Depositional environment, sequence stratigraphy and geochemistry of Lower Cretaceous carbonates (Fahliyan Formation), south-west Iran

Journal of Asian Earth Sciences 39 (2010) 148–160

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Depositional environment, sequence stratigraphy and geochemistry of Lower Cretaceous carbonates (Fahliyan Formation), south-west Iran M.H. Adabi a, M.A. Salehi a,*, A. Ghabeishavi b a b

Department of Geology, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran Department of Geology, National Iranian South Oil Company (NISOC), Ahwaz, Iran

a r t i c l e

i n f o

Article history: Received 31 July 2008 Received in revised form 9 February 2010 Accepted 23 March 2010

Keywords: Fahliyan Formation Lower Cretaceous Zagros Depositional environment Sequence stratigraphy Geochemistry

a b s t r a c t The Fahliyan Formation is a carbonate sequence of Lower Cretaceous (Berriasian–Hauterivian) age and was deposited in the Zagros sedimentary basin, Iran. In this investigation, the Fahliyan Formation at the type section and in the subsurface has been studied. Facies analysis and petrographic studies led to the recognition of 10 microfacies that were deposited in three facies belts: lagoon, shoal and open marine. The observed facies patterns indicate a carbonate rimmed-shelf depositional environment. Based on field observations, microfacies analysis and sequence stratigraphic concepts, two-third-order sequences in the type section and three-third-order sequences in the subsurface section were identified. The transgressive deposits display a predominance of deep subtidal facies, while highstand deposits show shallow subtidal facies. Some petrographic evidence such as an abundance of aragonite skeletal and non-skeletal components shows that this formation was deposited in a sub-tropical environment with original aragonite mineralogy. Geochemical evidence such as high Sr/Na ratios also support original aragonite mineralogy. d18O and d13C values suggests that alteration occurred during burial diagenesis, in a closed system, with low water/rock interaction. Palaeotemperature calculation, based on the heaviest oxygen isotope value in micritic samples of the Fahliyan limestone, shows that ambient water temperature was around 24 °C during the deposition of this formation. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction The Fahliyan Formation, a thick carbonate sequence of the Lower Cretaceous, is mainly a shallow marine carbonate succession that was deposited on the Arabian plate, the NE passive margin of Gondwanaland (James and Wynd, 1965; Motiei, 1993; Alavi, 2004). The Fahliyan Formation is present throughout the Zagros Basin, but it is best developed in the northern Persian Gulf area (Setudehnia, 1978) (Fig. 1A and B). The widespread development of reservoirs in the platform facies and proximity to organic-rich, deepwater Garu Formation and other source rocks have made the Fahliyan Formation one of the important exploration targets in the Persian Gulf region and the onshore Oil Fields of SW Iran (Motiei, 1993; Lasemi and Nourafkan Kondroud, 2008). Stratigraphy of the Fahliyan Formation have been studied by several authors (e.g. Wells, 1965; James and Wynd, 1965; Shakib, 1994; Hashemi-Hosseini, 2006) and sedimentary environments by Moallemi (2000). Recent facies analysis have revealed that the

Fahliyan Formation in some surface and subsurface sections of SW of Iran contain both shallow (restricted marine, barrier, and open marine) and deep marine (plagic and calciturbidite) facies which were deposited on a carbonate shelf platform and intrashelf basin, respectively (Khazaei, 2003; Lasemi et al., 2003; Mohammad-Khani, 2003; Lasemi and Feyzi, 2007). The sedimentation patterns and paleogeography of the Zagros Cretaceous strata including the Fahliyan Formation have been studied by Gaumet et al. (2002). In the above study the geodynamical basin evolution is analysed based on paleogeographical and isopach maps. The aim of this study are microfacies analysis, reconstruction of sedimentary environment, sequence stratigraphy framework and recognition of original carbonate mineralogy of the Fahliyan Formation in the study area. The present study uses petrographic, elemental and oxygen and carbon isotope evidence for determination of the original carbonate mineralogy and diagenesis of the Fahliyan limestones.

2. Geological setting * Corresponding author. Present address: Department of Geology, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran. Tel.: +98 21 2990 2617; fax: +98 21 2243 1690. E-mail addresses: [email protected] (M.H. Adabi), [email protected] (M.A. Salehi). 1367-9120/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2010.03.011

The Zagros fold–thrust belt is the deformed state of the Zagros sedimentary basin. This basin extendes over the northeastern (present coordinates) Afro-Arabian continental margin and was

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Fig. 1. Location map of the study area. (A) General map of Iran showing eight geologic provinces. The study area is located in the Zagros province (modified from Heydari et al., 2003). (B) Main structural subdivisions of the Zagros fold–thrust belt (modified from Farzipour-Saein et al. (2009)). (C) Fahliyan anticline and Gachsaran Oil Field have been shown in the structural map of Zagros Basin (modified from Sherkati and Letouzey (2004)).

affected by Early Cretaceous to present Zagros orogeny (Alavi, 2007). This belt is one of the important tectonic units of Iran and has a length of more than 1500 km, and width between 100 and 300 km (Motiei, 1993; Alavi, 2004) (Fig. 1A). During the Palaeozoic, Iran, Turkey and the Arabian plate (which now has the Zagros belt situated along its NE border), together with Afghanistan and India, made up the long, very wide and stable passive margin of Gondwanaland bordering the Palaeo-Tethys Ocean to the north (Berberian and King, 1981). The final closure of Palaeo-Tethys in the Tethyan domain took place during the Triassic to Jurassic (Stampfli and Borel, 2002; Golonka, 2004). The Neo-Tethys opened from the Late Carboniferous to the late Early Permian, beginning in the east of Australia and progressing to the eastern Mediterranean area (Stampfli and Borel, 2002). The Zagros orogen is a product of the closure of the Neo-Tethys that involved three major sequential geotectonic

events (Alavi, 1994, 2004, 2007): (1) subduction of the Neo-Tethyan oceanic crust beneath the Iranian plates, (2) emplacement of slivers of oceanic crust over the Afro-Arabian continental margin, and (3) collision of the Afro-Arabian continental margin with the Iranian plates. The sedimentary column in the Zagros is estimated to be 7–12 km. This is classified into four groups of rocks accumulated in different tectonosedimentary environments through Latest Neoproterozoic to Phanerozoic (Alavi, 2004, 2007). The Lowermost Jurassic to upper Turonian strata (third group) form a number of megasequences that accumulated on a shallow continental-shelf, facing N and NE toward Neo-Tethys in a palaeoequatorial setting (Alavi, 2004). Facies and thickness changes in a short distance have been reported at various stratigraphic sections in the Lower Cretaceous sediment in the Zagros sedimentary basin (Setudehnia, 1978;

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Sepehr and Cosgrove, 2004). Alavi (2004) concluded that facies variation, from SW to NE of Zagros basin through the Upper Jurassic to Lower Cretaceous, is due to reactivation of pre-existing, predominantly NS trending structural elements (Izeh and Kazeroun basement faults) inherited from underlying Neoproterozoic Pan-African belts. The Zagros fold–thrust belt, is divided into several zones (Lurestan, Izeh, Dezful Embayment, Fars, High Zagros) (Fig. 1B) that differ according to their structural style and sedimentary history (Falcon, 1961; Berberian and King 1981; Motiei, 1993). The study area is located in NE of the Dezful Embayment and SE of the Izeh zone (SW Iran) (Fig. 1B and C). The Izeh zone and Dezful Embayment are separated from Lurestan and Fars by the Balarud and Kazerun faults, respectively (Falcon, 1974; Motiei, 1993). 3. Lithostratigraphy The Lower Cretaceous of Zagros basin is composed of platform carbonate deposited in a shallow marine environment and is subject to notable variations in thickness (Ghazban, 2007). The thickness of the Fahliyan Formation at type section, near the village of Fahliyan on the south flank of Fahliyan anticline in SE of Izeh zone, was measured 283 m (Fig. 1C). This formation is composed of grey to brown, massive, oolitic to pellety limestone with minor brecciation in the basal portion (James and Wynd, 1965) (Fig. 2A). The basal contact, with sedimentological evidence of subaerial exposure that is represented by sucrosic, dark brown brecciated dolomites of the Surmeh, is unconformable and the upper contact with marls and thin limestones of the Gadvan Formation is conformable (Fig. 2B). This formation at subsurface section (Gachsaran Oil Field, well number 55) consists of 582 m of limestone which is overlain by marl and shale of the Gadvan Formation, and unconformably overlies anhydrite of the Hith Member.

used. For sequence stratigraphic interpretation, the concepts developed by many investigators (Posamentier et al., 1988; Emery and Myers, 1996; Catuneanu, 2006) were used. Powders of 38 micritic samples were dissolved in 1 Mole HCl for two hours and analyzed by Atomic Absorption Spectrometry for Ca, Mg, Sr, Na, Mn and Fe, at the Geology Department of the Shahid Beheshti University, Tehran, Iran, using 0.125 g samples. Because of low permeability, Micrite samples are more reliable components for geochemical analysis (Asmerom et al., 1991; Adabi, 2004b). The level of precision in the analysis was ±5 ppm for Sr, Na, Mn and Fe (Robinson, 1980). Standard samples were used to monitor the precision. Samples with more than 10% insoluble residue were not used in trace element interpretation. For oxygen and carbon isotope analysis, 15 mg of the powder of 13 limestone samples were allowed to react with anhydrous phosphoric acid in reaction tubes in a vacuum at 25 °C. The CO2 extracted from each sample was analyzed for d18O and d13C values, using VG STRA Series II at the Central Science Laboratory, University of Tasmania, Australia. The precision of data was established with duplicate analysis for both oxygen and carbon, is ±0.1‰. The maximum, minimum and mean of the chemical and isotopic analysis are given in Table 1 and 2. 5. Microfacies analysis Petrographic analysis led to recognition of 10 microfacies that are related to three depositional facies belts. These microfacies are described from deep to shallow environment.

Table 1 Major (in wt.%) and minor element compositions (in ppm) of carbonate rocks of the Fahliyan Formation, Iran.

4. Methods Samples were collected near significant lithologic changes within the stratigraphic succession of the Fahliyan type section and subsurface section. 160 uncovered polished thin sections were prepared for petrographic and sedimentological analysis of the Fahliyan carbonates. In addition, 400 subsurface thin sections were studied for facies analysis. Petrographic analysis was carried out on stained thin sections (Alizarin Red-S and potassium ferricyanide solutions) (Dickson, 1965) in order to distinguish ferroan and non-ferroan calcite and dolomite. The petrographic classification for carbonates is based on Dunham limestone classification (Dunham, 1962). Wilson (1975) and Flügel (2004) facies belts and sedimentary models were also

Max. Min. Mean

Ca (%)

Mg (%)

Sr (ppm)

Mn (ppm)

Na (ppm)

Fe (ppm)

39.34 38.36 38.93

0.81 0.24 0.39

1429 381 609

154 21 38

440 154 256

211 73 117

Table 2 Carbon and oxygen isotope compositions of carbonate rocks of the Fahliyan Formation, Iran. d13C Max. Min. Mean

1.16 0.87 0.54

d18O 2.77 5.93 4.73

Fig. 2. (A) Field view of the Fahliyan Formation at type section; brecciation in the basal portion is the sedimentological evidence of subaerial exposure; (B) limestone outcroups at the top of the Fahliyan Formation (type section).

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5.1. Open marine Two microfacies were recognized in this environment and composed mainly of radiolarian, sponge spicule and peloid. The open marine microfacies exist at the basal portion of subsurface section. These microfacies from distal to proximal environment include: 5.1.1. Radiolarian sponge spicule wackestone (MF 1) This microfacies is dominated by sponge spicule and radiolaria (20%). Spicules often appear circular in transverse sections. Peloids are scarce in this microfacies (Fig. 3A). 5.1.2. Fine-grained calciturbidite (MF 2) This microfacies is made up of alternation of fine peloid packstone and grainstone. The predominant grain in this microfacies

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is peloid (80%). (Fig. 3B). Fining upward cycles of calciturbidites facies which are with intercalation of deep open marine facies has been well-documented in the lower depositional sequence of Fahliyan Formation in some surface and subsurface sections of Izeh zone and South Dezful Embayment (Khazaei, 2003; Lasemi et al., 2003; Mohammad-Khani, 2003; Lasemi and Feyzi, 2007).

5.2. Interpretation In MF 1, the occurrence of radiolaria indicates that this microfacies belongs to the deepest part of the open marine environment (Flügel, 2004; Ghabeishavi et al., 2010). Abundant radiolaria shows very deep, cold water environment (Casey, 1993). In MF 2, calciturbidite is considered as resedimentation of carbonate particles of platform interior in preplatform basins. Calciturbidites generally

Fig. 3. Microfacies variations in the Fahliyan Formation. Scale bar represents 0.5 mm; (A) Radiolarian sponge spicule wackestone (MF 1) representing deep open marine; (B) fine-grained calciturbidite (MF 2) deep open marine; (C) intraclast ooid grainstone (MF 3) shoal facies; (D) ooid aggregate grainstone (MF 4) shoal facies; (E) bioclast peloid grainstone (MF 5) shoal facies; (F) intraclast grainstone (MF 6) shoal facies.

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comprise packstone or grainstone, coarse grains at the base and fining upwards into periplatform muds (Droxler and Schlager, 1985). These evidences poorly exist in the cutting samples of subsurface section, but have been detected in the outcrop and some other subsurface sections in Izeh zone and South Dezful Embayment, nearby to the subsurface section of this study (Khazaei, 2003; Lasemi et al., 2003; Mohammad-Khani, 2003; Lasemi and Feyzi, 2007). It has been noted that turbidites are more frequent in highstand intervals (highstand shedding, e.g., Reijmer et al., 1992; Schlager et al., 1994; Andresen et al., 2003; Schlager, 2005). Studies of calciturbidite composition showed that highstand turbidite contain more non-skeletal grains (like MF 2) derived from platform interior (Haak and Schlager, 1989; Reijmer et al., 1992). 5.3. Shoal Four microfacies were recognized in this environment and composed mainly of ooid, peloid and intraclast. 5.3.1. Intraclast ooid grainstone (MF 3) Concentric ooids are the dominate component (30%) and are evenly dispersed within the sparry calcite cement. Most ooids are commonly subspherical to subelongate, ranging in size from 0.2 to 0.8 mm, with an average size of 0.5 mm. The cortex thickness of ooids is inversely related to the size of the nucleus. Intraclasts are common in this microfacies and are often rounded. Oncoid, benthic foraminifera and gastropods are rare in this facies. In this microfacies, ooids have undergone intensive micritization (Fig. 3C). This microfacies mainly occurs in the type section of the Fahliyan Formation, although only a thin unit of this microfacies is present in the subsurface section. 5.3.2. Ooid aggregate grainstone (MF 4) This microfacies is characterized as aggregate. These grains consist of different component such as ooids, green algae and benthic foraminifers occurred within micritic materials ranging in size from 1 to 8 mm (Fig. 3D). Composite ooid, peloid and green algae are scarce in this microfacies. Intergranular and vug porosities are filled with equant sparry calcite cement. 5.3.3. Bioclast peloid grainstone (MF 5) This microfacies is characterized by a high aboundance of peloids (50%). These grains are spherical, structureless and wellsorted. Subordinate benthic foraminifera and intraclast are also present (20%) (Fig. 3E). 5.3.4. Intraclast grainstone (MF 6) Intraclasts are the main component of this micofacies. Intraclasts are generally polymodal in size, ranging from 0.5 to 8 mm, with an average of 4.5 mm. Most of the intraclast are subangular to angular. Some intraclasts are internally homogeneous and consist of micrites, while others display internal compositions such as pelloids and fossils. Micrite is rare but locally forms a packstone (Fig. 3F). 5.3.5. Interpretation Grainstone texture, concentric ooids, aggregate and well-sorted components in these facies belt are indicators of high energy environment (Wright and Burchette, 1996; Flügel, 2004; Reolid et al., 2007). Such high energy deposits are typically associated with carbonate shoals and bars on or near the seaward edge of platforms (Wilson, 1975; Van Buchem et al., 2002; Flügel, 2004). Intraclast grainstones are often interpreted as deposits formed by storm wave erosion, tidal currents and reworking of various sediment types occurring in shallow-marine environments (Flügel, 2004).

Based on facies characteristic, MF 6 is considered as high energy shoal facies. 5.4. Lagoon Four microfacies were recognized in this environment, and are composed mainly of peloid, green algae and benthic foraminifera (e.g. Miliolid and Trocolina). 5.4.1. Bioclast peloid packstone (MF 7) Peloids (40%) are the most abundant components in this microfacies. Most peloids are uniform in size and range from 100 lm to 0.5 mm, with an average size of 300 lm. Other components are dasyclads, echinoid debris, benthic foraminifera and lithocodium (Fig. 4A). 5.4.2. Benthic foraminifera dasyclads wackestone (MF 8) The main component in this microfacies is dasyclads green algae. Benthic foraminifera such as Trocolina, Pseudocyclaminia and Miliolids are the common skeletal components. Sponge spicule is present, but it is rare in this microfacies (Fig. 4B). 5.4.3. Fossiliferous mudstone (MF 9) This microfacies generally consists of about 90% to 100% lime mud, with 0–10% allochems. Bioclasts such as fragments of green algae, benthic foraminifera, echinoid and bivalves are very rare (Fig. 4C). This microfacies is very thick in subsurface section. 5.4.4. Silt size quartz grain lime mudstone (MF 10) In this microfacies, scattered silt to sand size quartz grains are present (Fig. 4D). Calcite pseudomorph after evaporates is the other feature of this microfacies. 5.4.5. Interpretation Microfacies in this width facies belt were recognized by fossil and sedimentary fabrics. MF 7 is interpreted to be located in the outer facies of a lagoon. The presence of sparry calcite cement in some parts of MF 7, indicate that this microfacies is deposited in a moderate to high energy environment influenced by tidal currents near shoal. MF 8 and MF 9 with benthic foraminifera and green algae, with wackestone and mudstone texture, shows lagoon depositional environment (Flügel, 2004; Husinec and Sokac, 2006; Bachmann and Hirsch, 2006). The components in MF 9 are similar to MF 8 but the associated skeletal grains (e. g. benthic foraminifera) are less diverse and not as abundant as in MF 8. Foraminifera diversity generally decreases from open to restricted environments (Amodio, 2006). Terrigenous sand which is present in MF 10 is common in platforms attached to land (Flügel, 2004; Schlager, 2005). Calcite pseudomorph after evaporates form in near coastal lagoon environment (Flügel, 2004). 6. Depositional environment Based on different facies associations recognized in the Fahliyan Formation, a carbonate rimmed-shelf model is proposed for this formation. High energy shoal facies belt formed a barrier at the platform margin and protected a very wide lagoon. The presences of calciturbidite facies, high energy shoal facies, protected lagoon, are all evidence of carbonate sequence deposition on a carbonate rimmed-shelf platform (Wright and Burchette, 1996; Flügel, 2004; Schlager, 2005). The Fahliyan Formation in the study area has many different sub-depositional environments from lagoon to deep open marine. The high thickness of lagoonal and shoal facies shows that the

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Fig. 4. (A) Bioclast peloid packstone (MF 7) lagoon facies; (B) benthic foraminifera dasyclads wackestone (MF 8) lagoon facies; (C) fossiliferous mudstone (MF 9) lagoon facies; (D) silt size quartz grain lime mudstone (MF 10) lagoon facies.

inner shelf was dominant in the study area. A depositional profile of Fahliyan Formation at the type section and subsurface section (Gachsaran Oil Field, well number 55), was reconstructed by using the relative position of facies belts (Fig. 5). Paleogeographical map which have been constructed for the Early Cretaceous of Zagros shows the presence of a shallow carbonate platform in the S and SW (Gaumet et al., 2002). At the base of the Cretaceous, a narrow N–S radiolarianrich deep-marine trough cuts through this platform probably following the Jurassic Gotnia basin orientation. Later, this trough is filled up and the platform progrades to the N and NE (Gaumet et al., 2002). Therefore, presence of the deep marine microfacies in the subsurface section of the Fahliyan Formation is in agreement with existence of a narrow N–S deep-marine trough through the platform. Based on microfacies analysis and mentioned paleogeographic map, the type section is located on shallow carbonate platform and the subsurface section on deeper depositional setting (‘‘marine trough” or ‘‘intra-shelf basin”). 7. Sequence stratigraphy The sedimentary facies characteristics of the Fahliyan Formation show a distinctive number of sequence boundaries, systems tracts and depositional sequences. The Fahliyan Formation in the type section consists of two third-order sequences, while in the subsurface section it consists of three third-order sequences that were deposited through Berriasian to Hauterivian. The type section and subsurface section of the Fahliyan Formation will be discussed in further detail for sequence stratigraphy. In the description of sections, deepening trends are considered to be a transgressive system tract (TST), shallowing trends are interpreted as a highstand system tract (HST); the change from deepening towards

shallowing is interpreted as maximum flooding zone (mfz). The sudden superposition of transgressive beds upon prograding beds is though to present a sequence boundary (SB) (Emery and Myers, 1996). The term ‘‘highstand shedding” means that a depositional system sheds most sediment into the adjacent basins during highstands of sea level (Schlager et al., 1994).

7.1. Type section (depositional sequence) The two-third-order depositional sequences at the type section are:

7.1.1. Sequence 1 (FA 1) This sequence is 84 m thick and its facies association can be grouped in transgressive and highstand systems tracts. The basal part of sequence 1 (TST), consists of different facies such lagoon and shoal. Above this package, the strata show an increase in deeper microfacies of shoal environment and are equivalent to the maximum flooding zone (mfz). Sediments that overlie the mfz mainly consist of lagoon, with thin interval of shoal microfacies (HST). The basal boundary in sequence 1 is sharp and clearly defined as type I sequence boundary, due to the presence of subaerial exposure that is represented by brecciated dolomites (Fig. 2A). In the most of the Zagros basin region the Fahliyan Formation rest unconformable either on carbonates of the Jurassic Surmeh Formation or on the Hith anhydrite (Ghazban, 2007). The upper boundary of this sequence is defined as a type II sequence boundary (basinward shift in facies), that shows no clear evidence of subaerial exposure (Fig. 6).

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Fig. 5. Schematic diagram of carbonate shelf with lateral facies relationship of the Fahliyan Formation at the type section and the subsurface section (Gachsaran Oil Field, well number 55).

7.1.2. Sequence 2 (FA 2) The thickness of this sequence is around 199 m. The lower part of sequence 2 (TST) consists of relatively thick bed of shoal facies. Mfz is placed at top of this unit and consists of intraclast ooid grainstone. The HST of sequence 2 mainly consists of lagoonal facies with thin interval of shoal facies. The lagoonal microfacies are mainly composed of green algae and benthic foraminifera (e.g. Trocolina). The diversity of skeletal component shows a gradual decrease towards the upper part of this system tract. The upper sequence boundary is sharp and easily traced, by retrogradation of deeper marine limestone and marls of the Gadvan Formation (Figs. 2B and 6). 7.2. Subsurface section (depositional sequence) The three-third-order depositional sequences at the subsurface section are: 7.2.1. Sequence 1 (GS 1) This sequence has a thickness of 179 m and directly overlays the anhydrite of Hith Member. Sequence boundary type I is recognized based on the existence of evaporite deposits, indicating sea level fall (Sarg, 2001). The basal part of sequence 1 (TST) consist of lagoonal microfacies with peloid and green algae. Above this package, the strata show an increase in deeper open marine microfacies (radiolarian sponge spicule wackestone), which is equivalent to mfz. The gama ray log along the stratigraphic column show the highest values in mfz (Fig. 7). The 37 meter thick of finegrained calciturbidite covers the mfz which were deposited during the high sea-level periods (‘highstand shedding’) (Droxler and Schlager, 1985; Schlager et al., 1994; Schlager, 2005). Grainstone and packstone of shoal and lagoon environment overlie the finegrained calciturbidite. These sediments are interpreted as HST

(Fig. 7). The boundary between sequences 1 and 2 is defined by retrogradation of shoal facies on lagoonal facies (type II) and shows no evidence of subaerial exposure (Fig. 7). 7.2.2. Sequence 2 (GS 2) The thickness of this sequence is around 249 m. The lower part of the sequence (TST) consists of shoal microfacies. The boundary between the TST and HST is marked by a change from shoal to lagoonal environment (Fig. 7). 7.2.3. Sequence 3 (GS 3) This sequence with the thickness of 154 m is developed on sequence boundary type II. The basal part of sequence 3 (TST) consists of lagoonal and shoal microfacies. The sediment that overlies mfz, consists mainly of lagoonal microfacies, which is interpreted as HST. Sequence boundary type II is recognized based on a change in depositional environment. This boundary is distinguished by upward-increasing gamma ray values and by retrogradation of deeper marine limestone and shale of the Gadvan Formation (Fig. 7). The sequence stratigraphic correlations are made along two sections (Fig. 8). The ages given for the third-order sequences are based on information from the literature, provided by the Hashemi-Hosseini (2006) for type section. 8. Determination of original carbonate mineralogy Studies of the original carbonate mineralogy during the Phanerozoic has been largely motivated by many researchers (e.g. Sandberg, 1983; Wilkinson et al. 1985; Hardie, 1996; Stanley and Hardie, 1998; Dickson, 2004). These workers believe that the carbonate mineralogy changes through the Phanerozoic, Mg/Ca ratio, PCO2 and sea level fluctuation are the main factors, although some other researchers (Nelson, 1988; Rao, 1991; Morse et al.,

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Fig. 6. Representative stratigraphic section showing the microfacies, depositional environments, relative sea-level curve and depositional sequence of the Fahliyan Formation at the type section.

1997; Adabi, 2004a) suggest that temperature is equally important. Carbonate mineralogy can vary in Recent shallow marine carbonates by changing sea water temperature (Morse et al., 1997; Adabi, 2004a).

Aragonite is the predominant mineral, along with some high-Mg calcite forming in modern sub-tropical warm-waters (Milliman, 1974). In modern temperate carbonates, high-Mg calcite predominates over low-Mg calcite and aragonite and in

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Fig. 7. Representative stratigraphic section showing the gamma ray profiles, microfacies, depositional environments, relative sea-level curve and depositional sequence of the Fahliyan Formation at Gachsaran Oil Field well number 55.

subpolar cold-water carbonates, the low-Mg calcite is dominate carbonate mineral (Rao, 1990; James and Clarke, 1997). Skeletal components in the Fahliyan Formation are mainly composed of green algae (Fig. 4A and B), and non-skeletal grains such

as peloids and ooids (Fig. 3C and E; Fig 4A), indicating that carbonates were deposited in a sub-tropical environment, similar to that in modern sub-tropical, warm shallow-marine waters which aragonite is primary mineralogy (Lees, 1975; Nelson et al., 2003).

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Fig. 8. Sequence stratigraphic correlation between two sections. Schematic diagram of carbonate shelf of the Fahliyan Formation is shown at the top.

Above evidence indicates that the carbonates were deposited in a sub-tropical environment, and aragonite was possibly the original carbonate mineralogy during deposition of the Lower Cretaceous Fahliyan Formation. 8.1. Geochemical evidence for original aragonite mineralogy 8.1.1. Trace elements Modern aragonite sediments in tropical warm shallow-marine waters have low Mn and Fe (20 ppm), moderate Na (2500 ppm) and high Sr (10,000 ppm) concentrations (Milliman, 1974). Modern, temperate shallow-marine calcite sediments on average have low Sr (3000 ppm) and high Na (5000 ppm), Mn (150 ppm) and Fe (1000 ppm) concentrations (Rao, 1990). Concentration of Sr (381–1429 ppm; mean 609 ppm), Na (154– 440 ppm; mean 256 ppm), Mn (21–154 ppm; mean 38 ppm) and Fe (73–211 ppm; mean 117 ppm) in the Fahliyan limestone suggest appreciable loss of Sr and Na, and a gain of Mn and Fe during moderate diagenesis when compared with modern tropical aragonite and temperate calcite carbonate. The plot of Sr–Mn values shows that most limestone samples fall within or close to the

warm-water sub-tropical aragonite fields of the Cretaceous Ilam Formation (Adabi and Asadi-Mehmandosti, 2008) (Fig. 9A). Modern and ancient tropical carbonates are differentiated from non-tropical counterparts by their Sr/Na ratio and Mn content (Rao, 1991; Adabi and Asadi-Mehmandosti, 2008). Modern tropical aragonite carbonates have low Mn and high Sr/Na ratio from 3 to 5, in contrast to modern temperate calcite, which have high Mn and low Sr/Na ratios, 1 (Adabi and Rao, 1991; Rao, 1991, 1996; Rao and Amini, 1995; Winefield et al., 1996; Adabi and AsadiMehmandosti, 2008). Sr/Na ratio in limestone of the Fahliyan Formation ranges from 1 to 4 (average 2.5) are similar to the Upper Cretaceous Ilam limestone (Adabi and Asadi-Mehmandosti, 2008) and Recent tropical aragonite (Milliman, 1974; Winefield et al., 1996) (Fig. 9B). The similarity of trace-element data with modern and ancient carbonates supports the interpretation that original carbonate mineralogy of the Fahliyan limestone was aragonite. 8.1.2. Oxygen and carbon isotopes Using oxygen and carbon stable isotope data, valuable information about palaeotemperature, diagenetic trend and recognition

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Fig. 9. (A) Comparison of the Sr and Mn of the Fahliyan limestone with recent tropical aragonite (A, Milliman, 1974) and altered aragonitic carbonates of the Upper Cretaceous Ilam Formation (Iran) (Adabi and Asadi-Mehmandosti, 2008). Note that most of the Fahliyan data falls within the sub-tropical Upper Cretaceous Ilam limestone field. (B) Mn versus Sr/Na variation in the Fahliyan limestone in comparison with the sub-tropical altered aragonitic carbonates of the Upper Cretaceous Ilam Formation (Iran) (Adabi and Asadi-Mehmandosti, 2008) and Recent tropical aragonite (Milliman, 1974) Note: Sr/Na of all Fahliyan samples are >1, indicating original aragonite mineralogy forming in a sub-tropical environment. (C) Comparison of d18O and d13C values of the Fahliyan limestone with the recent warm shallow-marine bulk carbonate field (Milliman and Müller, 1977), Cretaceous marine limestone (Kelth and Weber, 1964), altered aragonite carbonate of Ilam Formation Upper Cretacous (Iran) (Adabi and AsadiMehmandosti, 2008) and Tithonian–Berriasian Belemnites (Jenkyns et al., 2002). Note all limestone samples falls within Cretaceous marine limestones due to their similar age. (D) d18O versus Mn variation in Fahliyan limestone. Diagenetic trend for aragonite, high-Mg calcite (HMC) and low-Mg calcite (LMC) of Recent carbonate (R), Barlington limestone (CM) and Readbay limestone of Silurian (Canada) (Brand and Veizer, 1980) shows that the Fahliyan limestone have stabilized in the marine phreatic environment in a closed diagenetic system.

between different type of carbonate can be obtained (Marshall, 1992; Rao, 1996). Oxygen isotope values of the Fahliyan limestone vary between 2.77‰ PDB to 5.39‰ PDB (average 4.37‰ PDB) and carbon vary between 0.78‰ PDB to 1.16‰ PDB (average 0.54‰ PDB) (Table 2). In Fig. 9C, the isotopic values of the Fahliyan limestone have been compared with different isotopic carbonate fields. The isotopic fields include Recent warm shallow-marine bulk carbonate (Milliman and Müller, 1977), Cretaceous marine limestone (Kelth and Weber, 1964), altered aragonitic carbonates of the Upper Cretaceous Ilam Formation (Iran) (Adabi and Asadi-Mehmandosti, 2008) and Tithonian–Berriasian Belemnites (Jenkyns et al., 2002). Limestone samples of the Fahliyan Formation are plotted close to the field of Cretaceous marine limestone (Kelth and Weber, 1964) and altered aragonite carbonate of the Ilam Formation (Iran) (Adabi and Asadi-Mehmandosti, 2008). Oxygen and carbon isotope values of Tithonian–Berriasian Belemnites (Jenkyns et al., 2002) shows the LMC shell mineralogy has undergone less diagenetic effect, compared to the Fahliyan limestone aragonite samples. d18O and d13C values from the Fahliyan Formation suggest diagenetic alteration under a burial system (Fig. 9C). Variation of d18O versus Mn shows that the Fahliyan limestone has stabilized in a marine phreatic

environment in a closed diagenetic system (Brand and Veizer, 1980) (Fig. 9D). In a closed to semi-closed diagenetic system skeletal and non-skeletal carbonate grains with aragonite original mineralogy (like the Fahliyan limestones) would not show distinct dissolution and recrystallization or replacement features (James and Choquette, 1984). Palaeotemperature calculations, based on heaviest d18O ( 2.77‰ PDB) value of micrite sample and d18O value for Cretaceous seawater 1‰ SMOW (Shackelton and Kennett, 1975; Barron, 1983; Lecuyer and Allemand, 1999; Veizer et al., 1999; Gröcke et al., 2003; McArthur et al., 2004), using the equation of Anderson and Arthur (1983) indicates that minimum ambient water temperature was around 24 °C during deposition of this formation. The calculated temperature (24 °C) indicates that the Fahliyan limestones formed in a shallow marine sub-tropical environment, with original aragonite mineralogy similar to the Holocene carbonate in Persian Gulf. This interpretation is supported with the trace element and petrographic evidences.

9. Conclusions The Fahliyan Formation is a carbonate sequence of Lower Cretaceous age located in the Zagros sedimentary basin.

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Microfacies analysis has led to the recognition of 10 microfacies. Microfacies vary in lateral and vertical distribution, and show that carbonates were deposited on a carbonate rimmed-shelf platform. Distribution of facies in the subsurface section indicates that carbonates were deposited in a deeper depositional setting than the type section. Variation in relative sea level led to the deposition of two-thirdorder sequences in the type section and three-third-order sequences in subsurface section with sequence boundary types I and II in the Fahliyan Formation. The facies patterns clearly indicate relative sea level variations. Diverse skeletal and non-skeletal grains with abundant calcareous green algae present in the Fahliyan Formation, shows that these carbonates are similar to those of modern warm-water shallow-marine carbonates. Elemental and isotopic studies of carbonate rocks of the Fahliyan Formation at the type section indicate that these carbonates were deposited in a sub-tropical environment. The high Sr/Na ratios of more than 1 (average 2.5) confirm original aragonite mineralogy. Stable isotope and trace element values of the Fahliyan Formation show that the Fahliyan limestones were affected by fluids in a closed burial diagenetic system, with a low water/rock interaction. Palaeotemperature calculation based on the heaviest d18O value of micrite sample shows that minimum ambient water temperature was around 24 °C during deposition of the Fahliyan limestone. Acknowledgments The authors would like to thank National Iranian South Oil Company, especially Dr. Hassan Amiri-bakhtiyar and Dr. Hormoz Ghalavand for logistic support, Mr. Rohallah Shabafrooz and Ali Rahmani for their assistance in field studies, Mrs. Pourandokht Shojaei for elemental analysis at the Geology Department, Shahid Beheshti University, Iran and the Central Science Laboratory, University of Tasmania, Australia for isotope analysis. We also appreciate Dr. Benoit Vincent, Dr. Ali Moallemi, Dr. Reza Moussavi-Harami and Dr. Asadollah Mahboubi for their useful comments on the manuscript; Mr. Alireza Ghasempour and Tony Collings for editing the text; JAES chief and associate editors, and reviewers for their excellent review that improved our manuscript significantly. This research was financially supported by Shahid Beheshti University, Iran. References Adabi, M.H., 2004a. A re-evaluation of aragonite versus calcite seas. Carbonates and Evaporites 19, 133–141. Adabi, M.H., 2004b. Geochemistry of Sedimentary Rocks. Ariyan-Zamin Publication, Tehran (in Persian), 448 p. Adabi, M.H., Asadi-Mehmandosti, E., 2008. Microfacies and geochemistry of the Ilam Formation in the Tange-E Rashid area, Izeh, S.W. Iran. Journal of Asian Earth Sciences 33, 267–277. Adabi, M.H., Rao, C.P., 1991. Petrographic and geochemical evidence for original aragonitic mineralogy of Upper Jurassic carbonate (Mozduran Formation), Sarakhs area, Iran. Sedimentary Geology 72, 253–267. Alavi, M., 1994. Tectonics of the Zagros orogenic belt of iran: new data and interpretations. Tectonophysics 229 (3–4), 211–238. Alavi, M., 2004. Regional stratigraphy of the Zagros fold–thrust belt of Iran and its proforeland evolution. American Journal of Sciences 304, 1–20. Alavi, M., 2007. Structures of the Zagros fold–thrust belt in Iran. American Journal of Sciences 307 (9), 1064–1095. Amodio, S., 2006. Foraminifera diversity changes and paleoenvironmental analysis: the Lower Cretaceous shallow-water carbonates of San Lorenzello, Campanian Apennines, southern Italy. Facies 52, 53–67. Andresen, N., Reijmer, J.J.G., Droxler, A.W., 2003. Timing and distribution of calciturbidites around a deeply submerged carbonate platform in a seismically active setting (Pedro Bank, Northern Nicaragua Rise, Caribbean Sea). International Journal of Earth Sciences 92, 573–592. Anderson, T.F., Arthur, M.A., 1983. Stable isotopes of oxygen and carbon and their application to sedimentologic and paleoenvironmental problems. In: Stable

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