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18 3-lntegratedStrati11raphyofthelゝ/liocene5;(?(nlence,lゝ/liocenefromJapan (WGkaderMasakiTakahash沁-lmi1:masak返)lg・1j・gojp) /zzzEgrαなj∫なα哨iz‘αρ/zyθ/’治εルfiθa?zze謂αΓ加e∫ε9zzEzzce加治ε7・αzlαgzμ゛ααΓεα /VθΓ治eαsljαραzz MasakiTakahashi lntc5gratedbio,andchlonostratigraphyResealchGI oup,lnstituteofGeo5;cience,AISTsiteC-7. 1-1-IHigashi,Tsukuba,lbarak□()5-8567,Japan;msk.ta1こlltlashi@11汝.go.jp YukioYanaがsawa lrltc5gratedbio-andchronostlatigraphyRc5seafchGroup,lnstituteofGeo如ence.AISTsite(:ンフ. 1↓1Rllashi,ljukuba,lbafaki305-8567,Japan;y・yanagisawa@11i・;t.gojp HirokiHayashi lnstituteofGe2010gyandl)ale(mtology丿ohokuun沁ersity. Aoba,.4J llmaki,Aoba-ku,Sendaj,Japan;nn@mail.(:clohoku.ac.jp Hidekilwano KyotoFissiorl-TrackCo.Ltd.;44-4Min21rni-Tajiricho,Kita-ku,Kyoto603-8832,Japan;kyot()-ft@mbJew油.ne.jp Toshinoriokada Hiruzenlnstittltefor(11eologyandChl`alology;161-ISaiJkayama703-8242,Japan;hiruzen@oran卯.ocn.nejp Summary lntegTatedstratigraPhicstudyalongtheMiocenemarinesequenceintheTanaguraarea,Nor- theastJaPanjsbrienyintroduced. Radiometric芒萍werel・e(;entlydcうtenTlinedontwovolcaniclasticl’ocks(Kt-1andKt-フ)inter- t)(iidde(lintheloRz(:5standupPermostPartoftheMioceneKubotaForm21tioninthe’I’al’1aguraarea. (:)btainedK-Arage(10,6士0,2Ma)andf`issiontrackage(10√7士0,2Ma)oftheKt-1Tuffshowgood agTeementwitheachother,whichsuggeststh21tthescagesl゛ePIでsenttheeruptionageoftheKt-1 pyrodastics.ThefissiontrackageoftheKt`フPumicetuffwas&lerminedon100zirconl;I’ains(10,6 土0,3Ma),which(;()incideswiththeradiometric昭esoftheKt-1Tuff. Highsedim(うntaccumulationrate(25,5cm/1000aor:!55m/Ma)issuggestedbasedonK-Ar andfissiontrackl;eochronology.Pre;vi()usly5tablishedmi(;I-ofossilbiostratigraPhyarew(う11consis- tentwiththegeochr()nology・ Keywords:M沁cenc≒lntegratedstrat址raphy,}C嶋lag(≒fissiontrack211gらbiostratigraphy,calcareousnannofossn, planktonicforalninifera,radiol函a,diatom 1.1ntroduction Biostratigraphyisoneofthemostcommonandusefultoolsforagedeta`minationofmarine sediments,althoughitindicatesonlystratigraphicrelations.lfweneednumericalagesofstratabased onyieldedmicrof;()ssns,wecanestimatetheageusingatimescale,whichis(氾nstrllctedbyacombi- nationofmEt即et()-biostratigraPhyandgeomagneticPolaritytimescale(GPTS).The51‘ef()I’e,thereha- bilityofthetimescaledependsonthea(;(;uracyoftheGPTSandvalidityofcorrelationbetween mahnemagneticanomalyP旅em(GPTS)andmagne杖)stratigraphy。 RecentlyBerggreneta1.(1995)constructedagx)logictimescalebasedonCande&Kent (四95)’sGPTS,11hhoughBaksiG993),Wd(1995)andTakahashi&Danhara(1997)havel:)ointed outseriousproblemsontheGPTS.Thc51‘ef()rejtis basedontheradiometricdatingwiththemagneto, (力viouslyeffectivet()illtegratethechl7onology and/orbk)stl・姐graPhy.SomeNeogenemarine seqllellcesherbeddiltablevolcaniclastic121yersinJapan,whichisagoodoPportunityforcollstruct` 姐gatimescale(Takahashi&Oda,1997), ----------------------------------------nextpage一一,-----------一一-一一,--,------------------------------->> Fig.ISml)kl()cationonage()logicalmapoftheNUocenemarinest5(luenceintheTanaguraarea,NOI‘thealstJapan (mo(lifiedfroml;himamotoetlll・,1998). OdinG.S.(Ed.),PhanerozoicTimeScalらBuILLiai!Elnform.IUGSSubcomJeocllr()no1J6,0ffsetParis,2000

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183-lntegratedStrati11raphyofthelゝ /liocene5;(?(nlence,lゝ /liocenefromJapan

(WGkaderMasakiTakahash沁-lmi1:masak返)lg・1j・gojp)

/zzzEgrαなj∫なα哨iz‘αρ/zyθ/’治εルfiθa?zze謂αΓ加e∫ε9zzEzzce加治ε7・αzlαgzμ゛ααΓεα

/VθΓ治eαsljαραzz

MasakiTakahashilntc5gratedbio,andchlonostratigraphyResealchGI‘oup,lnstituteofGeo5;cience,AISTsiteC-7.

1-1-IHigashi,Tsukuba,lbarak□()5-8567,Japan;msk.ta1こlltlashi@11汝.go.jpYukioYanaがsawa

lrltc5gratedbio-andchronostlatigraphyRc5seafchGroup,lnstituteofGeo如ence.AISTsite(:ンフ.1↓1Rllashi,ljukuba,lbafaki305-8567,Japan;y・yanagisawa@11i・;t.gojp

HirokiHayashilnstituteofGe2010gyandl)ale(mtology丿ohokuun沁ersity.

Aoba,.4J‘llmaki,Aoba-ku,Sendaj,Japan;nn@mail.(:clohoku.ac.jpHidekilwano

KyotoFissiorl-TrackCo.Ltd.;44-4Min21rni-Tajiricho,Kita-ku,Kyoto603-8832,Japan;kyot()-ft@mbJew油.ne.jpToshinoriokada

Hiruzenlnstittltefor(11eologyandChl` alology;161-ISaiJkayama703-8242,Japan;hiruzen@oran卯.ocn.nejp

Summary

lntegTatedstratigraPhicstudyalongtheMiocenemarinesequenceintheTanaguraarea,Nor-theastJaPanjsbrienyintroduced.

Radiometric芒萍werel・e(;entlydcうtenTlinedontwovolcaniclasticl’ocks(Kt-1andKt-フ)inter-t)(iidde(lintheloRz(:5standupPermostPartoftheMioceneKubotaForm21tioninthe’I’al’1aguraarea.(:)btainedK-Arage(10,6士0,2Ma)andf`issiontrackage(10√7士0,2Ma)oftheKt-1TuffshowgoodagTeementwitheachother,whichsuggeststh21tthescagesl ePIでsenttheeruptionageoftheKt-1

pyrodastics.ThefissiontrackageoftheKt` フPumicetuffwas&lerminedon100zirconl;I’ains(10,6土0,3Ma),which(;()incideswiththeradiometric昭esoftheKt-1Tuff.

Highsedim(うntaccumulationrate(25,5cm/1000aor:!55m/Ma)issuggestedbasedonK-Arandfissiontrackl;eochronology.Pre;vi()usly5tablishedmi(;I-ofossilbiostratigraPhyarew(う11consis-tentwiththegeochr()nology・

Keywords:M沁cenc≒lntegratedstrat址raphy,}C嶋lag(≒fissiontrack211gらbiostratigraphy,calcareousnannofossn,

planktonicforalninifera,radiol函a,diatom

1.1ntroduction

Biostratigraphyisoneofthemostcommonandusefultoolsforagedeta` minationofmarinesediments,althoughitindicatesonlystratigraphicrelations.lfweneednumericalagesofstratabasedonyieldedmicrof;()ssns,wecanestimatetheageusingatimescale,whichis(氾nstrllctedbyacombi-nationofmEt即et()-biostratigraPhyandgeomagneticPolaritytimescale(GPTS).The51‘ef()I’e,thereha-bilityofthetimescaledependsonthea(;(;uracyoftheGPTSandvalidityofcorrelationbetweenmahnemagneticanomalyP旅em(GPTS)andmagne杖)stratigraphy。

RecentlyBerggreneta1.(1995)constructedagx)logictimescalebasedonCande&Kent

(四95)’sGPTS,11hhoughBaksiG993),Wd(1995)andTakahashi&Danhara(1997)havel:)ointedoutseriousproblemsontheGPTS.Thc51‘ef()rejtisbasedontheradiometricdatingwiththemagneto,

(力viouslyeffectivet()illtegratethechl7onologyand/orbk)stl・姐graPhy.SomeNeogenemarine

seqllellcesherbeddiltablevolcaniclastic121yersinJapan,whichisagoodoPportunityforcollstruct`姐gatimescale(Takahashi&Oda,1997),

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Fig.ISml)kl()cationonage()logicalmapoftheNUocenemarinest5(luenceintheTanaguraarea,NOI ‘ thealstJapan

(mo(lifiedfroml;himamotoetlll・,1998).

OdinG.S.(Ed.),PhanerozoicTimeScalらBuILLiai!Elnform.IUGSSubcomJeocllr()no1J6,0ffsetParis,2000

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20AMiocenemarinesequenceiswellexposedintheTanaguraarea,NortheastJapan(FigJ).

ThissequcnceisdividedintotheAkasakaandKubotaFormations.TheKubotaFormationyieldsrl励1‘ofossilsdolllinantly.AitaGS)88)andTaketani&Aka(1991)inve・;tigatedthePlanl(t()nicly()rami-111feralandradiol訂ianbiostratigraPhyofthef ormation,resPectively.Shimamotoetal.(1998)alsoreportedbiostratigraPhicstudyl7esultsofPlanktonicforaminifera,calcalcousnannofossilandradiola-ria.R・:・centlyHayashieta1ぺ2000)rev汝dtheplanktonicforall″1岫feralbio・;tratigraPhy.Diatombios-t心組raphywas丘rstl` ePortedbyY皿agis21waeta1ぺ::!000).

Therearesevensetsofvolcanidastic121yersintheKubotaFormation,asreportedbyShima-motoeta1.(1998)jnwhichwedatedtwovolcanidastiく:;1ayers(尺1-7and瓦z’7).lnthisPaPer,weshowbriefintroductionofthegeology,1itho-andbiostratigraphyandK-Arandfissiontrackagesobtained.ThestratigraphicrelationsbetwecnsomeimPortantmicl ofossilbiohorizonsandradiometricagesoftwopyl‘odasticrocksarealsod返ctlssed.

2.Geologyandstl`atigraphy

TheTanaguraareaissituatedonthesouthernPartofNortheastJapan(F址.1),wheretheMiocenem肛inesequencciswellexPosed.ThissequenceunconformablyoverliesthePre“Neogene

graniticandmcうtamorPhicrocksalongtheeastemmargin,andiscoveredbyaPHoceneconglomerateoftheN張ogiFormationhorizontally.TheMiocenesedimeりntaryrocksaregentlytiltednorthw(iりsterly,buttheysteePlyincnneeasterlyalongthewestemmarがnalthru・;tfault(ShilllamotoetllL,1998).

TheMiocenesequenceismorethan500minthkknessanddividedintothefollowingtwoformations.TheAkasakaFomlationiscomposedofconglom‘こrate,medium-tocoarse“gTainedsands-toneandsntstone.Cross-bed(hngs;tructureisoftenobservedthroughoutthisf ormation.MJjnemo1-柚scsareyielded,whichindicatesashallowsedimentaryenvironment.Benthic柘raminifersarerePortedfromtheAkasakaFormation,whichsuggestsaninnersublittoralzoneenvironment

(shallowerthan50m).

lncontrast,fine-grainedsandstoneandsiltstonewithfrequentintercalationofthinvolcani-clasticlayersdominatetheKubotaFormation,whichconformablycoverstheAkasakaFormation.Shimamotoet11L(195j)8)rePortedthebie;tratigraPhyofthePlanktonicforarllinifera,calcareousnan-1^ 10170ssilandl‘a(1101aria.TheyconcludedthatmostoftheKubotaFormationwascorrelativetozoneN,16ofthePlanktonicfo17amjiniferalzoneofBlowGS)69)orCN6toCNフofcalcareousnannofossilzonesofokada&Buk叩(1980).

RecentlyHayashietal.(2000)re-investigatedtheplanktonicforaminiferalbiostratigraPhy,andrecognizedakeysPeciesofyV・E?∂g/∂か∂‘7Majr加aαcど)μα四∫む,whosefirstoccuITence(FO)definesthelowerlimitofzoneN.16jnthelowestpaltoftheKubotaFormation(Fig.2).

CalcareousnannofossilkeysPeciesofC沢加附記rc∂α治心,whoseFOdefinesthebaseofCN6,andCとzr加心記7イα/yc❹心,whoseFOdefinesthebaseofCNフ,werealsorePorted(Fig.2).

Radiolariacontinuouslyoccun` edfromthemiddlepartoftheKubotaFormation.ShimamotoetaL(1998)condldedthatthemiddleFu‘toftheKubotal:;ormationwascorr(jativetothe句c・/z4aaz-z2θz77α澗αg/7‘2cθz71陥2ZoneofMotoyama&Mal°uyama(1998).

DiatombiostratigTaPhyoftheKubotaFon‘nationwaばecentlyestablishedbyYanaがsawaetaL(200(球ThestratigraphkJintervalfromtheKt-1tothemiddleoftheKt-4BandKt-4Ctufi’levelsiscoalativetothe7’加友・出回沢7y功とiZon球NPD5C)ofYanagisawa&Akiba(1998),becauseoftheoccurrenceofboth£)四面μ/卯吋∫∫加∂だa?ffand£)飢面£z/叩∫なvμなぶ忿andtheabsenceof£)四面❹∂一

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ヤ 1 犬 φ → や 二i : : : | : : ; ; ; ; l; 1 : ; ; : : ; ; : ; ‘`r…7TTT‥y……,…4-一一,:,一一,F-一一で.

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:÷

FOΛ/.∂coslaer7s/・s

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□ 呵 ふ 7 晶 ぐ

「¬FOD.内びgかesy

0.6±0.2Ma0.7t0.2Ma

(K-A「」(FT)

し‘|().6±0.3Ma(FT)

FOC.c∂/ycぴ/us10.7Ma

Radiolaria(ShimamotoetEII・,1998)

K M 1 4 … … … … …

卵 3 … … … … …

KM12………ijk4i

KM11‥‥‥‥‥‥‥

NK44

KM10‥‥‥‥‥‥

KM9‥‥‥‥‥‥‥aMa7こ…… l

乞……lテャT

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-

9WM9

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心Q’、’`Qa久司哨q巴Q゛XQト’-レーー::’-

SコSQQミーQq’`りQtxQ十:二………

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NK34

NK33,

NK32

NK31

NK30NK29

NK28

NK26

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EコミsSEI心だxuコ山

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NK18NK17NK16NK15

NK14

NK13

NK12NK11NK10NK9NK8’

NK7-NK6-NK5‘NK4

j . . . d .

NK21

II一一II

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II111111-I-I-I

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Biohorizons&theirestimatedages(E3er99renetal・,1995)

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23

3.Rad沁metricages

Morethantwentyvolcani(;1astictufflayers肛einterbeddedintheKubotaFormation,althoughonlytheKt-1TuffcontainssufficientmineralsforconventionalK-yxJ4dating.TheKt-1Tuffisbiotite-I ichco21rse-grainedl:tlf fasshowninFig.2.Takahashktaけ2001)madeadetailedcolumnarsectionoftheKt-ITuff,andcoUectedsample・;(about5kl;)fromitsmiddlePart(Fig.2)。

Coarse-§;rainedtuff5;EtmPleswerecrushedandsievedintothe48-80meshsizefTI・21ctionsforK-Aldatinlyょ・xfterultrasonicrinsing,themagneticfractionofthesaTlplewassep四回蚤dusinganiso,dynamicseParator.FreshbiotitefractionsxverePurifiedusingaheavyliquidtechnique.QuantitativeamUysisofargonandPotassiumwasPerformedatHiruzcnlnstittltcforGeologyandGeochl“:)nolo-

gy,tlsingtheanalyticalPro(;el(1ureass111ggestedbyltayaetaL(1991),The2sultsare・listedinTablelandtheweigllted-meanagewith1.u以氾rtaintyiM111;ocalculated.

Tablel.lくンν・XI-biotiteageoftheKt-ITuffinthelowestpartoftheKtlbotaFo皿ation.T’heK-Ar昭esヽ xzclrecalc山

tedwiththeSOGconvention21川eca)y(;onstants侈te呼r&Jag(・rj977).

Sample

(「111neral」

Kt-1

(biotite)

Potassium

(wt.%)-

4.56710.091

Radio9enicar9on40

(10’8ccSTP/9)

189.7土2.4

185.5±2.4

K-Arage

(Ma)

10.6710.25

10.4410.25

weりhtedmeanl0.5610.18

Table2Fi!;siontrackdataofzil・consfromtheKt,landKt-7TumintheKubotaFormation.

Sampe

code

Kt-1(EDI)

Kt-1(ED2)

No.of Spontaneous

(垂錫)

(扨{5cm’2〉

30

30

Kt-7(EDI)100

lnduced

p / ( 頂 菰

(106cm’2)

2.40(3289)3.09

1,56(1136)4.33

crystalsps

P(X2)

(%)

Dosimeter

p d ( / V の

(104cm’2)

NonRadiogenicAr(%)

10.6±0.3

19.9

20.6

U - A g e

(;ontent(=tl(y)

(ppm)(Ma)

OdinG.S.(Ed.)、Phanel‘ozoicTimeScalQButljai・Elnfom.IUGSSubcom.G(・()(;hronol・、16j)171;etParis、2000

(1)pand/Varetheciensityandtotalnumberoffissiontrackscc,11nted,respectively.

(2)Analysesweremadebythe(・xtel・naldetectormethod(Gleadow,1981)usinggeometryfactorsof0,5for2π/4π(ED1)andlforS!π/2π(ED2).

(3)Ageswer(・calculatedusingadosim(・t(・rglas・;SRM612andagaallrationfEtorsC(ED1)=352=t3(lwano&Dan-hara,1997)andこ(ED2)=372±5(Danharaetal。1991).

㈲/:)(χ2)isthel)1’obabintyofobtaininμheχ2-valueforv(陶reesoffreedom(wherev=numb2rofcrystals-1).(5)risacor2lationcoefFficientbetw(lenpsand匪(6)SampleswereirradiatedintheTRIGAMARKHnuclearrea(1()ratStJaul’suniversit)z(RikkyoDaigaku),Japan.

(4232)07.909

(3149)267.909

(2430)0.85431010.8±0.3

(2430)0.89844010.610.4

weightedmean10.710.2

1.59(4286)2.14(5779)28.130(2498)0.902210

24

SufficientzircongTainswereconcentratedbysieving,PanningバmdstandardmagneticandheavyliquidseParatingtechniques.ThesePajratedzirconswerered,euhedralandveryhomogeilleous,and,dlcうrefデorらregardedas(:り5;5;ential.Fissiontrackdatingwascarriedoutusingthec5xternaldc5tectormethod,whichisaPpliedtoboth泊terrla1(pohshed)surfacesandextemalsurfacesofjrcongrains

(EDlandED2methods,resP(iうctively:GleadoNv,1981).TwosetsofdatawereobtainedindePende-ntlyind廿fel` entsystems(observers).Ageswere(JctllatedfollowingthezetaaPProach(Hurford,1990).TheexPerimenta1Proceduresand町stemc£111brationsweredet訟edelsewhere(seeDanharaet111・,1991;lwano&Danhara,1997).Thaesultsar(・listedinTable2。

Onlythefis三.ontrackdatingwasappliedfortheKt-フpyl’o(:;lastics,becausethetuffdoesnotcontainbiotitenorsumcienthomblendemineralsforK-Ar(lating.Wemeasuredfissiontrack百啓oftheKt-7Tuffonmorethan100zircongr21㎞lsthroughEDlmcthod.Thefissiontrackagewascalcu-1atedfrom100grains,whichwereregardedas(う5;sentialbasedonthecarefulobservationthroughm沁1°ol;coPe.TheresultsarelistedonTable2・

4.Discussion

WeobtainedtheK-Art)iotiteagc5sof10,67土0,25Maand10,44土0,25Maforthe幻-j,and10,56士0,18Maasw(:り毎hted-meana§;(:り(Table1).As;f` orf7issiontrackages,|:11eresultilrltages

(10,8士0,3Maand10,6士0,4Ma)ar(・consistentwitheachoth(2r,butthedatafromintemalzircon

・;l.Hfaces(EDnf` ailedtotheZ2-te・;tatthe5%1evel(Table2).DanharaetaL(1991)(うxplaine(lthatthemainsourceofthenon-PoissonvariationinEDldatacouldbeattributedtotheh婁価箆ogeneityofura-niumcontentsaboveandbelowtheobservcdintcmalsurfacesusingagestandardswithnocontami-nationofdetritallgrains,Takahashietal.(2001)concludedthatthemainreasonfortheresultofthe

x2-testisnotthecontaminationofde❼talgrainsbutthehetero即neityofm‘aniumcontents;insidethe

grain.lnfact,theED2datapassedthe万一test.ThcうI elyore,weadoPtedaweう塘11ted-meanvalueof10√7士O,2Maforthef7issiontrackageoftheKt-1Pyrodastic・;(’rable2).ThisagewellcoillcideswiththeK-Arageofthesametuff・Con坦三quently,wecanconcludethattheobtainedagesarel‘eiliableandindicatetheeruptionageoftheKt-1pyrodastics。

0nlyfissiontrack昭ewasobtainedforthe尺1-7Pumicetuf’HayerintheupPerpartoftheKubotaFormation.Thefissiontrackageof10,6士0,3Maindic包皿salmostequaltoboththeK-AraswellasfissiontrackageoftheKt-1Pyr()dastics;jntcit)eddedabout200mbelowtheKt-フTuff.ThusthehighsedimentaccumulationratefortheKubotaFormationissuggestedbychronostl` atigTaPhybasedonK-Arand/orfissiontrackages。

11{うcentlyHaya如etal,(2()00)recognized旭昭/と)Z7・フ・7μ乙❹心aαc∂皿7四臨fromalmostthesamehorizonasthedatedKt-1Tun’(Fig.2).TheFOof7VJ・:ヽ乙7J芯凹斑definc・sthelowe・stlimitofzoncN.16,whichisestimatedas10,9Ma(Bcrggrenetal.,1995)or11,0(Montilnarietal.1997,zonclN.1c),wnlcnlsestlmateclas八J,ソMa(,bcrggreneta払1ソソ匈or11,UりVlontananetal.1ソソ/,P・613).Thel` (5fore,theageoftheKt-1Tuf’f(10,6-10,7Ma)is1:)沁stratil;I’aPhicallyconstrainedtobe10,9Maoryounger.ThusthestratigraPhicl` elationbetweenthePlanktonicforamini弛ralbiostratigTa-

Phyandobtainedradiome❼cagesisveryconsistent(Fig.3).

Asforthecalcareousrlannofossilbiostratigraphy,someimPortantbiohorizonswererecogni“5;1111Tlamotoet11L(1998).TheFOofa加7yr・アノイと)❹泣ら(jefiningaCN5b/CN6boundary,zedbyl;hilrnamotoet21L(1998).TheFOof(:フ乙・rj回心・’

estimatedaO1,y10,9Ma(Beilyラ毒芸net;1L,1995)J/;lsrcうc()gnizedatthemiddlelevelt){うtweentheKt-3飢dKt-4tu汀horizolls,about30mabovetheljCt-1.TheFOof(:フ面z7乙7之rcαか観4/z4s,defh‘1ingthebaseofCN7whoseagewasestimatedas10,7Ma(or11,2MaMontanarietal・,p.613),wa5;1゛epor-tedattheuPperPartoftheKubotaFormation汀heotherimPortantbiollorizonofth{う唵stoccul‘rence

(LO)ofoccり/隔心加叩ε必が四百estimatedan1,0-10,8MaDslocatedattheuPpermostpartoftheKubotaFormation,about10mbelowtheKt-フpyroclastics(10,6±0,3Ma).ThusthccstimatedagesofeachbiohorizonalmostcoincidewiththeradiometricageoftheKt’1andKt‘7tufflayers.ThesechronostratigraPhicaswellasbiostratigraPhicag(5-estimatesstronglysuggestthatabout200“mthicklTlarinesedimentsoftheKubotaFormationmaybedePositedduringaveryshorttimeinterva1(theuncertaintyofthaadiometricages;ca.0,4Ma),indicatingahighsedimentationrate(>25,5(j‘n/100011に5j5m/Ma).

OdinG.S.(Ed.),Phanel` ozoicTimeScalらBull.Liais.lrlform.IUGSSubcomJeochlonol.,16,○治etParis,2000

25

Magneticpolarity/ChronI C 4 A r l C 5 n l C 5 「 |I I ■ | ㎜ ㎜ ■ ||

Calcareousnannofossil CN8a

CN71CN6NN5b S5

Planktonicforaminifera N.16 N.15 N.14eつ:Z

A g e ( M a ) j , I l 0 , 1 1 1 , 1 §

KamitoyoSectionNishikawa

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Kt-7

● ● ● ● ● ● ● ● ■ ¬ ・ I

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j l j ・ ■ ■ 1

j●●帰●●●●●・●●●●●●●●●●●●●●●●●●●●●●●●●●●●●偏偏偏●●■■■■■

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♂ ・ ♂ % ♂ ゝ ♂ む / ~ l i 箆 ご こ 「 ` ` ″ l ` 」 . ` 」 ` ゛ ・ ヽ 」 ” I ° ぃ リ 7 ‾ 7 r ‾ T T ‾ 7 :`

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FO:firstoccurrenceln・|きeヤnββHrr凸nr、凸

■ ■ ● ● ● ● ● ● ■

■ ● ● ● ● ● ● ● ● ■

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■ ● ■ ● ■ ● ● ● ■ ■

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● ● ● ● ・ 蒔 ● 帰 ● ・

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1・「

Fig.3Stmtigraphicrelationshipbetx4’(・enthedatedt1Jffsandbiostrtaigmphkzones(illckldingsomeimportantbiohori-

zons).Theobtainedageilsofthe幻-jand尺1-7Tuffsandthenumcう雨alageofeachb沁horiz()n、estimatedonthebasis

ofthel3erggrenet11L(1995)’stim(5scale、afe出oindicated.

OdinG.S.(Ed.),PhanerozoicTim(・5;calらBu1LLiai!Jnform.IUGSSubcom.Geo(;11rono1・,16,()ffsetPil心,2000

5.Conclusion

26

K-Arand/orfissiontrackdatingwereappliedfortheKt-landKt-7Tuffs,in回箆beddedinthelowestPartanduPPermostPartoftheMioceneKubotaFormationintheTanaguraarea,Nol・theastJaPan。

TheK-Arbiotitellges(10,6士0,2Ma)andfTissiontrackalg6(10,7士0,2Ma)oftheKt-1

Pyrodasticscoincidewitheachother,suggestingthel°(うliabilityoftheagesobtained.ThefissiontrackageoftheKt-フpumicetuff(10,6士0,3Ma)弛111mostequaltotheKJUa.ndf`i∬iontrackagesoftheKt-1Tuffl

Theseradiometric啓発sshowgoodagTeementwiththel)17eviouslyestabHshedcalcaJ°eousand5;iliceoえlsmicl` ofossnbiostratigraPhy・

AcknowledgmentsThisworkhasbeenrealizedundertheaeがsoftheworkinggTouplntegratedStratigraPhyof

theMioceneSequence:MiocenefromJaPan,whichisundertheauspicesoftheSubcolllmissiononGeochronology(S.0.G.),lntemationa1CommissiononStratigraPhyofthel.U.G.S.lnthiscontext,山elong-standingconal:)orationwithProf.G.S.0dinofthePierreetMarieCurieuniversity(Paris)hasbeencうspeciallyhelpful.

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BuII.FukushimaMuseum,(2)13-27.*

Baksi,A.K.,1993.AgeomagneticpolaritytimescalefortheperiodO-17Ma,basedon40Ar-39Arplateauagesfor

selectedfieldrevel・sals.Geophys.R(う・Lett.,20:1607-1610.

BerggrenJV.A・,Kent,D.V・,Swisher,C.C・III&Aubry,M.-P.,1995.ArevisedCenozoicgeochronologyandchro-

nostratigraphyjnBerggren,XV.A・,etal・,eds・,Geochronology,timescalesandglobalstratigraphiccorrelation,

SEPX/ISpecia1Pub.,54:129-212.

Blow,XV.H.,1969.LateMiocenetoRecentplanktonicforaminiferalbiostratigraphy.lnBrうnnimann,P.andRenz,H.

H.,eds.,Pro(氾(5dil唱softheF汀stlrltemationalConkrenceonPlanktonicMicrofossils,Geneva1967,1:199-422,

E.J.Brill,Leiden.

Cande,S,C.&Kent,D.V.,1992.Anewgeomalgneticp()1aritytimescalefortheLateCretaceousandCenozoic.Jour.

Geophys.Res.,97:13917-13951.

Cande,S.C.&Kent,D.V.,1995.RevisedcalibrationofthegeomagneticpolaritytimescalefortheLateCretaceous

andCenozoic.Jour.Geophys,Res.,100:6093-6095.

DaJlhara,T..Kasuya,M,&Yamashita,T・,1991.Fission-trackagecalibrationusingintemalandextemalsurfacesof

zircon.Jour.Geo1.Soc.Japan,97:977-985.

Gleadow,A.J.XV.,1981.Fission-trackdatingmethod:what訂etherealaltemative?Nucl.TrackDetection,2:105-

117.

Hayashi,H.,Yalllaguchi,T.&Takahashi,M。2000.Planktonicforarlliiferalbiostra唯raphyoftheMioceneKub()ta

Formationinthe}ligashitana卯ra町ea,FukushimaPr(うfecture.Abstr.2000AnnuaIMeet.,Paleontol.So(;.Japan,

31.**

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