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Microfacies and depositional environment of the Cenomanian of the Bangestan anticline, SW Iran
Journal of Asian Earth Sciences 37 (2010) 275–285
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Microfacies and depositional environment of the Cenomanian of the Bangestan anticline, SW Iran Ali Ghabeishavi a, Hossein Vaziri-Moghaddam a,*, Azizolah Taheri b, Farid Taati c a
Department of Geology, Faculty of Sciences, University of Isfahan, Isfahan 81746-73441, Iran Geology Department, Faculty of Earth Science, Shahrood University of Technology, Shahrood, Iran c National Iranian Oil Company, Exploration Directorate, Tehran, Iran b
a r t i c l e
i n f o
Article history: Received 25 May 2008 Received in revised form 3 August 2009 Accepted 26 August 2009
Keywords: Bangestan anticline Sarvak Formation Sedimentary environment Cenomanian Zagros basin
a b s t r a c t The Sarvak Formation (Albian to Turonian in age) of the Zagros basin is a thick sequence of shallow-water carbonates. This work focuses on the microfacies and sedimentary environment of the margin of the Cenomanian intrashelf basin. In the study area (southwest of Iran), the Sarvak Formation is subdivided into 12 microfacies that are distinguished by petrographic analysis on the basis of their depositional textures and fauna. In addition, four major depositional environments were identified in the Sarvak Formation. These include shelf lagoon, platform margin, slope and basin environmental settings, which are interpreted as a carbonate shelf without an effective barrier separating the platform from the open ocean. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The Sarvak Formation, which is of Albian to Turonian age (Motiei, 1993; Wynd, 1965; James and Wynd, 1965), is one of the most prolific oil reservoirs in southwest Iran (Fig. 1). It was deposited on a carbonate platform developed across the elongated Zagros basin (Figs. 2 and 3). Studies of the Sarvak Formation have focused mainly on lithostratigraphy, biostratigraphy and palaeontology (e.g., Wynd, 1965; James and Wynd, 1965); however, detailed sedimentological and microfacies work is still needed. Sarvak sedimentation in the north and south of Khuzestan (Izeh and Dezful Embayment) took place under ‘‘shelf” conditions, while in the middle, it was separated by a ‘‘graben”-like feature in which the sedimentation of the thin-bedded oligostegina limestone accrued (Hart, 1970b) (Fig. 4). This ‘‘graben”-like feature was later interpreted as an intrashelf basin (Van-Buchem et al., 2006). During Aptian–Albian–Cenomanian times, the eastern part of the Arabian Plate (southwest Iran) was characterized by large intrashelf basins surrounded by shallow-water platforms. The sediments of the Sarvak Formation were deposited on platforms and within the intrashelf basin on the passive margin of the Arabian Plate (Ziegler, 2001). The accumulation of rudist debris at the shelf margin (around the intrashelf basin) of Sarvak is one of the oil prospects in the Izeh * Corresponding author. Tel.: +98 311 7932160; fax: +98 311 7932153. E-mail address: [email protected] (H. Vaziri-Moghaddam). 1367-9120/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2009.08.014
and Dezful Embayment. Thus, analyzing the facies distribution and sedimentary environments of the Sarvak shelf margin around the intrashelf basin has important implications. This study considers the Sarvak Formation in the Bangestan anticline (Fig. 3) in the northern margin of the Cenomanian intrashelf basin (Fig. 4). The aims of this paper are to describe and interpret the different microfacies using both field and petrographic observations and recognition of the depositional environment of the Sarvak Formation. To achieve these aims, one section at Tang-e Sarvak in the Bangestan anticline (the northern margin of the Cenomanian intrashelf basin, Sarvak type section) was selected. 2. Methods and study area Field and petrographic studies were carried out for facies analysis and palaeoenvironmental reconstruction of the Sarvak Formation. Facies definition was based on microfacies characteristics, including depositional texture, grain size, grain composition and fossil content. The classification of carbonate rocks followed the nomenclature of Dunham (1962) and Embry and Klovan (1971). The study area (Bangestan anticline) is located about 7 km from the town of Likak. A section was measured in detail at 30°58.980 N and 50°7.840 E (Fig. 3). More than 400 samples from the Sarvak Formation were studied. Some samples from the underlying Kazhdumi Formation were also analyzed for comparison.
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Fig. 1. Correlation chart of Cretaceous strata, Zagros basin, Iran (adapted from James and Wynd, 1965).
Fig. 2. Location of the study area. (A) General map of Iran showing eight geologic provinces. The Bangestan anticline is located in the Zagros (adapted from Heydari et al. (2003)). (B) Subdivisions of the Zagros province (adapted from Farzipour-Saein et al. (2009)).
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Fig. 3. Geological map of the studied section, Bangestan anticline, and southwest of Iran.
Fig. 4. Palaeogeography of the Cenomanian in the Dezful Embayment (adopted from Hart, 1970b). The Sarvak Formation in the north and south of Khuzestan (Izeh and Dezful Embayment) was deposited on a shallow shelf environment, while in the middle it was separated by a deeper environment (outer shelf). The direction of transition from the shallow environment to the deeper environment is from northeast to southwest.
3. Geological setting The NW-SE-trending Zagros fold-and-thrust belt extends for about 1800 km from the Taurus Mountains, about 300 km SE of the East Anatolian Fault in SE Turkey, through northern Iraq and SW Iran to the Strait of Hormuz, where the north–south-trending Oman Line separates the Zagros belt from the Makran accretionary prism (Falcon, 1974; Haynes and McQuillan, 1974). The Palaeo-Tethys Ocean was situated between the Laurussia (North America, Baltica and Siberia) and Gondwana (Africa, Arabia, Lut and other Iranian terranes) (Golonka, 2004). During Late Triassic–Early Jurassic times, several microplates were sutured to the Eurasian margin, closing the Palaeo-Tethys Ocean (Golonka, 2004).
The final closure of Palaeo-Tethys took place during the Triassic to Jurassic (Stampfli and Borel, 2002).The Neo-Tethys Ocean originated during the Permian as a result of the Carboniferous-earliest Permian rifting of the Cimmerian plates (Golonka et al., 1994; Sengör and Natalin, 1996). The evolution of the Zagros orogen is summarized by Alavi (2004) as: 1. Subduction of the Neo-Tethyan oceanic crust beneath the Iranian lithospheric block during the Early to Late Cretaceous. 2. Obduction of Neo-Tethyan ophiolite sheets over the Arabian continental margin in the Late Cretaceous (Turonian to Campanian).
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3. Collision of the Afro-Arabian continental margin with Iranian plates during the Late Cretaceous and later times. The sedimentary column in the Zagros is estimated to be up to 10 km thick (Alavi, 2004; Sherkati and Letouzey, 2004). These sediments were deposited in front of the evolving Zagros orogen (Alavi, 2004). North–south faults have controlled the facies and thickness of Phanerozoic sedimentary rock at least from the middle Cretaceous (Koop and Stoneley, 1982). The study area was a tectonically passive carbonate shelf from the Permian to about the Middle Cretaceous (Glennie, 2000). Development of the Zagros basin began in the Late Cretaceous as a result of fore-deep subsidence in front of the Zagros Suture (Haynes and McQuillan, 1974; Berberian and King, 1981; Alavi, 1994, 2004). During the Cenomanian, the study area was a part of the passive margin of the Arabian Plate (Ziegler, 2001; Alavi, 2004). The Zagros fold-and-thrust belt can be divided into a number of zones (Lurestan, Izeh, Dezful Embayment, Fars, High Zagros), which differ according to their structural style and sedimentary history (Berberian and King, 1981; Falcon, 1974; Motiei, 1993; Stöcklin, 1968). The study area is located at the boundary of the Dezful Embayment and Izeh zone (SW Iran) (Fig. 2). The Izeh zone lies across a sharp topographical break to the southwest of the High Zagros fault. This zone consists of a variety of structures of variable sizes and geometric characteristics (Sherkati and Letouzey, 2004). The boundary of the Izeh zone coincides with the Balarud, Kazerun, Mountain Front and High Zagros Faults. The Izeh zone and Dezful Embayment are separated by the Mountain Front Fault. In the southwest of the Mountain Front Fault, the Dezful Embayment corresponds to a low lying alluvial plain passing into dissected foothills generally less than 1000 m high and entirely covered by Tertiary sediments. It shows a sharp topographic difference with the Izeh zone across the Mountain Front Fault (Sherkati and Letouzey, 2004). The Izeh zone and Dezful Embayment are separated from Lurestan and Fars by the Balarud and Kazerun faults, respectively (Falcon, 1974; Motiei, 1993).
4. Lithostratigraphy The Sarvak Formation is interpreted to have been deposited in a shallow marine environment during the Albian to Turonian (Motiei, 1993), which passed into a lower-energy setting towards Fars and the Persian Gulf. However in the northwestern Lurestan area and toward Iraq, the Sarvak Formation interfingers with the Garau Formation. The nature of the upper boundary of the Sarvak Formation is variable (Fig. 1). The presence of conglomerates, breccia and iron-bearing sediments together with disconformities in the upper part of the Sarvak Formation are evidence of local uplift during the Late Cenomanian to Turonian. The lower boundary of the Sarvak Formation with the Kazhdumi Formation is conformable and gradational and is marked by a change from limestone (Sarvak Formation) to the shalier Kazhdumi Formation below (James and Wynd, 1965). According to James and Wynd (1965) and Setudehnia (1978), the type section in the study area is composed of the following three units: a lower 200-m thick fine-grained, nodular and thinto medium-bedded limestone with small ammonite impressions; a middle 120-m thick, massive limestone with chert nodules; and an upper 380-m thick, massive- to thick-bedded limestone with abundant rudist debris. In the lower parts of this unit, low angel clinoform is also present (Fig. 5). The Sarvak Formation in the studied section conformably overlies the Kazhdumi Formation with a transitional contact. Due to erosion or a lack of deposition, the Turonian sediments are not
Fig. 5. Clinoform at the studied section. The direction of the progradation is from northeast to southwest. (A) Outcrop photo of the clinoform. (B) Sketch of the clinoform at the studied section.
present in the studied section. The contact with the overlying marl and shale of the Gurpi Formation (Campanian–Maastrichtian) is sharp (Bourgeois, 1969; Hart, 1970a). 5. Biostratigraphy Wynd (1965) established a biostratigraphic zonation for the Sarvak Formation, which was reviewed by Khalili (1976). This revision showed a ‘‘general agreement” with the original zonation. This biozonation is commonly used in NIOC (National Iranian Oil Company). Based on the fauna, the Sarvak Formation was subdivided into three biostratigraphic zones (Fig. 6). 1. Assemblage 1 consists of: Favusella washitensis, Globigerineloides spp., Ticinella sp., oligosteginids, and Rotalipora sp. These microfauna correspond to the Favusella washitensis zone of Wynd (1965). Based on Wynd (1965) and Khalili (1976), the age of this biozone ranges from Albian to Lower Cenomanian. 2. Microfauna of this assemblage include Oligostegina (Bonetocardiella conoidea, Pithonella sphaerica, Pithonella ovalis, Pithonella trejoi) and Globigerineloides spp. This assemblage is correlated with the Oligostegina zone of Wynd (1965). The main development of this biozone occurs within the Albian to Turonian interval. Oligostegina still occur, but less frequently, throughout the Coniacian to Maastrichtian (Wynd, 1965; Khalili, 1976; Bolz, 1977). In the study area, this assemblage is attributed to the Cenomanian based on its stratigraphic position.
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Fig. 6. Biostratigraphy chart of the studied section. In the biozones column. 1: Favusella washitensis zone of Wynd (1965). 2: Oligostegina zone of Wynd (1965). 3: Nezzazataalveolinids assemblage zone of Wynd (1965).
3. This assemblage is characterized by the presence of Nezzazata spp., Praealveolina sp., Ovalveolina sp., Cisalveolina sp., Multispirina iranensis, Edomia sp., Taberina bingistani, Rabanitina sp., Dicyclina sp., Cuneolina sp., Dictyoconus sp., Orbitolina sp., Trocholina sp., textularids and miliolids. This assemblage is equivalent to the Nezzazata-alveolinids Assemblage zone of Wynd (1965). The age of this biozone is Cenomanian to Turonian (Wynd, 1965; Khalili, 1976). Bourgeois (1969) stated that the upper boundary of this biozone is limited to the uppermost Cenomanian. The upper boundary of this biozone is marked by an unconformity over almost the entire Iranian Zagros Basin. In the studied section, the top of this biozone is overlain by the Maastrichtian Gurpi Formation (Wynd, 1965; James and Wynd, 1965; Bourgeois, 1969). 6. Microfacies analysis The primary depositional features discernible in thin sections of the rock, including textures, microfossils and sedimentary structures, led to the recognition of 12 facies.
MF.1. Radiolaria packstone (Fig. 7A): This facies is dominated by radiolarian tests and is restricted to the upper part of the Kazhdumi Formation. Interpretation: This facies was deposited in an open marine environment, as indicated by abundant radiolaria as well as stratigraphic relationships with the MF.2. In accordance to the standard microfacies type described by Flügel (2004), MF.1 is interpreted to have been deposited below the storm wave base in a basin environment. MF.2. Planktonic foraminifera wackestone–packstone (Fig. 7B): The main components of this facies are planktonic foraminifera accompanied by oligosteginids. This microfacies is lime mud-dominated and lacks a shallow-water neritic fauna. Ammonites are the dominant macrofauna of this facies. Interpretation: The low-energy hydrodynamic regime indicates deposition below the normal wave base (Wilson, 1975; Geel, 2000; Flügel, 1982, 2004). The abundance of planktonic foraminifera, ammonites and the fine-grained matrix suggest an outer shelfbasin environment. MF.3. Oligosteginids wackestone–packstone (Fig. 7C): The predominant skeletal grains are oligosteginids and non-keeled planktonic
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Fig. 7. (A) Microfacies 1. Radiolaria packstone. (B) Microfacies 2. Planktonic foraminifera packstone. (C) Microfacies 3. Oligosteginids packstone. (D) Microfacies 4. Sponge spicules oligosteginids packstone. Abbreviations: Rad, radiolaria; Pl, planktonic foraminifera; Oli, oligosteginids; Sp, sponge spicules.
Fig. 8. (A) Microfacies 5. Echinoid oligosteginids packstone. (B) Microfacies 6. Peloidal packstone. (C) Microfacies 7. Fine-grained shell fragment (rudist or echinoid) packstone. (D) Microfacies 8. Rudist floatstone. Abbreviations: Oli, oligostegina; Ech, echinoid debris; Pelo, peloid; Rud, rudist debris.
foraminifera. Other bioclasts include small echinoid debris, sponge spicules, ostracods, unidentified small benthic foraminifera and very rare peloids. These deposits are abundant in the first 250 m
of the Sarvak Formation. Distinct chert nodules can be recognized in the field. They are restricted to upper part of this interval. The matrix is fine-grained micrite.
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Fig. 9. (A) Microfacies 9. Rudist grainstone. (B) Microfacies 9. Rudist grainstone with cortoid. (C) Microfacies 10. Benthic foraminifera rudist packstone. (D) Microfacies 11. High diversity benthic foraminifera wackestone. (E) Microfacies 11. High diversity benthic foraminifera packstone. (F) Microfacies 12. Low diversity benthic foraminifera wackestone. Abbreviations: Rud, rudist debris; Cor, cortoid; Mil, miliolids; Ben, benthic foraminifer; Nez, Nezzazata; Nezz, Nezzazatinella; Ost, ostracods; Orb, orbitolinids; Alv, alveolinids.
Interpretation: The presence of planktonic foraminifera and oligosteginids with very rare platform-derived material suggests a low-energy environment below the fair-weather wave base in a distal middle shelf to outer shelf. The abundance of oligosteginids and non-keeled planktonic foraminifers indicates eutrophic conditions (Arthur et al., 1987; Luciani and Cobianchi, 1999; Aguilara-Franco and Hernández Romano, 2004). MF.4. Sponge spicules oligosteginids wackestone (Fig. 7D): The components are dominated by oligosteginids and sponge spicules. Planktonic foraminifera, peloids, small echinoid debris, unidentified small benthic foraminifera and Nezzazata are subordinate. This facies is common in the lower part of the Sarvak Formation. Chert nodules are common in all intervals with this microfacies. Interpretation: This facies was deposited in a low- to mediumenergy, open marine environment. This interpretation is supported by the presence of oligosteginids, planktonic foraminifera, echinoids, sponge spicules and stratigraphic relationships with MF.3. Schulze et al. (2005) and Aguilara-Franco and Hernández Romano (2004) considered similar facies as representative of an open marine environment.
MF.5. Echinoid oligosteginids wackestone–packstone (Fig. 8A): This facies is found in the thick to massive beds of the lower part of the Sarvak Formation. It is dominated by oligostegina and echinoid debris. Undetermined small foraminifera, sponge spicules and peloids are also present. This microfacies has a fine-grained matrix. Interpretation: Faunal components, textural features, and stratigraphic relationships with the MF.3 suggest that this microfacies formed in a low- to medium-energy, open marine environment. MF.6. Peloidal packstone (Fig. 8B): Peloids are the most abundant components. Subordinate components are small benthic foraminifera, oligosteginids, non-keel planktonic foraminifera and shell fragments. This microfacies has a dark micritic matrix. This facies restricted to thick to massive beds of the lower part of the section that contains chert nodules. Interpretation: The biotic features, stratigraphic relationships with MF.5 and MF.7 (adjacent facies), sedimentological data (micritic matrix, absence of ooids or intraclasts) and abundance of peloids in this microfacies indicate that sedimentation took place in an open marine environment below the fair-weather wave base, with low-energy background conditions.
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Fig. 10. Depositional model of the Sarvak Formation at the Bangestan anticline. The interpretation is based on Flügel (2004). Abbreviations: FWWB, Fair-weather wave base; SWB, Storm wave base.
MF.7. Fine-grained shell fragment (rudist or echinoid) wackestone– packstone (Fig. 8C): The main characteristics of this microfacies are fine grains of rudist and echinoid debris, peloids and undifferentiated small benthic foraminifera in mud-supported textures. Minor particles include sponge spicules, oilgosteginids, gastropod debris, green algae, large benthic foraminifera (alveolinids and orbitolinids), Nezzazata and Nezzazatinella. The degree of fragmentation in the large benthic foraminifera is relatively high. Interpretation: The depositional environment is interpreted as the lower part of a carbonate slope. This interpretation is supported by the faunal components, wackestone–packstone texture, the small size, high degree of fragmentation of bioclasts and stratigraphic relationships with MF.6 and MF.8. This facies reflects an off shore transport rudist into distal middle shelf environment. Rudists obviously derived from the platform margin. MF.8. Rudist floatstone (Fig. 8D): Large rudist fragments and echinoid debris are the main components. The non-skeletal components consist of peloids. Subordinate components include cortoids; benthic foraminifera include alveolinids, orbitolinids, miliolids, Nezzazata, small Rotalia and gastropods. The degree of fragmentation and micritization in the benthic foraminifers is relatively high. Textures are floatstone with a bioclastic wackestone– packstone matrix. Interpretation: The faunal components, textural features, stratigraphic relationship with MF.7 and the reworked characteristics of the rudist fragments suggest that this microfacies formed in an upper slope environment under low to medium-energy. MF.9. Rudist grainstone (Fig. 9A and B): This facies is dominated by large rudist fragments. Subordinate components are peloids, echinoid debris, cortoids, small benthic foraminifera and intraclasts. Large benthic foraminifera such as alveolinids and orbitolinids are very rare. Fragmentation of the larger foraminifera is high. Interpretation: Due to the dominance of coarse and thick shelled rudists, grain-supported texture, well-sorted grains and a lack of lime mud, this facies is interpreted to have been deposited in high energy environments above the fair-weather wave base. In accordance with the standard microfacies types described by Wilson (1975) and Flügel (2004), MF.9 is interpreted as a shoal environment that was located at the platform margin, separating the open marine from the more restricted marine environment. MF.10. Benthic foraminifera rudist wackestone–packstone (Fig. 9C): This facies is characterized by the occurrence of rudist debris and benthic foraminifera. Benthic foraminifera include miliolids, alveolinids, orbitolinids and Nezzazata. Rare peloids, aggregate grains,
bivalves, gastropods, green algae, echinoids and sponge spicules are also present. Interpretation: The stratigraphic relationships with MF.8, co-occurrence of platform margin bioclasts and lagoonal biota suggest deposition at the lagoonal end of a platform margin. MF.11. High diversity benthic foraminifera wackestone–packstone (Fig. 9D and E): The main characteristic of this microfacies is the diverse benthic foraminifera in mud-supported textures. Benthic foraminifera include miliolids, alveolinids, orbitolinids and Nezzazata. Rare to common green algae are also present. Other components such as echinoids, rudists, sponge spicules, peloids, gastropods and bivalves are subordinate. Interpretation: This facies represents deposition in an open lagoon, low-energy environment, as indicated by the fine grain size (textures), stratigraphic relationships with MF.10 and MF.11 and diverse faunal association of the benthic foraminifera. The diversity of the fauna shows that the primary environment had good water circulation and normal salinity and oxygen content within the water column and the sediment surface. The existence of green algae indicates good aeration and light penetration (Zhicheng et al., 1997). MF.12. Low diversity benthic foraminifera wackestone (Fig. 9F): This facies is characterized by the dominant presence of small benthic foraminifera (miliolids and Nezzazata). Other components such as gastropods, shell fragments, green algae and sponge spicules are subordinate. Rare peloids are present. The matrix is fine-grained micrite. Interpretation: This facies was deposited in restricted low-energy lagoonal environments, as indicated by low-diversity skeletal fauna and the stratigraphic position. The limited diversity of bioclasts and dominance of micrites indicate deposition in a low-energy lagoonal environment with poor connection with the open marine environment. The low biotic diversity of the foraminifera indicates a high-stressed habitat in very shallow restricted areas, where great fluctuations in salinity and temperature probably occurred. 7. Sedimentary environment The Sarvak Formation represents sedimentation on a carbonate shelf on the basis of the distribution of the biota, textures and vertical facies relationships. The carbonate shelf environments are separated into: (1) the inner shelf, (2) the middle shelf and (3) the outer shelf (Flügel, 2004) (Fig. 10 and 11).
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Fig. 11. Stratigraphic column of the studied section in the Bangestan anticline. In the columns facies and sedimentary environment, numbered 1–12, refer to microfacies cods. See the text for explanation of microfacies 1–12. See Fig. 6 for explanation of the column biozones.
The inner shelf facies types are highly variable but contain abundant imperforated tests of foraminifera (e.g., miliolids, alveolinids and Nezzazata), dasycladacean and gastropods. The inner
shelf deposits represent a wider spectrum of marginal marine deposits indicating open lagoon and protected lagoon conditions. Open shallow subtidal environments are characterized by
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microfacies types (MF.10 and MF.11) that include high-diversity of benthic foraminifers, dasycladacean and rudist fragments. The fauna and flora support the interpretation of this association as deposited in warm, euphotic, shallow water under low-energy conditions, in an inner shelf setting. Restricted shallow subtidal environments of deposition are characterized by low-diversity benthic foraminiferal assemblages (MF.12). The foraminiferal associations are commonly dominated by imperforated foraminifera. Restricted conditions are suggested by lack of normal marine biota and the presence of restricted biota (imperforated foraminifera) (Reiss and Hottinger, 1984; Hottinger, 1997). The platform margin is represented by rudist grainstone. Rudist grainstone is interpreted to represent a shoal in a shallow subtidal zone, characterized by the winnowing of coarse-grained and sorted rudist fragments. The predominantly coarse and well-sorted grain size indicates deposition in a well-circulated environment in a shallow subtidal zone (Schulze et al., 2005). The middle shelf can be divided into a proximal and a distal middle shelf. The proximal middle shelf is characterized by coarse grained skeletal packstone-floatstones (MF.8). Skeletal grains are dominantly rudist and echinoid fragments. Deposition took place in shallow water near the fair-weather wave base. The distal middle shelf sediments are dominated by fine-grained skeletal wackestone–packstones (oligosteginids, fine-grained rudists and echinoids fragments) (MF.7). The distal middle shelf facies are differentiated from the proximal middle shelf by the smaller size of components, the greater amount of micritic matrix and the presence of oligosteginids. Indicators of the outer shelf deeper water facies are high amounts of intact tests of planktonic foraminifers. Abundant oligosteginids, siliceous sponge spicules and ammonites (MF.2–6) also indicate deeper water and fully marine conditions. The chert nodules are common in the proximal part of outer shelf. The basin is characterized by abundant oligosteginids, planktonic foraminifers and ammonites (MF.1 and MF.2). Based on the direction of progradation (Fig. 5) and the palaeogeographic map of Hart (1970b) (Fig. 4), the direction of transition from the shallow marine environment to a deeper marine environment is concluded to be from northeast to southwest (Fig. 10) 8. Conclusion The sedimentological analysis shows that the Sarvak Formation was formed by a carbonate shelf bordering an intrashelf basin, with abundant rudists in the mid-shelf environment and pelagic facies (oligostegina, radiolaria and planktonic foraminifera) in the outer shelf and basin environments. The sedimentation of the Sarvak Formation took place on a shallow carbonate shelf setting, in a facies belt consisting of an inner shelf, middle shelf, outer shelf and basin. In the inner shelf, the most abundant microfacies are wackestone–packstone with benthic foraminifera (such as alveolinids, orbitolinids and miliolids) and rudist fragments. The shoal facies are marked by rudist grainstone. The proximal middle shelf is dominated by floatstones with large rudist fragments, while wackestone–packstone with sponge spicules, oligosteginids and fine-grained rudists and echinoids are present in the distal middle shelf. The outer shelf is represented by wackestone–packstones with oligostegina and planktonic foraminifera. The basin environment is dominated by radiolaria packstone. Acknowledgments The authors wish to thank the reviewers for their helpful and constructive comments. The manuscript has been significantly improved by comments from Prof Dr Boris Natalin. We also would
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Plate tectonic evolution of the southern margin of Eurasia in the Mesozoic and Cenozoic. Tectonophysics 381, 235–273. Golonka, J., Ross, M.I., Scotese, C.R., 1994. Phanerozoic paleogeographic and paleoclimatic modeling maps. In: Embry, A.F., Beauchamp, B., Glass, D.J. (Eds.), Pangea: Global Environment and Resources, Memoir-Canadian Society of Petroleum Geologists, vol. 17. pp. 1–47. Hart, B.B., 1970a. The Kuh-e Bangestan/Kuh-e Safid stratigraphical survey, Iranian Oil Operating Companies Report 1162. unpublished. Hart, B.B., 1970b. Upper Cretaceous palaeogeography, structural history and prospect of the Khuzestan province. Iranian Oil Operating Companies Report 1162. Unpublished Haynes, S.J., McQuillan, H., 1974. Evolution of the Zagros Suture Zone, Southern Iran. Geological Society of America Bulletin 85, 739–744. Heydari, E., Hassanzadeh, J., Wade, W.J., Ghazi, A.M., 2003. Permian-Triassic boundary interval in the Abadeh section of Iran with implications for mass extinction. Part 1. Sedimentology. Palaeogeography, Palaeoclimatology, Palaeoecology 193, 405–423. Hottinger, L., 1997. Shallow benthic foraminiferal assemblages as signals for depth of their deposition and their limitations. Bulletin de la Societe Geologique de France 168, 491–505. James, G.A., Wynd, J.G., 1965. Stratigraphic nomenclature of Iranian oil consortium, agreement area. American Association of Petroleum Geologists Bulletin 49, 2182–2245. Khalili, M., 1976. The biostratigraphic synthesis of the Bangestan Group in southwest Iran. Iranian Oil Operating Companies Report 1219. unpublished. Koop, W.J., Stoneley, R., 1982. Subsidence history of the Middle East Zagros Basin, Permian to Recent. Philosophical Transactions of the Royal Society of London Series A 305, 149–168. Luciani, V., Cobianchi, M., 1999. The Bonarelli Level and other black shales in the Cenomanian–Turonian of the northeastern Dolomites (Italy): calcareous nannofossil and foraminiferal data. Cretaceous Research 20, 135–167. Motiei, H., 1993. Treatise on the geology of Iran: Stratigraphy of Zagros. Geological Survey of Iran, Tehran, 497p (in Persian). Reiss, Z., Hottinger, L., 1984. The Gulf of Aqaba: Ecological Micropaleontology. Ecological Studies 50. Springer-Verlag, Berlin. 354 p. Schulze, F., Kuss, J., Marzouk, A., 2005. Platform configuration, microfacies and cyclicities of the upper Albian to Turonian of west-central Jordan. Facies 50, 505–527.
A. Ghabeishavi et al. / Journal of Asian Earth Sciences 37 (2010) 275–285 Sengör, A.M.C., Natalin, B.A., 1996. Paleotectonics of Asia: fragment of a synthesis. In: Yin, Ann., Harrison, T.M. (Eds.), The Tectonic Evolution of Asia. Cambridge University Press, pp. 486–640. Setudehnia, A., 1978. The Mesozoic sequence in southwest Iran and adjacent area. Journal of Petroleum Geology 1, 3–42. Sherkati, S., Letouzey, J., 2004. Variation of structural style and basin evolution in the central Zagros (Izeh zone and Dezful Embayment, Iran). Marine Petroleum Geology 21, 535–554. Stampfli, G.M., Borel, G.D., 2002. A plate tectonic model for the paleozoic and mesozoic constrained by dynamic plate boundaries and restored synthetic oceanic isochrones. Earth and Planetary Science Letters 196, 17–33. Stöcklin, J., 1968. Structural history and tectonics of Iran, a review. American Association of Petroleum Geology Bulletin 52, 1229–1258.
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Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes
Microfacies and depositional environment of the Cenomanian of the Bangestan anticline, SW Iran Ali Ghabeishavi a, Hossein Vaziri-Moghaddam a,*, Azizolah Taheri b, Farid Taati c a
Department of Geology, Faculty of Sciences, University of Isfahan, Isfahan 81746-73441, Iran Geology Department, Faculty of Earth Science, Shahrood University of Technology, Shahrood, Iran c National Iranian Oil Company, Exploration Directorate, Tehran, Iran b
a r t i c l e
i n f o
Article history: Received 25 May 2008 Received in revised form 3 August 2009 Accepted 26 August 2009
Keywords: Bangestan anticline Sarvak Formation Sedimentary environment Cenomanian Zagros basin
a b s t r a c t The Sarvak Formation (Albian to Turonian in age) of the Zagros basin is a thick sequence of shallow-water carbonates. This work focuses on the microfacies and sedimentary environment of the margin of the Cenomanian intrashelf basin. In the study area (southwest of Iran), the Sarvak Formation is subdivided into 12 microfacies that are distinguished by petrographic analysis on the basis of their depositional textures and fauna. In addition, four major depositional environments were identified in the Sarvak Formation. These include shelf lagoon, platform margin, slope and basin environmental settings, which are interpreted as a carbonate shelf without an effective barrier separating the platform from the open ocean. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The Sarvak Formation, which is of Albian to Turonian age (Motiei, 1993; Wynd, 1965; James and Wynd, 1965), is one of the most prolific oil reservoirs in southwest Iran (Fig. 1). It was deposited on a carbonate platform developed across the elongated Zagros basin (Figs. 2 and 3). Studies of the Sarvak Formation have focused mainly on lithostratigraphy, biostratigraphy and palaeontology (e.g., Wynd, 1965; James and Wynd, 1965); however, detailed sedimentological and microfacies work is still needed. Sarvak sedimentation in the north and south of Khuzestan (Izeh and Dezful Embayment) took place under ‘‘shelf” conditions, while in the middle, it was separated by a ‘‘graben”-like feature in which the sedimentation of the thin-bedded oligostegina limestone accrued (Hart, 1970b) (Fig. 4). This ‘‘graben”-like feature was later interpreted as an intrashelf basin (Van-Buchem et al., 2006). During Aptian–Albian–Cenomanian times, the eastern part of the Arabian Plate (southwest Iran) was characterized by large intrashelf basins surrounded by shallow-water platforms. The sediments of the Sarvak Formation were deposited on platforms and within the intrashelf basin on the passive margin of the Arabian Plate (Ziegler, 2001). The accumulation of rudist debris at the shelf margin (around the intrashelf basin) of Sarvak is one of the oil prospects in the Izeh * Corresponding author. Tel.: +98 311 7932160; fax: +98 311 7932153. E-mail address: [email protected] (H. Vaziri-Moghaddam). 1367-9120/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2009.08.014
and Dezful Embayment. Thus, analyzing the facies distribution and sedimentary environments of the Sarvak shelf margin around the intrashelf basin has important implications. This study considers the Sarvak Formation in the Bangestan anticline (Fig. 3) in the northern margin of the Cenomanian intrashelf basin (Fig. 4). The aims of this paper are to describe and interpret the different microfacies using both field and petrographic observations and recognition of the depositional environment of the Sarvak Formation. To achieve these aims, one section at Tang-e Sarvak in the Bangestan anticline (the northern margin of the Cenomanian intrashelf basin, Sarvak type section) was selected. 2. Methods and study area Field and petrographic studies were carried out for facies analysis and palaeoenvironmental reconstruction of the Sarvak Formation. Facies definition was based on microfacies characteristics, including depositional texture, grain size, grain composition and fossil content. The classification of carbonate rocks followed the nomenclature of Dunham (1962) and Embry and Klovan (1971). The study area (Bangestan anticline) is located about 7 km from the town of Likak. A section was measured in detail at 30°58.980 N and 50°7.840 E (Fig. 3). More than 400 samples from the Sarvak Formation were studied. Some samples from the underlying Kazhdumi Formation were also analyzed for comparison.
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Fig. 1. Correlation chart of Cretaceous strata, Zagros basin, Iran (adapted from James and Wynd, 1965).
Fig. 2. Location of the study area. (A) General map of Iran showing eight geologic provinces. The Bangestan anticline is located in the Zagros (adapted from Heydari et al. (2003)). (B) Subdivisions of the Zagros province (adapted from Farzipour-Saein et al. (2009)).
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Fig. 3. Geological map of the studied section, Bangestan anticline, and southwest of Iran.
Fig. 4. Palaeogeography of the Cenomanian in the Dezful Embayment (adopted from Hart, 1970b). The Sarvak Formation in the north and south of Khuzestan (Izeh and Dezful Embayment) was deposited on a shallow shelf environment, while in the middle it was separated by a deeper environment (outer shelf). The direction of transition from the shallow environment to the deeper environment is from northeast to southwest.
3. Geological setting The NW-SE-trending Zagros fold-and-thrust belt extends for about 1800 km from the Taurus Mountains, about 300 km SE of the East Anatolian Fault in SE Turkey, through northern Iraq and SW Iran to the Strait of Hormuz, where the north–south-trending Oman Line separates the Zagros belt from the Makran accretionary prism (Falcon, 1974; Haynes and McQuillan, 1974). The Palaeo-Tethys Ocean was situated between the Laurussia (North America, Baltica and Siberia) and Gondwana (Africa, Arabia, Lut and other Iranian terranes) (Golonka, 2004). During Late Triassic–Early Jurassic times, several microplates were sutured to the Eurasian margin, closing the Palaeo-Tethys Ocean (Golonka, 2004).
The final closure of Palaeo-Tethys took place during the Triassic to Jurassic (Stampfli and Borel, 2002).The Neo-Tethys Ocean originated during the Permian as a result of the Carboniferous-earliest Permian rifting of the Cimmerian plates (Golonka et al., 1994; Sengör and Natalin, 1996). The evolution of the Zagros orogen is summarized by Alavi (2004) as: 1. Subduction of the Neo-Tethyan oceanic crust beneath the Iranian lithospheric block during the Early to Late Cretaceous. 2. Obduction of Neo-Tethyan ophiolite sheets over the Arabian continental margin in the Late Cretaceous (Turonian to Campanian).
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3. Collision of the Afro-Arabian continental margin with Iranian plates during the Late Cretaceous and later times. The sedimentary column in the Zagros is estimated to be up to 10 km thick (Alavi, 2004; Sherkati and Letouzey, 2004). These sediments were deposited in front of the evolving Zagros orogen (Alavi, 2004). North–south faults have controlled the facies and thickness of Phanerozoic sedimentary rock at least from the middle Cretaceous (Koop and Stoneley, 1982). The study area was a tectonically passive carbonate shelf from the Permian to about the Middle Cretaceous (Glennie, 2000). Development of the Zagros basin began in the Late Cretaceous as a result of fore-deep subsidence in front of the Zagros Suture (Haynes and McQuillan, 1974; Berberian and King, 1981; Alavi, 1994, 2004). During the Cenomanian, the study area was a part of the passive margin of the Arabian Plate (Ziegler, 2001; Alavi, 2004). The Zagros fold-and-thrust belt can be divided into a number of zones (Lurestan, Izeh, Dezful Embayment, Fars, High Zagros), which differ according to their structural style and sedimentary history (Berberian and King, 1981; Falcon, 1974; Motiei, 1993; Stöcklin, 1968). The study area is located at the boundary of the Dezful Embayment and Izeh zone (SW Iran) (Fig. 2). The Izeh zone lies across a sharp topographical break to the southwest of the High Zagros fault. This zone consists of a variety of structures of variable sizes and geometric characteristics (Sherkati and Letouzey, 2004). The boundary of the Izeh zone coincides with the Balarud, Kazerun, Mountain Front and High Zagros Faults. The Izeh zone and Dezful Embayment are separated by the Mountain Front Fault. In the southwest of the Mountain Front Fault, the Dezful Embayment corresponds to a low lying alluvial plain passing into dissected foothills generally less than 1000 m high and entirely covered by Tertiary sediments. It shows a sharp topographic difference with the Izeh zone across the Mountain Front Fault (Sherkati and Letouzey, 2004). The Izeh zone and Dezful Embayment are separated from Lurestan and Fars by the Balarud and Kazerun faults, respectively (Falcon, 1974; Motiei, 1993).
4. Lithostratigraphy The Sarvak Formation is interpreted to have been deposited in a shallow marine environment during the Albian to Turonian (Motiei, 1993), which passed into a lower-energy setting towards Fars and the Persian Gulf. However in the northwestern Lurestan area and toward Iraq, the Sarvak Formation interfingers with the Garau Formation. The nature of the upper boundary of the Sarvak Formation is variable (Fig. 1). The presence of conglomerates, breccia and iron-bearing sediments together with disconformities in the upper part of the Sarvak Formation are evidence of local uplift during the Late Cenomanian to Turonian. The lower boundary of the Sarvak Formation with the Kazhdumi Formation is conformable and gradational and is marked by a change from limestone (Sarvak Formation) to the shalier Kazhdumi Formation below (James and Wynd, 1965). According to James and Wynd (1965) and Setudehnia (1978), the type section in the study area is composed of the following three units: a lower 200-m thick fine-grained, nodular and thinto medium-bedded limestone with small ammonite impressions; a middle 120-m thick, massive limestone with chert nodules; and an upper 380-m thick, massive- to thick-bedded limestone with abundant rudist debris. In the lower parts of this unit, low angel clinoform is also present (Fig. 5). The Sarvak Formation in the studied section conformably overlies the Kazhdumi Formation with a transitional contact. Due to erosion or a lack of deposition, the Turonian sediments are not
Fig. 5. Clinoform at the studied section. The direction of the progradation is from northeast to southwest. (A) Outcrop photo of the clinoform. (B) Sketch of the clinoform at the studied section.
present in the studied section. The contact with the overlying marl and shale of the Gurpi Formation (Campanian–Maastrichtian) is sharp (Bourgeois, 1969; Hart, 1970a). 5. Biostratigraphy Wynd (1965) established a biostratigraphic zonation for the Sarvak Formation, which was reviewed by Khalili (1976). This revision showed a ‘‘general agreement” with the original zonation. This biozonation is commonly used in NIOC (National Iranian Oil Company). Based on the fauna, the Sarvak Formation was subdivided into three biostratigraphic zones (Fig. 6). 1. Assemblage 1 consists of: Favusella washitensis, Globigerineloides spp., Ticinella sp., oligosteginids, and Rotalipora sp. These microfauna correspond to the Favusella washitensis zone of Wynd (1965). Based on Wynd (1965) and Khalili (1976), the age of this biozone ranges from Albian to Lower Cenomanian. 2. Microfauna of this assemblage include Oligostegina (Bonetocardiella conoidea, Pithonella sphaerica, Pithonella ovalis, Pithonella trejoi) and Globigerineloides spp. This assemblage is correlated with the Oligostegina zone of Wynd (1965). The main development of this biozone occurs within the Albian to Turonian interval. Oligostegina still occur, but less frequently, throughout the Coniacian to Maastrichtian (Wynd, 1965; Khalili, 1976; Bolz, 1977). In the study area, this assemblage is attributed to the Cenomanian based on its stratigraphic position.
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Fig. 6. Biostratigraphy chart of the studied section. In the biozones column. 1: Favusella washitensis zone of Wynd (1965). 2: Oligostegina zone of Wynd (1965). 3: Nezzazataalveolinids assemblage zone of Wynd (1965).
3. This assemblage is characterized by the presence of Nezzazata spp., Praealveolina sp., Ovalveolina sp., Cisalveolina sp., Multispirina iranensis, Edomia sp., Taberina bingistani, Rabanitina sp., Dicyclina sp., Cuneolina sp., Dictyoconus sp., Orbitolina sp., Trocholina sp., textularids and miliolids. This assemblage is equivalent to the Nezzazata-alveolinids Assemblage zone of Wynd (1965). The age of this biozone is Cenomanian to Turonian (Wynd, 1965; Khalili, 1976). Bourgeois (1969) stated that the upper boundary of this biozone is limited to the uppermost Cenomanian. The upper boundary of this biozone is marked by an unconformity over almost the entire Iranian Zagros Basin. In the studied section, the top of this biozone is overlain by the Maastrichtian Gurpi Formation (Wynd, 1965; James and Wynd, 1965; Bourgeois, 1969). 6. Microfacies analysis The primary depositional features discernible in thin sections of the rock, including textures, microfossils and sedimentary structures, led to the recognition of 12 facies.
MF.1. Radiolaria packstone (Fig. 7A): This facies is dominated by radiolarian tests and is restricted to the upper part of the Kazhdumi Formation. Interpretation: This facies was deposited in an open marine environment, as indicated by abundant radiolaria as well as stratigraphic relationships with the MF.2. In accordance to the standard microfacies type described by Flügel (2004), MF.1 is interpreted to have been deposited below the storm wave base in a basin environment. MF.2. Planktonic foraminifera wackestone–packstone (Fig. 7B): The main components of this facies are planktonic foraminifera accompanied by oligosteginids. This microfacies is lime mud-dominated and lacks a shallow-water neritic fauna. Ammonites are the dominant macrofauna of this facies. Interpretation: The low-energy hydrodynamic regime indicates deposition below the normal wave base (Wilson, 1975; Geel, 2000; Flügel, 1982, 2004). The abundance of planktonic foraminifera, ammonites and the fine-grained matrix suggest an outer shelfbasin environment. MF.3. Oligosteginids wackestone–packstone (Fig. 7C): The predominant skeletal grains are oligosteginids and non-keeled planktonic
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Fig. 7. (A) Microfacies 1. Radiolaria packstone. (B) Microfacies 2. Planktonic foraminifera packstone. (C) Microfacies 3. Oligosteginids packstone. (D) Microfacies 4. Sponge spicules oligosteginids packstone. Abbreviations: Rad, radiolaria; Pl, planktonic foraminifera; Oli, oligosteginids; Sp, sponge spicules.
Fig. 8. (A) Microfacies 5. Echinoid oligosteginids packstone. (B) Microfacies 6. Peloidal packstone. (C) Microfacies 7. Fine-grained shell fragment (rudist or echinoid) packstone. (D) Microfacies 8. Rudist floatstone. Abbreviations: Oli, oligostegina; Ech, echinoid debris; Pelo, peloid; Rud, rudist debris.
foraminifera. Other bioclasts include small echinoid debris, sponge spicules, ostracods, unidentified small benthic foraminifera and very rare peloids. These deposits are abundant in the first 250 m
of the Sarvak Formation. Distinct chert nodules can be recognized in the field. They are restricted to upper part of this interval. The matrix is fine-grained micrite.
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Fig. 9. (A) Microfacies 9. Rudist grainstone. (B) Microfacies 9. Rudist grainstone with cortoid. (C) Microfacies 10. Benthic foraminifera rudist packstone. (D) Microfacies 11. High diversity benthic foraminifera wackestone. (E) Microfacies 11. High diversity benthic foraminifera packstone. (F) Microfacies 12. Low diversity benthic foraminifera wackestone. Abbreviations: Rud, rudist debris; Cor, cortoid; Mil, miliolids; Ben, benthic foraminifer; Nez, Nezzazata; Nezz, Nezzazatinella; Ost, ostracods; Orb, orbitolinids; Alv, alveolinids.
Interpretation: The presence of planktonic foraminifera and oligosteginids with very rare platform-derived material suggests a low-energy environment below the fair-weather wave base in a distal middle shelf to outer shelf. The abundance of oligosteginids and non-keeled planktonic foraminifers indicates eutrophic conditions (Arthur et al., 1987; Luciani and Cobianchi, 1999; Aguilara-Franco and Hernández Romano, 2004). MF.4. Sponge spicules oligosteginids wackestone (Fig. 7D): The components are dominated by oligosteginids and sponge spicules. Planktonic foraminifera, peloids, small echinoid debris, unidentified small benthic foraminifera and Nezzazata are subordinate. This facies is common in the lower part of the Sarvak Formation. Chert nodules are common in all intervals with this microfacies. Interpretation: This facies was deposited in a low- to mediumenergy, open marine environment. This interpretation is supported by the presence of oligosteginids, planktonic foraminifera, echinoids, sponge spicules and stratigraphic relationships with MF.3. Schulze et al. (2005) and Aguilara-Franco and Hernández Romano (2004) considered similar facies as representative of an open marine environment.
MF.5. Echinoid oligosteginids wackestone–packstone (Fig. 8A): This facies is found in the thick to massive beds of the lower part of the Sarvak Formation. It is dominated by oligostegina and echinoid debris. Undetermined small foraminifera, sponge spicules and peloids are also present. This microfacies has a fine-grained matrix. Interpretation: Faunal components, textural features, and stratigraphic relationships with the MF.3 suggest that this microfacies formed in a low- to medium-energy, open marine environment. MF.6. Peloidal packstone (Fig. 8B): Peloids are the most abundant components. Subordinate components are small benthic foraminifera, oligosteginids, non-keel planktonic foraminifera and shell fragments. This microfacies has a dark micritic matrix. This facies restricted to thick to massive beds of the lower part of the section that contains chert nodules. Interpretation: The biotic features, stratigraphic relationships with MF.5 and MF.7 (adjacent facies), sedimentological data (micritic matrix, absence of ooids or intraclasts) and abundance of peloids in this microfacies indicate that sedimentation took place in an open marine environment below the fair-weather wave base, with low-energy background conditions.
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Fig. 10. Depositional model of the Sarvak Formation at the Bangestan anticline. The interpretation is based on Flügel (2004). Abbreviations: FWWB, Fair-weather wave base; SWB, Storm wave base.
MF.7. Fine-grained shell fragment (rudist or echinoid) wackestone– packstone (Fig. 8C): The main characteristics of this microfacies are fine grains of rudist and echinoid debris, peloids and undifferentiated small benthic foraminifera in mud-supported textures. Minor particles include sponge spicules, oilgosteginids, gastropod debris, green algae, large benthic foraminifera (alveolinids and orbitolinids), Nezzazata and Nezzazatinella. The degree of fragmentation in the large benthic foraminifera is relatively high. Interpretation: The depositional environment is interpreted as the lower part of a carbonate slope. This interpretation is supported by the faunal components, wackestone–packstone texture, the small size, high degree of fragmentation of bioclasts and stratigraphic relationships with MF.6 and MF.8. This facies reflects an off shore transport rudist into distal middle shelf environment. Rudists obviously derived from the platform margin. MF.8. Rudist floatstone (Fig. 8D): Large rudist fragments and echinoid debris are the main components. The non-skeletal components consist of peloids. Subordinate components include cortoids; benthic foraminifera include alveolinids, orbitolinids, miliolids, Nezzazata, small Rotalia and gastropods. The degree of fragmentation and micritization in the benthic foraminifers is relatively high. Textures are floatstone with a bioclastic wackestone– packstone matrix. Interpretation: The faunal components, textural features, stratigraphic relationship with MF.7 and the reworked characteristics of the rudist fragments suggest that this microfacies formed in an upper slope environment under low to medium-energy. MF.9. Rudist grainstone (Fig. 9A and B): This facies is dominated by large rudist fragments. Subordinate components are peloids, echinoid debris, cortoids, small benthic foraminifera and intraclasts. Large benthic foraminifera such as alveolinids and orbitolinids are very rare. Fragmentation of the larger foraminifera is high. Interpretation: Due to the dominance of coarse and thick shelled rudists, grain-supported texture, well-sorted grains and a lack of lime mud, this facies is interpreted to have been deposited in high energy environments above the fair-weather wave base. In accordance with the standard microfacies types described by Wilson (1975) and Flügel (2004), MF.9 is interpreted as a shoal environment that was located at the platform margin, separating the open marine from the more restricted marine environment. MF.10. Benthic foraminifera rudist wackestone–packstone (Fig. 9C): This facies is characterized by the occurrence of rudist debris and benthic foraminifera. Benthic foraminifera include miliolids, alveolinids, orbitolinids and Nezzazata. Rare peloids, aggregate grains,
bivalves, gastropods, green algae, echinoids and sponge spicules are also present. Interpretation: The stratigraphic relationships with MF.8, co-occurrence of platform margin bioclasts and lagoonal biota suggest deposition at the lagoonal end of a platform margin. MF.11. High diversity benthic foraminifera wackestone–packstone (Fig. 9D and E): The main characteristic of this microfacies is the diverse benthic foraminifera in mud-supported textures. Benthic foraminifera include miliolids, alveolinids, orbitolinids and Nezzazata. Rare to common green algae are also present. Other components such as echinoids, rudists, sponge spicules, peloids, gastropods and bivalves are subordinate. Interpretation: This facies represents deposition in an open lagoon, low-energy environment, as indicated by the fine grain size (textures), stratigraphic relationships with MF.10 and MF.11 and diverse faunal association of the benthic foraminifera. The diversity of the fauna shows that the primary environment had good water circulation and normal salinity and oxygen content within the water column and the sediment surface. The existence of green algae indicates good aeration and light penetration (Zhicheng et al., 1997). MF.12. Low diversity benthic foraminifera wackestone (Fig. 9F): This facies is characterized by the dominant presence of small benthic foraminifera (miliolids and Nezzazata). Other components such as gastropods, shell fragments, green algae and sponge spicules are subordinate. Rare peloids are present. The matrix is fine-grained micrite. Interpretation: This facies was deposited in restricted low-energy lagoonal environments, as indicated by low-diversity skeletal fauna and the stratigraphic position. The limited diversity of bioclasts and dominance of micrites indicate deposition in a low-energy lagoonal environment with poor connection with the open marine environment. The low biotic diversity of the foraminifera indicates a high-stressed habitat in very shallow restricted areas, where great fluctuations in salinity and temperature probably occurred. 7. Sedimentary environment The Sarvak Formation represents sedimentation on a carbonate shelf on the basis of the distribution of the biota, textures and vertical facies relationships. The carbonate shelf environments are separated into: (1) the inner shelf, (2) the middle shelf and (3) the outer shelf (Flügel, 2004) (Fig. 10 and 11).
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Fig. 11. Stratigraphic column of the studied section in the Bangestan anticline. In the columns facies and sedimentary environment, numbered 1–12, refer to microfacies cods. See the text for explanation of microfacies 1–12. See Fig. 6 for explanation of the column biozones.
The inner shelf facies types are highly variable but contain abundant imperforated tests of foraminifera (e.g., miliolids, alveolinids and Nezzazata), dasycladacean and gastropods. The inner
shelf deposits represent a wider spectrum of marginal marine deposits indicating open lagoon and protected lagoon conditions. Open shallow subtidal environments are characterized by
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microfacies types (MF.10 and MF.11) that include high-diversity of benthic foraminifers, dasycladacean and rudist fragments. The fauna and flora support the interpretation of this association as deposited in warm, euphotic, shallow water under low-energy conditions, in an inner shelf setting. Restricted shallow subtidal environments of deposition are characterized by low-diversity benthic foraminiferal assemblages (MF.12). The foraminiferal associations are commonly dominated by imperforated foraminifera. Restricted conditions are suggested by lack of normal marine biota and the presence of restricted biota (imperforated foraminifera) (Reiss and Hottinger, 1984; Hottinger, 1997). The platform margin is represented by rudist grainstone. Rudist grainstone is interpreted to represent a shoal in a shallow subtidal zone, characterized by the winnowing of coarse-grained and sorted rudist fragments. The predominantly coarse and well-sorted grain size indicates deposition in a well-circulated environment in a shallow subtidal zone (Schulze et al., 2005). The middle shelf can be divided into a proximal and a distal middle shelf. The proximal middle shelf is characterized by coarse grained skeletal packstone-floatstones (MF.8). Skeletal grains are dominantly rudist and echinoid fragments. Deposition took place in shallow water near the fair-weather wave base. The distal middle shelf sediments are dominated by fine-grained skeletal wackestone–packstones (oligosteginids, fine-grained rudists and echinoids fragments) (MF.7). The distal middle shelf facies are differentiated from the proximal middle shelf by the smaller size of components, the greater amount of micritic matrix and the presence of oligosteginids. Indicators of the outer shelf deeper water facies are high amounts of intact tests of planktonic foraminifers. Abundant oligosteginids, siliceous sponge spicules and ammonites (MF.2–6) also indicate deeper water and fully marine conditions. The chert nodules are common in the proximal part of outer shelf. The basin is characterized by abundant oligosteginids, planktonic foraminifers and ammonites (MF.1 and MF.2). Based on the direction of progradation (Fig. 5) and the palaeogeographic map of Hart (1970b) (Fig. 4), the direction of transition from the shallow marine environment to a deeper marine environment is concluded to be from northeast to southwest (Fig. 10) 8. Conclusion The sedimentological analysis shows that the Sarvak Formation was formed by a carbonate shelf bordering an intrashelf basin, with abundant rudists in the mid-shelf environment and pelagic facies (oligostegina, radiolaria and planktonic foraminifera) in the outer shelf and basin environments. The sedimentation of the Sarvak Formation took place on a shallow carbonate shelf setting, in a facies belt consisting of an inner shelf, middle shelf, outer shelf and basin. In the inner shelf, the most abundant microfacies are wackestone–packstone with benthic foraminifera (such as alveolinids, orbitolinids and miliolids) and rudist fragments. The shoal facies are marked by rudist grainstone. The proximal middle shelf is dominated by floatstones with large rudist fragments, while wackestone–packstone with sponge spicules, oligosteginids and fine-grained rudists and echinoids are present in the distal middle shelf. The outer shelf is represented by wackestone–packstones with oligostegina and planktonic foraminifera. The basin environment is dominated by radiolaria packstone. Acknowledgments The authors wish to thank the reviewers for their helpful and constructive comments. The manuscript has been significantly improved by comments from Prof Dr Boris Natalin. We also would
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