the eifelian givetian boundary (middle...

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23rd Annual Keck Symposium: 2010 Houston, Texas

INTRODUCTION AND PURPOSETheDevonianSystem(418.1±3.0–365.7±2.7Ma)issubdividedintothreeepochs,Lower,Middle,andUpper,whicharefurthersubdividedintosevenstag-es.TheMiddleDevonianissubdividedintotheEif-elian(391.9±3.4–388.1±2.6Ma)andtheGivetian(388.1±2.6-383.7±3.1Ma;Kaufmann,2006).ThestageboundaryisdefinedbythefirstappearanceofPolygnathushemiansatusattheEifelian-GivetianStageGlobalStratotypeSectionandPoint(GSSP),whichisasectionatJebelMechIrdaneintheTifilaltofMorocco(Walliseretal.1995).Atsomesections,theappearanceofthegoniatiteMaenioceras undu-latum hasbeenusedasaproxyfortheboundary(KutcherandSchmidt,1958).TheEifelian-GivetianstageboundaryoccursjustaboveagloballyrecognizedhypoxicepisodecalledtheKaĉákEvent(House,1995).Duetoitsassocia-tionwiththeDacryoconaridNowakiaotomari,theKaĉákEventisalsoreferredtoastheKaĉák-otomariEvent.Cricketal.(2000)discoveredaperiodoflowmagneticsusceptibilityvaluesassociatedwiththeKaĉák-otomariEvent,reflectingalowerdetritalironconcentrationwithinthesedimentaryrecord.Lowerconcentrationsofdetritalironareinterpretedastheresultofatransgressiveepisodeandconse-quentmigrationofclasticinputlandward.TheendoftheKaĉák-otomariEventcoincideswithasharpriseinmagneticsusceptibilitylevelsroughlyconcur-rentwiththeEifelian-Givetianboundary(Cricketal.2000).Travisetal.(2009)haveusedthepositivemagneticsusceptibilityspiketoapproximatetheEif-elian-GivetianboundaryineasternNorthAmerica.Thepurposeofthispaperistoanalyzeandapproxi-matetheEifelian-GivetianstageboundaryoftheTsakhirSection,whichislocatednearShineJinstintheGoviAltaiRegionofsouthernMongolia.

GEOLOGIC SETTINGBadarchetal.(2002)arguedthatMongoliaconsistsofnumerousterranesthatwereaccretedontosmallPrecambriancratonicblocksintheHangayRegionduringthePaleozoicandMesozoic.Thefocusofthisinvestigationarestratainaregionrecognizedaspartofoneoftheseaccretionarywedges,whichisreferredtoastheGobiAltaiTerrane(Figure1;Ba-darchetal.,2002;MinjinandSoja,2009a).Badarchetal.(2002)characterizedtheGobiAltaiTerraneasabackarcbasin,asevidencedbyabundantvolcano-clasticsedimentaryrocks.ThevolcanicswithintheGobiAltaiTerranearebelievedtobederivedfromaprehistoricislandarcrepresentedbytheMandalo-vooTerrane,whichwasactivethroughouttheLowerandMiddleDevonian(Badarchetal.,2002).

ThestudyareanearShineJinst(Fig.1)representsaseriesofrelativelycontinuousandmildlydeformedrockstrata,rangingfromtheOrdoviciantothe

THE EIFELIAN GIVETIAN BOUNDARY (MIDDLE DEVONIAN) AT TSAKHIR, GOVI ALTAI REGION, SOUTHERN MONGOLIA

NICHOLAS SULLIVANStateUniversityofNewYorkatGeneseoFacultyAdvisor:D.JeffreyOver

Figure 1 – Simplified tectonostratigraphic terrane map for Mongolia with the Gobi Altai and Mandalovoo Terranes high-lighted. (modified from Badarch et al. 2002)

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Carboniferous(MinjinandSoja,2009b).Wangetal.(2005)arguedthattheEifelian-Givetianbound-arylieseithernearthetopoftheTsagaankhaalgaFormation,ornearthebottomoftheoverlyingGovialtaiFormation,suggestingthepossibilityofagapinthestratigraphicrecordthatincludesthestageboundary.Alekseeva(1993)describedtheTsagaankhaalgaFormationaspredominantlymassive,beddedlimestonesandassociatedconglomeratesandsand-stonelayers.Wangetal.(2005)reportedthecon-odonts Caudicriodus angustus cauda,Caudicriodus stelcki,andCaudicriodus unwoschmidti,withintheTsagaankhaalgaFormation,therebyconstrainingitsagetotheEifelian.Alekseeva(1993)subdividedtheGovialtaiForma-tionintotwomembers.ThelowerorTentaculiteMemberischaracterizedbyblacksiltstonesandclaystoneswithoccasionalsandstoneandsandylimestonelayers.TheupperorKharMemberischaracterizedbyvolcanoclasticsedimentswithoccasionalinterbeddedbasalts.ConodontsoftheUpperensensisZonewerereportedbyAlekseeva(1993)intheTentaculiteMember,therebycon-strainingitsagetotheGivetian.TheEifelian-GivetianStageboundaryisbelievedtofallwithintheupperTsagaankhalgaorlowerGovi-altai.Alekseeva(1993)suggestedthattheTsakhirsectionisincompleteandthetwoformationsareunconformable,asentimentsharedbyWangetal.(2005).LITHOLOGY AND LOCATION OF THE TSAKHIR SECTION

From3August2009to6August2009,thesectionatTsakhirwasmeasuredandsampled.Theap-proximatelocationofthesectionis44º22’53”Nand99º28’46”E.Theoutcropliesonaridgethattrendsroughlynorthwest-southeastbetweentwodrainagedividesthatcontaindirtroads.Thecrestoftheridgeconsistsofmassivelybedded,fossiliferouscarbonates.Asmallsampleofthesecarbonatebeds,believedtocorrespondtothebaseofUnit18from

Alekseeva(1993),wascollectedformicrofossilsandthinsectionproduction.

Theprimaryfocusofthisstudywasanoutcropthatcanbefoundonthethirdhilleastofthedrainagedividethatcrossestheridgetothewest(Fig.2).Thestratawerefoundtohaveatrendof058withadipof50tothesouth.However,theyweredeterminedtobeoverturned;indicatingstratigraphicupwastothenortheast.

A103metersectionofrockwasstudiedwithzeroestablishedatthelevelwherealightpinkishgraysiltstoneisoverlainbyalightgraysiltyshale(Fig.3).The3.6m–25.0m,28.8m-43.0m,and49.0m–53.0msectionintervalswereobscuredbyalluviumandthereforewerenotrecorded.Magneticsusceptibil-itysampleswerecollectedathalfmeterintervalsthroughthesection.Fivebulksampleswerecol-lectedforthinsectionsandmicrofossilanalysisat65.2m,68.8m,76.5-77.0m,87.0mand96.0-97.0m(Fig.3).

Figure 2 – Location of the Tsakhir section (outlined in the black box) on a simplified geologic map of the study area. Faults are indicated by red lines. Drainage channels are indicated by dotted blue lines. The Tsakhir Well is represented by a red dot. Geologic units are labeled below (modified from Wang et al. 2005; and Minjin and Soja, 2009).

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Thestratigraphicsectionconsistsprimarilyoffinegrainedsiliclastics;mostlydarkbluetodarkgraysiltyshales.Alsopresentaregreenishgraytuffaceousconglomerates,mediumgreentograyfeldspathicrhyolitesiliciousbeds,andinterbeddedcarbonateswhichweresampledformicrofossils.

MICROFOSSIL DATA AND SYSTEMATICSThecarbonatebedsampledat68.8m,76.5-77.0m,87.0m,and96.0-97.0mwereprocessedforcon-odontsusingasolutionofformicacid.Conodontswererecoveredfromthe68.8mintervalandimagedusingascanningelectronmicroscope(Fig.4).

Icriodus sp.Remarks:ThemainP1elementofIcriodusischar-acterizedbythreelongitudinalrowsofnodeswithsomeformsincludingtransverseridges(Clarketal.,1981).ThestratigraphicrangeisbetweenthePridolianandtheFamennian.Specimen4AisaP1elementdisplayingbythethreelongitudinalrowsofnodescharacteristicofIcriodus.Thenodesofthecentralrowareconnectedbyalongitudinalridge.Transverseridgesarepresent,runningbetweenlateralandcentralnodes.

Polygnathus linguiformis linguiformisRemarks:P. linguiformislinguiformisisdefinedbyanelongateplatewithoneextremityproducedintoatonguelikeprojectionwhichismarkedbytrans-verseridges.Theanteriorportionoftheplatformischaracterizedbyadcarinaltroughsstraddlingtheblade(Hinde,1879).ThistaxonisknowntohaveastratigraphicrangebetweenthelateEifelianandearlyGivetian(Ziegleretal.,1979).AlthoughFigure

Figure 3a – Stratigraphic column compiled from data collected from the Tsakhir Section. Figure 3b – Key to lithologies

Figure 4 – Conodonts from the sample collected at 68.8 m car-bonate bed. Images were acquired using the scanning electron microscope. White bar represents two millimeters.Icriodus sp.A; Upper view of TSEG 01726, (long axis; 1.19 mm), Polygnathus linguiformis linguiformisB; Upper view of TSEG 01734 (long axis; 1.45 mm)Polygnathus sp.C; Upper view of TSEG 01730 (long axis 0.84 mm)D; Upper view of TSEG 01736 (long axis 0.72 mm), most likely the posterior end of sample TSEG 01730Polygnathus ensensis E; Upper view of TSEG 01736 (long axis 0.77 mm)F; Oblique view of TSEG 01731 (long axis 0.77 mm)Polygnathus xylus xylusG; Upper view of TSEG 01735 (long axis .78 mm)

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4Bdoesnotrepresentacompleteandundamagedconodont,itisintactenoughtoviewthetonguelikeprojectionandlateralridgescharacteristicofP. linguiformis linguiformis.

Polygnathus sp.Remarks:ThegenusPolygnathusisdefinedbycar-miniplanateP1elements.P1elementsofPolygnathusconsistofaflatplatformextendingawayfromtheposteriorsectionofablade.Figures4Cand4Darebelievedtorepresenttwofragmentsofthesameconodont.Thespecimenswerenotidentifiedtothetriviallevel.ThestraightoutermarginsofthebladeanddistinctadcarinaltroughsarereminiscentofP. linguiformis.However,thespecimenslackthelateralridgesandtonguelikeprojections.Relativelyflatbutdiscreteprotrusionsontheplatformarealsovisible.Thespecimenbearsresemblancetospeci-mensidentifiedasPolygnathus pseudofoliatusbyWalliser(1996,Fig.4A,4B)fromMorocco.

Polygnathus ensensis Remarks:P. ensensis(synonymouswithP. xylus ensensis)isdefinedbydistinctlyserratedplatformmarginsjustposteriortothegeniculationpoint(Zeigleretal.,1976).Polygnathus ensensisfirstap-pearsintheupperEifelian,rangingintotheearlyGivetian(BultynckandHollevoet,1999).Figure4Eisfragmentedandthebladeisnotintact.However,thedistinctlyserratedmarginsintheposteriormar-ginoftheplatformarevisible.Figure4Fshowsanobliqueviewoftheplatform.

Polygnathus xylus xylus Remarks:P. xylus xylusischaracterizedbyplatformmarginsjustposteriortothegeniculationpointthatarenotsignificantlyserrated.ThetaxonappearsintheupperEifelian,rangingintotheLower varcus-SubzoneoftheearlyGivetian(Ziegleretal.1976).Figure4GshowsthespecimenwhichdisplaystheunserratedmargincharacteristicofP. xylus xylus.Thespecimenalsoshowsanelongatedbladewhichisalsocharacteristicofthetaxon.

MAGNETIC SUSCEPTIBILITY DATA

MagneticsusceptibilitydatawasacquiredusinganAGICPKappabridgehousedintheSUNYGeneseoDepartmentofGeology.DatawerecompiledandorganizedinMicrosoftExcelandmagneticsuscep-tibility(m3/kg)wasplottedagainststratigraphicthicknessinmeters(Fig.5A).Magneticsusceptibil-itydatashowanoverallpositivetrendmovingupthesection.Therearethreemajorpositivespikesinmagneticsusceptibilityatapproximately0.0m,47.0m,and63.0m.

Thesepositivespikesovershortintervals,combinedwiththeoverallpositivetrendofmagneticsuscepti-bilitymovingupsection,issimilartodatacollectedfromtheEifelian-GivetianGSSPatJbelMechIrdaneinMorocco(Cricketal.,2000)andfromMiddleDevonianstratasampledinNewYork(Travisetal.,2009).

Figure 5 –Magnetic susceptibility (m3/kg) of samples collected at Tsakhir Section plotted against stratigraphic thickness in me-ters. Blue line represents magnetic susceptibility of individual samples. Red curved line represents mean magnetic susceptibil-ity for sets of eight samples.

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CONCLUSIONS

ThesiliclasticandvolcanicsedimentsattheTsakhirSectionareassignedtotheTentaculiteMemberoftheGovialtaiFormation.ThisisconsistentwiththegeologicmapproducedbyWangetal.(2005)andtheobservationsofAlekseeva(1993).

Thelithologyofthesectionshowsmanysignifi-cantsimilaritiestoMiddleDevonianoutcropsinNorthAmerica.ThemassivecarbonatesoftheTsagaankhaalgaFormationoverlainbythemorefinelybeddedsiliclasticshalesoftheGovialtaiFor-mationarestrikinglysimilartooutcropsineasternNorthAmericawherecarbonatesoftheOnondagaFormationareoverlainbytheMarcellusandSka-neatelesshales.Theabundanceofvolcanoclasticsthroughoutthesectionisconsistentwiththecon-clusionsofBadarchetal.(2002)whodefinedtheShineJinstareaasabackarcterranethathadformedbehindavolcanicislandarcthatwasactiveduringtheDevonian.ThepresenceofPolygnathus xylus xylusandPolyg-nathus linguiformis linguiformisindicatethatthecar-bonatebedfoundat65.8mostlikelyrepresentstheLowervarcus-ZoneofthelowerGivetian.TheEif-elian-Givetianboundaryistentativelyplacedabovethepositivemagneticsusceptibilityshiftobservedatapproximately0.0meters,wherethemeasurementsappeartostabilizeatalevelbetween6.00x10-8and9.00x10-8m3/kg.However,biostratigraphiccontrolisnotsufficienttoconstraintheboundaryintervalandthickintervalsofthesectionareobscuredbyalluvium,thuslimitingtheresolutionofmagneticsusceptibilitytrends.

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