Post on 07-Mar-2021
BullejtinoftheGec)1ogic1211SurveyofJapan,x/01.5mO),I⊃バ77-484,2000
FissiontrackageoftheOSIド|フu汀intheMiocenemarinesequenceofthe
ArakawaGroupintheKarasuyamallrea,centraIJapan
MasakiTAI弦HAsHIlandHidekiTwANo2
MasakiTA皿11AsHlandllide1!ilwANo⑤00)li` issiontrackageoftheog1TuffintheMiocenenlarine
s5(?quenceoftheAI-akawaGroupintheKarasuymaareaぺentra□21panJ❹/.Gθ/.&mへ,/叩自,vo1.51
㈲),p.477-4と34パ5figiら1table.
Atlstract:ZirconfissiontrEkagewasdeterminedfortheoglTuffinterbeddedinthelowesn〕artof
theMioceneOganeFormationintheKarasuyamaarea.Twofissiontrackageswere()btainedas11.6±
0.5Ma(EDlmc,t1!od)and11.7±0.6Ma(ED2m(,thod)andthew(2泌htedmeanagewasca1(;し11atedas
11.6±0.4MafortheOglp17rodastics.Thisagecoinddeswithapl-c?viousK-Arageofthesametu汀sam-
ple(11.8±0.2Ma).Thel-diableagesgiveage()(jlr()n()1ogicconstraintontheCN5a/CN5bboundaryofca1(こ21reousnannofossilzones.
1.1ntrodudion
Con!biningthegreoch170nologyxvithmagnetostl’21tigra-
phyand/ol-L310str21tigraphyalongthesamesectionwi11
presentimpoMantconst17aintsongeologictimescale,t:)(2causesomes3(3riousprobkn!shavebeenpointedout
onCandeandKent’s(1995)geomagneticpolaritytime
sca硲(13aksi,1993;Wei,1995;Tak21hashiandDanhara,
1997).A乱1lereisverylimitedd;1tablevolcaniclasUclay-
erswithinthedeepseacoresamp1(?s,thex/olcarli-sedi-
mentarys3c?ctionsofsomeNeogenemarineseqし1c?ncesin
,1211)anplayanimportantr()1eforintc,S丁I-at(2(lstl-aliigral⊃hicxvork(T誠2thashi皿dOdaj997)。
ThemarinesequenceoftheArakawaGroup(Sakai,
1986),(j)cposedintheKarasuyamaareajsoneofthe
mosts山tablemarineseqt.1encesfortheMioceneinte-
gl` atedstratiSJIニaphy.Thisss(?(luenceyieldsbothcalcare-ousand函1iceousmicro4万〇ssils,andinterbedsmany
pyrodast叱121yers,someofwhichs3upplysし1汪ablemate-l・圈回orradi()metricdating。
lnthispaper,welE)l-esentnewlyobtainedfissiontrack
昭eoftheog1″I` tム1ff,intel-beddedillt1-1e1()wes5tr)artoftheoganeFormatior亘S5akai,1986),andd沁cussits
stralligraphicsign函canceforconstl-uctingatimescale.
2.G42010gy,stratigraphyandsample
Thelぐ211-astJyamaareaisssitual:edinthenort1-1el-npart
c)吋(・ntr211Jap21n印ig.1),,,dleretherlliddleMiocene
marineseqしlence,namedtheArakawaGroupjswe11
e?jcl⊃osed.Thisss(?quenceunG)❹ormablyoverliesthepre-Neogenet)21s5ementandlowerMioceneへ/olcanicrocks
IMieralandFuelResourcesDepal-tn!ent,GSJ.
2KyotoFisl;ion-TrackCo.,Ltd.44-41xylinarlli-Tajil’icho,Kita-
ku,Kyoto(503-88;32,Japan
alongthe(?asternmargin.ltiscoveredbythe
lニ)1eistoceneconglomerateoftheSakaibayashi
Formation.TheMiocenesedirl!c?ntaryrocksaregently
tiltedandonly回ewfaし11tswereobs5(?rv(?(1thl’ot,lghout
tharea(T21rlakaandT21kllhaslj,1998).″rhemorethan700n!-th沁kMiocenesequenceis
dividedintofourformations;Kobana,0gane,T21nokura
andlrienoFormationsinasc(jndingord(?lてSak21i,1986).
TheKobanaFormationiscomposedofconglom(2rate,
mc?dium-tofine-Srl-ainedc211careouss5皿dstoneandsilt-
stone,w此h30keytしLfflayers.0n】ythelower凶rtis
chal-21cterizedbyf()resetbedding.MarinemoHuscsare
oftenyielded,andca1G汀eousm弛l-()4テossilsareweHpre-
s;(2rved,howeversiliceousmic17()fossilswere(1沁solved
duetotheoverburden.TheK-Ar昭esofsixtuf171ayers
(Kb1,Kb2,Kb15,Kb20,Kb23andKb29)weredeter-rnined(T21k2111as1-1id❹丿9S)9,2000).
TheoganeFormation,conformablycoveringthe
KobanaFormation,ischracterizedbydarkgraysilt-
stone.WeH-pr(?s5ervedcalcareousmicrofossilsareyield-edfromitslowerpart,andsiliceousmicl’c)foss面from
theupperpart.S511cty-onekeytu汀setsareirll:erbedded
inthisf()rmation,andtheOglandog25weredatedby
theK-Armethod(Takahasllida/.,1999).
lncontrast,theuppertwounits(Tanokし1raand
lrienoFormations)aredominatedbydiatom21ceoussilt-
stone.Morethanfortytu狸setsareint(jrcalatedinthe
TanokuraFormation.Anlongthem,Tn18,Tn20,Tn28,
Tn38andTn翁.5keytuぎ1ayerscontainbiotite.RecentlyTanakaandTahhashi(1S)S)8)e!5tablishedthe
(2alcareousnannofossilbiostratigraphyfortheKobana
andlowestpartoftheoganeFormationl;(Fig.3).They
recognizedtwoimp(:)rtantt)iohorizons;1astoccし11-rences
K(乃rwords:Mioce)化,fissiontrack21ge,c;alcar(2ousnannofos-siいn咄gratedstrati£lraphy,ArakawaGroし1p,cent1-211Japan
- 4 7 7 -
40°3 ’
42’
41
) 4 0 ’ N
…i‥・A
700
400
KeyBed
呂一,ヽ・11-|rl
Tn40
332520
1 1 5 iU951 o 121 ol o
S』コ)・OC.wト
Tn6Tn5
Og52
208625
443332
gggggg
m
m
l003
茲知≒ご
091100〇
Column
40°4IE
j❹/dj?げf/lfGぐ7£J/tJgllc・a/S11jwyQ/・.Zど卵皿VIθ/.j7,A/θ.加,2卯θ
51 6’ 140°7’E
j?べ
LEGEND
□]Riverterrace,natura目evee
[二回Loamdeposits
[ニコSakakjbayashiFm.
mlrienoFm.
FこニりITanokuraFm.
匡目OganeFm.
写否応冊obanaFm.
r27:iNakagawaGroup
l75列YamizoGroup
1
Sea
f;・/・りa畷
づ(6
Jll
Ka
0 1 0 0 0 2 0 0 0 mら
91
-
1・
Ocean
゜381N
371N
36’
……………:…………:……zぺ丿バタ。゙ぷ5i;・:・,5SI,言ふ≒ヽトs=j:………y.:。4=1………ェ・:I.Il……l………::i.:`………::::::・i=-====・………=r=ェll聊言yj.=,::lil:iliiiyallj,i。;l:;ヽ:=1゙ s゙:ェ;:;i3・i;・jil=ii;”iミミ;SIij……辺|。ェェ==i==` Qヽド`-‾`J::で
Fig.1Sampl(?1ocationonthege()1()gicalmapoftheMio(こenemarinesc?cluenceoftheArakawaGroLlp,(|汝ributedinthe
lくaral;uyamaar(!a,centl‘aljFapan(modi屁dfromTochigiF)refectureomcら1988,1991)。
- 4 7 8 一 一
f’,・。,・5,1θ,11md碍押り/’・f/jどθg/77,!が,・j□/jど沿lm51り・α。,rl,s’でり11e’jcer7i,尨1加1,s・/lh7,j,/7,。・α,jaJ
Fig.2?,/lapshowingtllc,lo❼ityofdate?dtu削topograph叱lnap:“さJiita乙sc山lj/25,000bytheGeogr211:)hicalS3111・veylnstitute).
(LOS)Of知/j。。rJ㈲。,。,・/7./。,-a。,rJ叩/H,sand{う・t・/心,r即/仙。s
/ZrJ・・た/回心.Thef()rmerandlatt(jrdefinetheCN4/CN5a皿dCN5a/CN5bbourldariesofthecalcal-eousrlarlnofos-
silzo皿s((⊃kadaandBしtkry,1980),IJe,sr)(jctively.Among
山em,theCN5a/CN5bbot.lndarywaspr(?dselydeter-
I-ninedinthc?fine-IJrainedssandstorl(?1ay(?lフバ1tonemeter
belowtheog1丁し1ff,wh沁hwasdatedby出elTission
trackmethodt1-1lぺ川lghthiswork.
:11?issiontrackdating
TheOg1Tし置ischaracterizedbybiotiteandhorn-
blend(2一石chmedium噌rainedtし唾,wllichis5cmthick.
Bot.hlowerandし1pperboundariesofthispyl-oclastics
areverys5harp,andnohthicfragnTlentwithinthetu丘
canbeobs5erved.ThicknessoftheOg1TIJffisconstant
tllroughout出eKarasuyamaarea.WecoHected3kg
sa17npleoftheog1Ttj汗f(Figs.1&2),andcarriedoutthe
(mnventionaしircon53(≧I:)arationbysievinSらpanning,and
m昭neticandheavyliquidseparatingtechrliques.The
og1Tuffyieldednlanyzircons,whichwerereddish,
(?uhedralandhadnozonalstructuresinside。
Fissiontrackdatingwasdonebyusingthec?xtemal
de脆ct:ormethodthatwasappliedtoboththeinterna1
(p❼ished)ande?xternals5tll‘facesofzirconSrmins(EDlandED2n万1(・thodlらlrespe(:tively:Gleadow,1981).The
函|:ernalstム11‘fac(2shaveanadvantageofbeingfreefrom
the(?jcterna1(ごffect,whichisthecont:aminationoffis-siontracksoriginatingexternaHytothedatedzircon.
0ntheotherhand,thectxterna回しlrfac(?shaveanadvan-
tageofeasiertrackidentificationbecauseallthe
()bservedtracksarelongerthanthehalf-1engthofa(Jnlpletetrack.TheED2methodisc?s;r)eciallys51.11table
fordatingvolcanicashortuffthatindudesnloreorless
- 4 7 9 -
thickness(m)
lol 101 1 Mi ol oll E」o山Qcsao
6
5
6
0
。
5
5
7111//C111jリ/・//1む{r辿θ/fJgllal/S111・Fり’r?/し/叩てljtl々 j/.j/、Λ/a./θ、2吻θ
Og5,6094093
5o]116±04M呉玲
45
40ml 1 511 311 11 ol E」OL
35
19 1 il ml l ijl lnl 1 5 ili一
30
11.8±0.2MaK-Ar’
Kb301Kb294JI
12.1±0.2Ma(K-Ar)↑卜Kb28ニ
Kb27
Kb26
Kb25Kb24
Kb23一価・・-‘12.4±0.2Ma(K-Ar)
Kb22Kb21
Kb2012.6±0.2Ma(K-Ar)
Kb15-e‘12.6±0.2Ma(K-Ar)
- 4 1
一 一 4 1
r『tr11
-57-
’ 5 6 ’
l/一 一
- 一
一 -
一 一 ’ 3 0 1 - -
- 2 9 9
- 2 9 8
. _ 2 9 7
- 2 9 6
一 一 5 5
二 5 4 二5c-一一52
44
43
4 l ohttp://www..l.l. 01011 11 1 1 1 111 http://www.g.netpl//www..o5.cohttp://www.il.1. 111 1 i16 l ol 1111 illiollli
(EJol&おI)
EI 11 E yjJ http://www.i.l. 1 11 li U
W111 oyn3j ol il11 iil l 1114 1 11 1 1 11 o5 151 551 o5 01111 o5 l il 1111 ol iill 5 ll l J1119 9 51 1 5
won
M
5 o lll III ll I I l 1 1 11 o l ll 0991 l ll ll l l ll ll lll i lll o 31 ol ll l o l 1 l l 2 0911 l l 3
o l l 01111 l til 19 l l 21 5http://www..l. l o 11111 911511101 i 111011 l li 111 l l 1111g 1111111 11 1011 l li
1 1 ohttp://www.. l 1 1 151 1 1101tlhttp://www.. 1 11 1 1115j13111110 l l l 9 li 111 l lil l ol
- - 一 一
- 一 一
8Zハ)
HZH
LOofCyc//car9o//治ljsf/or/d∂nus
LEGEND
Eコverycoarse-mediumsandstoneE∃fine,3EIrldstorle,sandys;iltstone圖§lsiltstoneE∃coarsetuff[コfine-clayeytuff㎜lhjn-beddedfinetuff[∃tuffElceous[互回purTli[]eous阿ilshell[匡凹bioturbation巨Ξ]concretion
Kb28tkeybed-408samplepositionfor
microfossils
レレド卜
barren
distinctboundary
indistinctboundary
no-exposure
hard
soft
RadiometricagesFく-Ar:Takahashietal.(1999)
HayEI・5hietal.(2000)FT:thiswork
CalcareousnannofossilTanakaandTakahashi(1998)
Fig.3StratilJraphicd心tri1⊃utions()t:15electc?d(:alc;lreousnanll〔jfossil!51〕ecic?sonthe(lc?tailedstratil;raphic(:(jlnposite(到umnof
山e叩ljel-partoftheKobanatolowestpal-toftheO卯neFormationsいI⊃artlyl・c?visedfromTanakaandTali2lhashi,1998).
StTatiSか?phicpos雨onswithagesoftheog1TしばfandprevioLlsly1-eporteddatedv❼caniclasticsarealsos5110wn.
- 4 8 0 -
10
Number
5
0
160
Ns120
80
40
0
p S 5
(cm’2)
2
1×
jr711.。‘,J,バ几?dど収frが・1/1,?θが7jがj,□加尺ど7,m司・£jgしi5’り’1ε”cごf7i沃む1カ心/ja。1ど//。¥。,司
EDI(a)Tト{ist()9ram
黄重竃毒
2oT(Ma)
0 5 1 0 1 5 2 0
(b)Ns-Nicol‘l’elation
0 40 80 120 160Ni
(c)ps。X:・h;orrelation
1×1052 5 1 × 1 0 6 2 5 1 × 1 0
pi(cm’2)
(broken)
- 4 8 1 -
ED2(a)Tト{ist・}9ram10
Number
5
p S 5
(cm’2)
2
|×
0
0
1×1062
40 80
0 5 1 0 1 5 2 0
(b)Ns-Nに・)「r・llation
120 160Ni
(c)ps.picorr。lation
51 × 1 0 7 2 51×108
X)i(cm’2)
25
Figバ|【:;rai】1一回-grainplotsoHiss函n-traddataf()l-zil-c()11fromtheoglT❼f1Jsingthe(2xternald(式ect()rnTlethod21ppliedtothe
illtelmalsし11ィaにe圧D1:1eft)ancle3ltモ1-nalsElrface(ED2:right)バa)Histc)gl-amsofgrain-ageバ21bove2)anditsanjyti(ニalerro1バt⊃elow),
(b)co1Telation()脳p()11taneoustrackcounts(Ns)an(linclucedtrackco皿ts(i` Hヽ),and((2)col-relation017spo1UElneous5tra(211density
(卜)andinducedtrackd(jnsity(,j)aregivejn.Statisti❼valuesofcc)ITelationc(ymcient凪Standarddejviatic)n(syx)2111dper-
(1(?ntage()1デV211うatic)n(P)are出oshown.Thesl1-211n一昭edist1-il⊃utionandlin(?arityofNs/Niand戸/,。icorrelationd(jmonstrate
thttheog1Tし府ziFconhasasinglc?21gep()pし11ationxvithoutacontalllinationofdetrit211(olde1うgrains,
/畑//,がらりμ加Gt?rj/ql‘cヽ ❹&.lj・1,1り’げ。/印皿,1Kθ/.j/,A/θ。/θ,2θ卯
ConfinedTrack Semiご「rack
(2625)0.825120
(2625)0.550160
weりhtedmean
11.6士O。5
11.7士0.6
11.6士O。4
EDI
ED2
U-
content
(ppm)
AgeMethod
(±1(J)
(Ma)
「
0.932 28.3士1.0
28.5±1.2
8。61(2645) EDI270
15
1
0
5
WEi0.0Z
0
15
1
0
5
WUi
90Z
0
Standard(FishCanyonTuff)
2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5
Track❻gth(μm)
091
2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5
1’「a(lklength(μm)
ひ)jと
§卜-○
j
100
80
0
0
0
6
4
2
0
120
0
0
0
0
0
108642
jQ心」ご0.0Z
0
Standard(FishCanyonTuff)
1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5
Tracklength(μm)
091
1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5
丁旧[]klength(μm)
Fjg.5ComparisonoHengthdぼrit〕utionsofspont2lrleoustl-acksh!zil・(2onfol・confinedtl・2lck(left)andsellli-tl・ackon(,xtel-na)
lsし11面ces(right)betweentheog1T❼andFishCanyonTイ(st2mdard).
Tab】elF岫siontr2犯kdataofzi17consfromtheoglTuffinthelowestpartoftheoganeFol-mation.
SampeMineralNo.of
c o d e c r y s t a l s
091
091
EDI
ED2
ZIrCOn
zircon
30
30
F7s/7C∂ηyonアひが
FCTEDlzircon20
FCTED2zircon20
Sponlaneous
芦(/\/s)
(106cm’2)
1.01
0.680
5.44
3.22
(1512)
(772)
(3567)
lnduced
μ(/\//)
(106cm’2)
1.31
1.65
(1965)
(1874)
2。90(1903)
P(χ2)
(%)
70
83
50
Dosimeter
p d ( 重 彭
(104cm“2)
8.54
8.54
(1ハ。andjv21rethed(?nsityancltotalnumber()皿ssi()11trackwountedげt2spectjvely.
②Analysesweremadebytheejxtem211detectornlethodusingasJ(j()metl-yfactorofOyfor2π/4π(ED1)ancにfor2π/2π
(ED2).
③Ageswere❼culatedusingadol;imet(jl‘glassS5RM612and21ge(12111brati皿仙ctorsnED1)=352±3andレED2)=334±4
(沁21noandD皿halJa,1997).
㈲P壮士sthelっ1-obabilityc,f()1y)taillingthe;(にvahleforンde,greeso討reedom似dlereレ=rl皿!ber(jfcrゝ 715taly1).
⑤,・is;thec(よ1-relationcc)(jmci(jntbetw(?(?n,t7sand皿
郎Sampleswc,1-eirradiat(!dし1singTRIGAMARKTTnucle211-react()rofhPaullsU❹ve,rsity(RikkyoD211gaktl丿a即n.
(7汀heref(ごrence21ge21doptc?dfortheFishCanyonTulテfis27.8±0.2Ma旧u汀ordandH21mmers(lhmidt,1985).
- 4 8 2 -
77j5jQ・jf・7・ch尽白?/’1/jcθが7’・!/7j・・/ノ,Ejl1。・6,り・a。ja,y叩。c。t・・げほf,/,r,。,/,絞。,6/711・α,j司
cjetrita1Fainsb{2causedetrital£ll-ain引で1avea5;horter
s1.1rfa(:e?-tracklengthdistributioncElsedbyth(?rma1
11eating,andcanbeeasily旧(mt汀ied.Age-c2111bration
w21s5madeonthebasisofthezetaapl:)roachre?com-
111endedbytheIUGSSubcommissiononGeodlr()nology
(1-lurford,1990).EXI)erimentalp圃(:edし『esandsystem(咀1iL)rati{⊃nswerec?}cr)1ainedindetJilinD21nharacに/.
(1991)andlwanoandD21nhar21(1997).TheFishCanyon’I` t.lff,oneoftheageは皿dardmatc・l’ials(I-lurford,1990)
wasalsol:)l?I⊃aredtochecktheage(:2111brati皿andcom-
I:)Elrethetracklcmgthdistl・ibution.
4.RcsullsandDkussioll
ThesspontalleoustrackdensityoftheOglzil・conwas
homogGleoし1samonggrainsforeachdatedmoしmt(EDl
andED2).ln21ddition,nor3ossibledetrit11ぽraill,;having
llighertr皿kdensityandshortertFacklengthonthe
(?)ctema15s町faceswererecognized.Thirty町ainswere
randomlys;electedandmeasuredforeachmethod.
(:;l‘211n-t:)y-grain21nalyticaldataareshowninFig.4.We
canseethattheOglzirconshaveasingleagepopula-
tionwithnodetritalgrains.
The1-c?sultingfissiontrackagesare11.6±0.5Mafor
EDland11.7±0.6MaforED2,andconjstentwith
e皿hother.Bothdatapassthroughtheλ丿一testata5%
1eve1.Thel-c?sultsoftheagestandardalsoshowedgood
agr(2el’nentwiththe17e?fcうrencevalue,i111plyil噌thatboth
EDlanclED2caに)Fationswerereas5omlblyexecuted
(Table1).Mc)l-(,over,notrackl;hortenil堪wasfoEmdfor
(?ithertheco❹inedtracksGrltemals3t.11イace)orsemト
tJacks(externalst.1rface)inzirc()n(Fig.5).’T` hisindト
catesthat山eoglzirconshaveneverbeen計fected
fromthermalheating,andsshouldl-ecordthef()rmation
ageofthistdfsample.nla・efore,weadoptedaweighレ
edmeanageof11.6±0.4Mafortheoglpy1-oclastics
(Table以Kasuya(198;7)firstreported171ssiontrackageofthe
oglTurfas11.8±1.0Ma.However,thisageisregard-
edas函validoneatpl^ c?sent,because山ezetacalibra-
tionwasnotadoptedinKasuya(1987).Rc!cently,I゙` 211d!ai5hしに/.(1999)determinedtheK-/yl-biotiteage
as11.8±0.2Ma.0urnewlyobtained4Tissiontrackage
(11.6±0.z1M21)(x)inci(leswiththeK-Ar21ge,whi(Jhsug-
geststh2lttheseagesarereliablealldl-(?I⊃resenttheageoftheOglpy1-oclasticseruption.
TheLOofC./7r7/‘/乙7皿心ddiningtheCN5a/CN5b
boundary,islocatedonemeterbelowthedatedtしlff
(TanakaandT21k2thaslli,1998).Asthenleansediment
21(2(mmulationrateatthelowestpartoftheOganeFom121tionwasestimatedas1.6(ml/1000ylてT21k21hashi
da/・,1999),thenurnericalageoftheCN5a/CN5b
boundarycanberegardedasthesameastheradiomet-
ricageoftheOg1Tu削1L6-11.8Ma).’T` hisestimationis
weHconcc)rdantwithBe司grenlバ1995)timescale
(CN5a/CN5b=11.8Ma).
Acknowledgnlents:Weare£ll’atefultoTomoakiSumiiof
theG(,ologic211Surveyol丿apanforhiscriti(1211readingofthem21n匹cript.WealsothankTohruDanharaofKyoto
l7ission-TrackCo.,Ltd.andTojiNumataofFAPTG
RadiationRe?sc?archLaboratory,Ltd.fortheirhelpwith
neutronirradiationatSt.Pau1’s(Rikkyo)University
1‘(?actor.ThiswoFkhasbeenrealizedunderthew()rking
groupofthelntegratedStratigraphyofMioceneSequence:N410cellefromJapan,whichisundertheaus-
picesoftheSubconlmissiononGeochronology,lnternationa1CommissiononStratigraphyoftheIUGS.
lnthiscontext,thelong-standingcollaborationwithDr.
G.S.0dinofthePierreetMarieCurieuniversity
(P211-is)hasbeenc,specia115zll(jlpfu1.
llcferences
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*in、lapanesewithEnglish211〕stract.
**inJapanese.
lleceivedJune22,2000
/xcceptedJuly:2?6,2000
栃木県烏山地域に分布する中新世海成層(荒川層群)に挟在する091凝灰岩のフィッショントラック年代
高橋雅紀・岩野英樹
要 旨
栃木県烏山地域に分布する中新世海成層荒川層群(z:)大金丿削Eと下部に挟在する珪長質凝灰岩(Og1)より抽
出lしたジルコンについてフィッショントラック年代を測定し,11.6±0.4Ma(1,7e,rror)の年代値を得た.
同一試料について測定されたK-AI-年代値(11.8±0.2Ma;Takahshしに/.,1999)とよい一致を示すことか
ら,得られたフィッション年ィ切直はOg1凝灰岩の噴出1年代を示すと判断される.年代決定されたOg1凝
灰岩の1メートル下位に石灰質ナンノ化石の(ヤ/yc7a,・即/fr/に./7り・‘・1/回川の終産出が確認されていることから,得られた年代は,同生層準により規定されるCN5a/CN5b境界(OkadaandBukry,1980)年代を示すと
判断される.
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