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Sedimentary facies and sequence stratigraphy of the Asmari Formation at Chaman-Bolbol, Zagros Basin, Iran
Journal of Asian Earth Sciences 29 (2007) 947–959 www.elsevier.com/locate/jaes
Sedimentary facies and sequence stratigraphy of the Asmari Formation at Chaman-Bolbol, Zagros Basin, Iran Mahnaz Amirshahkarami a, Hossein Vaziri-Moghaddam a,¤, Azizolah Taheri b a
Geology Department, Faculty of Sciences, Esfahan University, Esfahan 81746-73441, Iran b Geology Department, Faculty of Earth Science, Shahroud Technology University, Iran
Received 21 October 2005; received in revised form 17 May 2006; accepted 14 June 2006
Abstract The Oligocene–Miocene Asmari Formation of the Zagros Basin is a thick sequence of shallow water carbonate. In the study area, it is subdivided into 14 microfacies that are distinguished on the basis of their depositional textures, petrographic analysis and fauna. Based on the paleoecology and lithology, four distinct depositional settings can be recognized: tidal Xat, lagoon, barrier, and open marine. The Asmari Formation represents sedimentation on a carbonate ramp. In the inner ramp, the most abundant lithofacies are medium grained wackestone–packstone with imperforated foraminifera. The middle ramp is represented by packstone–grainstone to Xoatstone with a diverse assemblage of larger foraminifera with perforate wall, red algae, bryozoa, and echinoids. The outer ramp is dominated by argillaceous wackestone characterized by planktonic foraminifera and large and Xat nummulitidae and lepidocyclinidae. Three third-order depositional sequences are recognized from deepening and shallowing trends in the depositional facies, changes in cycle stacking patterns, and sequence boundary features. © 2006 Elsevier Ltd. All rights reserved. Keywords: Asmari Formation; Sequence stratigraphy; Microfacies; Larger benthic foraminifers; Ramp; Early Miocene; Iran
1. Introduction The Asmari Formation is part of the Tertiary deposits (Oligo–Miocene) of southwest Iran. Lithologically, the Asmari Formation at the type section consists of 314 m of mainly limestones, dolomitic limestones, and argillaceous limestones (Motiei, 1993). It was deposited on a carbonate platform developed across the Zagros Basin (Fig. 1). Little work has been done on the eVects of relative sea level changes during deposition of the Oligo–Miocene carbonate sediments in the Zagros Basin. The main objectives of this paper are to (1) describe and interpret the depositional environments represented by the Asmari Formation and (2) describe and interpret the origin of sequences that devel-
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oped in the study area mainly based on the distribution of the foraminifera. 2. Geological setting The Zagros Basin is the second largest basin in the Middle East with an area of about 553,000 km2. It extends from Turkey, northeastern Syria and northeastern Iraq through northwestern Iran and continues into southeastern Iran. This foreland Basin developed with the disappearance of Neotethys as suturing began in the northwest and migrated southeast during the Middle to Late Eocene. The suturing was accompanied by crustal thickening and movement along the originally passive margin of the Arabian plate and is related to the spreading movements in the Red Sea– Gulf of Aden (Hemplton, 1987). The Zagros Basin is divided into three zones, The Zagros fold-thrust zone, the imbricated zone, and the Urumieh–Dokhtar magmatic
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Fig. 1. Correlation chart of the Tertiary of southwest Iran (adopted from Ala, 1982).
zone (Alavi, 2004). The study area is in the Zagros fold– thrust zone (Fig. 2).
west of Fahlian in southwest Iran (Fig. 3). The section was meseared in detail at 30°19⬘29⬙N, 51°14⬘41⬙E.
3. Methods and study area
4. Previous work
This study involves one stratigraphic section from the Asmari Formation that was measured bed by bed and investigated sedimentologically. The rocks were classiWed in the Weld using a hand lens and their depositional fabric described (Dunham, 1962), followed by sampling for thin section analysis. More than 220 thin sections were examined. Some samples from the underlying Pabdeh Formation were also analysed for comparison. The study area is located at Chaman–Bolbol village, about 26.4 km north-
Interest in the study of the paleontology, stratigraphy, and sedimentary environment of the Asmari Formation has been largely motivated by the exploration for oil and gas, because it contains more than 90% of Iran’s oil. The Asmari Formation was named after the Kuh-eAsmari in Khozestan province by Busk and Mayo (1918) and referred to as a sequence of Cretaceous-Eocene age. Richardson (1924), Van Boeckh et al. (1929) and Lees (1993) dated the Asmari Formation as Oligocene–Miocene. Thomas (1948) deWned it as Oligocene–Burdigalian in age. Biostratigraphy and lithological characteries of the Asmari Formation were studied by James and Wynd (1965), Adams and Bourgeois (1967), Wells (1967), Kalantari (1986), Jalali (1987), SeyraWan et al. (1996) and SeyraWan (1981). Hamedani et al. (1997), SeyraWan and Hamedani (1998, 2003), SeyraWan and Mojikhalifeh (2005), SeyraWan (2000) and Vaziri-Moghaddam et al. (2006) studied the depositional environment of the Asmari Formation. 5. Biostratigraphy Biostratigraphic criteria of the Asmari Formation were established by Wynd (1965) and reviewed by Adams and Bourgeois (1967) in unpublished reports only. Based on the foraminiferal assemblages (Table 1), the Asmari Formation is divided into lower, middle, and upper units. From base to top, two foraminiferal assemblages were recognized in the study area:
Fig. 2. Subdivisions of the Zagros orogenic belt: OL, Oman line; UDMA, Urumieh-Dokhtar magmatic arc; ZDF, Zagros deformational front; ZFTB, Zagros fold-thrust belt; ZIZ, Zagros imbricate zone; ZS, Zagros suture (after Alavi, 2004).
(1) Assemblage I is characterized by the presence of Miogypsinoides spp., Valvulinid sp., Archaias asmaricus, Elphidium sp., Austrotrillina asmariensis, Spiroclypeus blankenhorni, Eulepidina spp., Nephrolepidina spp., Lepidocyclina sp., Operculina sp. and Heterostegina
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Fig. 3. Location and geological map of the study area at Chaman– Bolbol, southwest Iran.
sp. This microfauna correspond to the Miogypsinoides–Archaias–Valvulinid sp. assemblage zone of Adams and Bourgeois (1967). The faunal assemblage of this zone suggests a Aquitanian age. (2) Assemblage II consists of Borelis melo, Meandropsina iranica, Dendritina rangi, Peneroplis farsensis, and miliolids and represents the Borelis melo-Meandropsina iranica assemblage zone of Early Miocene (Burdigalian) age (Adams and Bourgeois, 1967). Table 1 Biozonation of the Asmari Formation modiWed after Adams and Bourgeois (1967) Biozones
Rock units
Age
Borelis melo-Meandropsina iranica Elphidium sp. 14-Miogypsina Archaias asmaricus-Archaias hensoni Eulepidina-NephrolepidinaNummulites
Upper Asmari
Burdigalian
Upper middle Asmari Late Aquitanian Lower middle Asmari Early Aquitanian Lower Asmari
Oligocene
6. Facies description and depositional environment The primary depositional features discernible in thin sections, including textures, microfossils, and sedimentary structures, have allowed the recognition of 14 facies. Systematic sampling and Weld observation at the study area show that the lowermost outcrops of the Asmari Formation interWnger with the uppermost exposed layers of the Pabdeh Formation (Oligocene). For comparison, some samples from the uppermost Pabdeh Formation have been studied as well. Microfacies A (see below) is recognized from the Pabdeh Formation. 6.1. Microfacies A—bioclast, pelagic foraminifera wackestone–packstone (Pl. 1/1) This facies is characterized by an association of pelagic foraminifera, small benthic foraminifera and fragments of molluscan shells. The texture is characterized a wackestone–packstone.
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The dominance of Wne grained sediments, abundance of planktonic foraminifera and the lack of abraded detritus indicate a very low energy depositional environment, probably an outer slope environment. The low energy hydrodynamic character indicates deposition below the normal wave base (Wilson, 1975; Flugel, 1982; Geel, 2000). The absence of photo symbiont bearing taxa suggests that this facies was deposited below the photic zone (Geel, 2000; Cosovic et al., 2004). Pedley (1996), Vaziri-Moghaddam et al. (2006), Okhravi and Amini (1998) considered similar facies as representative of an outer slope environment. 6.2. Microfacies B—bioclast wackestone (Pl. 1/2) These sediments contain sparse non-diagnostic fauna of ostracoda, pelagic foraminifera, and shell fragments. No sedimentary features indicative of shallow water or highenergy sedimentation were observed. The lime-mud dominated lithology, presence of pelagic foraminifera, and stratigraphic position indicate that deposition took place in a low energy deep water environment below storm wave base (Corda and Brandano, 2003). 6.3. Microfacies C—nummulitidae, pelagic foraminifera, bioclast wackestone (Pl. 1/3) This microfacies is composed of wackestone with pelagic foraminifera (globigerinids and globorotalids) and nummulitidae. Numulitidae are represented as prodominantly elongate and thin walled forms. Other bioclasts are small detrital bryozoa, corallinacean, and ostrea. This facies is characterized by the simultaneous occurrence of planktonic foraminifera and larger benthic foraminifera. The matrix is Wne-grained micrite. This facies is distinguished from facies A by the presence of larger benthic foraminifera. The abundance of normal marine biota (larger benthic foraminifera with perforate walls) accompanied by pelagic foraminifera suggests a platform slope depositional setting between the normal wave base and the storm wave base for this microfacies. (Corda and Brandano, 2003; Romero et al., 2002). Similar sediments were reported from the deeper shelf by Geel (2000). The simultaneous occurrence of the larger perforated foraminiferal tests, such as large and Xat sembiont bearing nummulitidae, with planktonic foraminifera are representative of the deepest portion of the lower limit of the photic zone, most likely in a slope environment (Geel, 2000; Romero et al., 2002). 6.4. Microfacies D—bioclast, nummulitidae, lepidocyclinidae wackestone–packstone (Pl. 1/4-6) The predominate fauna are larger benthic foraminifera with perforate walls (nummulitidae and lepidocyclinidae). Among the larger foraminifera, the nummulitidae are rep-
resented by Operculina, Heterostegina and Spiroclypeous. This microfacies has a Wne grained matrix. Locally, packstones occur with tightly packed, Xat lepidocyclinidae Plates 1–4. Other bioclasts include bryozoa, Ostrea, mollusca, echinoid, ostracoda, and small benthic foraminifera. This facies was deposited in a low-medium energy, open marine environment. This interpretation is supported by the abundance of typical open marine skeletal fauna including Xat and large nummulitidae, lepidocyclinidae, bryozoa, and echinoidea (Romero et al., 2002). The presence of large and Xat foraminifera such as lepidocyclinidae and nummlitidae in comparison with analogues in the modern platform (Hottinger, 1980, 1983; Hohenegger, 1996; Hallock, 1999; Reiss and Hottinger, 1984; Leutenegger, 1984) allowed us to interpret this facies as having been deposited in the lower photic zone. 6.5. Microfacies E–nummulitidae, Amphistegina, bioclast packstone (Pl. 1/8) This microfacies has a high diversity of benthic biota including forminifera (Amphistegina and Heterostegina), corallinacean, bryozoa, and echinoid. There are a few quartz grains in the limestone. Depositional textures are represented by packstone. The presence of red algae and larger foraminifera such as Amphistegina and Heterostegina indicate that this environment was situated in the oligophotic zone (Corda and Brandano, 2003; Brandano and Corda, 2002). Larger foraminifera like Amphistegina and Heterostegina live in tropical to subtropical environments over a wide bathymetric range, but are particulary abundant between 40 and 70 m (Hottinger, 1997, 1983; Hallock and Glenn, 1986). Due to changes in the type of faunas in some thin sections, the name of this facies changes to Nummulitidae, Neorotalia, bioclast, echinid wackestone–packstone (Pl. 2/2) orNeorotalia, bioclast packstone (Pl. 2/1). Microfacies E represents deposition on a shallower slope environment. This interpretation is supported by the abundance of typical open-marine skeletal fauna including benthic foraminifera with perforate walls, echinoidea, bryozoa, abundant micrite matrix, and a stratigraphic position below the platform margin sediments. Sediments dominated by nummulitidae, Amphistegina, Neorotalia specimens, mollusca, and corallinaceans characterize the middle ramp environment (Pomar, 2001b; Cosovic et al., 2004). 6.6. Microfacies F–nummulitidae, Miogypsinoides, Neorotalia, coral, corallinacean packstone-Xoatstone (Pl. 2/ 3–5) These sediments are characterized by an abundance of larger calcareous foraminifera, including Xat nummulitidae,
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Plate 1. Fig. 1. Microfacies A: Bioclast, pelagic foraminifera packestone (Sample No. Z1, Pabdeh Formation). Fig. 2. Microfacies B: Bioclast wackestone (Sample No. Z16). Fig. 3. Microfacies C: Nummulitidae, pelagic foraminifera, bioclast packstone (Sample No. Z32). Fig. 4. Microfacies D: Bioclast, nummulitidae, lepidocyclinidae packstone (Sample No. Z14). Fig. 5. Microfacies D: Bioclast, nummulitidae, lepidocyclinidae packstone (Sample No. Z17). Fig. 6. Microfacies D: Bioclast, nummulitidae, lepidocyclinidae packstone (Sample No. Z33). Fig. 7. Microfacies E: Nummulitidae, Amphistegina, bioclast wackestone–packstone (Sample No. Z49).
Miogypsinoides, and Neorotalia. Other bioclasts include coral, corallinacean, and echinoid fragments. Textures are packstone or Xoatstone. This facies is formed in an open marine environment under normal marine salinity conditions with open water circulation and medium hydrodynamic energy. Evidence for this interpretation includes abundant open marine skeletal fauna and stratigraphic position. The presence of corallinacean and larger foraminifera suggest a middle ramp position and indicate oligotrophic conditions (Pedley, 1996; Pomar, 2001a).
6.7. Microfacies G–bioclast, corallinacean, coral packstone– grainstone-Xoatstone (Pl. 2/6–7) This facies is characterized by abundant corallinacean and coral. Other bioclasts are rare but include bryozoa and mollusca. The textures are packstone–grainstone or Xoatstone. The depositional environment is interpreted as the upper part of a carbonate slope. This interpretation is supported by the mm size debris of reef-derived particles, in particular coralline red algae and corals. This microfacies is distinguished from the reef facies by an abundance of angular chips of cor-
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Plate 2. Fig. 1. Microfacies E Nummulitidae, Neorotalia, bioclast packstone (Sample No. Z73). Fig. 2. Microfacies E: Neorotalia, bioclast, echinoid packstone (Sample No. Z62). Fig. 3. Microfacies F: Nummulitidae, Miogypsinoides, Neorotalia, coral, corallinacean Xoatstone (Sample No. 71). Fig. 4. Microfacies F: Nummulitidae, Miogypsinoides, Neorotalia, coral, corallinacean packstone–Xoatstone (Sample No. Z68). Fig. 5. Microfacies F: Nummulitidae, Miogypsinoides, Neorotalia, coral, corallinacean Xoatstone (Sample No. Z72). Fig. 6. Microfacies G: Bioclast, corallinacea, coral Xoatstone (Sample No. Z60). Fig. 7. Microfacies G: Bioclast, corallinacea, coral Xoatstone (Sample No. Z70). Fig. 8. Microfacies H: Bioclast packstone–grainstone (Sample No. Z12).
alline red algae and shell debris, and the absence of in-place boundstone fabrics. A similar microfacies was reported by Wilson (1975), Longman (1981), Flugel (1982), Riding et al. (1991) and Melim and Scholle (1995).
margin, separating the open-marine from the more restricted marine environment. 6.9. Microfacies I–bioclast, nummulitida, miliolids wackestone–packstone (Pl. 3/1–2)
6.8. Microfacies H–bioclast packstone–grainstone (Pl. 2/8) The main bioclastic components are echinoidea, mollusca, bryozoa, and small benthic foraminifera. This facies has a packstone–grainstone texture. The features of these facies indicate moderate to high-energy shallow water conditions with signiWcant movement and reworking of bioclasts. In accordance with the standard microfacies types described by Wilson (1975) and Flugel (1982), microfacies H is interpreted as a shoal environment that was located at the platform
This facies is composed of wackestone–packstone with micritic bioclastics. Skeletal grains include echinoid, nummulitidae (Operculina,Heterostegina and Spiroclypeus) and miliolids. The minor taxa are Borelis, Austrotrillina, lepidocyclinidae and Neorotalia. The occurrence of perforate benthic foraminifera and imperforate benthic foraminifera indicate that sedimentation took place in a shelf lagoon (Romero et al., 2002).
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Plate 3. Fig. 1. Microfacies I: Bioclast, nummulitida, miliolids packstone (Sample No. Z46). Fig. 2. Microfacies I: Bioclast, nummulitida, miliolids packstone (Sample No. Z74). Fig. 3. Microfacies J: Intraclast, miliolids, bioclast packstone–grainstone (Sample No. Z83). Fig. 4. Microfacies K: Bioclast, imperforated foraminifera wackestone–packstone (Sample No. Z221). Fig. 5. Microfacies K: Bioclast, imperforated foraminifera wackestone–packstone–grainstone (Sample No. Z193). Fig. 6. Microfacies K: Bioclast, imperforated foraminifera packstone–grainstone (Sample No. Z84). Fig. 7. Microfacies K: Bioclast, imperforated foraminifera packstone–grainstone (Sample No. Z89). Fig. 8. Microfacies L: Mudstone (Sample No. Z178).
A similar microfacies was reported from a shelf lagoon environment by Nebelsick et al. (2001) and Corda and Brandano (2003). 6.10. Microfacies J–intraclast, miliolids, bioclast packstone– grainstone (Pl. 3/3) The main biotic components consist of miliolids and echinoidea. Intraclasts are also present. Corallinacean, Neorotalia and Miogypsinoides, and small benthic foraminifera such as Discorbis are the minor elements. Textures reXect poorly sorted packstone–grainstone. Some of the grains have been partially micritized. Co-occu-
rence of normal marine bioclasts and lagoonal biota with intraclasts suggest deposition at the lagoonal end of the platform margin (Hallock and Glenn, 1986). A similar facies with normal marine fauna and lagoonal biota was reported by Lasemi (1995) from the lagoonward side of a platform margin of the Upper Jurassic Kopet Dagh Basin, NE Iran. 6.11. Microfacies K–bioclast, imperforated foraminifera wackestone–packstone–grainstone (Pl. 3/4-7) The abundant components of this microfacies are benthic foraminifera with imperforated walls that include:
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Plate 4. Fig. 1. Microfacies M: Fenestrate mudstone with evaporate psuedomorphs (Sample No. Z142). Fig. 2. Microfacies M: Fenestrate mudstone with birdseye structures (Sample No. Z142). Fig. 3. Microfacies N: Stromatolitic boundstone (Sample No. Z169, Location is between sequence 2 and sequence 3). Fig. 4. Polished rock surface of stromatolite layer (Sample No. Z169).
Archaias, Peneroplis, Dendritina, Meandropsina, Borelis, Austrotrillina and miliolids. Other biota are corallinacean, coral, bryzoa and shell fragments. These deposits include diVerent textures ranging from wackestone to packstone and grainstone. The occurrence of a large number of imperforate foraminifera tests indicates that the sedimentation took place in a shelf lagoon setting (Geel, 2000; Romero et al., 2002). 6.12. Microfacies L–mudstone (Pl. 3/8) Lime mud generally comprises from 90 to 100 percent of this rock while carbonate and non-carbonate grains are rare. These sediments contain sparse non-diagnostic fauna. This facies was deposited in a restricted lagoon. Evidence for this interpretation includes the paucity of fauna both in diversity and abundance, lack of subaerial exposure features, and a stratigraphic position below the lagoonal facies (Wilson and Evans, 2002). 6.13. Microfacies M–fenestrate mudstone (Pl. 4/1–2) This microfacies consists of Wne grained microcrystalline limestone. Bioclasts are lacking, fenestrate structures are well developed, and evaporate psuedomorphs are rare. Birdseye or fenestral structures are typical products of shrinkage and expansion, gas bubble formation, and air escape during Xooding, or may even result from burrowing activity of worms or insects (Shinn, 1983). These vuggy structures are typical of a tidal Xat zone (Shinn et al., 1965; Shinn, 1968, 1983; Ginsburg and Hardie, 1975). The Wne grained nature of this facies, lack of fauna, and presence of fenestral fabric suggest that deposition occurred in a tidal Xat environment.
6.14. Microfacies N–stromatolitic boundstone (Pl. 4/3–4) These deposits are represented by mud-supported lithotypes formed by millimeter thick laminae, generally without fossils, irregularly undulating and laterally continuous (stromatolitic type cryptoalgal laminae). The cyanobacteria with their Wlamentous features trapping and binding of sedimentary particles produced a laminated sediment or stromatolite. This facies type is common in tidal Xat sediments (Flugel, 1982; Hardie, 1986; SteinhauV and Walker, 1996; Lasemi, 1995; Hernandez-Romano, 1999; Aguilera-Franco and Hernandez-Romano, 2004). Today, Xat laminated structures of microbial origin are found in intertidal settings. In regions with an arid climate (e.g. Persian Gulf or Shark Bay) stromatolites with smooth mats are located in the lower intertidal zone (Davies, 1970a,b; Kinsman and Park, 1976; HaVman, 1976). 7. Sedimentary model The gradual transitions between lithofacies and biofacies types, and the depositional proWle of the shelf transect investigated, indicate a very low gradient (Fig. 4). Owing to the generally similar morphology and hydrodynamic setting of shelf systems, the most appropriate model appears to be that of a very low-gradient, homoclinal carbonate ramp (Burchette and Wright, 1992). The absence of a shelfbreak is also corroborated by the lack of re-sedimented lowstand deposits. By comparing the textures and fossil assemblages of the Asmari Formation with those of recent settings, it seems that the Asmari Formation ramp can be compared to the present day homoclinal ramp of the Persian Gulf (Read, 1985; Jones and Desrochers, 1992).
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Fig. 4. Depositional model for the platform carbonates of the Asmari Formation at Chaman–Bolbol area, Zagros Basin, SW Iran. The interpretation is adopted from Hottinger (1997), Pomar (2001b), Wilson et al. (1999), and Rasser and Nebelsick (2003).
The tidal Xat sediments are composed of fenestral lime mudstone and stromatolitic boundstone. In the inner ramp, the most abundant lithofacies are medium to coarse-grained larger foraminifer with imperforate wall– bioclast wackestone-packestone (Fig. 4). The presence of imperforate foraminifera that include Archaias, Peneroplis, Dendritina, Meandropsina, Borelis, Austrotrillina, and miliolids indicates a low-energy, upper photic, shallow shelf lagoon depositional environment. Generally the upper photic zone is dominated by porcellaneus larger foraminifera predominantly living in symbiosis with dinophyceans, chlorophyceans or rhodophyceans (Leutenegger, 1984; Romero et al., 2002). The middle ramp setting is characterized by the medium to Wne grained foraminiferal–bioclastic packestone-grainstone-Xoatstone dominated by assemblages of larger foraminifera with perforate walls such as Lepidocyclina, Eulepidina, Nephrolepidina, nummulitidae (Operculina, Heterostegina, Spiroclypeous), Amphistegina, Miogypsina, and Miogypsinoides (Fig. 4). The faunal association suggests that the depositional environment was situated in the mesophotic to oligophotic zone (Hottinger, 1997; Pomar, 2001). The lower photic zone is dominated by large, Xat, and perforated foraminifera (such as lepidocyclinids) associated with symbiont-bearing diatoms (Leutenegger, 1984; Romero et al., 2002). Lower slope facies are diVerentiated from upper slope by the greater amount of micritic matrix, an increase in the Xatness, and size of the perforate foraminifera and presence of planktonic foraminifera. The outer ramp was characterized by low energy conditions and deposition of mudstone. The lack of bioclasts and
the appearance of planktonic foraminifera indicates deep water (Corda and Brandano, 2003). 8. Sequence stratigraphy The general vertical facies succession through the Asmari Formation and the stacking patterns and upward changes in the parasequences permit the identiWcation of sequences and their component systems tracts within the succession. Sequence boundaries are the key to determine third-order depositional sequences and these boundaries are dependent on the evidence from vertical facies changes (Fig. 5). On this basis, sequence stratigraphy terminology (i.e., maximum Xooding surface, sequence boundary, Highstand Systems Tract, Transgressive Systems Tract) has been applied to the sequences recognized in the Chaman Bolbol section. The transgrassive or upward-deepening portion of sequences (Transgressive Systems Tract, TST) ends with the maximum Xooding surface that records the deepest water depths attained for each sequence. The regressive or upward-shallowing portion of sequences (Highstand Systems Tract, HST) ends at the sequence boundary, which records the eVects of minimum longterm accommodation (Nicols, 1999; Emery and Myers, 1996). 8.1. Sequence 1 This sequence is 169.15 m thick and its facies associations can be grouped into Transgressive and Highstand Systems Tracts. The basal part of a basinal facies is inter-
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Fig. 5. Vertical facies distribution and sequences of the Asmari Formation at Chaman–Bolbol section, Zagros. TST: transgressive systems tract; EHST: early highstand systems tract; LHST: late highstand system tract; mfs: maximum Xooding surface; SB2: sequence boundary type 2. (For symbols see Fig. 4.)
preted as the Transgressive Systems Tract (TST) of the sequence 1 (Fig. 5). These sediments consist of marl with planktonic foraminifera and larger benthic foraminifera with perforated walls. Above this package, the strata show an increase in deeper water fauna and this bed is equivalent to the mfs.
Wackestone and packstone with perforated larger benthic foraminifera overlie the mfs. These sediments were mostly deposited in a middle and upper slope environment and are interpreted as the EHST (Early HST). The upper part of the HST (Late HST or LHST) is characterized by wackestone– packstone with abundant imperforate foraminifera, which
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indicate a change from open marine to a restricted palaeoenvironment. The boundary between sequence1 and sequence 2 is characterized by fenestrate mudstone. The boundary between sequence 1 and sequence 2 is a type 2 of sequence boundary (SB2), because the sequence boundary shows no clear evidence of subaerial exposure. 8.2. Sequence 2 The thickness of sequence 2 is nearly 20 m. The lower part of sequence 2 (TST) consists of open shelf lagoon deposits that are mostly characterized by the simultaneous occurrence of both perforated and imperforated foraminifera. The upper part of sequence 2 (HST) is composed of limestone with imperforate foraminifera. The boundary between sequence 2 and sequence 3 is characterized by stromatolitic boundstone (SB2). 8.3. Sequence 3 Sequence 3 is entirely composed of 30.1 m lagoonal facies with no evidence of deepening. They have an aggradational stacking pattern. This sequence is overlain by the Gachsaran Formation of Middle Miocene age. There is a disconformity (SB1) between the Asmari Formation and the Gachsaran Formation. 9. Comparison between the Chaman–Bolbol and Lali sections From all the sections logged in the platform of the Zagros Basin, the Lali section is the most representative and for this reason was selected for comparison with the studied section. The Asmari Formation in the Lali section is Late Oligocene-Early Miocene in age, a determination that is mainly based on foraminifera. However, an Early Miocene age has been concluded for our section. Vaziri-Moghaddam et al. (2006) described 10 diVerent types of microfacies from the Lali section that are similar to those found in our study area. Microfacies 1, 2, 3, and 4, which were deposited in an open marine environment, are very similar to microfacies A, B, C, and D of our study area. Microfacies 7, 8, and 9 described in the above mentioned article characterizes a lagoon environment, like, L, M, and N dose here. In both regions, the sequence boundaries are not associated with long periods of exposure and there is an unconformity (SB1) between the Asmari Formation and the succeeding Gachsaran Formation. In both sections, on the basis of the distribution of the foraminifera, four major depositional environments are identiWed. These include tidal Xat, shelf lagoon, shoal, and open marine environments. 10. Conclusions The Asmari Formation represents sedimentation on a carbonate ramp. Fourteen microfacies are recognized
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within this carbonate platform section. They are grouped into four depositional environments representing tidal Xat, shelf lagoon, shoal, and open marine. Three third-order depositional sequences are identiWed from shallowing and deepening trends of depositional facies and changes in the cycle stacking pattern. Sequence 1 is composed of transgressive and highstand systems tracts. Transgressive systems tracts are associated with planktonic, smaller benthic and large perforated foraminifera. Highstand systems tracts are Wrst characterized by large perforated foraminifera (EHST) and then imperforated foraminifera (LHST). Sequence 2 is composed of transgressive and highstand systems tracts. The transgressive systems tract consists of open shelf lagoon deposits that are mostly characterized by the simultaneous occurrence of both perforated and imperforated foraminifera. The upper part of the sequence 2 or highstand systems tract is composed of limestone with imperforated foraminifera. The boundary between sequence 2 and sequence 3 is characterized by stromatolitic boundstone (SB2). Sequence 3 is characterized by aggredational lagoonal facies. Acknowledgments The authors appreciate Iran Oil OVshore Company (IOOC) and A. Ghobishavi for help in the Weld. This study was completed at the University of Esfahan and supported by oYce of graduate studies. The authors are grateful to the oYce for their support. The authors would also appreciate the reviewers for their helpful comments. References Adams, C.G., Bourgeois, E., 1967. Asmari biostratigraphy. Geological and Exploration Div., Iranian Oil OVshore Company Report 1074, Unpubl. Aguilera-Franco, N., Hernandez-Romano, U., 2004. Cenomanian-Turonian facies succession in the Guerrero-Morelos Basin, Southern Mexico. Sedimentary Geology 170, 135–162. Ala, M.A., 1982. Chronology of trap formation and migration of hydrocarbons in Zagros sector of southwest Iran. American Association of Petroleum Geologists Bulletin 66, 1536–1542. Alavi, M., 2004. Regional stratigraphy of the Zagros fold-thrust belt of Iran and its proforeland evolution. American Journal of Science 304, 1–20. Brandano, M., Corda, L., 2002. Nutrients, sea level and tectonics: constrains for the facies architecture of a Miocene carbonate ramp in central Italy. Terra Nova 14, 257–262. Burchette, T.P., Wright, V.P., 1992. Carbonate ramp depositional systems. Sedimentary Geology 79, 3–57. Busk, H.G., Mayo, H.T., 1918. Some notes on the geology of the Persian oilWelds. Journal Institute Petroleum Technology 5, 5–26. Corda, L., Brandano, M., 2003. Aphotic zone carbonate production on a Miocene ramp Central Apennines, Italy. Sedimentary Geology 61, 55–70. Cosovic, V., Drobne, K., Moro, A., 2004. Paleoenvironmental model for Eocene foraminiferal limstones of the Adriatic carbonate platform (Istrian Peninsula). Facies 50, 61–75. Davies, R.G., 1970a. Carbonate bank sedimentation, Eastern Shark Bay, Western Australia. In: Logan, B.V., et al. (Eds.), Carbonate Sedimentation and Environments Shark Bay, Western Australia. Am. Assoc. Pet. Geol. Mem. 13, pp. 85–168.
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Davies, R.G., 1970b. Algal-laminated sediments, Western Australia environments, Shark Bay, Western Australia. American Association of Petroleum Geological Memories 13, 169–205. Dunham, R.J., 1962. ClassiWcation of carbonate rocks according to their depositional texture. In: Ham, W.E. (Ed.), ClassiWcation of Carbonate Rocks. A Symposium: AAPG Bulletin, pp. 108–121. Emery, D., Myers, K.J., 1996. Sequence Stratigraphy. Blackwell Science, Oxfordp. 297 pp. Flugel, E., 1982. Microfacies Analysis of Limestone. Springer, Berlin-Heidelberg, New york. pp. 633. Geel, T., 2000. Recognition of stratigraphic sequences in carbonate platform and slope deposits: empirical models based on microfacies analysis of Paleogene deposits in southeastern Spain. Palaeogeography, Palaeoclimatology, Palaeoecology 155, 211–238. Ginsburg, R.N., Hardie, L.A., 1975. Tidal and storm deposits Northwestern Andros Island, Bahamas. In: Ginsburg, R.N. (Ed.), Tidal Deposits. pp. 201–208. Hallock, P., 1999. Symbiont-Bearing Foraminifera. In: Gupta, B.K.Sen. (Ed.), Modern Foraminifera. Kluwer Academic, Dordrecht. Hallock, P., Glenn, E.C., 1986. Larger foraminifera: A Tool for Paleoenvironmental analysis of Cenozoic carbonate depositional facies. Palaios 1, 55–64. Hamedani, A., Torabi, H., Piller, W., Mandic, O., Steininger, F. F., Wielandt, U., Harzhauser, M., Nebelsick, J. H., Schuster, F., 1997. Oligocene/ Miocene sections from Zagros foreland basins of Central Iran. Abstr., 18th IAS Regional Meeting of Sedimentol., Heidelberg, pp. 155–156. HaVman, P., 1976. Stromatolite morphogenesis in Shark Bay, Western Australia. In: Walter, M.R. (Ed.), Stromatolites. Dev. Sedimentol. vol. 20, pp. 381–388. Hardie, L.A., 1986. Ancient carbonate tidal Xat deposits Quaterny. Journal of the Colorado School of Mines 81, 37–57. Hemplton, M.R., 1987. Constraints on Arabian plate motion and extensional history of Red Sea. Tectonics 6, 687–705. Hernandez-Romano, U., 1999. Facies stratigraphy and diagenesis of the Cenomanian-Turonian of the Guerrero-Morelos Platform, southern Mexico: Reading, Postgraduate Research Institute for Sedimentology. University of Reading, UK, Ph.D thesis, p. 322. Hohenegger, J., 1996. Remarks on the distribution of larger foraminifera (Protozoa) from Palau (western Carolines). In: Aoyama, T. (Ed.), The Progress Report of the 1995 Survey of the Research Project, Man and the environment in Micronesia. Kagoshima University Research Center for the PaciWc Islands, Occasional Papers 32, pp. 19–45. Hottinger, L., 1997. Shallow bentihic foraminiferal assembelages as signals for depth of their deposition and their limitations. Bull. Soc. Geol. France 168/4, 491–505. Hottinger, L., 1980. Repartition comparee des grands foraminiferes de la mer Rouge et de l,Ocean indien. Ann. Univ. Ferrara 6, 35–51. Hottinger, L., 1983. Processes determining the distribution of larger foraminifera in space and time. Utrecht Micropaleont. Bull 30, 239–253. Jalali, M.R., 1987. Stratigraphy of Zagros Basin. National Iranian Oil Company. Exploration and Production Division Report 1249, 1072. James, G.A., Wynd, J.G., 1965. Stratigraphic nomenclature of Iranian oil consortium agreement area. AAPG Bulletin 49, 2182–2245. Jones, B., Desrochers, A., 1992. Shallow platform carbonates. In: Walker, R.G., James, N.P. (Eds.), Facies Models Response to Sea Level Changes. Geol. Assoc. Canada, St. Jones, Newfoundland, pp. 277–303. Kalantari, A., 1986. Microfacies of carbonate rocks of Iran. National Iranian Oil Company, Geological Laboratory Publication, Tehran, vol. 11, 520 pp. Kinsman, D.J.J., Park, R.K., 1976. Algal belt and coastal sabkha evolution, Trucial Coast, Persian Gulf. In: Walter, M.R. (Ed.), Stromatolites. Dev. Sedimentol. vol. 20, pp. 421–433. Lasemi, Y., 1995. Platform carbonates of the Upper Jurassic Mozduran Formation in the Kopet Dagh Basin, NE Iran-facies, palaeoenvironments and sequences. Sedimentary Geology 99, 151–164. Lees, G.M., 1993. Reservoir rocks of persian oil Welds. AAPG Bulletin 17, 224–240.
Leutenegger, S., 1984. Symbiosis in benthic foraminifera, speciWcity and host adaptations. J. Foram. Res. 14, 16–35. Longman, M.W., 1981. A process approach to recognizing facies of reef complex. In: Toomey, D.F. (Ed.), European Fossil Reef Models, 30. Soc. Eocn. Paleontol. Mineral. Spec. Publ., pp. 9–40. Melim, L.A., Scholle, P.A., 1995. The forereef facies of the Permian Capitan Formation: the role of sediment supply versus sea-level changes. J. Sediment. Res. B 65, 107–119. Motiei, H., 1993. Stratigraphy of Zagros. In: Treatisie of Geology of Iran, 1. Iran Geol. Surv., Publ., pp. 281–289. Nebelsick, J.H., Stingl, T.V., Rasser, M., 2001. Autochthonous Facies and Allochthonous Debris Flows compared: Early Oligocene carbonate facies patterns of the lower Inn valley (Tyrol, Austria). Facies 44, 31–46. Nicols, G., 1999. Sedimentary and Stratigraphy. Blackwell Science, Oxford. 355 pp. Okhravi, R., Amini, A., 1998. An example of mixed carbonate–pyroclastic sedimentation (Miocene, Central Basin, Iran). Sediment Geol 118, 37–57. Pedley, M., 1996. Miocene reef facies of Pelagian region (Central Mediterranean region). In: Franseen E.K, Esteben, M., Ward W.C, Rouchy J.M. (Eds), Models for Carbonate Stratigraphy from Miocene Reef complexes of Mediterranean Regions. SEPM Concept Sediment Paleontol, vol. 5, pp. 147–259. Pomar, L., 2001a. Types of carbonate platforms: a genetic approach. Basin Research 13, 313–334. Pomar, L., 2001b. Ecological control of sedimentary accomodation: evolution from a carbonate ramp to rimmed shelf, Upper Miocene, Balearic Islands. Palaeogeography, Palaeoclimatology, Palaeoecology 175, 249–272. Rasser, M.W., Nebelsick, J.H., 2003. Provenance analysis of Oligocene autochthonous and allochthonous coralline algae: a quantitative approach towards reconstructing transported assemblages. Palaeogeography, Palaeoclimatology, Palaeoecology 201, 89–111. Read, J.F., 1985. Carbonate platform facies models. AAPG Bulletin 69/1, 1–21. Richardson, R.K., 1924. The geology and oil measures of southwest Persia. Journal Institute Petroleum Technology 10, 256–283. Riding, R., Martin, T.M., Braga, T.C., 1991. Coral-stromatolite reef framework, Upper Miocene, Almeria, Spain. Sedimentology 38, 799–818. Romero, J., Caus, E., Rossel, J., 2002. A model for the palaeoenvironmental distribution of larger foraminifera based on Late Middle Eocene deposits on the margin of the south Pyrenean basin(SE Spain). Palaeogeography, Palaeoclimatology, Palaeoecology 179, 43–56. Reiss, Z., Hottinger, L., 1984. The Gulf of Aqaba: Ecological Micropaleontology. Springer, Berlin. 354 pp. SeyraWan, A., 1981. Geological study of the Siah Makan Field (Asmari Formation). National Iranian Oil Company, Report., 37–45, unpublished. SeyraWan, A., 2000. Microfacies and depositional environments of the Asmari Formation, at Dehdes area (A correlation across Central Zagros Basin). Carbonates and Evaporites 15, 22–48. SeyraWan, A., Hamedani, A., 1998. Microfacies and depositional environment of the Upper Asmari Formation (Burdigalian), north-central Zagros Basin, Iran. N. Jb. Geol. Palaont. Abh. 210/2, 129–141. SeyraWan, A., Hamedani, A., 2003. Microfacies and palaeoenvironmental interpretation of the lower Asmari Formation (Oligocene), NorthCentral Zagros Basin, Iran. N. Jb. Geol. Palaont. Mh. 3, 164–174. SeyraWan, A., Mojikhalifeh, A.R., 2005. Biostratigraphy of the Late Paleogene-Early Neogene succession, north-central border of Persian Gulf, Iran. Carbonates and Evaporites 20/1, 91–97. SeyraWan, A., Vaziri-Moghaddam, H., Torabi, H., 1996. Biostratigraphy of the Asmari Formation, Burujen area, Iran. J. Sci. 7/1, 31–47. Shinn, E., 1968. Practical signiWcance of birdseye structures in carbonate rocks. J. Sediment. Petrol. 38, 215–223. Shinn, E., 1983. Tidal Xats. In: Scholle, P.A., et al. (Eds.), Carbonate Depositional Environments. Am. Assoc. Pet. Geol. Mem. 33, pp. 171–210. Shinn, E., Ginsburg, R.N., Llyod, R.M., 1965. Recent supratidal dolomite from Andros Island, Bahamas. In: Pray, L.C., Murray, R.C. (Eds.),
M. Amirshahkarami et al. / Journal of Asian Earth Sciences 29 (2007) 947–959 Dolomitization and Limestone Diagenesis, A Symposium. Soc. Econ. Paleontol. Mineral. Spec. Publ. 13, pp. 112–123. SteinhauV, D.M., Walker, K.R., 1996. Sequence stratigraphy of an apparently non-cyclic carbonate succession: recognizing subaerial exposure in a largely subtidal, Middle Ordovician stratigraphic sequence in eastern Tennessee. In: Witzke, G.A., Ludvingson, J.E., Day, B.J. (Eds), Paleozoic Sequence Stratigraphy: Views from the North American Craton. Special Paper-Geological Society of America, vol. 306, pp. 87–115. Thomas, A.N., 1948. The Asmari Limestone of southwest Iran. ALOC Report 706, unpublished. Van Boeckh, H.D.E., Lees, G.M., Richardson, F.D.S., 1929. Contribution to the Stratigraphy and Tectonic of Iranian ranges. The Structure of Asia, London. 58–177. Vaziri-Moghaddam, H., Kimiagari, M., Taheri, A., 2006. Depositional environment and sequence stratigraphy of the Oligocene-Miocene
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Sedimentary facies and sequence stratigraphy of the Asmari Formation at Chaman-Bolbol, Zagros Basin, Iran Mahnaz Amirshahkarami a, Hossein Vaziri-Moghaddam a,¤, Azizolah Taheri b a
Geology Department, Faculty of Sciences, Esfahan University, Esfahan 81746-73441, Iran b Geology Department, Faculty of Earth Science, Shahroud Technology University, Iran
Received 21 October 2005; received in revised form 17 May 2006; accepted 14 June 2006
Abstract The Oligocene–Miocene Asmari Formation of the Zagros Basin is a thick sequence of shallow water carbonate. In the study area, it is subdivided into 14 microfacies that are distinguished on the basis of their depositional textures, petrographic analysis and fauna. Based on the paleoecology and lithology, four distinct depositional settings can be recognized: tidal Xat, lagoon, barrier, and open marine. The Asmari Formation represents sedimentation on a carbonate ramp. In the inner ramp, the most abundant lithofacies are medium grained wackestone–packstone with imperforated foraminifera. The middle ramp is represented by packstone–grainstone to Xoatstone with a diverse assemblage of larger foraminifera with perforate wall, red algae, bryozoa, and echinoids. The outer ramp is dominated by argillaceous wackestone characterized by planktonic foraminifera and large and Xat nummulitidae and lepidocyclinidae. Three third-order depositional sequences are recognized from deepening and shallowing trends in the depositional facies, changes in cycle stacking patterns, and sequence boundary features. © 2006 Elsevier Ltd. All rights reserved. Keywords: Asmari Formation; Sequence stratigraphy; Microfacies; Larger benthic foraminifers; Ramp; Early Miocene; Iran
1. Introduction The Asmari Formation is part of the Tertiary deposits (Oligo–Miocene) of southwest Iran. Lithologically, the Asmari Formation at the type section consists of 314 m of mainly limestones, dolomitic limestones, and argillaceous limestones (Motiei, 1993). It was deposited on a carbonate platform developed across the Zagros Basin (Fig. 1). Little work has been done on the eVects of relative sea level changes during deposition of the Oligo–Miocene carbonate sediments in the Zagros Basin. The main objectives of this paper are to (1) describe and interpret the depositional environments represented by the Asmari Formation and (2) describe and interpret the origin of sequences that devel-
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1367-9120/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2006.06.008
oped in the study area mainly based on the distribution of the foraminifera. 2. Geological setting The Zagros Basin is the second largest basin in the Middle East with an area of about 553,000 km2. It extends from Turkey, northeastern Syria and northeastern Iraq through northwestern Iran and continues into southeastern Iran. This foreland Basin developed with the disappearance of Neotethys as suturing began in the northwest and migrated southeast during the Middle to Late Eocene. The suturing was accompanied by crustal thickening and movement along the originally passive margin of the Arabian plate and is related to the spreading movements in the Red Sea– Gulf of Aden (Hemplton, 1987). The Zagros Basin is divided into three zones, The Zagros fold-thrust zone, the imbricated zone, and the Urumieh–Dokhtar magmatic
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Fig. 1. Correlation chart of the Tertiary of southwest Iran (adopted from Ala, 1982).
zone (Alavi, 2004). The study area is in the Zagros fold– thrust zone (Fig. 2).
west of Fahlian in southwest Iran (Fig. 3). The section was meseared in detail at 30°19⬘29⬙N, 51°14⬘41⬙E.
3. Methods and study area
4. Previous work
This study involves one stratigraphic section from the Asmari Formation that was measured bed by bed and investigated sedimentologically. The rocks were classiWed in the Weld using a hand lens and their depositional fabric described (Dunham, 1962), followed by sampling for thin section analysis. More than 220 thin sections were examined. Some samples from the underlying Pabdeh Formation were also analysed for comparison. The study area is located at Chaman–Bolbol village, about 26.4 km north-
Interest in the study of the paleontology, stratigraphy, and sedimentary environment of the Asmari Formation has been largely motivated by the exploration for oil and gas, because it contains more than 90% of Iran’s oil. The Asmari Formation was named after the Kuh-eAsmari in Khozestan province by Busk and Mayo (1918) and referred to as a sequence of Cretaceous-Eocene age. Richardson (1924), Van Boeckh et al. (1929) and Lees (1993) dated the Asmari Formation as Oligocene–Miocene. Thomas (1948) deWned it as Oligocene–Burdigalian in age. Biostratigraphy and lithological characteries of the Asmari Formation were studied by James and Wynd (1965), Adams and Bourgeois (1967), Wells (1967), Kalantari (1986), Jalali (1987), SeyraWan et al. (1996) and SeyraWan (1981). Hamedani et al. (1997), SeyraWan and Hamedani (1998, 2003), SeyraWan and Mojikhalifeh (2005), SeyraWan (2000) and Vaziri-Moghaddam et al. (2006) studied the depositional environment of the Asmari Formation. 5. Biostratigraphy Biostratigraphic criteria of the Asmari Formation were established by Wynd (1965) and reviewed by Adams and Bourgeois (1967) in unpublished reports only. Based on the foraminiferal assemblages (Table 1), the Asmari Formation is divided into lower, middle, and upper units. From base to top, two foraminiferal assemblages were recognized in the study area:
Fig. 2. Subdivisions of the Zagros orogenic belt: OL, Oman line; UDMA, Urumieh-Dokhtar magmatic arc; ZDF, Zagros deformational front; ZFTB, Zagros fold-thrust belt; ZIZ, Zagros imbricate zone; ZS, Zagros suture (after Alavi, 2004).
(1) Assemblage I is characterized by the presence of Miogypsinoides spp., Valvulinid sp., Archaias asmaricus, Elphidium sp., Austrotrillina asmariensis, Spiroclypeus blankenhorni, Eulepidina spp., Nephrolepidina spp., Lepidocyclina sp., Operculina sp. and Heterostegina
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Fig. 3. Location and geological map of the study area at Chaman– Bolbol, southwest Iran.
sp. This microfauna correspond to the Miogypsinoides–Archaias–Valvulinid sp. assemblage zone of Adams and Bourgeois (1967). The faunal assemblage of this zone suggests a Aquitanian age. (2) Assemblage II consists of Borelis melo, Meandropsina iranica, Dendritina rangi, Peneroplis farsensis, and miliolids and represents the Borelis melo-Meandropsina iranica assemblage zone of Early Miocene (Burdigalian) age (Adams and Bourgeois, 1967). Table 1 Biozonation of the Asmari Formation modiWed after Adams and Bourgeois (1967) Biozones
Rock units
Age
Borelis melo-Meandropsina iranica Elphidium sp. 14-Miogypsina Archaias asmaricus-Archaias hensoni Eulepidina-NephrolepidinaNummulites
Upper Asmari
Burdigalian
Upper middle Asmari Late Aquitanian Lower middle Asmari Early Aquitanian Lower Asmari
Oligocene
6. Facies description and depositional environment The primary depositional features discernible in thin sections, including textures, microfossils, and sedimentary structures, have allowed the recognition of 14 facies. Systematic sampling and Weld observation at the study area show that the lowermost outcrops of the Asmari Formation interWnger with the uppermost exposed layers of the Pabdeh Formation (Oligocene). For comparison, some samples from the uppermost Pabdeh Formation have been studied as well. Microfacies A (see below) is recognized from the Pabdeh Formation. 6.1. Microfacies A—bioclast, pelagic foraminifera wackestone–packstone (Pl. 1/1) This facies is characterized by an association of pelagic foraminifera, small benthic foraminifera and fragments of molluscan shells. The texture is characterized a wackestone–packstone.
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The dominance of Wne grained sediments, abundance of planktonic foraminifera and the lack of abraded detritus indicate a very low energy depositional environment, probably an outer slope environment. The low energy hydrodynamic character indicates deposition below the normal wave base (Wilson, 1975; Flugel, 1982; Geel, 2000). The absence of photo symbiont bearing taxa suggests that this facies was deposited below the photic zone (Geel, 2000; Cosovic et al., 2004). Pedley (1996), Vaziri-Moghaddam et al. (2006), Okhravi and Amini (1998) considered similar facies as representative of an outer slope environment. 6.2. Microfacies B—bioclast wackestone (Pl. 1/2) These sediments contain sparse non-diagnostic fauna of ostracoda, pelagic foraminifera, and shell fragments. No sedimentary features indicative of shallow water or highenergy sedimentation were observed. The lime-mud dominated lithology, presence of pelagic foraminifera, and stratigraphic position indicate that deposition took place in a low energy deep water environment below storm wave base (Corda and Brandano, 2003). 6.3. Microfacies C—nummulitidae, pelagic foraminifera, bioclast wackestone (Pl. 1/3) This microfacies is composed of wackestone with pelagic foraminifera (globigerinids and globorotalids) and nummulitidae. Numulitidae are represented as prodominantly elongate and thin walled forms. Other bioclasts are small detrital bryozoa, corallinacean, and ostrea. This facies is characterized by the simultaneous occurrence of planktonic foraminifera and larger benthic foraminifera. The matrix is Wne-grained micrite. This facies is distinguished from facies A by the presence of larger benthic foraminifera. The abundance of normal marine biota (larger benthic foraminifera with perforate walls) accompanied by pelagic foraminifera suggests a platform slope depositional setting between the normal wave base and the storm wave base for this microfacies. (Corda and Brandano, 2003; Romero et al., 2002). Similar sediments were reported from the deeper shelf by Geel (2000). The simultaneous occurrence of the larger perforated foraminiferal tests, such as large and Xat sembiont bearing nummulitidae, with planktonic foraminifera are representative of the deepest portion of the lower limit of the photic zone, most likely in a slope environment (Geel, 2000; Romero et al., 2002). 6.4. Microfacies D—bioclast, nummulitidae, lepidocyclinidae wackestone–packstone (Pl. 1/4-6) The predominate fauna are larger benthic foraminifera with perforate walls (nummulitidae and lepidocyclinidae). Among the larger foraminifera, the nummulitidae are rep-
resented by Operculina, Heterostegina and Spiroclypeous. This microfacies has a Wne grained matrix. Locally, packstones occur with tightly packed, Xat lepidocyclinidae Plates 1–4. Other bioclasts include bryozoa, Ostrea, mollusca, echinoid, ostracoda, and small benthic foraminifera. This facies was deposited in a low-medium energy, open marine environment. This interpretation is supported by the abundance of typical open marine skeletal fauna including Xat and large nummulitidae, lepidocyclinidae, bryozoa, and echinoidea (Romero et al., 2002). The presence of large and Xat foraminifera such as lepidocyclinidae and nummlitidae in comparison with analogues in the modern platform (Hottinger, 1980, 1983; Hohenegger, 1996; Hallock, 1999; Reiss and Hottinger, 1984; Leutenegger, 1984) allowed us to interpret this facies as having been deposited in the lower photic zone. 6.5. Microfacies E–nummulitidae, Amphistegina, bioclast packstone (Pl. 1/8) This microfacies has a high diversity of benthic biota including forminifera (Amphistegina and Heterostegina), corallinacean, bryozoa, and echinoid. There are a few quartz grains in the limestone. Depositional textures are represented by packstone. The presence of red algae and larger foraminifera such as Amphistegina and Heterostegina indicate that this environment was situated in the oligophotic zone (Corda and Brandano, 2003; Brandano and Corda, 2002). Larger foraminifera like Amphistegina and Heterostegina live in tropical to subtropical environments over a wide bathymetric range, but are particulary abundant between 40 and 70 m (Hottinger, 1997, 1983; Hallock and Glenn, 1986). Due to changes in the type of faunas in some thin sections, the name of this facies changes to Nummulitidae, Neorotalia, bioclast, echinid wackestone–packstone (Pl. 2/2) orNeorotalia, bioclast packstone (Pl. 2/1). Microfacies E represents deposition on a shallower slope environment. This interpretation is supported by the abundance of typical open-marine skeletal fauna including benthic foraminifera with perforate walls, echinoidea, bryozoa, abundant micrite matrix, and a stratigraphic position below the platform margin sediments. Sediments dominated by nummulitidae, Amphistegina, Neorotalia specimens, mollusca, and corallinaceans characterize the middle ramp environment (Pomar, 2001b; Cosovic et al., 2004). 6.6. Microfacies F–nummulitidae, Miogypsinoides, Neorotalia, coral, corallinacean packstone-Xoatstone (Pl. 2/ 3–5) These sediments are characterized by an abundance of larger calcareous foraminifera, including Xat nummulitidae,
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Plate 1. Fig. 1. Microfacies A: Bioclast, pelagic foraminifera packestone (Sample No. Z1, Pabdeh Formation). Fig. 2. Microfacies B: Bioclast wackestone (Sample No. Z16). Fig. 3. Microfacies C: Nummulitidae, pelagic foraminifera, bioclast packstone (Sample No. Z32). Fig. 4. Microfacies D: Bioclast, nummulitidae, lepidocyclinidae packstone (Sample No. Z14). Fig. 5. Microfacies D: Bioclast, nummulitidae, lepidocyclinidae packstone (Sample No. Z17). Fig. 6. Microfacies D: Bioclast, nummulitidae, lepidocyclinidae packstone (Sample No. Z33). Fig. 7. Microfacies E: Nummulitidae, Amphistegina, bioclast wackestone–packstone (Sample No. Z49).
Miogypsinoides, and Neorotalia. Other bioclasts include coral, corallinacean, and echinoid fragments. Textures are packstone or Xoatstone. This facies is formed in an open marine environment under normal marine salinity conditions with open water circulation and medium hydrodynamic energy. Evidence for this interpretation includes abundant open marine skeletal fauna and stratigraphic position. The presence of corallinacean and larger foraminifera suggest a middle ramp position and indicate oligotrophic conditions (Pedley, 1996; Pomar, 2001a).
6.7. Microfacies G–bioclast, corallinacean, coral packstone– grainstone-Xoatstone (Pl. 2/6–7) This facies is characterized by abundant corallinacean and coral. Other bioclasts are rare but include bryozoa and mollusca. The textures are packstone–grainstone or Xoatstone. The depositional environment is interpreted as the upper part of a carbonate slope. This interpretation is supported by the mm size debris of reef-derived particles, in particular coralline red algae and corals. This microfacies is distinguished from the reef facies by an abundance of angular chips of cor-
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Plate 2. Fig. 1. Microfacies E Nummulitidae, Neorotalia, bioclast packstone (Sample No. Z73). Fig. 2. Microfacies E: Neorotalia, bioclast, echinoid packstone (Sample No. Z62). Fig. 3. Microfacies F: Nummulitidae, Miogypsinoides, Neorotalia, coral, corallinacean Xoatstone (Sample No. 71). Fig. 4. Microfacies F: Nummulitidae, Miogypsinoides, Neorotalia, coral, corallinacean packstone–Xoatstone (Sample No. Z68). Fig. 5. Microfacies F: Nummulitidae, Miogypsinoides, Neorotalia, coral, corallinacean Xoatstone (Sample No. Z72). Fig. 6. Microfacies G: Bioclast, corallinacea, coral Xoatstone (Sample No. Z60). Fig. 7. Microfacies G: Bioclast, corallinacea, coral Xoatstone (Sample No. Z70). Fig. 8. Microfacies H: Bioclast packstone–grainstone (Sample No. Z12).
alline red algae and shell debris, and the absence of in-place boundstone fabrics. A similar microfacies was reported by Wilson (1975), Longman (1981), Flugel (1982), Riding et al. (1991) and Melim and Scholle (1995).
margin, separating the open-marine from the more restricted marine environment. 6.9. Microfacies I–bioclast, nummulitida, miliolids wackestone–packstone (Pl. 3/1–2)
6.8. Microfacies H–bioclast packstone–grainstone (Pl. 2/8) The main bioclastic components are echinoidea, mollusca, bryozoa, and small benthic foraminifera. This facies has a packstone–grainstone texture. The features of these facies indicate moderate to high-energy shallow water conditions with signiWcant movement and reworking of bioclasts. In accordance with the standard microfacies types described by Wilson (1975) and Flugel (1982), microfacies H is interpreted as a shoal environment that was located at the platform
This facies is composed of wackestone–packstone with micritic bioclastics. Skeletal grains include echinoid, nummulitidae (Operculina,Heterostegina and Spiroclypeus) and miliolids. The minor taxa are Borelis, Austrotrillina, lepidocyclinidae and Neorotalia. The occurrence of perforate benthic foraminifera and imperforate benthic foraminifera indicate that sedimentation took place in a shelf lagoon (Romero et al., 2002).
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Plate 3. Fig. 1. Microfacies I: Bioclast, nummulitida, miliolids packstone (Sample No. Z46). Fig. 2. Microfacies I: Bioclast, nummulitida, miliolids packstone (Sample No. Z74). Fig. 3. Microfacies J: Intraclast, miliolids, bioclast packstone–grainstone (Sample No. Z83). Fig. 4. Microfacies K: Bioclast, imperforated foraminifera wackestone–packstone (Sample No. Z221). Fig. 5. Microfacies K: Bioclast, imperforated foraminifera wackestone–packstone–grainstone (Sample No. Z193). Fig. 6. Microfacies K: Bioclast, imperforated foraminifera packstone–grainstone (Sample No. Z84). Fig. 7. Microfacies K: Bioclast, imperforated foraminifera packstone–grainstone (Sample No. Z89). Fig. 8. Microfacies L: Mudstone (Sample No. Z178).
A similar microfacies was reported from a shelf lagoon environment by Nebelsick et al. (2001) and Corda and Brandano (2003). 6.10. Microfacies J–intraclast, miliolids, bioclast packstone– grainstone (Pl. 3/3) The main biotic components consist of miliolids and echinoidea. Intraclasts are also present. Corallinacean, Neorotalia and Miogypsinoides, and small benthic foraminifera such as Discorbis are the minor elements. Textures reXect poorly sorted packstone–grainstone. Some of the grains have been partially micritized. Co-occu-
rence of normal marine bioclasts and lagoonal biota with intraclasts suggest deposition at the lagoonal end of the platform margin (Hallock and Glenn, 1986). A similar facies with normal marine fauna and lagoonal biota was reported by Lasemi (1995) from the lagoonward side of a platform margin of the Upper Jurassic Kopet Dagh Basin, NE Iran. 6.11. Microfacies K–bioclast, imperforated foraminifera wackestone–packstone–grainstone (Pl. 3/4-7) The abundant components of this microfacies are benthic foraminifera with imperforated walls that include:
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Plate 4. Fig. 1. Microfacies M: Fenestrate mudstone with evaporate psuedomorphs (Sample No. Z142). Fig. 2. Microfacies M: Fenestrate mudstone with birdseye structures (Sample No. Z142). Fig. 3. Microfacies N: Stromatolitic boundstone (Sample No. Z169, Location is between sequence 2 and sequence 3). Fig. 4. Polished rock surface of stromatolite layer (Sample No. Z169).
Archaias, Peneroplis, Dendritina, Meandropsina, Borelis, Austrotrillina and miliolids. Other biota are corallinacean, coral, bryzoa and shell fragments. These deposits include diVerent textures ranging from wackestone to packstone and grainstone. The occurrence of a large number of imperforate foraminifera tests indicates that the sedimentation took place in a shelf lagoon setting (Geel, 2000; Romero et al., 2002). 6.12. Microfacies L–mudstone (Pl. 3/8) Lime mud generally comprises from 90 to 100 percent of this rock while carbonate and non-carbonate grains are rare. These sediments contain sparse non-diagnostic fauna. This facies was deposited in a restricted lagoon. Evidence for this interpretation includes the paucity of fauna both in diversity and abundance, lack of subaerial exposure features, and a stratigraphic position below the lagoonal facies (Wilson and Evans, 2002). 6.13. Microfacies M–fenestrate mudstone (Pl. 4/1–2) This microfacies consists of Wne grained microcrystalline limestone. Bioclasts are lacking, fenestrate structures are well developed, and evaporate psuedomorphs are rare. Birdseye or fenestral structures are typical products of shrinkage and expansion, gas bubble formation, and air escape during Xooding, or may even result from burrowing activity of worms or insects (Shinn, 1983). These vuggy structures are typical of a tidal Xat zone (Shinn et al., 1965; Shinn, 1968, 1983; Ginsburg and Hardie, 1975). The Wne grained nature of this facies, lack of fauna, and presence of fenestral fabric suggest that deposition occurred in a tidal Xat environment.
6.14. Microfacies N–stromatolitic boundstone (Pl. 4/3–4) These deposits are represented by mud-supported lithotypes formed by millimeter thick laminae, generally without fossils, irregularly undulating and laterally continuous (stromatolitic type cryptoalgal laminae). The cyanobacteria with their Wlamentous features trapping and binding of sedimentary particles produced a laminated sediment or stromatolite. This facies type is common in tidal Xat sediments (Flugel, 1982; Hardie, 1986; SteinhauV and Walker, 1996; Lasemi, 1995; Hernandez-Romano, 1999; Aguilera-Franco and Hernandez-Romano, 2004). Today, Xat laminated structures of microbial origin are found in intertidal settings. In regions with an arid climate (e.g. Persian Gulf or Shark Bay) stromatolites with smooth mats are located in the lower intertidal zone (Davies, 1970a,b; Kinsman and Park, 1976; HaVman, 1976). 7. Sedimentary model The gradual transitions between lithofacies and biofacies types, and the depositional proWle of the shelf transect investigated, indicate a very low gradient (Fig. 4). Owing to the generally similar morphology and hydrodynamic setting of shelf systems, the most appropriate model appears to be that of a very low-gradient, homoclinal carbonate ramp (Burchette and Wright, 1992). The absence of a shelfbreak is also corroborated by the lack of re-sedimented lowstand deposits. By comparing the textures and fossil assemblages of the Asmari Formation with those of recent settings, it seems that the Asmari Formation ramp can be compared to the present day homoclinal ramp of the Persian Gulf (Read, 1985; Jones and Desrochers, 1992).
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Fig. 4. Depositional model for the platform carbonates of the Asmari Formation at Chaman–Bolbol area, Zagros Basin, SW Iran. The interpretation is adopted from Hottinger (1997), Pomar (2001b), Wilson et al. (1999), and Rasser and Nebelsick (2003).
The tidal Xat sediments are composed of fenestral lime mudstone and stromatolitic boundstone. In the inner ramp, the most abundant lithofacies are medium to coarse-grained larger foraminifer with imperforate wall– bioclast wackestone-packestone (Fig. 4). The presence of imperforate foraminifera that include Archaias, Peneroplis, Dendritina, Meandropsina, Borelis, Austrotrillina, and miliolids indicates a low-energy, upper photic, shallow shelf lagoon depositional environment. Generally the upper photic zone is dominated by porcellaneus larger foraminifera predominantly living in symbiosis with dinophyceans, chlorophyceans or rhodophyceans (Leutenegger, 1984; Romero et al., 2002). The middle ramp setting is characterized by the medium to Wne grained foraminiferal–bioclastic packestone-grainstone-Xoatstone dominated by assemblages of larger foraminifera with perforate walls such as Lepidocyclina, Eulepidina, Nephrolepidina, nummulitidae (Operculina, Heterostegina, Spiroclypeous), Amphistegina, Miogypsina, and Miogypsinoides (Fig. 4). The faunal association suggests that the depositional environment was situated in the mesophotic to oligophotic zone (Hottinger, 1997; Pomar, 2001). The lower photic zone is dominated by large, Xat, and perforated foraminifera (such as lepidocyclinids) associated with symbiont-bearing diatoms (Leutenegger, 1984; Romero et al., 2002). Lower slope facies are diVerentiated from upper slope by the greater amount of micritic matrix, an increase in the Xatness, and size of the perforate foraminifera and presence of planktonic foraminifera. The outer ramp was characterized by low energy conditions and deposition of mudstone. The lack of bioclasts and
the appearance of planktonic foraminifera indicates deep water (Corda and Brandano, 2003). 8. Sequence stratigraphy The general vertical facies succession through the Asmari Formation and the stacking patterns and upward changes in the parasequences permit the identiWcation of sequences and their component systems tracts within the succession. Sequence boundaries are the key to determine third-order depositional sequences and these boundaries are dependent on the evidence from vertical facies changes (Fig. 5). On this basis, sequence stratigraphy terminology (i.e., maximum Xooding surface, sequence boundary, Highstand Systems Tract, Transgressive Systems Tract) has been applied to the sequences recognized in the Chaman Bolbol section. The transgrassive or upward-deepening portion of sequences (Transgressive Systems Tract, TST) ends with the maximum Xooding surface that records the deepest water depths attained for each sequence. The regressive or upward-shallowing portion of sequences (Highstand Systems Tract, HST) ends at the sequence boundary, which records the eVects of minimum longterm accommodation (Nicols, 1999; Emery and Myers, 1996). 8.1. Sequence 1 This sequence is 169.15 m thick and its facies associations can be grouped into Transgressive and Highstand Systems Tracts. The basal part of a basinal facies is inter-
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Fig. 5. Vertical facies distribution and sequences of the Asmari Formation at Chaman–Bolbol section, Zagros. TST: transgressive systems tract; EHST: early highstand systems tract; LHST: late highstand system tract; mfs: maximum Xooding surface; SB2: sequence boundary type 2. (For symbols see Fig. 4.)
preted as the Transgressive Systems Tract (TST) of the sequence 1 (Fig. 5). These sediments consist of marl with planktonic foraminifera and larger benthic foraminifera with perforated walls. Above this package, the strata show an increase in deeper water fauna and this bed is equivalent to the mfs.
Wackestone and packstone with perforated larger benthic foraminifera overlie the mfs. These sediments were mostly deposited in a middle and upper slope environment and are interpreted as the EHST (Early HST). The upper part of the HST (Late HST or LHST) is characterized by wackestone– packstone with abundant imperforate foraminifera, which
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indicate a change from open marine to a restricted palaeoenvironment. The boundary between sequence1 and sequence 2 is characterized by fenestrate mudstone. The boundary between sequence 1 and sequence 2 is a type 2 of sequence boundary (SB2), because the sequence boundary shows no clear evidence of subaerial exposure. 8.2. Sequence 2 The thickness of sequence 2 is nearly 20 m. The lower part of sequence 2 (TST) consists of open shelf lagoon deposits that are mostly characterized by the simultaneous occurrence of both perforated and imperforated foraminifera. The upper part of sequence 2 (HST) is composed of limestone with imperforate foraminifera. The boundary between sequence 2 and sequence 3 is characterized by stromatolitic boundstone (SB2). 8.3. Sequence 3 Sequence 3 is entirely composed of 30.1 m lagoonal facies with no evidence of deepening. They have an aggradational stacking pattern. This sequence is overlain by the Gachsaran Formation of Middle Miocene age. There is a disconformity (SB1) between the Asmari Formation and the Gachsaran Formation. 9. Comparison between the Chaman–Bolbol and Lali sections From all the sections logged in the platform of the Zagros Basin, the Lali section is the most representative and for this reason was selected for comparison with the studied section. The Asmari Formation in the Lali section is Late Oligocene-Early Miocene in age, a determination that is mainly based on foraminifera. However, an Early Miocene age has been concluded for our section. Vaziri-Moghaddam et al. (2006) described 10 diVerent types of microfacies from the Lali section that are similar to those found in our study area. Microfacies 1, 2, 3, and 4, which were deposited in an open marine environment, are very similar to microfacies A, B, C, and D of our study area. Microfacies 7, 8, and 9 described in the above mentioned article characterizes a lagoon environment, like, L, M, and N dose here. In both regions, the sequence boundaries are not associated with long periods of exposure and there is an unconformity (SB1) between the Asmari Formation and the succeeding Gachsaran Formation. In both sections, on the basis of the distribution of the foraminifera, four major depositional environments are identiWed. These include tidal Xat, shelf lagoon, shoal, and open marine environments. 10. Conclusions The Asmari Formation represents sedimentation on a carbonate ramp. Fourteen microfacies are recognized
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within this carbonate platform section. They are grouped into four depositional environments representing tidal Xat, shelf lagoon, shoal, and open marine. Three third-order depositional sequences are identiWed from shallowing and deepening trends of depositional facies and changes in the cycle stacking pattern. Sequence 1 is composed of transgressive and highstand systems tracts. Transgressive systems tracts are associated with planktonic, smaller benthic and large perforated foraminifera. Highstand systems tracts are Wrst characterized by large perforated foraminifera (EHST) and then imperforated foraminifera (LHST). Sequence 2 is composed of transgressive and highstand systems tracts. The transgressive systems tract consists of open shelf lagoon deposits that are mostly characterized by the simultaneous occurrence of both perforated and imperforated foraminifera. The upper part of the sequence 2 or highstand systems tract is composed of limestone with imperforated foraminifera. The boundary between sequence 2 and sequence 3 is characterized by stromatolitic boundstone (SB2). Sequence 3 is characterized by aggredational lagoonal facies. Acknowledgments The authors appreciate Iran Oil OVshore Company (IOOC) and A. Ghobishavi for help in the Weld. This study was completed at the University of Esfahan and supported by oYce of graduate studies. The authors are grateful to the oYce for their support. The authors would also appreciate the reviewers for their helpful comments. References Adams, C.G., Bourgeois, E., 1967. Asmari biostratigraphy. Geological and Exploration Div., Iranian Oil OVshore Company Report 1074, Unpubl. Aguilera-Franco, N., Hernandez-Romano, U., 2004. Cenomanian-Turonian facies succession in the Guerrero-Morelos Basin, Southern Mexico. Sedimentary Geology 170, 135–162. Ala, M.A., 1982. Chronology of trap formation and migration of hydrocarbons in Zagros sector of southwest Iran. American Association of Petroleum Geologists Bulletin 66, 1536–1542. Alavi, M., 2004. Regional stratigraphy of the Zagros fold-thrust belt of Iran and its proforeland evolution. American Journal of Science 304, 1–20. Brandano, M., Corda, L., 2002. Nutrients, sea level and tectonics: constrains for the facies architecture of a Miocene carbonate ramp in central Italy. Terra Nova 14, 257–262. Burchette, T.P., Wright, V.P., 1992. Carbonate ramp depositional systems. Sedimentary Geology 79, 3–57. Busk, H.G., Mayo, H.T., 1918. Some notes on the geology of the Persian oilWelds. Journal Institute Petroleum Technology 5, 5–26. Corda, L., Brandano, M., 2003. Aphotic zone carbonate production on a Miocene ramp Central Apennines, Italy. Sedimentary Geology 61, 55–70. Cosovic, V., Drobne, K., Moro, A., 2004. Paleoenvironmental model for Eocene foraminiferal limstones of the Adriatic carbonate platform (Istrian Peninsula). Facies 50, 61–75. Davies, R.G., 1970a. Carbonate bank sedimentation, Eastern Shark Bay, Western Australia. In: Logan, B.V., et al. (Eds.), Carbonate Sedimentation and Environments Shark Bay, Western Australia. Am. Assoc. Pet. Geol. Mem. 13, pp. 85–168.
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