis farsaensis, Rotalia viennoti, and Sphaerogypsina globusa. We have used the Adams and Bourgeois (1967) foraminiferal zones in this study that are applicable for the Asmari Formation (Oligo-Miocene in age) in the Zagros basin. The B. melo curdica is one of the most significant Early Miocene species and its first occurrence indicates lower boundary of biozone-1 of Adams and Bourgeois (Vaziri 1987; VaziriMoghadam et al. 2006; Daneshian Dana and Ramadan 2007; Ranjbaran et al. 2007; Amirshahkarami et al. 2010; Seyrafian et al. 2011). Some other studies such as Rahaghi (1980) and Seyrafian et al. (1996) have used the first occurrence of B. melo curdica as the lower boundary marker of the Burdigalian age. Benthic and planktonic foraminiferal distribution is shown in Fig. 4. At the Chahestan section, the first occurrence of B. melo is according to the base of the section. Based on Adams and Bourgeois studies (1967), Elphidium sp.14 indicates the boundary of subzones 2a and 2b in sample no. 10. Miogypsina has been reported in the Asmari Formation with Aquitanian age and we mostly have found Miogypsina in Aquitanian sediments in sample no. 12. According to Adams and Bourgeois (1967), occurrence of Rotalia viennotti and Elphidium sp. 14 is not together and with B. melo curdica. Daneshian and Dana Ramadan (2007) have found them altogether in the Qom Formation in Central Iran geological zone. We also have found them altogether in the Burdigalian deposits so it is similar to Central Iran zone. The last occurrence of R. viennotti and Elphidium sp. 14 was late and possibly in Burdigalian age at the Chahestan section that is similar to Central Iran (Daneshian and Ramezani Dana 2007). Based on examination of benthic foraminifera, the lower part of the Chahestan section from the base up to 85 m is according to B. melo group—M. iranica assemblage zone of Adams and Bourgeois (1967) that is equivalent to the upper part of the Asmari Formation in the Zagros basin (Fig. 4, Table 1; Ranjbaran et al. 2007; Amirshahkarami et al. 2010; Seyrafian et al. 2011). The second biozonation that is used in this study is of Laursen et al. (2009), which determined biozones along with their precise age that could be effective for sequence stratigraphy studies. The lower part of the Guri Member at the Chahestan section is according to B. melo cordica–B. melo melo biozone of Laursen et al. (2009), which shows a Burdigalian age for this part (Table 1; Fig. 4).


Fig. 4 Stratigraphy section and the distribution chart, occurrence and range of foraminifera. It contains biostratigraphic zonations of the Guri Member limestone at the Chahestan section


Plate 1 1 R. viennoti, 2 R. viennoti, 3 Textularia sp., 4 Triloculima trigonula, 5 M. irregularis, 6 Miogypsina camplanatus, 7 Triloculina sp., 8 M. anahensis, 9 A. howchini, 10 N. tournoueri, 11 B. melo curdica, 12 Sphaerogypsina globolus, 13 Ammonia bessari, 14 Elphidium sp., 15 Elphidium sp., 16 P. evolutus, 17 Trilocolina trigonula, 18 Pyego sp., 19 A. operculiniformis. Scale bar 250 μm

Marl and shale samples of the Chahestan section were washed. They have rather high diversity and abundance of planktonic foraminifera. A total of 23 spices and nine genus of

planktonic foraminifera in the upper part of the Guri Member in washed samples of the Chahestan section were found (Fig. 4, Plate 3): Anomalina sp., Discorbinella sp.,

Author's personal copy Arab J Geosci Plate 2 1 P. evolutus, 2 Bigerina sp., 3 Lepidocyclina sp., 4 Amphistegina lesoni, 5 A. rotula, 6 Operculina complanata, 7 A. hensoni, 8 Valvinid sp., 9 Quinqueculina sp., 10 Globigerina sp., 11 D. rangi, 12 A. kirkokensis, 13 Meanderopsina iranica, 14 Borelis pygmea, 15 P. farsensis. Scale bar 250 μm

Entosanella sp., Entosolevia sp., Globorotalia sp., Marssonella sp., Proboscidea sp., Textularia sp., Uvigerina sp., Anomalinoides fasciatus, Anomalinoides vitrinodus, Discorbinella complanata, Globigerina woodi woodi, Globigerina woodi connecta, Globigerina brazier, Globigerina insitatus, Globoquadryna dehiscens, Gyroidina zelandicus, Globigerina obesa, Globigerina borealis, Globigerina regularis, Globigerina labiacassata, Lagena elangata, Lagena hispida, Langena substriata, Oolina globosa, Rectouvigerina vesca, and Uvigerina auberiana.

Index planktonic foraminifera were not found in studied samples; however, some planktonic foraminifera species such as G. woodi woodi, G. regularis, Globoquadrina dehiscence, and Textularia subconica were identified on the upper part of the Chahestan section that could determine Middle Miocene age and mark the beginning of the Langhian age for this section (Bolli et al. 1987). Based on planktonic foraminifera, with a thickness of 85 m up to the end of the Chahestan section have been precipitated in Langhian age (Middle Miocene).


Plate 3 SEM photographs of recognized forminiferas of the Guri Member

Considering foraminifera of the Chahestan section led to five results. (1) A total of 18 spices and 12 genus of benthic foraminiferal were found in the studied section. (2) The most significant species are Elphidium sp. 14, Miogypsina, A. hensoni, P. evolutus, and P. farsensis. Based on the above species, the age of the lower part of the Guri Member in the Chahestan section is Early

Miocene (Burdigalian age). (3) Adams and Bourgeois (1967), Laursen et al. (2009), Wynd (1965), and the Cahuzac and Poignant (1997) biozonations of benthic foraminifera (Oligocene–Miocene) in the Zagros basin in SW Iran were utilized for Chahestan section in SW Zagros basin. (4) A total of 23 spices and nine genus of planktonic foraminiferal were recorded in the studied section. (5) Index planktonic


forams were not found in the studied samples; however, some planktonic spices such as G. woodi woodi, G. regularis, and G. dehiscence indicate Middle Miocene age (Langhian) for the upper part of the Guri Member at the Chahestan section. Biostratigraphy of the Guri Member at the Dorahi–Homag section The thickness of the Guri Member at the Dorahi–Homag section is 478 m and it includes limestone with some interbeds of marl. Planktonic foraminifera are rare at this section and standard biozonation based on them is not possible. There are a large number of benthic foraminifera that biozonation was made based on them. Thin section and washed marl samples studies led to recognition of 25 spices and 23 genus of benthic foraminifera at this section (Fig. 5, Plates 1, 2, and 3): Asterigerina sp., Meandropsina anahensis, Spiroloculina sp., Quinquecolina sp., Dentalina basipelanta, R. viennoti, Nonionia sp., Lepidocyclina sp., Elphidium sp., Rotalia sp., Spirolina sp., Archaias sp., A. beccarii, Miogypsina sp., A. hensoni, Textularia sp., Operculina compelanata, Amphistegina sp., M. irregularis, Pyrgo sp., Triloculina trigonula, Meandropsina sp., Dendritina rangi, Triloculina sp., Operculina sp., Miogypsinoides complanatus, B. melo curdica, A. operculiniformis, Miliola sp., A. howchini, Bigenerina sp., Nephrolepidina tournoueri, Sphaerogypsina globulus, Globigerina sp., Asterigerina rotula, Amphistegina lessoni, Eulepidina cf elephantina, Hetrostegina sp., Sigmolina sp., Peneropelis evolutus, Valvinid sp., Spirolina cylindracea, Borelis pigmaea, A. kirkokensis, Peneropelis farsansis, M. iranica, Psudo lituonella richeli, Eulepidina cf dilatata, and Amonia sp. The most significant recorded benthic foraminifera of this section are subdivided into two groups including: (1) Elphidium sp. 14, Miogypsina, A. hensoni, A. howchini, P. evolutus, and P. farsensis that were found in the lower part of the section, which were found in the lower part of the section; and (2) B. melo curdica and M. iranica, which were found in the upper part of the section. According to most of the biozonation studies of the Zagros basin (e.g., Laursen et al. 2009; Wynd 1965; Cahuzac and Poignant 1997), the first group reflects sedimentation in Aquitanian age and the second group proves Burdigalian age. In the Dorahi–Homage section, the first occurrence of B. melo curdica in the Aquitanian–Burdigalian boundary marker was found in sample no. 32 and the boundary of the lower and upper parts of this section is defined based on it. The thickness of the lower part is from the base of the section up to the thickness of 90 m and the upper part is from 90 m up to the end of the section that its thickness is 380 m. Based on Adams and Bourgeois (1967), the lower part of this section is according to biozone 2a and 2b (Table 1; Fig. 5). A. hensoni that belongs to

biozone 2a was recorded in sample no. 11 and Elphidium sp. 14 was recorded from sample no. 16 that indicates the beginning of biozone 2b. Therefore, most of the proportion of the lower part thickness is according to biozone 2b from Adams and Bourgeois (1967). Miogypsina was found in Burdigalian age in this section that is similar to the Asmari Formation (Ranjbaran et al. 2007; Amirshahkarami et al. 2010; Seyrafian et al. 2011) and different from the Qom Formation (OloigiMiocene of Central Iran) that was reported in Aquitanian age (Daneshian and Ramezani Dana 2007). The Guri Member at the Dorahi–Homag section was compared to other bizonations of the Zagros basin and its lower part is based on some of the benthic foraminifera such as Miogypsina, Elphidium sp., and P. farsensis (Table 1; Fig. 5) is according to assemblage zone 1 of Laursen et al. (2009). The presence of A. howchini and P. evolutus is equal to zone 59 from Wynd (1965). Also, A. howchini and Miogypsina were found in the lower part of this section that is according to biozone 1 from Cahuzac and Poignant (1997; Table 1). Based on the occurrence of B. melo curdica and M. iranica, the upper part of this section is according to the assemblage zone 2 of Laursen et al. (2009), zone 61 of Wynd (1965), and assemblage zone 2 of Cahuzac and Poignant (1997; Table 1). As well as, based on Society for Sedimentary Geology (SEPM) charts, the last occurrence of A. howchini was in 18.80 Ma on SEPM charts 1998 (based on Cahuzac and Poignant (1997)), which corresponds to the top of the Foram Zonse M3 and N6 (Berggren et al. 1995). Also, the last occurrence of N. tournoueri happened at 17.30 Ma on SEPM charts immediately after A. howchini, which correspond to the base of the Foram Zones M4 and N6 (Berggren et al. 1995). At the Dorahi section samples, they are similar and occur in the upper part which indicates their age. The results of this section are summarized below. (1) A total of 25 spices and 23 genus of benthic foraminiferal were recognized in the studied sections; (2) The most significant species are B. melo curdica, M. anahensis, Miogypsina, M. iranica, A. howchini, Elphidium sp. 14., P. farsensis, P. evolutus, D. rangi, and T. trigonula. According to these species, Early Miocene (Aquitanian–Burdigalian) age was determined for the Guri Member at the Dorahi– Homag section. (3) We have used Adams and Bourgeois (1967), Laursen et al. (2009), Wynd (1965), and Cahuzac and Poignant (1997) biozonations of benthic foraminifera (Oligocene–Miocene) of the Zagros basin for this section in SE Zagros basin. Microfacies Based on petrographic studies, 28 carbonate microfacies were recognized in the Guri Member limestone. They are deposited in four facies associations including A, B, C, and


Fig. 5 Stratigraphy section and the distribution chart, occurrence, and range of foraminifera. It contains biostratigraphic zonations of the Guri Member limestone at Dorahi–Homag section

Author's personal copy Arab J Geosci Fig. 6 Microfacies and main biotic and abiotic grains of the Guri Member. 1 Dolostone microfaciel; 2 conglomerate lithofacies; 3 sandstone lithofacies; 4 bioclast mudstone; 5 Miliolid wackstone–packstone microfacies; 6 large benthic bioclast floatstone; 7 peloid bioclast packstone–grainstone; 8 large benthic red algae floatstone; 9 a large gastropod in microfacies B5; 10 coral floatstone–framestone; 11 intraclast bioclast floatstone– rudstone; 12 Miogypsina lepidocyclina flutstone– rudstone; 13 Operculina bioclast flutstone–rudstone; 14, 15 echinoids in echinoid bioclast mudstone–wackstone– packstone; 16, 17, 18 red algaes in bryozoan bioclast flutstone– framestone, red algae benthic wackstone–flutstone and bioclast mudstone–wackstone–flutstone; 19, 20, 21 bryozoans in bioclast mudstone–wackstone–flutstone, bryozoan bioclast flutstone– framestone, and bioclast mudstone–wackstone–flutstone; 22 bioclast mudstone– wackstone–flutstone; 23 brachiopod along with bryozoan in brachiopod bioclast flutstone– rudstone; 24 planktonic wackstone–mudstone; 25 annelids (Disturba) in planktonic wackstone–mudstone microfacies; 26 bioclast planktonic wackstone– packstone–flutstone. RA Red Algae, Bz Bryozoan, D Disturba, Br Brachiopod, Pl Planktonic foraminifera, Ec Echinoid, BE bird eye, Oc Operculina

Author's personal copy Arab J Geosci Table 2 Oxygen and carbon stable isotopes data of the Guri Member No.

δ18O (VPDB)

δ13C (VPDB)

Micrite 4 32 38 41-9 41-10 41-11 41-12

−2.67 −3.75 −4.78 −6.07 −6.68 −5.92 −5.37

1.25 0.55 2.24 2.70 2.60 2.70 2.66

41-13 42 48 52 54 63-1 73-2 81 83-2 84 121 122 135 165-1 171-1 171-10 171-11 171-12 171-13

−5.41 −4.56 −3.26 −3.27 −3.49 −3.36 −2.25 −0.72 −1.21 −3.06 −5.13 −4.30 −2.71 −1.68 −1.92 −5.26 −5.39 −5.16 −5.91

2.68 2.95 2.02 0.83 1.06 2.18 2.08 1.47 2.24 3.26 3.57 4.26 1.65 0.45 1.38 −1.52 −1.33 −1.44 −1.22

171-14 Dolomite 4 5 9 Red algae 12-3 83-1 100-2

−5.53

−1.11

1.43 1.31 1.23

2.16 2.10 2.21

−2.78 −2.19 −2.01

1.00 2.33

116-2 171-2 Miogipsina 63-2 165-2 465-2 469-2 Brachiopod 109-2

−1.87 −1.12

2.11 2.22 1.45

−2.38 −1.90 −1.92 −2.01

2.29 0.81 2.12 2.17

−1.61

3.77

109-3 109-4 161-2 168-2

−1.55 −1.43 −1.72 −1.5

3.44 3.02 3.33 3.25

Table 2 (continued) No. Coral 97-1 97-2 130-2 131-2

δ18O (VPDB)

δ13C (VPDB)

−6.11 −6.16 −6.18 −5.88

2.26 2.32 2.21 2.12

D. The facies associations were interpreted as supratidal (A), lagoon (B), coral reef (C), and open marine (D). Facies are interpreted based on Wilson (1975) and Flugel (2010) and then are subdivided into facies associations and the paleoenvironments were interpreted and depositional model was depicted based on Read (1985) and Einsele (2000). Facies association A (supratidal) This facie association consists of medium to thin beds of conglomerate, sandstone, mudstone, and dolomudstone that are mostly brown, red, and yellow to gray in color. This association was subdivided into four microfacies: Microfacies A1: Dolomudstone consists of fine crystals dolomite in a background of limy mudstone. This microfacies includes fenestral and bird eyes fabrics in two macro and micro-scales (1 of Fig. 6). There are some fine quartz grains as well, while skeletal and nonskeletal grains are absent (1 of Fig. 6). More than 50 % of fine crystal minerals were not stained by alizarin red in the Dickson (1966) method. The heaviest values of oxygen carbon that can be caused by dolomite mineral belong to this microfacies (Table 2). Microfacies A2: Mudstone consists of limy mudstone without skeletal and nonskeletal grains. It includes fenestral and bird eyes fabrics. This facies petrographically is the same to the previous one but it was stained by alizarin red that reflects limy micrite of this microfacies. Isotope analyses show that oxygen isotope is lighter than previous (Table 2). Microfacies A3: Conglomerate is composed of conglomerate that is mostly oligomictic and contains sedimentary components and some fossil shells (2 of Fig. 1 and 2 of Fig. 6). The cement of this facies is mostly composed of calcite. It is located at the base of the stratigraphy sections (2 of Fig. 1). Microfacies A4: Sandstone and pebbly sandstone is composed of medium- to coarse-grain sands and in some cases, it contains about 20 % pebble grains. The most important cement is calcite. Most of the


components are sedimentary grains and minors components are quartz and chert grains (3 of Fig. 1). Sedimentary environment of facies association A Based on absent or low abundant and diversity of marine biota, fenestral and bird eyes fabrics, dolomudstone (Alsharhan and Kendall 2003; Flugel 2010; Brasier et al. 2011; Immenhauser et al. 2012), conglomerate, and sandstone, this facies association indicate sedimentation in a tidal flat environment (Fig. 7). Facies association B (Lagoon) This facie association mostly consists of medium to thick beds of limestone that are buff to gray in color. This association was subdivided into nine microfacies: Microfacies B1: Bioclast mudstone: The percentage of biotic grains in this facies is less than 10 %, which mostly are 1 % bivalve fragments, 1 % Miliolid, and 1 % Ammonia (4 of Fig. 6). The background is micrite which stained after staining by alizarin red that proves a lack of dolomite. Microfacies B2: benthic bioclast wackstone: composed of 10 % Miliolid, 10 % porcelaneous large benthic foraminifera (PLF; including Archaias, Meandropsina, and Borelis), and 4 % Ammonia. The background is mostly micrite and in some cases contains about 5 % of fine grain quartz. Microfacies B3: Miliolid Wackstone–Packstone: the main grain of this facies is miliolid of 30–40 % abundance. Miliolids are floated in micrite background (5 of Fig. 6). Microfacies B4: large benthic bioclast floatstone: this facies is composed of 15 % Archaias, 12 % Meandropsian, 10 % Borelis, 2 % coral, 2 % echinoid, 2 % bivalve, and 2 % red algae in a matrix of micrite (6 of Fig. 6). Microfacies B5: bioclast wackstone–floatstone: composed of about 5 % Miliolid, 2 % bivalve, 4 % PLF including Archaias and Meandropsina 2 %, ostracods, 2 % Ammonia, 4 % red algae, 2 % gastropod (9 of Fig. 6), 2 % brachiopod, and 5 % peloid. In some cases, it contains up to 5 % fine grain quartz. Sometimes, the percentage of PLFs increase, which mostly are larger than 2 mm. The matrix of this facies is mainly composed of micrite. Microfacies B6: peloid bioclast packstone–grainstone: the main grains of this microfacies are abiotic grains including 60 % peloid and 8 % ooid (7 of Fig. 6). Other grains are red algae 4 %, brachiopod 2 %, bryozoan 2 %, and echinoid 2 %. The background is composed of 70 % calcite cement and 30 % micrite. Microfacies B7: large benthic red algae floatstone: this facies is composed of 20 % PLF (including 10 % Archaias, 6 % Meandropsina, and 4 % Borelis) and 20 % red algae (8 of Fig. 6). More than 80 % of grains

are larger than 2 mm. The matrix of this facies is composed of 80 % micrite and 20 % clear calcite cement. Microfacies B8: benthic coral wackstone–floatstone: composed of 10 % Miliolid, 5 % Archaias, and 20 % coral. The matrix is composed of 60 % micrite and 40 % calcite cement. Increasing cement reflect the higher energy of sedimentary environment. In addition, the presence of corals is an indicator of increasing water flow (Schlager 2005). Microfacies B9: crab bioclast wackstone: composed of 20 % large-scale crabs (4 of Fig. 2), 5 % bryozoan, 10 % PLF (including Meandropsina, and Archaias), 5 % coral, and 5–8 % Miliolid. Crabs are studied and suggest lower Miocene age (Heidari et al. 2012). This microfacies in all of cases is vertically located at the underneath of the coral frameworks that reveal the coexisting of corals and crabs. Sedimentary environment of facies association B This facies association characterized by high micrite content that is indicative of low-energy environments; in addition, distribution of skeletal and nonskeletal grains help to interpret sedimentary environments. The abundance of Miliolid, Ammonia, Elphidium, Archaias, Meandropsina, and Borelis show low-energy condition of restricted water (e.g., Adachi et al. 2004; Penneya and Racey 2004; Papazzoni and Trevisani 2006; Tasli et al. 2006; Boudagher-Fadel 2008; Koukousioura et al. 2011; Blazquez and Usera 2010). Abundant peloid with mudsupported fabric reflect lagoon condition as well (Flugel 2010). Mixed open marine bioclasts (such as red algae, echinoid, coral, and brachiopod) and protected bioclasts (such as mentioned benthic foraminifera) in this association may have been deposited in semirestricted lagoon condition (Fig. 8). The presence of crabs along with euryhaline biotic allochems and overlain by corals reflect the back reef in lagoon with higher sea water flow than restricted parts of lagoon (4 of Fig. 2; Heidari et al. 2012). Facies association C (coral patch reef) This association consists of medium to thick bedded limestone, which are light brown to dark gray in color. Microfacies C1: coral benthic floatstone: The main allochems of this microfacies are corals that have an abundance of 40–50 %. Other grains along with corals are 5 % Miliolid, 5 % Archaias, 5 % Meandropsina, and some Ammonia and Elphidium. The grains of this microfacies are mainly floated in a matrix of micrite and in some cases it has up to 40 % clear calcite cement.


Fig. 7 Facies and sequence stratigraphy section, distribution, abundant of biotic and abiotic grains in the Chahestan Section. Sea level changes and sea level curve of the Guri Member is compared with global sea level of Hardenbol et al. 1998


Fig. 8 Facies and sequence stratigraphy section, distribution, abundant of biotic and abiotic grains in the Dorahi–Homag section. Sea level changes and sea level curve of the Guri Member is compared with global sea level of Hardenbol et al. (1998)


Microfacies C2: coral floatstone–framestone: this facies is composed of 60–90 % coral grains that mostly are filled by calcite cement. In some instances, corals are filled by micrite and blocky calcite cements (10 of Fig. 6). Early cement are effective in protect corals from compaction in burial stage of diagenesis. Silicification and dissolution are two dominant features of this facies that occurred in coral bodies and matrix. Microfacies C3: coral bioclast floatstone–framestone composed of 50 % coral, 10 % ooid, 5 % red algae, 5 % brachiopod, 5–8 % bryozoan, 5 % echinoid, 2 % Miliolid, 5 % PLF (including Archaias, and Meandropsina) 2–5 % Operculina and Amphistegina, and 5 % Miogypsina. The background is calcite cement and in some cases is micrite. High-energy water led to washing micrite but micrite amounts increase in cases with higher amounts of PLF and Miliolid. Sedimentary environment of facies association Coral reefs are effective for sensitive records of tectonics, paleoclimate, sea level changes, and paleoecology conditions of the environmental settings (Gwirztman and Buchbinder 1978; Dabrio 1990; Mankiewicz 1995; Flecker et al. 1995; Perrin et al. 1995; Hayward et al. 1996; Karabiyikoglu et al. 2005). Therefore, they are useful for interpreting stratigraphic framework, paleoenvironmental setting, basin configuration, and structural evolution of sedimentary basin (Karabiyikoglu et al. 2005). Almost all of the coral-bearing intervals of the Guri Member in both stratigraphy sections are overlain by a bivalve fragmental bed that is debris of talus situations. Most of the coral succession pinch out laterally and their width have a range between 50 and 100 m that indicate expanding of patch reefs between open sea and lagoon. Low quantity of mud, the presence of marine biota in coarse-grained skeletal (such as bryozoans, red algae, echinoid, and brachiopod) indicate sedimentation of this facies association in high-energy coral reef environments (e.g., Masse et al. 2003; Flugel 2010; Mazumder et al. 2012). Facies association D (open sea) This association consists of horizontally medium to thick bedded limestone that is buff and gray to dark gray in color. It contains red algae, brachiopod, bivalve, echinoid, bryozoans, and intraclast. This facies association was subdivided into 12 microfacies as follow: Microfacies D1: intraclast bioclast floatstone–rudstone: About 30–40 % intraclast (11 of Fig. 6) is the most prominent grain of this microfacies. It also contains 5 % bryozoan, 5 % red algae, 5 % brachiopod, and 2 % bivalve in a domain micrite matrix. In some samples, the background is calcite cement that mostly is blocky cement type.

Microfacies D2: red algae benthic wackstone–floatstone: Contains 40–60 % different types of red algae (such as 6–8, 6–17), 10 % PLF (including Archaias, Meandropsina, and Borelis), 5–8 % Miliolid, 5 % Operculina, 2–5 % coral, and 2–5 % Miogypsina and Lepidocyclina in a matrix of more than 85 % micrite and less than 15 % calcite cement. Red algae grains are detectable by unequipped vision and appear as centimeter scale bright spot in a dark background of micrite. Microfacies D3: Miogypsina floatstone–rudstone: This facies composed of 50–60 % Miogypsina (12 of Fig. 6), 2 % Operculina, 2 % brachiopod, 1 % Asterigerina and Amphistegina, and 1 % bryozoan. Most of allochems are greater than 2 mm in a blend of micrite and calcite cement. Microfacies D4: Operculina bioclast floatstone–rudstone: Composed of 50–60 % Operculina (13 of Fig. 6), 5 % Asterigerina, 5 % Amphistegina, 5 % bryozoan, 5 % red algae, 5 % Miogypsina, 2 % echinoid, 2 % brachiopod, 1 % Triloculina, and 1 % textularia. Most of the grains are larger than 2 mm in a matrix of calcite cement and micrite. Microfacies D5: echinoid bioclast mudstone–wackstone–packstone: Echinoids with 15–30 % abundance are the most important biotic allochem of this facies (14 and 15 of Fig. 6). This facies also includes 2 % brachiopod, 2 % red algae, 2 % bivalve, 5 % coral, 2 % bryozoan, 5 % Miliolid, and 5 % Archaias in a matrix of micrite. Microfacies D6: bivalve bioclast floatstone: Composed of 40–50 % bivalve, 5 % brachiopod, 5 % red algae, 5 % bryozoan, and 2 % echinoid. This microfacies mostly lied on coral intervals that reflect the debris fragments of talus. The main genus of bivalves in studied carbonate rocks is Kuphus that are very large and in some instances are larger than 1 m (3 of Fig. 1). They mostly are vertical to bedding. Microfacies D7: red algae bioclast floatstone: This facies is composed of 40–50 % red algae (16 and 18 of Fig. 6), 5 % bryozoan, 5 % brachiopod, 5 % bivalve, 2 % echinoid, and 5 % Operculina and Miogypsina. The main different of this facies and D2 is the presence of euryhaline bioclast including PLFs and Miliolid in D2 microfacies that reflect more lagoon condition of D2 microfacies. Most of the red algae grains are larger than 2 mm in a matrix of 85 % micrite and 15 % calcite cement. Microfacies D8: bryozoan bioclast floatstone–framestone: The main allochem of this facies is bryozoan (19–21 of Fig. 6) that have an abundance of 50–70 %. Other grains with lower abundance are brachiopod 2– 5 %, echinoid 2 %, red algae 5 %, and Operculina and Miogypsina 5 %. The background of this facies mostly is limy mud but sometime it includes more than 80 % blocky calcite cement. Most of bryozoans are filled by


blocky calcite cement (21 of Fig. 6) and some other are filled by micrite (20 of Fig. 6). Microfacies D 9 : bioclast wackstone–floatstone: Composed of 5 % brachiopod, 5 % red algae, 5 % echinoid, 5 % Operculina, 5–8 % coral, 5 % Miogypsina, 5 % bryozoan, 5 % bivalve, 2 % gastropod, and 2 % Triloculina in a background of micrite (22 of Fig. 6). In some samples, most of the grains are larger than 2 mm. Microfacies D10: brachiopod bioclast floatstone–rudstone: Brachiopod grains have an abundance of 50– 60 % that are the main allochems of this facie (23 of Fig. 6). Most of the brachiopod shells are larger than 2 mm and are along with about 10 % bryozoan, 5 % bivalve, 5 % coral, 2 % Operculina, 2 % red algae, and 2 % Miogypsina. Microfacies D11: bioclast planktonic wackstone–packstone: Composed of 10 % planktonic foraminifera, 5– 10 % red algae, 5–10 % bryozoan, 5 % brachiopod, 2 % bivalve, and 2 % Ammonia in a matrix of micrite (24 of Fig. 6). Microfacies D12: planktonic wackstone–mudstone: This facies contain 10 % planktonic foraminifera, 4 % annelids (25 of Fig. 6), 2–4 % Ammonia (26 of Fig. 6), 2 % red algae, and 2 % bryozoan in a matrix of micrite. In some samples, amount of grains is less than 10 % and including about 10 % of fine grain quartz. Sedimentary environment of facies association D euryhaline organisms (such as brachiopod, red algae, bryozoans, echinoid, and some bivalves) indicate deposition in the open marine environments (Flugel 2010; Bitner and M-Dekova 2005; Bachmann and Hirsch 2006; Holcova and Zagorsek 2008; Carcel et al. 2010; Chen et al. 2011; Saber 2012). In addition, high content of limy mud reflects rather low energy conditions of sedimentation (Adachi et al. 2004). In this association, the grain-supported microfacies may have been deposited in a relatively high-energy environment above the wave base and close to coral reefs, while the mud-supported microfacies may have been formed underneath of the wave base. Planktonic foraminifera-bearing facies in a micrite matrix with some other foraminifera such as Ammonia reflect the deepest part of this facies association. Presence of Ammonia, which prefer shallower water conditions, in deepest facies of this facies association indicate that Miocene age sea in study area have not been very deep.

have been precipitated in four facies association including supratidal, lagoon, coral reef, and open sea (Fig. 7). The proposed sedimentation model for the Guri Member in this study illustrates inner ramp to outer ramp sedimentary environments. It contains a large number of foraminifera and other biotic and abiotic grains (Fig. 8). Redistributed lowstand sediments were not observed in the study area that can prove non-existent of shelf break (e.g., Sadeghi et al. 2011). A wide range of sediments were deposited in the inner ramp including marginal marine facies that reflect high-energy sedimentary environment of reef (facies association C), lagoon (MF B4 to B9), and more restricted lagoon (MF B1 to B 4 ). Some benthic foraminifera such as Archaias, Peneroplis, Austrotrillina, Miliolids, Dendritina, Borelis, Ammonia, and Elphidium were recognized in lagoon facies association (FA B) that reflect sedimentation in calm environments. The margin of the Guri carbonate platform is marked by coral Framestone facies association (FA C). The middle ramp sedimentary environments are characterized by increasing marine organisms and medium-sized grains of foraminifera wackstone–packstone (MF D1 to D10). The main larger foraminifera are hyaline and perforate wall types including Operculina, Asterigerina, Amphistegina, Heterostegina, Miogypsina, and Lepidocyclina (Fig. 7). Marl and marly limestone facies that contain planktonic foraminifera (MF D11 and D12) indicate sedimentary environments of the outer ramp (Fig. 7). Abundant mud and absence of wave and current structures indicate low-energy environment underneath the wave action surface (Wilson 1975; Burchette and Wright 1992). Sequence stratigraphy The sequence stratigraphy techniques and concepts developed by many workers have been used in this study to differentiate sequence boundaries, parasequences, and system tracts based on marine flooding surfaces and unconformities (e.g., Haq et al. 1987; Vail et al. 1991; Hardenbol et al. 1998; Catuneanu et al. 2010). Based on facies variations, sea level changes were interpreted for Lower to Middle Miocene interval and sequence stratigraphic framework established in this study. The Guri Member in the study area was calibrated to the Ap11 tectonostratigraphic megasequence and is coincidence to Ng10 to Ng20 maximum flooding surface (MFS) in Dorahi–Homag stratigraphy section and N20 to N30 MFS of Neogene successions of the Arabian plate that was studied by Sharland et al. (2001).

Sedimentary environment modeling Chahestan section Based on lateral and vertical studies of facies associations, the Guri Member has been deposited during Aquitanian– Burdigalian–Langhian (Lower–Middle Miocene) in a shallow carbonate ramp with several patch reefs. Carbonates

Biostratigraphic studies show the age of Burdigalian (Lower Miocene) to Langhian (Middle Miocene) for the Guri Member at Chahestan section (Figs. 4 and 9). The Guri


Fig. 9 The rest of Fig. 8


Member at this section is composed of two depositional sequences. The lower boundary of this unit is supersequence are type 2 (SB2) type and carbonate rocks overlies siliciclastic rocks of Razak Formation (1 of Fig. 1). The upper boundary is characterized by deposition of gray to green marl and shale sediments of the Mishan Formation. The sequence boundaries within this SB2. Ds1 This DS composed of an alternation of medium to thin beds of carbonate and siliciclastic rocks. Sedimentation in this depositional sequence has been started with a 2 m bed of conglomerate (2 of Fig. 1). It is composed of sedimentary clasts such as carbonate and sandy clasts that have been derived from older Formations of the Zagros basin (such as Razak and Jahrom Formations). The conglomerate have been overlaid by sedimentation of lagoonal microfacies that contain PLFs such as Archaias, Peneroplis, Borelis, and some smaller benthics such as Miliolid and Rotalia along with some marine organisms such as red algae, brachiopod, and bryozoan that reflect high flow of sea water in semirestricted lagoon environments. Presence of some PLFs such as B. melo, Peneroplis, and large Miliolid indicate more open sea flows in a lagoon that is close to open sea and reefs in Neogene (Boudagher-Fadel 2008). Lagoonal microfacies were followed by D8 microfacies that is one of the deepest marine microfacies of considered carbonate rocks and indicate a fast transgression of the sea level and is supposed as MFS of this DS. It is followed by shallowing upward parasequences of coral reef, lagoonal, and finally dolostone and siliciclastic sediments of supratidal that reflect a regression and are assume as Highstand System Tract (HST) of this DS (Fig. 9). DS2 This DS is composed of alternations of thick to medium beds of limestone and green marls (Figs. 4 and 9). It is differentiated with a SB 2 from depositional sequence (DS1). After sedimentation of HST and the regressive trend of sea level, the beginning sedimentation of marine microfacies that contain brachiopods and some large benthic forams that indicate a transgression and their boundary with lower unit is defined as TS and is a SB2 (Fig. 9). Marine organisms such as red algae, Operculina, Miogypsina, bryozoans, brachiopods, and echinoids increased upward and in upper parts of this system tract planktonic foraminifera appears and increases that indicate a transgression of sea level. Maximum abundance and diversity in sample no. 61 indicate the MFS of this DS (e.g., Nagy 2005). Therefore, sedimentation under this surface is supposed as the transgressive system tract (TST) of this DS. This system tract

composed of limestone and marl; their washed samples are also rich with planktonic forams and their distributions are shown in Fig. 4. A decreasing trend in abundance and diversity of planktonic forams upward and occurrence of fine benthic forams such as Miliolids and Ammonia along with peloids (that indicate very restricted water; BoudagherFadel 2008), prove a regression in sea level and is typical HST for the Chahestan section of this DS (Fig. 9). In this section, except for the lowermost part, it shows a good correlation with global sea level curves (Hardenbol et al. 1998); the rest of the section is not according to it (Fig. 9). Activities of the Zagros foreland and salt diapirs in southeast of Zagros basin in Lower to Middle Miocene are two important local controls on the Guri carbonate ramp (Alavi 2007) that caused differences between the Zagros basin sea level and global sea level curves in Lower to Middle Miocene. Dorahi–Homag section Sequence stratigraphy analysis was done based on field observations and petrographic studies. Supersequence of the Lower Miocene Guri Member in the study area is bounded by SB2. The lower boundary of this supersequence is SB2 type and carbonate rocks overlies siliciclastic rocks of Razak Formation (1 of Fig. 2). The upper boundary is characterized by deposition of gray to green marl and shale sediments of Mishan Formation (3–2). This interval is subdivided into three depositional sequences in Dorahi–Homag area. DS1 This DS1 is composed of thick bed marl with medium to thin beds of limestone at the lower part and medium to thick beds of limestone with some interbeds of green marl. It started with deposition of an oligomictic conglomerate unit that its clasts mostly have been originated from elder Formations. Conglomerate bed has followed by mudstone, dolostone, and benthic mudstone–wackstone which contain restricted foraminifera such as Miliolid that reflect low circulation of open sea water and low amounts of oxygen (Geel 2000). Deposition of limy–mudstone and fine crystalline dolomite in the lower parts of the DS1 is similar to deposition in salina environment that has been described by other workers (e.g., Warren 1999; Al-Sharhan and Kendall 2003). Therefore, dolostone and mudstone beds indicate supratidal and lagoon sedimentary environments that are followed by more marine facies of open sea environment such as D2 to D5. It finally concluded to D8 facies that is characterized by abundant large brachiopod, bryozoans, and other euryhaline organisms. It is supposed as MSF because it is overlaid by restricted sedimentary facies of lagoon (Figs. 10 and 11).


Fig. 10 Sedimentary model of carbonate rocks of the Guri Member that show sedimentation in inner, middle, and outer parts of a ramp

Accordingly, from the base of the section up to the above, the MFS was deposited in TST. Based on Sharland et al. (2001), studies on sequence stratigraphy of the Arabian 5.00 4.00 3.00

Micrite Brachiopod

2.00 δ13C

Plate Early Miocene is according to a TST that is related to global sea level curves (Haq et al. 1987). First, recognized TST could be coincidental to the recorded transgression in the Arabian Plate and the global rising of sea level (Haq et al. 1987; Hardenbol et al. 1998). The dominant microfacies after sedimentation of D8 facies that was known as MFS are lagoonal microfacies that reflect a fall in sea level and we referred to these shallowing upward parasequences as HST that is according to a global sea level fall (Fig. 10) (Haq et al. 1987; Hardenbol et al. 1998).

Red algae 1.00

Miogypsina

DS2

Coral -8.00

-6.00

-4.00

-2.00

0.00 0.00

2.00

Dolomite

-1.00

δ18O

-2.00

Fig. 11 Oxygen vs. carbon stable isotope in the Guri Member limestone

The second depositional sequence (DS2) in the Dorahi– Homag section mostly composed of medium to thick bed limestone. There is no exposure surface between DS1 and DS2, so they have been differentiated by a SB (Fig. 10). DS2 have been started with deposition of marine carbonate (microfacies D5 that contains echinoids, brachiopods, bryozoans, bivalves, and Hyaline Larger Benthic Foraminifera


(HLF)) and have been defined as the TS boundary of this DS during the second transgression of the Lower Miocene Sea in SE Zagros basin. Lagoonal facies of last HST (DS1) are overlaid by a carbonate interval of marine facies with marine organisms. It indicates transgression of sea level, so it is defined as the TST of this depositional sequence (Fig. 10). Deposition of brachiopod bioclast floatstone is according to MFS in this TST because it is one of the deepest facies of the Guri Member and also is followed by shallowing upward parasequences, which are mostly deposited in restricted water of lagoonal environments and are supposed as HST carbonate sediments. The dminant facies after sedimentation of MFS are B1–B6 microfacies of the lagoon facies association that are supposed as HST of DS2 and contain many of restricted organisms such as PLFs (including Meandropsina, Archaias, and Peneroplis), Miliolid, Elphidium, Textularia, and Ammonia. There are some interbeds of marine facies, which have vanished at most upper part of DS2 and most restricted microfacies such as B1 that contains about 2–5 % very restricted foraminifera such as Miliolids and reflect lowest boundary of sea level regression. DS3 This DS is composed of thick and some medium bed limestone with some interbeds of green marl. Lagoonal microfacies of the last HST have finished by sedimentation of D3 microfacies that contains Operculina and Miogipsina which reflect increasing in oxygen and open sea flows (Geel 2000) and is defined as sequence boundary type 2 between DS2 and DS3 and TS between last HST and the TST of this depositional sequence. The carbonate interval that was supposed as the TST of this DS contains marine and coral reef microfacies. Coral reefs mostly containing many marine organisms such as brachiopods, red algae, and bryozoans indicate high influx of open sea water. Planktonic foraminifera appear in this TST and in some cases are abundant up to about 10 % (Fig. 11). Sedimentation of a coral unit at the top of this system tract is along with the development of the lagoonal microfacies that were attributed to the HST of this DS (Fig. 11). Although, this HST contains many marine facies, decreasing abundance and disappearing of planktonic foraminifera and the presence of restricted organism such as PLFs along with marine facies led to introducing this part as HST of this DS. Paleoecology Large benthic foraminifera are very useful in reconstructing paleoecology. Their modern forms are present in tropical carbonate platforms in the upper part of the light zone (Barattolo et al. 2007). Most of the biofacies show a specific accumulation of foraminifera that depend on environmental conditions that have accumulated sediments. Distribution and

abundance of foraminifera in modern and ancient sedimentary environments depend on some interbasinal conditions including nutrients, temperature, salinity, depth, light, substrate, and water energy (Geel 2000; Carannante et al. 1988; Hottinger 1983, 1997; Hohenegger 2004; Murray 1991, 2006; Beavington-Penny and Racey 2004; Pomar et al. 2004; Romero et al. 2002). Most of the larger foraminifera are not limited to one sedimentary environment and ecology, but they always are more important in particular environments. They are not necessarily ever occupying the same ecological niches (Gaemers 1978). The lower part of the Guri limestone in the Chahestan section and the entire thickness of the Dorahi– Homag section contain large benthic foraminifera with high diversity and abundance. In the Chahestan section, the upward section number of benthics have reduced and the number of planktonic foraminifera increased, so foraminifera content can be used in interpreting paleoecology in this study. According to Hottinger (1983), light is the most important factor in the distribution of foraminifera. Changes in water depth could affect environmental changes such as sedimentation rate, turbulence, light intensity, organic changes, water temperature, and dissolved oxygen (Altenbach et al. 2003; Leckie and Olson 2003). Large benthic foraminifera species, due to being sensitive to water depth in the past, were used to determine the depth of water. Some authors introduced food and oxygen as two very important factors for limiting benthic foraminifera (e.g., Jorissen 1987, 1988; Altenbach et al. 1999; Van der Zwaan et al. 1999). Diversity and abundance of large benthic foraminifera depend on some effective factors that are described here. Paleotemperature Different organisms live in certain temperature ranges. For example, coral reefs thrive in temperatures of 23–25 °C. Large benthic foraminifera live in the tropical and subtropical water environments with average temperatures of 18–20 °C. Many of them live in the summer at the temperature of 25 °C as well (Adams and Mackenzie 1998). Some of the large benthic foraminifera such Borelis, Lepidocyclina, and Archaias are restricted to tropical areas (Betzler et al. 1997; Brandano et al. 2009; BouDagher-Fadel and Wilson 2000). Miogypsinoid, Operculina, and Amphistegina live in tropical to subtropical environments in a wide range of depth, but they are more abundant in depths of 40–70 m (Hottinger 1997, 1983; Hallock 1988; Hallock and Glenn 1986). In order to calculate paleotemperature of the Guri Member, sedimentary environment total of 45 samples were analyzed for oxygen and carbon stable isotopes (Table 2; Fig. 12). Six groups of samples including micrite, dolomite, coral, brachiopod, red algae, and Miogypsina were considered. The heaviest samples can be used for calculating paleotemperature that are closer to sedimentation conditions. Several formulae are proposed for estimating ambient water paleotemperature (e.g.,


Fig. 12 Diameter/thickness (D/T) in HLF shells of Guri Member in SE Zagros Basin. From right to left (shallowing trend) D/T ratio decrease as a respond to decreasing water depth and increasing water energy. Shallow marine foraminifera in wave-influenced environments

show D/T ratio ranging from 2.44 to 2.84, while deeper foraminifera, which are along with planktonics and live in oligophotic water, D/T ratio is about 7.42 (Beavington-Penney 2002)

Shackleton and Kennett 1975; Friedman and O’Neil 1977; Anderson and Arthur 1983). We have used the Anderson and Arthur (1983) equation for calculation of paleotemperature in this study:

(Hohenegger 1994), while Miogypsina, Operculina, and Lepidocyclina prefer lagoon to subtidal environments with low to medium water energy. However Miogypsina, Lepidocyclina, Operculina, and Archaias can tolerate troubled water as well. Modern Heterostegina are also predominant in calm water (Kumar and Saraswati 1997). Many of the examined corals in this study were branch type (4 of Fig. 1 and 3 of Fig. 2) that indicates high energy and shallow turbulent water (Schlager 2005; p. 16).

T ¼ 16 4:14ðdc dwÞ þ 0:13ðdc dwÞ2 Where T is temperature, δc is the calcite oxygen isotope ratio relative to PDB, and δw is the water oxygen isotope ratio relative to SMOW. Oxygen value for Lower Miocene has been calculated and show a value of 2 ‰ (Zachos et al. 2001). Brachiopod shells have δ18O range from −1.5 to −1.72 ‰ (mean, −1.56) and δ 13 C between 3.02 and 3.77 ‰ (mean, 3.36) that indicate a temperature of 25 °C of seawater in the surface water of Lower Miocene sea in the study area that indicates warm waters of tropical areas (Schlager 2005). The calculated temperature is consistent with the estimated values by large benthic foraminifera. Sediment type and substrate Based on the studies of Hottinger (1977), Operculina on soft substrates, Heterostegina on hard substrates, and Sorites could be seen in various types of substrate and Borelis is more abundant in carbonate-rich environments. Lepidocyclina, Archaias, Heterostegina, and Operculina can live in environment with about 40 % of clastic influx, but increasing in clastic influx lead to decreasing their number. Miogypsina greatly suffers clastic influx and is less affected (Kumar and Saraswati 1997). Operculina is mostly recorded in limestone and marl; they only appear in low sand contents. Water energy Large and mainly flat shells are not suitable for dynamic environments because even small turbulence of water can float most large foraminifera (Rasser et al. 2005). Heterostegina mainly can be seen in lagoon environments

Light Some species require less light that are introduced as oligophotic. They live in shadow zones and deeper parts of platform (Pomar et al. 2004). Lower surfaces of photic zone are related to penetration of light in water and are situated at about 130 m deep in clear water (Renema and Troelstren 2001). Red algae and some of the larger foraminifera are indicators of this environment. Some foraminifera such as Operculina and Amphistegina are also documented in environments with light intensity levels of only 5 % (BeavingtonPenny and Racey 2004). Soritides are habitants of tropical waters of Neogene. Their modern types live in oligotrophic, shallow, and tropical to subtropical environments, where the light intensity for a light interceptor coexists with foraminifera is high enough. So they choose dark surrounding leaves and seaweed beds (Hottinger 1997, 2001; Richardson 2001). Size, benthic foraminifer’s shells flatness levels, and wall condensation can also provide information about the environment (Hallock and Glenn 1986; Langer and Hottinger 2000; Geel 2000). Flatness and large size usually occurs within the lower parts of photic zone. For example, under photic zone, Lepidocyclina and Nummulitid are found in open sea, but small- to medium-sized and lens shaped Nummulitids live along with Alveolina in the upper part of photic zone in the inner platform (Langer and Hottinger 2000; Geel 2000).


Green algae and reef building corals are the marker of photic zone in modern seas (Geel 2000); also PLFs in the upper part of photic zone (0–50 m) are abundant (BeavingtonPenny and Racey 2004). The presence of PLFs such as Archaias, Peneroplis, Borelis, and Miliolid indicate photic zone in studied sections. HLFs such as Lepidocyclina, Operculina, Heterostegina, and Amphistegina with red algae are oligophotic area index (Bosence 1983). Nutrition one of the most important factors that controls on variety of organisms (lead to various facies) is the amount of available food (Langer and Hottinger 2000). Benthic organisms indicate marine nutrient levels. Carbonate-secreting benthos prefers living in oligotrophic and mesotrophic environments. Particularly, coral reefs develop in submarine environments with lowest level of nutrients and they easily demise with excess nutrients (Schlager 2005, p. 10). Increasing of nutrients lead to thriving red algae and reducing of nutrient causes coral growth and development (James et al. 1999). Abundant foraminifera such as Miliolid indicate a restricted lagoon environment with rather high levels of nutrient and high salinity (hypersaline). Depth some HLFs such as Lepidocyclina and large flat HLFs coexistence with red algae indicate shallow parts of open sea. Some foraminifera such as Borelis and Austrotrilina are rare in very shallow waters and live in shelter form habitats such as patch reefs with moderate water energy (BouDagher-Fadel 2000; Murray 2006). A. beccarii and Elphidium sp. species are indicative of shallow waters (Pippèrr and Reichenbacher 2010). Based on the above studies, the Guri Member in the Chahestan section have started with shallow water environments that has led to sedimentation of alternations of coral reef and back reef facies (Fig. 7). Those alternation show tropical to subtropical regions, oligotrophic to mezotrophic, and low to medium levels of water energy. Second part is sedimentation in fore-reef microfacies and successions that have been deposited in mezotrophic to oligotrophic in low energy condition that is confirmed by presence of red algae along with some forams such as Operculina, Amphistegina, and Heterostegina. Abundant and high diversity of planktonic foraminifera in upper parts of the Chahestan section indicate sedimentation in deeper water of open sea. Nutrient levels have changed from oligotrophic and mezotrophic into eutrophic in upper part because of influx of siliciclastic, which were seen in thin section as quartz grains and fossil fragments. This change have caused occurrence and development of smaller benthic and planktonic forams instead of larger benthic forams and reef building corals that are consistent with oligotrophic to mezotrophic (Boudagher-Fadel 2008; Pippèrr and Reichenbacher 2010).

At the Dorahi–Homag section, distribution and abundance of PLFs such as B. melo, Archaias, Peneroplis, and Meandropsina are summarized and simplified as Large Benthic Forams in sequence stratigraphy sections (Figs. 9, 10, and 11) of the Guri Member and some other HLFs such as Operculina and Miogipsina have distinguished. Distribution and abundance of PLF and HLF in this section display a close relationship with depositional sequences and their system tracts. As TSTs contain large amounts of HLF and depleted of PLF (Figs. 9, 10, and 11). Conversely, HSTs are full of PLF and depleted of HLF. Based on the presence of many of large benthic spices that are recognized and recorded in this study, temperature has been warmer than 20 °C in tropical to subtropical environments. In most cases, there is no any large benthic along with coral reefs that reflect decreasing nutrient level and indicate oligotrophic conditions. Conversely, plenty of red algae in this section reflect high levels of nutrient and eutrophic conditions. Too many changes in sedimentary environment at the Dorahi– Homag section inhibit stability of any factor including nutrient, salinity, light, substrate, etc. Most of the foraminifera species of this section reflect sedimentation in photic zone.

Conclusion At the Chahestan section, a total of 18 spices and 12 genus of benthic foraminiferal were recognized. The most significant species are Elphidium sp. 14, Miogypsina, A. hensoni, P. evolutus, and P. farsensis that indicate the age of Early Miocene (Burdigalian age) for lower part. A total of 23 spices and nine genus of planktonic foraminifera were recorded at the Chahestan section. Index planktonic forams were rare in studied samples. Although, some planktonic spices such as G. woodi woodi, G. regularis, and G. dehiscence indicate Middle Miocene age (Langhian) for upper part of the Guri Member at Chahestan section. A total of 25 species and 23 genus were recognized at the Dorahi–Homag section. The most significant species at the Dorahi–Homag section are B. melo curdica, M. anahensis, Miogypsina, M. iranica, A. howchini, Elphidium sp. 14., P. farsensis, P. evolutus, and D. rangi, and T. trigonula. According to the above species, Early Miocene (Aquitanian–Burdigalian) was determined for the Guri Member at this section. We have used Oligocene–Miocene biozonation of Asmari Formation in the Zagros basin for this section in SE Zagros basin. Four facies association (including A, B, C, D) are suggested for sedimentation of carbonate rocks of the Guri Member in study area. In a basin-ward trend, facies associations are interpreted as supratidal, lagoon, coral reef, and open sea that were precipitated in a ramp. This ramp in a seaward trend includes inner


ramp, middle ramp, and outer ramp. Carbonate rocks of Chahestan and Dorahi–Homag sections were deposited in two and three depositional sequences, respectively. Resting of the studied interval (marine facies) on siliciclastic (continental) sediments reflect a major transgression in sea level at Lower Miocene in study area and Zagros basin. The distribution and abundance of foraminifera reflect precipitation in tropical to subtropical, mesotrophic to oligotrophic, and eutrophic to oligotrophic conditions. According to large benthic foraminifera data, water temperature average was determined between 25 and 30 °C that is confirmed by analyzing oxygen and carbon stable isotopes. Finally, paleoecology and paleoenvironments and sea level changes were interpreted based on integration of petrographic, biostratigraphic, and stable isotopes studies that lead to modeling of study area at the Lower–Middle Miocene. Acknowledgments This paper is a part of the Ph.D dissertation by A. Heidari at Ferdowsi University of Mashhad and The University of Kansas. This study was supported by Ferdowsi University of Mashhad and The University of Kansas (USA). We would like to appreciate Fatemeh Zabihi and Narges Shokri from Ferdowsi University of Mashhad for recognition and pereparing foraminifera samples for study by SEM and Dr. Reazaei from Homozgan University. We are also thankful to Gregory Cane from University of Kansas Stable Isotope Laboratory and Mr. Javad Rabani, Hamed Mohammadian for their help during field work in Zagros Mountain. Marcelle Boudagher-Fadel, University College London, and Seyed Ali Moalemi, Reservoir Geology Division, Exploration and Production Department, Research Institute of Petroleum Industry, Tehran, Iran.

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