Biostratigraphy and paleoecology of the Qom Formation, Iran 555

ABSTRACT

A study of large benthic foraminifera from the 147 m-thick Qom Formation in the Chenar area (northwestern Kashan) is reported. One hundred and twelve thin sections were prepared and the distribution of benthic foraminifera was analyzed to reconstruct paleoenvironmental conditions. Study of these sections led to the identification of 28 genera and 38 species. On the basis of the recognized foraminifera, the section is comparable to Lepidocyclina- Operculina- Ditrupa Assemblage zone, and the age of the Qom Formation in the studied section is assigned to the Oligocene.

Evidence of sea level changes is observed from bottom to top of the studied section. On the basis of large benthic foraminifera assemblages and microfacies features, three major depositional environments (inner shelf, middle shelf and outer shelf) were defined. The inner shelf facies is characterized by wackstone-packstone, dominated by miliolid and small perforate foraminifera. The middle shelf facies is represented by packstone-grainstone with diverse assemblage of large perforate benthic foraminifera. Outer shelf facies is dominated by large perforate benthic foraminifera as well as planktic foraminifera. The distribution of the Oligocene large benthic foraminifera in the studied area indicates that shallow marine carbonate sediments of the Qom Formation were deposited in a photic zone of tropical to sub-tropical environments. Finally, the correlation between the study area and some other sections of Central Iran indicates that sedimentation of the Qom Formation is continued from Late Rupelian to Chattian in northwest and Late Rupelian to Aquitanian in southeast direction.

Key words: biostratigraphy, benthic foraminifera, Qom Formation, Oligocene. Aquitanian, Iran.

RESUMEN

Se reporta el estudio de foraminíferos bentónicos grandes de la Formación Qom, con 147 m de espesor, en el area de Chenar (noroeste de Kashan). Ciento doce secciones delgadas fueron preparadas y la distribución de foraminíferos bentónicos fue analizada para reconstruir las condiciones paleoamebietales. El estudio de esas secciones llevó a la identificación de 28 géneros y 38 especies. Con base en los foraminíferos identificados, la sección es atribuible a la zona de Lepidocyclina- Operculina- Ditrupa y la edad de la Formación Qom en la sección estudiada es asignable al Oligoceno.

Evidencias de cambios del nivel del mar pueden ser observados desde la base y hacia a cima de la sección estudiada. Con base en los ensambles de foraminíferos bentónicos grandes y en las características de las microfacies, tres ambientes de depósito fueron definidos (plataforma interna, media y externa). Las facies de plataforma interna se caracterizan por wackstone-packstone, dominada por miliólidos y

BiostratigraphyandpaleoecologyoftheQomFormationinChenararea(northwesternKashan),Iran

Elham Behforouzi* and Amrollah Safari

Department of Geology, Faculty of Sciences, University of Isfahan, Isfahan, Iran, 81746-73441.* elibehforouzi@yahoo.com

Revista Mexicana de Ciencias Geológicas, v. 28, núm. 3, 2011, p. 555-565

Behforouzi,E.,Safari,A.,2011,BiostratigraphyandPaleoecologyoftheQomFormationinChenararea(northwesternKashan),Iran:RevistaMexicanadeCienciasGeológicas,v.28,núm.3,p.555-565.

Behforouzi and Safari556

pequeños foraminíferos perforados. La plataforma media está representada por packstone-grainstone, con diversos ensambles de foraminíferos bentónicos grandes perforados. Las facies de plataforma externa están dominadas por foraminíferos bentónicos perforados grandes, así como por foraminíferos planctónicos. La distribución de los foraminíferos bentónicos grandes del Oligoceno indica que los sedimentos marinos carbonatados someros de la Formación Qom fueron depositados en la zona fótica de un ambiente tropical a sub-tropical. Finalmente, la correlación entre el área estudiada y otras secciones de Irán Central indica que la sedimentación de la Formación Qom fue continua en dirección noroeste desde el Rupeliano Tardío al Chattiano, y lo fue en dirección sureste del Rupeliano Tardío al Aquitaniano.

Palabras clave: bioestratigrafía, foraminíferos bentónicos, Formación Qom, Oligoceno, Aquitaniano, Irán..

INTRODUCTION

Inthisstudy,theforaminiferaassemblageinthestrati-graphicsectionlocatedatChenarvillage,50kmnorthwestofKashan,isdescribed.ThestudyareaexposesCenozoicsedimentsandis locatedat51°09’02”Elongitudeand34°05’37”Nlatitude(Figure1).

Duetothehighevolutionrate,abundance,widespreaddistributionandabruptextinctionofspecies,benthicforaminiferaareconsideredanimportanttoolforthestudyofthebiostratigraphyandevolutionofgeneraandspecies,relativedepthorpaleoenvironmentalreconstruction(Vaziri-Moghaddamet al.,2010).Consideringthedistributionofforaminiferaandanalyzingsomefactors likelight,temperature,nutrientsupply,substrate,hydrodynamicenergy, depth,water locomotion and symbiosis, thepaleoenvironmentcouldbereconstructed.TherearesomesimilaritiesbetweenthebenthicforaminiferaassemblagesofQomFormationandAsmariFormationandduetolackofaformalproposedbiozonationintheQomFormation,thebiozonationofferedbyWynd(1965)andLaursenet al.(2009)fortheAsmariFormationwereusedinthisinvestigation.

Geological investigationof theQomFormationinCentralIranstartedwiththeworkofTietze(1875).SubsequentworkerssuchasStahl(1911),Riben(1935),FuronandMarie(1939),Furon(1941),FurrerandSoder(1955),Gansser(1955),Dozy(1944,1955),Abaieet al.(1964)andBozorgnia(1966)reportedonthemarinesedimentsoftheQomFormation.AfewforaminiferalspecieswereillustratedbyFuronandMarie(1939),Furon(1941),FurrerandSoder(1955),Abaieet al.(1964)andBozorgnia(1966).Rahimzade(1994)collectedthenamesofmostresearchers,partofwhoseworkcoveredtheQomFormation.

FurrerandSoder(1955)dividedtheQomFormationintosixmembers;Abaieet al.(1964)increasedthenum-berofmembers to ten.Abaieet al. (1964)notedthat twomembers,c-1andc-3,werethemainobjectivesinexplorationwells,duetofracturedevelopmentenhancingporosity/permeabilityandhydrocarbonshows.Bozorgnia(1966)proposedandintroductiontenmembersfortheQomFormationanddistinguishedseverallocalbasinsofdeposi-

tioninCentralIran,onthebasisofitslithologicalcharacters.Rahaghi(1973,1976,and1980)suggestedOligo-MioceneagefortheQomFormation.OkhraviandAmini(1998)reconstructedpalaeoenvironmentofthef-memberoftheQomFormationbasedonmicrofaciesanalysis.Basedonechinodermsandmicrofosils,KhaksarandMaghfouri-Moghaddam(2007)proposedMiddletoLateOligoceneagefortheQomFormation.Aalaeobiogeographicrecon-structionoftheQomFormationwasproposedbyReuteret al.,2007.

METHODS AND STUDY AREA

FieldworkwasconcentratedonanoutcropoftheQomFormation,located10kmtothenorthofChenarvillage.A

CaspianSea

Persian Golf

Kashan

N44°

38°

24°

Aran-Bidgol

Kashan

Isfahan

GhamsarBarzak

Niasar

JoshaghanChenarJoshagh Ravand

N

34°5'

51° 9'

10 kmStudy area

I R A N

Oman Sea

64°

Figure1.LocationmapofthestudiedareaincentralIran.

Biostratigraphy and paleoecology of the Qom Formation, Iran 557

itscoevalcounterpartintheZagrosbasininsouthwestIran(AsmariFormation)(Stöcklin1952;Bozorgnia1966and Kashfi 1988). Therefore, biozonations established for theQomFormationinthisworkarelargelybasedonthebiozonationsofWynd(1965)andLaursenet al.(2009),whichwasusedfortheAsmariFormationthatcomprisesanOligocenetoEarlyMiocenecarbonatesequence(Table1).

Frombasetotopatwoforaminiferaassemblageswererecognizedinthestudiedsection(Figure3):

Assemblage1: Ischaracterizedby thepresenceofLepidocyclinasp., Eulepidinasp., Eulepidinadila-tata, Eulepidinaelephantina,Nephrolepidinacf. mar-ginata,Nephrolepidina cf. tournoueri,Nephrolepidinasp.,Operculina complanata, Operculina sp.,Sphaerogypsina globulusa, Haplophragmium slingri, Planorbulinasp.,Neorotalia viennoti, Neorotalia sp.,Valvulinid sp.,Amphistegina sp.,Amphistegina lessonii, Elphidium sp.,Bigenerina sp.,Textulariasp., Discorbis sp.,Quinqueloculinasp., Triluculina trigouenula, Globorotalia cf. nana, Globorotalia siakensis (Figures4and5).Thisassemblage

147mthicksectionwasmeasuredindetail,andatotalof112 specimens were sampled during the detailed field inves-tigation.Samplesweretakenfromthecarbonateandmarlylayersalmosteverymeteraccordingtofaciesvariation.Thinsectionswereprovidedforharderlitologieswhilstsofterli-tologiesweredisaggregatedandtheforaminiferapickedandanalyzed.Disaggregatedsampleswerewetsievedthrougha 151 μm. Thin sections were studied under the microscope fortheanalysisofbenthicforaminifera.Taxonomicclas-sification was based on Loeblich and Tappan (1988), Adams andBourgeois(1967)andAdams(1969).

GEOLOGICAL SETTING

Onthebasisofthesedimentarysequence,magmatism,metamorphism, structural setting and intensity ofdeformation,Iranianplateauhasbeensubdividedintoeightcontinentalfragments,including,Zagros,Sanandaj-Sirjan,Urumieh-Doktar,CentralIran,Alborz,Kopeh-Dagh,LutandMakran(Heydariet al.,2003).ThestudyareaislocatedintheCentralIranbasin(Figure2).

DuringtheEarlyPaleogene,theTethyanseawaywasawideoceanthatconnectedthetwomajoroceanicrealms,the Atlantic and the Pacific (Schustr and Wielandt, 1999). The subduction and final collision of the African-Arabian platearoundEocene-OligoceneboundarywasaccompaniedbythevanishingoftheTethyanseaway,thedisconnectionof the Atlantic and the Pacific Ocean and the birth of the IndianOceanandtheMediterranean.Asaconsequence,Central-IranianpaleogeographychangeddramaticallybythedevelopmentofavolcanicarcwhichseparatedaforearcfromabackarcbasinduringEocenetimes.MarinesedimentationoftheQomFormationbeganduringtheOligoceneandcontinuedtotheendoftheEarlyMioceneintheEsfahan-SirjanforearcandintheQombackarcbasin(SchustrandWielandt,1999).

BIOSTRATIGRAPHY OF FORAMINIFERA

LargerbenthicforaminiferaarewidelydistributedintheTertiarycarbonateplatformoftheQomFormation.Theydevelopedcomplicatedinternalstructureswhichcanbe identified when they are randomly thin sectioned. These organismscanprovidecompleteanddetailedevidenceforbiostratigraphicanalysisoftheshelflimestonebecauseof rapid diversification, abrupt extinction and abundance (Beavingtone-PennyandRacey,2004).

The sedimentary deposits of the study areayieldedabundantlargerbenthicforaminifera,therefore,biostratigraphiczonationisbasedontheseorganisms.Sofar,aformalbiostratigraphicframeworkhasnotyetbeenestablishedfortheQomFormation.However,basedonforaminiferalsimilaritiesageneralagreementexiststocorrelatetheQomFormation(CentalIranBasin)with

Zagros Province

Sanandaj-Sirjan Province

Urumieh-Dokhtar Province

Central Iran Province

Makran Province

Alborz Province

Lut ProvinceKopeh DaghProvince

Suture

Fault

Study area50° 60°

Persian Gulf

Pakistan

30°

Afghanistan 35°

Turkmenistan60°

Caspiansea

50°

Turkey

35°

Iraq

30°

Figure2.SubdivisionsoftheZagrosorogenicbelt(adoptedfromHeydariet al.,2003),andzonationinAsmariFormation(Laursenet al.,2009andWynd,1965).

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correspondstothe“Lepidocyclina-Operculina-Ditrupa assemblagezone”.AssemblagezoneofWynd(1965)andLaursenet al.(2009)appliedfortheAsmariFormation.ThisassemblageisattributedtotheOligoceneandiscorrelatedwiththelowerpartoftheAsmariFormation.

Assemblage2:Lepidocyclina-Operculina-Ditrupa assemblagezone.Theassemblagehasthisassociatedfauna:Planorbulinasp.,Eulepidina dilatata,Haplophragmium slingri,Rotalia viennoti,andalgae.ThiszonerangesfromtheRupelianintotheChattian(Laursenet al.,2009).

PALEOECOLOGY

Inthissection,anattempttoinvestigatefactorsthatinfluence the benthic foraminifera distribution in the Qom Formationispresented.Inthisregards,environmentalagentssuchasnutrientsupply,light,temperature,waterlocomotion,substrate,salinity,hydrodynamicenergy,depth and symbiosis are very significant. In terms of the paleoecology,paleoecologicalconstraintsporposedbyHallokandGlenn(1986)includethreefamiliesobservedinstudiedsection(Nummulitidea,AmphisteginideaandPeneroplidea).

Nutrient supply

Largebenthicforaminiferaarehighlyadaptedtostable, oligotrophic and nutrient-deficient conditions, but theycannotrespondcompetitivelywhennutrientresourcesbecomeabundant(Hallock,1985).Inorganic,“biolimiting”nutrientsentershallow-watercommunitiesprincipallybyturnoverorupwellingofdeeperwaters,byrun-offfromland

orbyadvectionfromareasofupwellingorrunoff(HallockandSchlager,1986).

Largesymbiont-bearingbenthicforaminiferaarecompatible to nutrient deficiency conditions and in such situation,thealgalsymbiontusesorganicwastesubstancesofthehostandCO2originatedfromhostrespirationforphotosynthesis(Beavingtone-PenneyandRacey,2004).Andso,theproducedsubstancessuppliesnutrientforthehost.

In the studied section, semi-restricted lagoon environmentsinferredbythepresenceofperforateandimperforateforaminiferaindicatemesotrophictoeutrophoticconditionsandshallowtodeepmarineenvironmentspointtomesotrophic tooligotrophicconditions.ShowthatLepidocyclinaharbouredendosymbionticalgaetoprovidesufficient light for the photosynthesis of the symbionts (Chaproniere,1975).NumerouschambersoftwentytestsofNephrolepidinacf.tournouer indicatedinsample(B16)(Figure6).

Light

TheabundanceofrobustandthicktestsofOperculinain the study area reflects increasing light intensity (Figure 7).Lightintensityandhydrodynamicforcecontroltestmorphologythroughsymbioticinteractions.Inshallow,well-lit waters the calcification rate in foraminiferal test isenhancedbyphotosyntheticsymbionts,inordertopre-ventphotoinhibitionofsymbioticalgaewithinthetestortestdamageinturbulentwater(Beavingtone-PenneyandRacey,2004).

Larger foraminifera are sensitive to changesinenvironmentalfactors includinglight intensityandhydrodynamicforcealongdepthgradientswhichare

Epoch Stage Biozonation of Laursen .et al(2009)

Biozonation of Wynd (1965)

Burdigalian Borelis melo curdica -Borelis melo melo Borelis melo curdicaAssemblage zone(zone 61)

Assemblage zone (zone 59)

Miocene

AquitanianMiogypsina-Elphidium sp. 14Peneroplis farsenensis Archaias

operculinoformisAssemblage zone

(zone 58)

Nummulitesintermedius-Nummulites vascusAssemblage zone

(zone 57)

ChattianArchaias hensoni-Miogypsinoidescomplanatus

Nummulites vascus-Nummulites fichteli

Lepidocyclina-Operculina-Ditrupa

Oligocene Rupelian

Globigerina-Turborotalia cerroazulensisHantkenia

Lepidocyclina-Operculina-DitrupaAssemblage zone (zone 56)

Globigerina .sppAssemblage zone (zone 55)

Table1.ZonationinAsmariFormation(Laursenet al.,2009andWynd,1965).

Biostratigraphy and paleoecology of the Qom Formation, Iran 559

Lithology

B1B4

B8

B15

B23

B26B28B30B32

B35

B41

B46

B50

B53

B57B63B65

B68

B72

B78

B83

B87

B90B94

B98

B102B107B110

B6

B11

Sampleno.

Stage

Formation

Qom

Rupelian-Chattian

B20

10

20

30

40

50

60

70

80

90

100

110

120

130

140

Thicknes(m)

Series

Oligocene

Eoce

ne

Volcanic

Quinq

uelocu

linasp.

miliolids

Amph

istegina

.sp

Ope

rculina

.sp

Textullaria

.sp

Bige

rina

.sp

Elph

idium

.sp

Lepido

cyclina

.sp

Trilu

culin

atrigou

enula

Nep

hrolep

idina

.sp

V alvulinidsp.

Globo

rotalia

siak

ensis

Globo

r otalia

nana

cf.

Hap

loph

ragm

ium

slinge

nDisco

rbissp

.

Pene

roplis

evolutus

Spha

erog

ypsina

sp.

Plan

orbu

linasp.

Neo

rotalia

vien

noti

Pyrgosp

.

Ditr

upasp

.Heteros

tegina

.sp

Neph

rolepidina

.margina

tacf

Eulepidina

sp.

Nep

hrolep

idina

tour

noue

ricf.

Ope

r culinaco

mplan

ata

Eulepidina

dilatata

Recent alluvium

Sandy limestone

Thick bedded limestone

Marly limestone

Medium bedded limestone

Gravely limestone

Thin bedded limestone

Volcanic

Figure3.LitostratigraphiccolumnandverticaldistributionofsomebenthicforaminifersinChenararea,northwestKashan.

Behforouzi and Safari560

Figure4.a)Neorotalia viennoti,subaxialsection(sampleNo.B72),b) Neorotalia viennoti,equatorial section(sampleNo.B102),c) Planorbulinasp.,subaxialsection(sampleNo.B98),d) Elphidium sp.,subaxialsection(sampleNo.B76),e) Sphaerogypsina sp.,axial section(sampleNo.B68),f) Ditrupasp.(sampleNo.B73),g) Haplophragmiumsp.,subaxialsection(sampleNo.B63),h)Amphistegina sp.,subaxialsection(sampleNo.B34).

Biostratigraphy and paleoecology of the Qom Formation, Iran 561

mirroredbychangesintestshapeandsizeofsymbiont-bearingforaminifera(Beavingtone-PenneyandRacey,2004).Theshallowdwellingmicroperforatedhyalineforaminiferaproducesmall,robustandovatetestsandincrease calcification rate which brings about a thick wall structuretopreventphotoinhibitionoftheendosymbiontwithinthetestinhighlyilluminatedshallowwaters,ortestdamagesinmobilesubstrates,whilehyalineforaminiferafromgreaterdepthsshifts to larger, thinnerandmoretransparenttesttothriveinstronglyreducedlightintensity(Beavingtone-PenneyandRacey,2004).Onthebasisofthedependenceofcarbonateproductiontolightpenetration,

20 µm 20 µm

50 µm

500 µm 20 µm

1 mm

20 µm

d)

a)

c)b)

e)

f)g)

Figure6.Lepidocyclinatest.Smallporesandlateral chamberlets,whichresultedfromalgalsymbiosisreplacement(sampleB16).

Figure5.a)Eulepidina cf.dilatata,axialsection(sampleNo.B80),b) Nephrolepidina cf.marginata,axialsection(sampleNo.B38),c)Nephrolepidinacf.tournoueri,axialsection(sampleNo.B65),d)Operculina sp.,axialsection(sampleNo.B55),e)Operculina sp.,equatorialsection(sampleNo.B70),f)Globorotalia siakensis, axialsection(sampleNo.B97),g) Globorotalia cf.nana, subaxialsection(sampleNo.B86).

Behforouzi and Safari562

threedifferentgroupsofbenthicorganismscouldbeconsidered.

RedalgaeandsomehyalinebenthicforaminiferasuchasHeterostegina,OperculinaandLepidocyclina identified intheQomFormationsamplesareindicatorsofdysphoticzones.Bassiet al.(2007)dividedthephoticzoneintoup-per and lower parts; in this classification, Neorotalialiveintheupperpartoftheupperphoticzone,andHeterostegina,OperculinaandLepidocyclinaaredominantinthelowerpartoftheupperphoticzone,whilstinthelowerphoticzoneLepidocyclinaisdeveloped.

Light-independentbiotaincludesbryozoans,mol-lusks,crinoids,brachiopodsandspongesthatarewidespreadinstudiedsection(sampleB12).Organismsthatcomposethe light-dependent biota identified include green algae, reef-buildercoralsandmiliolidsconstitutingindicatorsofphoticzone.

Severallargeforaminifersthathostalgalsymbiontspresentamoreevolvedskeletalstructurethanthosewithoutsymbiont.ThisprocessisalsoseeninLepidocyclina(Figure4).Largebenthicforaminifershostunicellulareukaryoticalgalsymbiontsthataredependentonlightandnutrientandarethereforerestrictedtoeuphoticzone(Romeroet al.,2002).Symbiont-bearinglargeforaminiferaarerestrictedtowarmwateroftropicalrealmswherewatertemperatureishigherthan14–18°Cinthecoldestmonthsoftheyear(Renema,2006).

Figure7.a)Operculinawiththicktest,whichisindicatorofshallowdepthandincreasinglightintensity(sampleB12);b)Operculinawiththintestfromhighdeepanddecreasinglightintensity(sampleB16).

Figure8.a)Amphistegina(sampleB12)withthicktestandsmallsizewhichisanindicatorofincreasinghydrodynamicregime;b)Amphistegina(sampleB17)withthintestandlargesizeindicatedecreasinghydrody-namicregime.

Water motion

This factor influences the test shape (diameter/thick-nessratio).Generally,lightintensityandwatermotionpromotestheformationofsecondarylayersinforaminif-eraltests.Onthecontrary,whenthelightintensityandthehydrodynamicsareweakthegrowthratedecreases.Inthestudiedarea,Amphistegina fromlagoonenvironmentwithhighenergyandintenselightarethickerthanthoseofdeeperenvironmentswithlowerenergy(Beavingtone-PenneyandRacey,2004)(Figure8).

Substrate nature

Substratenaturealsodependsonwaterturbulence.Foraminiferawhichliveonacoarsegrainsubstratehavethickertestsandarefusiform.Amphistegina alsopreferhardsubstrateswithhighenergy(Figure8),whileOperculinaliveonsoftandmuddysubstratesandhavethinshells(Figure8).

Salinity

Highsalinityplaysapreventativeroleingrowthandevolutionoflargeforaminifera,butmediumdegreeofsalin-ityisnotsoeffective.Insemi-restrictedlagoonenvironment,

Biostratigraphy and paleoecology of the Qom Formation, Iran 563

a) b)

c) d)

20 μm

20 μm

100 μm

50 μm

Figure9.a)Perforateforaminiferainassociationwithimperforateforaminifera(sampleB9toB12);b)Perforateforaminifera(sampleB13toB17);c)echinoid;d)bryozoans.

becauseoftheconnectionwithopenmarineenvironmentsandlowpercentofsalinity,perforateandimperforatefora-miniferscanbeobservedtogether(Figure9a),whereas,consideringnormalsalinityinshallowpartsofopenmarineofthestudiedsection,onlyperforateforaminifershavebeenfound(Figure9b).Thepresenceofstenohalineorganisms(echinoidandbryozoan)isanindicatorofmarinenormalsalinity(Figure9c,9d).

Hydrodynamic energy and depth

The occurrence of benthic organisms reflects their compatibilityinhighorlowenergyenvironments(Flugel,2004).Forinstance,inshallowenvironmentswithinthephoticzone,withincreasingwatermotion,benthicfor-eminifersshapechanges.Itmeansthathighenergycausesthetesttobethick(increasingincarbonateproduction)anddecreasesitsgrowthrateandeventuallyreducestheirtestsize(Beavingtone-PenneyandRacey,2004).Haynes(1965)statedthattheshapeoflargeforaminiferachangesunder the influence of hydrodynamic conditions and its symbioticrelationshipwithalgae.Theauthorexpressedthatspecieswithfreelife(non-epizoan)andsphericalspeciesare indicators of reef environment cleaned by flows, while

fragilethintestswithmaximumsurface/volumeratiocouldbefoundincalmenvironmentswithlowlight.Generally,foraminifersincompatibilitywithhighenergy,createslamellarandthicktestslikeAmphistegina.TheelongatedOperculinawiththintestfoundinthisworkisanindicatorofdeepenvironments(Beavingtone-PenneyandRacey,2004).Inthisresearch,acomparisonisshownbetweenthickAmphisteginawithlamellartestsinhighenergylagoonenvironmentandelongatedAmphisteginathatisindicatoroflowenergyenvironment(Figures10and11).

(1) (2) (3) (4) (5)

A. Lobifera A. Lessonii A. Papillosa

Increasing water energyand light intensity

Figure10.ChangeintheshapeandsizeofAmphistegina testduetochangeinlightintensityandhydrodynamicenergy(Beavingtone-PenneyandRacey,2004).

Behforouzi and Safari564

CONCLUSION

Onthebasisoftheforaminiferarecognizedinthestudiedsection,theQomFormationiscomparabletotheLepidocyclina-Operculina-Ditrupa AssemblageZone.TheageoftheQomFormationinthestudiedsectionisOligocene.

Thecarbonaterocksofthestudyareacontainaphoto-zoanassociationcomposedpredominantlyoflargebenthicforaminiferaandcorallineredalgaeinassociationwithcorals,heterotrophs.Thisphotozoanassemblageindicatesoligotrophicconditions.Moreover,theabundanceandas-sociationoflargebenthicforaminiferawithcorallineredalgaearereferredtoasforalgalfaciesdevelopedinshal-low,warmwaterenvironmentsofthephoticzonewhereoligotrophicconditionwasprevalent.

Basedonthepaleoecologyandlithology,threedistinctdepositionalsettingcanberecognized:innershelf,middleshelfandoutershelf.

Innershelffaciescontainabundantimperforatetestsofforaminifera.Middleshelffaciesarecharacterizedbyabundantlargeperforateforaminifertests.Towardthebasin,plankticforaminiferaandlargeforaminiferswithperforatetestsoccurcontemporaneously.Basin(outershelf)faciesismarkedbyhighplankticforaminiferacontentsembeddedinwackstone.

ACKNOWLEDGMENTS

Inparticular,wewouldliketoexpressourmostsinceregratitudetoProf.Dr.FranciscoVegaforhishelpfuladviceandguidance.Theauthorswishtothankthereviewersfortheirhelpfulandconstructivecomments.WealsothankDr.HosseinVaziri-Moghaddam,Dr.AliBahrami,Dr.AkbarGhazi-FardandDr.MohammadAliMakki-Zadeforhissup-portandthoughtfulcomments.Wealsowouldliketothankthe University of Isfahan for providing financial support. AlsowethanktheRevistaMexicanadeCienciasGeológicasreviewersfortheirconstructivecomments.

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Manuscriptreceived:December18,2010Correctedmanuscriptreceived:June16,2011Manuscriptaccepted:June26,2011