Revision of the Foraminiferal Biozonation Scheme in Upper Cretaceous Carbonates of the Dezful...

8/10/2019 Revision of the Foraminiferal Biozonation Scheme in Upper Cretaceous Carbonates of the Dezful Embayment Zagros Iran Integrated Palaeontological Sedime

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Revue de micropalontologie 57 (2014) 97116

Original article

Revision ofthe foraminiferal biozonation scheme in Upper Cretaceouscarbonates ofthe Dezful Embayment, Zagros, Iran: Integrated

palaeontological, sedimentological and geochemical investigation

Rvision des biozones de foraminifres du Crtac suprieur du rentrant de Dezful, dans le Zagros :

intgration des recherches palontologiques, sdimentologiques et gochimiques

Mahboobeh Omidvar a, Hamzeh Mehrabi b,d,, Fereshteh Sajjadi b, Hassan Bahramizadeh-Sajjadi c,Hossain Rahimpour-Bonab b, Alireza Ashrafzadeh c,d

a Department of Geology, Faculty of Sciences, University of Isfahan, Isfahan, Iran

bDepartment of Geology, Faculty of Sciences, University of Tehran, Tehran, IrancNational IranianOil Company (NIOC), Tehran, Iran

dMAPSAOil Company, Geology division, Tehran, Iran

Abstract

Cretaceous carbonate successions ofthe Bangestan Group, such as the Sarvak and Ilam formations, are among the most prolific hydrocarbonreserves ofthe Middle East. However, relatively little is known about their detailed palaeontology and biostratigraphy. Moreover, due to litho-logical similarity ofthese carbonate formations recognition oftheir boundaries in subsurface studies is problematic. To investigate these units,biostratigraphic analyses were carried out on nearly 1100 m ofcores, including core plug samples and thin sections prepared from five giant andsupergiant oilfields in the northern and southern Dezful Embayment, SW Iran. Accordingly, 59 species offoraminifera (assigned to 43 genera) aswell as 11 species ofnon-foraminifera (10 genera) were recognized. As a result, three biozones were identified, which in stratigraphic order are:Nezzazata-Alveolinids Assemblage Zone; Moncharmontia apenninica-Nezzazatinella-Dicyclina Assemblage Zone; and Rotalia skourensis-algaeAssemblage Zone. These are compared with the Wynds (1965) biozonation scheme, previously introduced in the Zagros area, and a revisedscheme is presented. Accordingly, a CenomanianTuronian age and a ConiacianCampanian age are envisaged for the Upper Sarvak and Ilamformations, respectively. In our new biostratigraphic scheme, the SarvakIlam formations boundary is considered to be located above theMonchar-montia apenninica-Nezzazatinella-Dicyclina Assemblage Zone (equivalent ofValvulammina-Dicyclina Assemblage Zone ofWynd, 1965), that isTuronian in age. This zone is bounded by two palaeoexposure surfaces, which correspond approximately to the CT boundary transitional intervaland a post-Turonian, which can be possibly assigned to the Coniacian. Significant sedimentological features of these disconformities includebauxiticlateritic horizons, karstified profiles and solution-collapsed breccias. Geochemical signatures ofthese meteorically altered surfaces arealso considered to calibrate biofacies and biozones. Finally, we compared our new biozonation scheme with other studies in neighboring areas ofSW Iran and the Middle East. 2014 Elsevier Masson SAS. All rights reserved.

Keywords: Foraminifera; Biostratigraphy; Cretaceous; Stable isotopes; Palaeoexposure; Dezful Embayment; Iran

Rsum

Les sries carbonates crtaces du Groupe de Bangestan, ou formations de Sarvak et dIlam, comptent parmi les plus riches rservesdhydrocarbures du Moyen-Orient. Nos connaissances palontologiques et biostratigraphiques restent cependant peu dtailles. En raison delithologies similaires, les limites de ces formations sont difficiles reconnatre en subsurface. Afin de pallier ce problme, des analyses biostrati-graphiques ont t effectues sur prs de 1100 m de carottes, dchantillons de modules de base et de lames minces, prpares partir de trios

Corresponding author. Department of Geology, Faculty of Sciences, University of Tehran, Tehran, Iran.E-mail address: [email protected] (H. Mehrabi).

http://dx.doi.org/10.1016/j.revmic.2014.04.0020035-1598/ 2014 Elsevier Masson SAS. All rights reserved.
http://www.sciencedirect.com/science/journal/00351598http://localhost/var/www/apps/conversion/tmp/scratch_7/dx.doi.org/10.1016/j.revmic.2014.04.002mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_7/dx.doi.org/10.1016/j.revmic.2014.04.002http://localhost/var/www/apps/conversion/tmp/scratch_7/dx.doi.org/10.1016/j.revmic.2014.04.002mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_7/dx.doi.org/10.1016/j.revmic.2014.04.002http://www.sciencedirect.com/science/journal/00351598http://crossmark.crossref.org/dialog/?doi=10.1016/j.revmic.2014.04.002&domain=pdf


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98 M. Omidvar et al. / Revue de micropalontologie 57 (2014) 97116

champs gants et supergants dans le nord et le sud du rentrant de Dezful (SW de lIran). Cinquante-neufespces de foraminifres (correspondant 43 genres), ainsi que 10 genres et 11 espces dautres microfossiles ont t reconnus. En consquence, trois biozones ont t identifies, qui sontdans lordre stratigraphique : Zone dassemblage Nezzazata-Alveolinides ; Zone dassemblage Moncharmontia apenninica-Nezzazatinella-Dicyclina ; et Zone dassemblage Rotalia skourensis-algues. Ces rsultats sont compars avec la biozonation tablie en 1965 par Wynd, dans lesmonts du Zagros. Un schma rvis est prsent. Des ges cnomaniensturoniens et coniacienscampaniens sont respectivement envisags pour lapartie suprieure des formations de Sarvak et dIlam. Dans ce nouveau cadre biostratigraphique, la limite des formations de Sarvak et dIlam semblepouvoir tre fixe au-dessus de la zone dassemblage Moncharmontia apenninica-Nezzazatinella-Dicyclina (quivalent de la zone dassemblage

Valvulammina-Dicyclina de Wynd, (1965)), dont lge est turonien. Cette zone est dlimite par deux surfaces dmersion ; lune se situantapproximativement autour de lintervalle transitoire de la limite CnomanienTuronien et lautre tant post-turonienne, couvrant probablement leConiacien. Ces discontinuits sont soulignes par des horizons bauxitiques et latritique, des profils karstifis et des brches de dissolutions. Lessignatures gochimiques de ces surfaces particulires sont galement prises en compte pour recalibrer les biofacis et les biozones. Enfin, nousavons compar ce nouveau schma de biozonation avec ceux des rgions voisines du SW de lIran et du Moyen-Orient. 2014 Elsevier Masson SAS. Tous droits rservs.

Mots cls : Foraminifres ; Biostratigraphie ; Crtac ; Isotopes stables ; Surfaces dmersion ; Rentrant de Dezful ; Iran

1. Introduction

The aim of this study is to improve the biostratigraphicknowledge of the CenomanianCampanian carbonate succes-sions (including the Sarvak and Ilam formations, BangestanGroup) in the Dezful Embayment, southwestern part of Iran.After the Oligo-Miocene Asmari Formation, these two rock-units represent the second most important reservoirs of southand southwest Iran. In most parts of the Dezful Embayment,carbonate sequences of the Ilam Formation disconformablyoverlie the carbonates ofthe Sarvak Formation. This lithologicalsimilarity causes some difficulties in their lithostratigraphic dif-ferentiation (especially in the subsurface studies). Together withsedimentological and geochemical investigations (Rahimpour-Bonab et al., 2012a, b, 2013; Mehrabi and Rahimpour-Bonab,

2014), biostratigraphy could provide a useful tool for rela-tive dating and differentiation of these carbonate sequences(Zampetakis-Lekkas et al., 2007). Although the Bangestan car-bonates are known as significant reservoirs in the Middle East,their detailed biostratigraphy is still ambiguous. As presentedin Fig. 9, various biostratigraphic schemes are currently avail-able for the SarvakIlam formations in the Zagros area; all areon the basis ofWynds (1965) biozonation scheme. Since 1965,this scheme is widely used by researchers in both academic andindustrial conventions. Due to the lack ofany index and applica-ble planktonic fauna for biostratigraphic zonation, Wynd (1965)uses the small and large benthic foraminifera and other ben-

thic fauna (e.g., algae) for his biozonation in the SarvakIlamformations.One ofthe most important problems in the above mentioned

studies is to define the boundary between the Sarvak and Ilamreservoirs based upon their fossil content. Precise definition ofthe boundary is critical in both geological and reservoir aspects(e.g., reserves volume estimation).

Inmost parts ofthe Zagros area (including the Dezful Embay-ment), a regionally traceable disconformity separates the Sarvakand Ilam formations. In many studies in Iran and other places inthe Middle East, this disconformity is dated (both relatively andabsolutely) in Middle to Late Turonian (Taghavi et al., 2006;Hollis, 2011; van Buchem et al., 2011).

In the Dezful Embayment, while many workers believe inonly one unconformity in the CenomanianCampanian succes-sions, they disagree on its stratigraphic position. Some (e.g.Wood and Lacassagne, 1965; Wynd, 1965) locate the bound-ary at the top ofbiozone 29 ofWynd (1965), while others (e.g.Bourgeois, 1969; Hart, 1970) favor a location at the base ofthisbiozone. Recently, Ghabeishavi and Rahmani (2006), based ontheir biostratigraphic and sequence stratigraphic analyses oftheBangestan reservoirs, set the boundary somewhere within thebiozone 29. However, Khalili (1976) assumes two major discon-formities in the SarvakIlam sequences, bounding the biozone29. Detailed characteristics of the CenomanianCampanianintervals are elaborated in this study in subsurface stratigraphicsections of the northern and southern Dezful Embayment.Finally, a revised biozonation scheme is presented for these

lucrative successions in the Zagros area.

2. Geological setting and stratigraphy

The Cretaceous sedimentary record of the Middle East(including the Dezful Embayment of southwest Iran andMesopotamian Basin ofIraq; Fig. 1) isone ofthe most importantpetroleum systems ofthe world (Alsharhan and Nairn, 1986). Itmainly consists ofshallow marine carbonates deposited on theramp-like platforms, surrounding intrashelfbasins in the north-eastern margin ofthe Arabian Plate (Setudehnia, 1978; Murris,1980; Alsharhan and Nairn, 1986, 1988). In the Dezful Embay-ment, neritic carbonates ofthe Sarvak and Ilam formations havedeveloped on these shallowramps from the Albian to Campanian(Motiei, 1993). As a result, benthic faunal assemblages are thebest selections for biostratigraphic zonation ofthese sequences.On the other hand, in the Lurestan province, where the intrashelfbasins have developed during the Cretaceous, pelagic fauna aremore usable for biozonation (Bahramizadeh-Sajjadi, 2012).

Tectonically, the Middle Cretaceous marks an importantperiod ofchange from a stable, passive margin, which had con-trolled sedimentation since the Permian, to an active margin withthe initial phase ofNeotethys closure and the onset ofAlpineOrogeny (Sadooni and Aqrawi, 2000; Glennie, 2000; Ziegler,


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M. Omidvar et al. / Revue de micropalontologie 57 (2014) 97116 99

Fig. 1. A. Principal geological and structural sub-divisions of southwestern Iran; the Dezful Embayment is labeled as grey. B. Location map of the studied oilfields(marked as black color) within the northern and southern Dezful Embayment. Some other important fields of these areas are also shown (green color).

2001; Sharland et al., 2001; Hollis and Sharp, 2011). At thistime, several palaeohighs (and troughs) existed in the SW partof Iran and some other areas ofthe Middle East (van Buchem

et al., 2002a, b; Sepehr and Cosgrove, 2005; Alavi, 2004, 2007;

Ahmadhadi et al., 2007; Rahimpour-Bonab et al., 2012a). Thesepalaeo-structures were formed by the reactivation ofbasementfaults and halokinetic movements (related to the Hormoz salt

series in SW Iran and the Persian Gulf) during the initiation of


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Neotethys subduction beneath the Central Iran and Oman andophiolite obduction on the NE margin ofArabian Plate (Hart,1970; Alsharhan and Nairn, 1988; Sepehr and Cosgrove, 2005).

The coupled imprints of tectonic activities, tropical cli-mate and high frequency sea-level fluctuations have led tothe recurring emersions and disconformity development inthe CenomanianCampanian sedimentary record of SW Iran(including the Sarvak and Ilam formations) and other placesin the Middle East (Emami et al., 2010; Verges et al., 2010;Sharp et al., 2010; Aqrawi et al., 2010; Razin et al., 2010;Hajikazemi et al., 2010, 2012; Hollis, 2011; Rahimpour-Bonabet al., 2012a, b, 2013; Mehrabi and Rahimpour-Bonab, 2014).On the Arabian Platform and in the Zagros area, the collec-tive effects of these events during the CenomanianTuronian,have led to three important phases ofemergence and erosionalhorizons as detailed below (Fig. 2):

1. Mid-Cenomaniandisconformity (95 Ma); which is correlatedregionally in the Arabian Platform and Zagros area. Seem-

ingly, this sea-level fall and subsequent emergence is eustaticin origin and is traceable on a global scale (e.g. Alsharhanand Nairn, 1997; Aqrawi et al., 1998; Sharland et al., 2001;Sharp et al., 2010; Hollis, 2011). This palaeoexposure sur-face is generally not drilled in the studied oilfields due to thelack ofreservoir quality in the lower Sarvak Fm;

2. Late CenomanianEarly Turonian (CT) disconformity(92Ma) is present in southwest Iran and in some parts ofthe Arabian Platform, where local salt diapirism or basementblock movements (epeirogenic and/or local tectonic uplifts)were active (e.g. Jordan et al., 1985; van Buchem et al., 1996,2002, 2011; Immenhauser et al., 2000; Sharp et al., 2010;

Vincent et al., 2010; Hollis, 2011; Hajikazemi et al., 2012;Rahimpour-Bonab et al., 2012a, 2013);3. Mid-Turonian disconformity (8690 Ma) is another emer-

gentphase that is recognized regionally (Videtich et al., 1988;Sharland et al., 2001; Razin et al., 2010; Sharp et al., 2010;vanBuchemet al., 2011; Hajikazemiet al., 2012; Rahimpour-Bonab et al., 2012a). It separates the upper Sarvak Fm. fromthe Ilam Formation (Fig. 2).

While evidence for the Mid-Cenomanian and Mid-Turoniandisconformities are regionally traceable over the ArabianPlatform and Zagros area, the CT disconformity occurs incon-sistently (Alsharhan and Nairn, 1986, 1988; Motiei, 1993;

Aqrawi et al., 1998, 2010). In some areas of the KhuzestanProvince, where the Laffan shale is present, only the later dis-conformity is recognizable at the upper part of the Turoniansequences.

Lithostratigraphically, the sedimentary record of theCenomanianCampanian in the Arabian Platform and Zagrosarea (8998.9 Ma) includes the Wara, Ahmadi, Rumaila andMishrifformations in Saudi Arabia, the Natih, Fiqa and Arumaformations in Oman, the Derder Formation in southeast Turkey,the Ahmadi, Rumaila, Mishrif and Gudair formations in Iraqand the Sarvak and Ilam formations (Bangestan reservoirs) inthe Zagros area ofIran (Fig. 2). Nearly one-third of the oil in

place (OIP) found in nearly 29 giant and supergiant oilfields

in southwest Iran are hosted by Bangestan Group reservoirs(Fig. 2).

The Sarvak Formation is present in south and southwest Iranand has been drilled by numerous exploration wells in many oiland gasfields ofthese areas (especially in the Dezful Embay-ment, the Mesopotamian Basin of Iraq and the Persian Gulf;Figs. 1 and 2). In its type section (Tang-e-Sarvak, Kuh-e Banges-tan), it conformably overlies the Kazhdumi Formation with atransitional contact, but its upper contact with the shales andmarls ofthe Gurpi Formation is sharp (Motiei, 1993). However,in many oilfields (including those discussed here), the SarvakFormation is overlain unconformably by carbonates ofthe IlamFormation. This lithological similarity with the overlying unitmakes recognition ofits upper boundary problematic, mainly inthe subsurfacestudies. From a biostratigraphic point ofview, wewill deal with this boundary in details.

The Ilam Formation, at its type section (Kabirkuh area,Lurestan province), is overlain the Surgah Formation and under-lain by the Gurpi Formation (Motiei, 1993; Fig. 2). However,

in most parts of the southwestern Iran, the Ilam Formationis generally represented by the shallow-water limestones thatunconformably overlie the carbonate sediments of the SarvakFormation and is conformably overlain by the shales and marlsof the Gurpi Formation (Fig. 2).

3. Materials and methods

This study is based on data from six wells drilledin five giant and supergiant oilfields (Abteymour, Ahwaz,Gachsaran, Rag-e-Safid and Marun) in the northern andsouthern Dezful Embayment, SW Iran. To investigate the

CenomanianSantonian successions at these fields, about1380m ofcore and core plug samples were examined. High res-olution palaeontological analyses (on almost 1000 thin sections)together with image and quantitative analyses of all compo-nents were used to characterize biozones and biofacies of theSarvakIlam formations. The biozones were used to date thestudied intervals and the palaeoexposure surfaces. Samplingintervals weregenerally between 0.3 and 1 m, but were tightenedat the vicinity ofbiozones boundaries and disconformities.

Dueto the absence ofwell-preserved (non-diagenetic) fossilsinthe studied successions, bulk samples were used for stable iso-tope analyses. For bulk sampling, precautions were taken duringsample preparation including the use ofa binocular microscope

to avoid stylolites and microfractures and examining thin sec-tions for precise sampling. A 0.5 mm tungsten carbide bit wasused for drilling. In total, 500 samples were powdered, preparedand then analyzed for oxygen and carbon stable isotopes. Geo-chemical analyses were carried out at Texas A&M Universityslaboratory using a Gas Bench online with a Finnigan Delta PlusXP. values are presented with reference to the PDB standardin permil.

4. Results

Biostratigraphic studies have been carried out on selected

subsurface sections of the upper Sarvak and Ilam formations


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Fig. 2. Generalized chronostratigraphy of the Cretaceous successions in various parts of the Middle East region, including the Sarvak and Ilam formations ofthe Bangestan Group. Three main disconformable surfaces are also shown. These are marked as 1: middle Cenomanian disconformity; 2: CenomanianTuroniandisconformity and 3: middle Turonian disconformity. In the studied wells, two and sometimes three of these disconformable boundaries are recorded.

in the northern and southern parts of the Dezful Embaymentincluding the Abteymour (AT-1), Ahwaz (AZ-1), Marun (MN-1), Rag-e-Safid (RS-1) and Gachsaran (GS-1 and GS-2) (Fig. 1)oilfields. As a result, 59 species of foraminifers (attributed to43 genera), 11 taxa ofnon-foraminifers (distributed among 10genera) and rudist debris are identified. Presence of individ-ual short-range index species, first observed occurrence (FOO)and/or the last observed occurrence (LOO) of taxa and co-occurrence of two or more taxa are used for biostratigraphicsubdivision ofthe examined intervals.

Accordingly, three biozones have been identified; some aredifferent from the biozonation scheme previously suggested byWynd (1965) for the Dezful Embayment and neighboring areasin the Zagros domain (Fig. 9). The first two biozones of thisbiozonation are known from the Sarvak Formation and the lastbiozone is known from the Ilam Fm. The Wynds biozonationadvocated zones and biozones whereas in reality, it was dis-cussing biofacies units, and consequently, in the area underdiscussion, the two kinds of interpretation were often inter-changed. In this study, two biofacies remain identified in theSarvak and Ilam formations, namely rudist debris biofacies (theequivalent ofWynds biozone No. 24) and Oligostegina bio-

facies (the equivalent ofWynds biozone No. 26). Moreover, the

other important difference ofour biozonation with the Wyndsscheme is about his biozone No. 29 (Valvulammina-Dicyclinaassemblage Zone). We introduced a new assemblage zone,namely Moncharmontia apenninica-Nezzazatinella-DicyclinaAssemblage Zone, in the uppermost part ofthe Sarvak Forma-tion in subsurface sections of the Dezful Embayment. In thefollowing parts, we will deal in detail first with these biozones,then with two biofacies.

4.1. Biostratigraphy

4.1.1. Nezzazata-Alveolinids assemblage zone

Known from the base of the Upper Sarvak Formation, thisbiozone is equivalent ofWynd

s biozone 25, and defined by theco-occurrence ofindex foraminifera, such asNezzazata simplex,Nezzazata conica, Ovalveolina ovum, Praealveolina cretaceaand Cisalveolinafallax(Figs. 3 and 4).

Other micro- and mega-fossils occurring in this biozone are:Praealveolina simplex Reichel, 1936; Nezzazata concava

Smout, 1956; Nezzazata gyra (Smout, 1956); Dicyclinaschlumbergeri MunierChalmas, 1887;Broeckina (Pastrikella)balcanica Cherchel et al., 1976; Cycledomia iranica (Hen-

son, 1948);Pseudolituonella reicheliMarie, 1955;Chrysalidina


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Fig. 3. Photographs of the foraminiferal and non-foraminiferal taxa encountered in this study in the SarvakIlam formations. 1,Nezzazata simplex(scale: 100m)2, Nezzazata conica (scale: 100m) 3, Nezzazata concave (scale: 100m) 4, Ovalveolina ovum (scale: 500m) 5, Praealveolina cretacea (scale: 500m) 6,Multispirinacretacea (scale: 500m)7, Pseudolituonella reicheli (scale: 100m)8, Chrysalidina cretacea (scale: 500m)9, Coxites zubranensis (scale: 100m)10, Dicyclina schlumbergeri (scale: 500m) 11, Rudist debris (scale: 500m).

gradata dOrbigny, 1839; Cuneolinapavonia dOrbigny, 1839;Pseudorhipidioninabingstani (De Castro, 1966); Pseudorhapy-dionina dubia (De Castro, 1966); Nezzazatinella picardi(Henson, 1948); Biconcava bentori Hamaoui, 1970; Biplanatapeneropliformis Hamaoui and Saint-Marc, 1970; Murgeinaapula (Lupertosinni); Trocholina arabica Henson, 1949;

Trochospira avnimelechi Hamaoui, 1965; Coxites zubairensisSmout, 1956; Charentia cuvillieri Neumann, 1965; Num-moloculina regularis Philippson, 1887; Spiroloculina cretaceaReuss, 1854; Pseudocyclammina rugosa Yabe and Hanzawa,1926; Hemicyclamina sigali Maync, 1953; Cyclolina cre-tacea dOrbigny, 1846; Multispirina iranensis Reichel, 1941;


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Fig. 4. Photographs of the foraminiferal and non-foraminiferal taxa encountered in this study in the SarvakIlam formations. 1:Mangashtia viennoti (scale: 100m)2: Nezzazatinella picardi (scale: 100m) 3: Cuneolina pavonia (scale: 100m) 46: Moncharmontia apenninica (scale: 100m) 7: Rotalia skourensis (scale:100m)8: Neomeris sp. (scale: 100m)9: Cympolia sp. (scale: 500m) 1012: Oligosteginids (scale: 100m).

Calcisphaerula innominata Bonet, 1956; Stomiosphaerasphaerica (Kaufmann, 1865); Ellipsactina sphaeractinoidesPfender, 1937; Nezzazata sp.; Dicyclina sp.; Choffatella sp.;Praealveolina sp.;Ovalveolina sp.;Dictyoconus sp.; Peneroplissp.;Lenticulina sp.;Rotalia sp.;Hedbergella sp.; miliolids; tex-tulariids; echinoid fragments; rudist debris; algal debris; corals;ostracods; sponge spicules; gastropods; oligosteginids and shellfragments.

This zone is equivalent to the Wynd

s biozone 25 and isCenomanian in age as indicated by the presence ofCisalve-olina (see BouDagher-Fadel, 2008) (Fig. 9). This zone occursin nearly all studied wells except the well MN-1 (Marun Oil-field). The thickness of this biozone varies from 38 m (inGachsaran-2) to 130 m (in Abteymour-1; Table 1). The thick-ness of this zone in the other studied wells is presented inTable 1.


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Table 1Thickness variations of the recognized biozones in the studied wells. These are attributed to the combined effects of the eustatic sea-level changes and tectonicactivities during deposition of the SarvakIlam formations, and afterward (refer to the text for more details).

Biozone Thickness in studied wells (m)

AT-1 AZ-1 MN-1 GS-1 GS-2 RS-1

(1)

Nezzazata-Alveolinids Assemblage Zone

130 94 100 38 76

(2)Rudist dabris biofacies

40 56 68 25

(3)Moncharmontiaapenninica-Nezzazatinella-DicyclinaAssemblege Zone

69 118 127 0 20 80

(4)Oligostegina biofacies

120 59 58

(5)Rotalia skourensis-algae AssemblegeZone

120 59 19 58 40 24

As the selected samples only represent the Upper part ofSarvak Formation, the lower limit of this biozone (as iden-tified herein) is no older than Cenomanian (see Bolz, 1977).Bourgeois (1969) considers the upper limit of this biozone asLate Cenomanian. It is also proved in some studies (e.g. Husinecet al., 2000) that all Alveolinids of this zone were diminishedat the Late Cenomanian due to a worldwide Oceanic AnoxicEvent, known as OAE2 (Husinec et al., 2000). It should benoted that a major disconformity known from various parts ofthe SW Iran and surrounding areas ofthe Middle East (e.g. vanBuchem et al., 2002, 2011; Hollis, 2011; Hajikazemi et al., 2010,2012; Rahimpour-Bonab et al., 2012a). This disconformity has

resulted froma combination ofrelative sea-level fall and tectonicactivities. It separates this biozone from the overlying biozone(Moncharmontia apenninica-Nezzazatinella-DicyclinaAssem-blage Zone) and dated relatively at the Late CenomanianEarlyTuronian (CT boundary).

Meteoric diagenetic features, such as intense dissolution,solution-collapse brecciation and meteoric cementation alongwith development of palaeosol horizons are recorded beneaththis disconformity in the studied wells. Samples collected belowthis disconformity have heavily depleted 18O (4 to 6PDB) and 13C (3 to 8 PDB) values (Figs. 5 and 6),owing to progressive water/rock reactions during meteoric dia-genesis (the inverted J pattern; Fig. 7). The 18O and 13C

ratios ofthese samples plot close to the freshwater limestone18O and 13C mean line (Keith and Weber, 1964) (Fig. 7).These depleted values are interpreted to be resulted from thehigh water/rock reactions in an open diagenetic system asso-ciated with prolonged subaerial exposure (Brand and Veizer,1981; Lohmann, 1988; Saller and Moore, 1991). The 13C ver-sus 18O plots for the four studied wells are presented in Fig. 7.In each plot, freshwater and marine calcite mean 13C (Keithand Weber, 1964) is shown together with 18O mean ofepigenicfreshwater calcite (Degens and Epstein, 1964; Murata et al.,1969). The isotopic values of all wells show an inverted Jpattern, geochemical pattern ofmeteorically affected carbonate

rocks (Lohmann, 1988), and three main groups ofsamples can

be distinguished (see Rahimpour-Bonab et al., 2013 for moredetails):

A. samples with stable isotope compositions close to those esti-mated for Cretaceous marine carbonates;

B. samples with some reduction in 18O values. These sam-ples are from the Upper Sarvak below the post-Turoniandisconformity (SarvakIlam formations boundary) and indi-cate the occurrence ofmeteoric diagenesis which resulted inlightening ofcarbon and oxygen isotope ratios;

C. samples with extremely reduced 13C and 18O which occuraroundthe intersection oftwo dashed lines ofepigenic fresh-water calcite 18O and mean 13C. These samples are fromthe Middle Sarvak Fm. below the CT disconformity andindicate the occurrence of long-term and recurring intensemeteoric diagenesis which resulted in significant decreasingof carbon and oxygen isotope values in an open diageneticsystem.

Correlation. Saint-Marc (1975) introduced the Pseudorha-pidionina laurinensis Zone from the Cretaceous sequencesof Lebanon based on this species and considered the ageof this zone to be Upper Cenomanian. Tasli et al. (2006)defined the Pseudorhapidionina dubia and Biconcava ben-tori cenozone in the Upper Cenomanian sequences of south

Turkey. Nezzazata simplex, Nezzazata conica, Merlingina cre-tacea, Chrysalidina gradata,Nezzazatinellapicardi, Biplanatapeneropliformis,Vidalina radoicicae,Cuneolinapavonia,Pseu-dolituonella reicheli, Ovalveolina sp., and Bolivinopsis sp. arethe other important foraminifera in this zone. The microfaunarecorded in this zone have a considerable similarity with ourbiozone 1 (Nezzazata-Alveolinids assemblage zone) assem-blage (Fig. 8). In SE Iraq, Sharbazhery (1999) introduced theParathalmanninella appenninica, Heterohelix globulosa Zonein the Cenomanian time interval (Fig. 8). Whittaker et al. (1998)introduced the Rotalipora cushmani and Whiteinella archaeoc-retacea zones in the Middle to Late Cenomanian ofthe Middle

East (Fig. 8).


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Fig. 5. Main diagenetic features that overprinted the Sarvak carbonates in the Abteymour Oilfield (AT-1 well) below the CenomanianTuronian and post-Turoniandisconformities. AG: Extensive meteoric dissolutions (karstification) in both macro- and microscopic scales. Isotopic ratios of both carbon and oxygen are alsoshown. Significantdepletions in both13Cand 18O below the palaeoexposure surfaces are attributedto the meteoric waters-carbonate rocksreactions during subaerialexposure.

4.1.2. Moncharmontia apenninica-Nezzazatinella-

Dicyclina assemblage zone

Wynd (1965) introduced the Valvulammina-Dicyclina

Assemblage Zone from the Uppermost part of the Sarvak

Formation in the Khuzestan, Fars and Lurestan provinces. Inhis scheme, this biozone was defined as that interval in whichValvulammina spp. andDicyclina spp. form the major microfau-

nal elements. He dated this biozone as Turonian in the Khuzestan


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Fig. 6. Photomicrographs of the main diagenetic features that recorded below the disconformable surfaces in the Gachsaran Oilfield (GS-1 and GS-2 wells). A, Band D. Silicification. C. Fe oxide staining in microscopic scale. EH. Solution-collapsebrecciation and lateritic horizons. 13Cand 18O compositions of these wellsare also shown. Depletion in both 18O and 13C is recorded below the palaeoexposure surfaces in the studied wells. These depleted values are interpreted to be dueto meteoric water/rock reactions associated with subaerial exposure.


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Fig. 7. 13C versus 18O plots for the four studied wells. In each plot, freshwater and marine calcite mean 13C (Keith and Weber, 1964) is shown together with18O mean of epigenic freshwater calcite (Degens and Epstein, 1964; Murata et al., 1969). The isotopic values of all wells, show an inverted J pattern, and threemain groups of samples can be distinguished: A.Samples with stable isotope compositions close to those estimated for Cretaceous marine carbonates. B.Sampleswith some reduction in 18O values. C. Samples with extremely light 13C and 18O, which occur around the intersection of two dashed lines of epigenic freshwatercalcite 18O and mean 13C (see text for details).

Fig. 8. Chart showing biostratigraphic zones, and correlation based on micro- and macrofossils from the CenomanianSantonian intervals of the Dezful Embaymentand other neighboring areas.


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province, where it has its best development. Afterward, stud-ies by other researchers mostly follow the Wynds biozonationscheme, but some changes in biozone 29 were suggested. Woodand Lacassange (1965) introduced the Valvulammina-DicyclinaZone and dated it as Santonian. Bourgeois (1969) named thisbiozone as Valvulammina Zone and dated it as Turonian. Bolz(1977) and Khalili (1976) in their biostratigraphic studies inthe Zagros area introduced the Valvulammina-Dicyclina assem-blage Zone from the Upper Sarvak Formation and dated it asTuronian.

In this study, we picked a new microfauna (Moncharmon-tia apenninica) that can be used, together with other indexforaminifera, for biozonation of the Upper Sarvak Formationin subsurface sections of the Dezful Embayment. This zoneoccurs in the uppermost part of the formation in nearly allstudied wells. It begins with the appearance ofMoncharmon-tia apenninica and an abundance ofNezzazatinellapicardi andDicyclina schlumbergeri. Moreover, the disappearance ofCeno-manian index taxa, such as alveolinid species (i.e. Cisalveolina

fallax, see BouDagher-Fadel, 2008) is another clue for recogni-tion ofthis biozone (Figs. 3 and 4). This zone ends whenRotaliaskourensis appears in the lowermost part ofthe Ilam Formation(Fig. 4). Other micro- and mega-fossils ofthis zone are:Spiroloculina cretacea Reuss, 1854; Cuneolina pavonia

dOrbigny, 1839; Mangashtia viennoti Henson, 1948; Num-moloculina regularis Philippson, 1887; Nummoloculina heimiBonet emend Coskin and Coskin, 1958; Trochospira avnim-elechi Hamaoui, 1965; Daxia cenomana Cuvillier and Szakall,1949; Pseudolituonella reicheli Marie, 1955; Thaumatoporellaparvovesiculifera Raineri, 1922; Aeolisaccus kotori Radoicic,1959; Calcisphaerula innominataBonet, 1956;Ammobaculites

sp.;Dicyclina sp.;Rotalia sp.; Whiteinella sp.;Hedbergella sp.;Miliolids; Textularids; echinoid fragments; rudist debris; algaldebris; corals; ostracods; sponge spicules; gastropods and shellfragments.

Based on these microfaunal contents, this zone is Turonian inage. There are some inconsistencies about the appearance age ofM. apenninica. For example, De Castro (1966) and Altiner andDecrouez (1982) considered the appearance ofM. apenninicain the Lower Senonian (i.e. ConiacianSantonian). On the otherhand, Bignot and Poisson (1974) reported this species fromthe Cenomanian, and Bignot and Guernet (1967) dated it asTuronian. Also, Korbar and Husinec (2003) considered theappearanceofM. apenninicaas Upper Turonian.Bahramizadeh-

Sajjadi (2012) in his recent correlation of the Haftkal-61,Mamatin-10, Zelooi-5 and Papileh-1 wells, considered this zoneas equivalent to the Wynds biozone 27 (Helvetoglobotruncanahelvetica/Clavihedbergella/Hedbergella assemblage zone), thatis Turonian in age.

This zone is recorded in all studied wells, except the GS-1 well in which the Turonian strata are totally absent. Thethickness ofthis zone in the studied wells is listed in Table 1.As it is evident, considerable variations in thicknesses of thiszone are recorded among the studied intervals. These are partlyascribed to the effects of well-known post-Turonian subaerialexposure and erosion of the uppermost intervals of the Sar-

vak Formation. Evidence for this palaeoexposure is recorded

in petrographic studies both in the core-scale and thin sec-tions in forms ofbauxiticlateritic horizons, karstic features andsolution-collapse breccias (Figs. 5 and 6).

Geochemically, reduction in both 18O (2 to 6) and13C (0 to 7) is recorded below this palaeoexposure sur-facein the studied wells (Figs. 5 and 6). These reduced values areinterpreted to be resulted from the meteoric water/rock reactionsin a semi-closed diagenetic system associated with subaerialexposure under a warm and humid climate (Fig. 7) (Brandand Veizer, 1981; Lohmann, 1988; Saller and Moore, 1991).This disconformable boundary is reported by many researchersfrom various parts of the Middle East region (including theDezful Embayment) and elsewhere globally (e.g. Setudehnia,1978; Alsharhan and Nairn, 1986, 1988; Aqrawi et al., 1998;van Buchem et al., 2011; Hollis, 2011; Rahimpour-Bonabet al., 2012a; Mehrabi and Rahimpour-Bonab, 2014). Detailedsedimentological (facies, diagenesis and sequence stratigra-phy) and geochemical studies of the studied intervals havebeen presented by Mehrabi and Rahimpour-Bonab (2014) and

Rahimpour-Bonab et al. (2013). In their study, they investi-gated the occurrence and development ofthese disconformitiesin subsurface sections ofthe Dezful Embayment.

Correlation. Even ifTasli et al. (2006) introduced the Ostra-coda andMiliolidae Interval Zone in the Turonian strata ofsouthTurkey, a better biozonation may be established in the MiddleEast. For instance, Saint-Marc (1975) reported the Cisalve-olina fallaxZone from uppermost parts ofthe Cenomanian andTuronian intervals ofthe Lebanon (Fig. 8). As mentioned ear-lier, Sharbazhery (1999) introduced the Nezzazata gr. simplexZone (Praealveolina cretacea, Cisalveolina lehneri Subzone)in CenomanianTuronian intervals ofSE Iraq. Whittaker et al.

(1998) introduced the Helvetoglobotruncana helvetica Zone inTuronian sequences of the Middle East (Fig. 8). Accordingly,Franco (2003) introduced an identical zone from the Turonianinterval ofMexico. Finally, Chiocchini and Mancinelli (2001)reported the Nezzazatinella cf. aegyptiacal-Nummoloculina cf.irregularis Zone in Turonian sequences ofItaly (Fig. 8).

4.1.3. Rotalia skourensis-algae assemblage zone

This zone is equivalent of the Wynd

s biozone 30 (Rotaliasp.22-algae Assemblage Zone; Fig. 9) and is the first biozone ofthe Ilam Formation that recorded in the lowermost parts ofthisformation in all studied wells. It is marked by the appearance ofRotalia skourensis and abundanceofalgaldebris, mainly includ-

ing Neomeris sp. and Cymopolia sp (Fig. 4). The thickness ofthis zone is listed in Table 1. The other micro- and mega-fossilsof this zone are:Calcisphaerula innominata Bonet, 1956; Stomiosphaera

sphaerica Kaufmann, 1865; Hedbergella planispira Tappan,1940; Hedbergella rischi Moullade, 1974; Moncharmontiaapenninica De Castro, 1967; Spiroloculina cretacea Reuss,1854; Nezzazatinella picardi Henson, 1948; Dicyclina sp.,Rotalia sp.,Marginotruncana sp.,Hedbergella sp.,Heterohelixsp.,Neomeris sp., Cympolia sp.,Halimeda sp., Salpingoporellasp. Saccocoma, miliolids, oligosteginids and textularids.

This zone is defined as that interval in which Rotalia sp. 22

(R.cf. skourensis) co-occurs with abundant algal, mollusc and


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Fig. 9. Biostratigraphic zonation schemes suggested by researchers (e.g., Wood and Lacassagne, 1965; Wynd, 1965; Bourgeois, 1969; Khalili 1976; Bolz, 1977) forthe CenomanianCampanian successions of the Dezful Embayment (SW Iran). They are used in this study as a basis for correlation.

echinoid debris (Wynd, 1965). It aged as ConiacianCampanianbased on its faunal content. Throughout most parts ofthe DezfulEmbayment, the combined effects ofeustatic sea-level changesand tectonics have resulted in the development ofa stratigraphicgap between the Turonian and Santonian. In such localities, thiszone is aged as SantonianCampanian (see Rahimpour-Bonabet al., 2013 for more details).

Correlation. Tasli et al. (2006) suggested theMoncharmontiacompressa andDicyclina schlumbergeri Zone for Lower Senon-ianintervals ofSE Turkey that includes some index foraminifers,such as M. compressa, M. apenninica, D. schlumbergeri and

algae, such as Thaumatoporellasp. andAeolisaccus sp. (Fig. 8).This zone is partly time equivalent of our biozone 5. Also,Chiocchini and Mancinelli (2001) introduced the Accordiellaconical-Rotorbinella scarsellai Zone in ConiacianSantonianbeds ofItaly (Fig. 8).

4.2. Biofacies

4.2.1. Rudistdebris biofacies

Rudist debris are the only recognizable fauna in this biofacies(Fig. 3), which is equivalent ofWynd

s biozone 24 (Fig. 9). Thisbiofacies has a wide time range (from Aptian to the end ofCre-

taceous) in neritic carbonates ofthe Zagros area. This biofacies

is recorded in most ofthe studied wells (including AT-1, AZ-1,RS-1 and GS-1 wells; Figs. 10 and 11). In the Abteymour Field,this biofacies is recorded as two 24 m- and 16 m-thick intervalswithin theNezzazata- alveolinids assemblage zone (biozone 1)and is dated as Cenomanian based on its stratigraphic position.This biofacies is mostly constrained within the other biozones(i.e. biozones 21, 22 and 25 ofWynd, 1965) and provides sometraceable and important reservoir intervals in drilled wells oftheDezful Embayment.

In the Ahwaz oilfield (AZ-1 well; Fig. 10), this biofacies isrecorded at two levels; the first, a 22 m-thick interval (depth

range: 35523530 m) constrained in biozone 1 (Nezzazata-Alveolinids assemblage zone) and the second, a 34 m-thickinterval(depthrange: 34723506 m) that falls in the range ofbio-zone 3 (Moncharmontia apenninica-Nezzazatinella-DicyclinaAssemblage Zone). These intervals are aged as Cenomanian andTuronian, respectively, based on their stratigraphic positions.

Inthe RS-1 well (Fig. 11), this biofacies is recorded as a 25 m-thick interval (depth range: 27472722 m) that overlies biozone1 and in the GS-1 well, as a 67 m-thick interval (depth range:26072540 m) within the biozone 1. In both wells, this biofaciesis aged as Cenomanian on the basis ofits stratigraphic position(Figs. 10 and 11). The thickness ofthis biofacies in other studied

wells is presented in Table 1.


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Fig. 10. The biozonation scheme for the studied intervals in the AZ-1 well showing the zones and stratigraphic distribution of the main microfossils (benthic andplanktic foraminifera and other important fauna). Two main disconformable surfaces (including CT boundary and post-Turonian disconformities) are marked asred jagged lines. They are discussed in detail in the text.


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Fig. 11. The biozonation scheme for the studied intervals in the RS-1 well showing the zones and stratigraphic distribution of the main microfossils (benthic andplanktic foraminifera and other important fauna). Two main disconformable surfaces (including CT boundary and post-Turonian disconformities) are marked asred jagged lines.

4.2.2. Oligostegina biofacies

This biofacies is equivalent ofthe Wynd

s biozone 26 (Fig. 9)and its main extension in the Zagros area is from the Albianto Turonian, although it may be as young as Maastrichtian.Due to the uncertainty of using oligosteginids (Fig. 4) astime indicators, this biofacies is aged only on the basis of itsstratigraphic position. Accordingly, this biofacies is recorded

in AT-1, AZ-1 and GS-1 wells as concurrent with biozone5 (Rotalia skourensis-algae assemblage Zone). The thicknessof this biofacies in the studied wells is presented in Table 1.This is aged as TuronianCampanian based on its stratigraphicposition. Oligosteginids are also reported from the LowerSarvak Formation both in the Lurestan and some parts of theKhuzestan provinces. The Lower Sarvak is not included in this


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study because ofthe absence ofrecovered cores in the studiedwells.

5. Discussion

Biostratigraphic correlation of the studied intervals is car-ried out to elaborate on the biozones extension in northernand southern parts ofthe Dezful Embayment (Fig. 12) Due tothe unavailability ofcomplete intervals ofthe Ilam and Sarvakformations, we are mainly focused on the thickness variationsof biozone 3 (Moncharmontia apenninica-Nezzazatinella-Dicyclina Assemblage Zone) as its whole interval was present.For this biostratigraphic correlation the disconformable surfaceat the CenomanianTuronian boundary is considered as a baselevel.

In the study area (SW Iran), the Sarvak Formation is dividedinto two parts known as the Lower and Upper Sarvak, based onthe presence ofa major disconformable surface in the middleparts ofthis formation, at the CenomanianTuronian boundary

(Fig. 2). In the studied wells, the lower Sarvak is defined by thebiozone 1 (Nezzazata-Alveolinids Assemblage ZoneBiozoneNo.25 ofWynd, 1965) that is recorded in nearly all studied wells(except for MN-1) with various thicknesses (Table 1). Faunalcontent ofthis zone shows a considerable diversity and indicatesa range from open marine, patch reefs and slope environments(see Mehrabi and Rahimpour-Bonab, 2014).

In most parts of the Dezful Embayment, a palaeoexposuresurface separates the Lower and Upper Sarvak. This surface cor-responds with the regionally-detected CenomanianTuronianboundary disconformity. This disconformity is recogniz-able both at the core and thin section scales by its

diagenetic features and geochemical signatures. Fe oxides stain-ing (bauxiticlateritic horizons), karstified profiles (meteoricdissolution vugs) and solution-collapse brecciation are amongthe most prominent diagenetic features of this palaeoexpo-sure surface (Figs. 5 and 6). Decreased values of18O (5to 7) and 13C (0 to 8) were recorded below thissurface in all of the studied wells (Figs. 5 and 6). In wellsAT-1, AZ-1, RS-1 and GS-2, some of the most importantchanges in faunal assemblages are detected around this bound-ary. The last occurrence of the Alveolinid species and otherCenomanian index taxa, such as Cisalveolina fallax, Mer-lingina cretacea, Biconcava bentori, Murgeina apula, Coxites

zubairensis, Chrysalidina gradate etc. are among the most

important examples for these changes (see BouDagher-Fadel,2008; Figs. 10 and 11). Above this boundary, Turonian strataare defined by the first appearance (FAD) ofMoncharmontiaapenninica, an abundance ofNezzazatinella and Dicyclina andthe presence ofMangashtia viennoti and Nummoloculina spp.Khalili (1976) believes that the CT and Mid-Turonian discon-formities constrain the Wynds biozone No. 29 (biozone 3 inthis study; Fig. 9). Also, Bahramizadeh-Sajjadi (2012) believesthat in the Khuzestan Province (including the Dezful Embay-ment), Turonian intervals are marked by the neritic facies ofWynd

s biozone No. 29 (Valvulammina-Dicyclina assemblageZone). He also stated that these neritic facies are time equiva-

lents ofthe Turonian pelagic facies in the Lurestan Province

defined as Wynds biozone 27 (Helvetoglobotruncana Hel-vetica/Clavihedbergella/Hedbergella Assemblage Zone).

The Upper Sarvak Formation is characterized by biozone 3(Moncharmontia apenninica-Nezzazatinella-DicyclinaAssem-blage Zone) in all of the studied wells (Fig. 12). This zoneis Turonian in age and its microfossils have a lower diver-sity than the underlying biozone (i.e., Nezzazata-Alveolinidsassemblage Zonebiozone No. 25 of Wynd, 1965). Thesemostly belong to the protected shallow marine environments(see Mehrabi and Rahimpour-Bonab, 2014 for more details).A considerable change in thickness of biozone 3 is recordedacross the studied wells; as it reaches to the minimum thick-ness in GS-1 and GS-2 wells (0 to 20 m). Maximum thicknessesof this zone are recorded in AZ-1 and MN-1 wells as 120 and125m, respectively. In the AT-1 well, its thickness increasesto 68 m (Table 1). As a result, this zone becomes thinnestfrom the Marun field toward its NW and SE sides (Fig. 12).These variations are attributed to the effects of the afore-said disconformable boundaries, which have been formed

by the combined effects of tectonic activities and high fre-quency sea-level fluctuations during the Upper Cretaceous ofthe Arabian plate (Sepehr and Cosgrove, 2005; Ahmadhadiet al., 2007; Rahimpour-Bonab et al., 2012a; Hajikazemi et al.,2012; Mehrabi and Rahimpour-Bonab, 2014). Intense meteoricdiagenesis, under warm and humid climatic conditions gov-erning the Arabian Plates Cretaceous platforms has resultedin considerable changes in synchronous carbonate sequences.Effects ofthese palaeoexposure surfaces on the facies character-istics, diagenesis history and reservoir quality oftheir underlyingcarbonatesequences have been the subject ofseveral studies bothin Iran and other neighboring areas (e.g. Taghavi et al., 2006;

Aqrawi et al., 1998, 2010; Razin et al., 2010; Sharp et al., 2010;Rahimpour-Bonab et al., 2012a, b; Mehrabi and Rahimpour-Bonab, 2014; Rahimpour-Bonab et al., 2013).

The Ilam Formation is characterized by biozone No. 5(Rotalia skourensis-algae Assemblage Zone) in all ofthe stud-ied wells (Figs. 9 and 12). The thickness of this zone variesconsiderably among the studied sections (Table 1). It reachesto its minimum thickness in RS-1 well (24 m) and in the AT-1 well, it shows the maximum thickness (120 m). In AT-1 andAZ-1 wells, biozone No. 4 (Oligostegina facies) is presentat lowermost part of the Ilam Formation, within the biozoneNo. 5. Oligosteginids are more dominant in AZ-1 well thanthe other studied wells (i.e. AT-1 and MN-1 wells). It shows

that the Ilam carbonate platform was shallower in NW andSE parts of the Dezful Embayment (Fig. 12). This shallowingof depositional environment reaches to its maximum in RS-1well, in which the oligosteginids are very rare and algae frag-ments (i.e. Neomeris sp. and Halimidae sp.) become dominantgrains (Fig. 11). However, toward the Gachsaran field, a deeperdepositional environment has been recorded. In this field, somepelagic taxa such as Heterohelixsp. and oligosteginids becomefrequent in upper parts ofthe Ilam Formation. Such intense vari-ations in depositional settings are completely compatible withthe active tectonic regime and high frequency sea-level fluctu-ations during the Upper Cretaceous ofthe NE parts ofArabian

Plate.


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Fig. 12. Biostratigraphic correlation across the studied wells. The main palaeoexposure surfaces (CT boundary and post-Turonian) are dated relatively based on thisbiozones are conspicuous (see text for details).


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6. Conclusions

Systematic studies ofmicrofossils from the Sarvak and IlamFormations in five giant and supergiant oilfields located inthe northern and southern Dezful Embayment have resultedin the recognition of 59 taxa and 43 genera of foraminiferaand 11 taxa and 10 genera of non-foraminifers. These aregrouped into three biozones and two biofacies as follows: IntheSarvak Formation:Nezzazata-AlveolinidsAssemblage Zone,Moncharmontia apenninica-Nezzazatinella-Dicyclina Assem-blage Zone and Rudist debris biofacies. In the Ilam Formation:Rotalia skourensis-algae Assemblage Zone and Oligosteginabiofacies.

This study showed that the boundary between the Sar-vak and Ilam Formations is located above the Moncharmontiaapenninica-Nezzazatinella-Dicyclina Assemblage Zone (bio-zone 3) that is equivalent of biozone No. 29 of Wynd (1965)and coincides with the worldwide post-Turonian disconformity.The lower limit ofthis zone is also marked by another palaeo-

exposure surface that corresponds with CenomanianTuronianboundary disconformity that is traceable regionally. Effectsof these important stratigraphic surfaces are measurable byusing the study of foraminiferal assemblages such as theAlveolinids.

Considering the sedimentological and geochemical charac-teristics (i.e., stable isotope values), the palaeoexposure surfacesare easily detectable in the Sarvak Formation using their asso-ciate diagenetic alterations and decreased values of stableisotopes (i.e., carbon and oxygen). Intense meteoric dissolu-tion (karstification), bauxiticlateritic horizons and collapsedbreccias are among the most prominent of these diagenetic

imprints.Post-Turonian emergence and erosion has resulted in theabsence ofMiddle/Upper Turonian to Coniacian strata in almostall parts ofthe Dezful Embayment (including the studied fields;Motiei, 1993; James and Wynd, 1965). Biostratigraphic cor-relation of the Upper Sarvak Formation across the studiedwells indicates thinning out ofthe Moncharmontia apenninica-Nezzazatinella-Dicyclina Assemblage Zone from the Marunfield towards its NW and SE sides. This zone reaches to itsmaximum thickness in AT-1 well (69 m) and to its minimumthickness in GS-1 and GS-2 wells (0 to 20 m). These variationsin thickness ofthe Upper Sarvak Fm. are attributed to the effectsof erosional phases during the CenomanianTuroniantime span.

Such variations are also recorded in the Ilam Formation.

Acknowledgements

The University of Tehran and the NISOC are both grate-fully acknowledged for kind provision of required data andfacilities. Our special thanks go to the two respected reviewers(Prof. A.Zambetakis-Lekkas and Prof. R. Coccioni) for theirvery useful comments and corrections which have consider-ably improved the original manuscript. Taniel Danelian, DanielVachard, Marcelle K. BouDagher-Fadel and Eric Armynot du

Chtelet are also thanked for their comments on the manuscript.

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Revision of the Foraminiferal Biozonation Scheme in Upper Cretaceous Carbonates of the Dezful...

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