Composition and Provenance of Clay Minerals inthe Northern Part of Lake Tonle Sap, Cambodia

著者 Okawara Masafumi, Tsukawaki Shinjijournal orpublication title

地學雜誌

volume 111number 3page range 341-359year 2002-01-01URL http://hdl.handle.net/2297/11702

地学雑誌

JournalofGeogfaphy lll(3)341-3592002

CompositionandProvenanceo缶Ｃ１ａｙＭｉｎｅｒａｌｓｉｎｔｈｅ

ＮｏｒｔｈｅｒｎＰａｒｔｏ丘ＬａｋｅＨｂｎＩｅＳａｐ,Ｃａｍｂｏｄｉａ

MasafUmiOKAWARA＊andShinjiTSUKAWAKI*＊

Abstract

UniquesedimentaryprocessesunderthecontrolofdrasticseasonalHuctuationsinits

waterlevelandareaareexpectedinLakeTonleSap,thelargestlakeinSoutheastAsia・Ａｓ

ａｐａｒｔｉｎａｓｅｒｉｅｓｏｎｐｕｒｓｕｉｎｇｔｈｅｓedimentaryprocessesandtheirtemporalchangesofthe

lake,sedimentsandsoilsｗｅｒｅcollectedandexaminedfromthenorthernpartofthelakeand

itsadjacentwatersystemsandareasinordertorevealcompositionandprovenanceofclay mineralsdistributedinthenorthernpartofthelake

lllite，kaolinminerals，smectiteandchloritewerecommonlyidentifiedfromthesurmce

andsuspendedsedimentsofthelake・Judgingfiomilliteandchloritearenotdetectedinthe

sedimentandsoilsamplesfromriversandtheAlluvialplainsaroundthelakebutthebottom

sedimentsfromthelbnleSapRiverthatconnectsthelakewiththeMekongRiveryielda

certainamountoｆｔｈｅｍ，ｉｔｉｓｓｔａｔｅdthatilliteandchloriterecognizedinthelakesediments

werederivedfromtheMekongRiver・Sinceilliteandchloritearenotdeｔｅｃｔｅｄｉｎｔｈｅｌｏｗｅｒ

ｐａｒｔｏｆｔｈｅｃｏｒｅｄsedimentsfromthenorthernpartofｔｈｅｌａｋｅｉｎｓｐｉｔｅｏｆｔｈｅｕｐｐｅｒｐａｒｔ

ｃｏntainsthem，appearanceofilliteandchloriteiｎｔｈｅｕｐｐｅｒｐａｒｔofthecoreindicatesthata

changeofsedimenttransportinwatersystemsaroundthelaketookplaceatabouｔ5,000years ＢＰＬ

Ｋｅｙｗｏｒｄｓ：Cambodia，LakeTbnleSap，MekongRiver，clayminerals，sedimentation

taryprocessesofthelakeFurther,thewater

areaofthelakeexpandsdrasticallyinralny

seasonsduetoagreatamountofwatersupply

mainlyfromtheMekongRiverthroughthe

lbnleSapRiverthatconnectsthelakewith

theMekongRiver・Suchsettingholdsouta

promisingprospectfbrinvestigationsofuni‐

quesedimentaryprocessesinthelakeunder

thecontrolofseasonalfluctuationsinwater

LIntroductiOm

LakeTbnleSap，thelargestlakein

SoutheastAsia，liesincentralCambodia・

Ｓｉｎｃｅｔｈｅｌａｋｅｉｓｋｎｏｗｎａｓ１１ｔｈｅｍｕｄｏｃｅａｎ１１

ｔｈａｔｒｅｆｂｒｓｔｏｄｅｅｐｂｒｏｗｎｃｏｌｏｕｒｏｆｔｈｅｗａｔｅｒ

ａｌｌｙｅａｒｒｏｕｎｄduetoalargeamountofmuddy

suspendedsedimentsinwater，clayminerals

wouldactanimportanｔｒｏｌｅｏｆｔｈｅｓｅｄｉｍｅｎ－

＊DepartmentofCivilandEnvironmentalEngineering,FacultyofEngineerlng,IwateUniversity6 ３－５Ueda4-chome,Morioka,O20-8551Japan．

*＊DivisionofEco-Ibchnology>InstituteofNatureandEnvironmentalLchnologZKanazawaUniversity6 c/oGeneralEducationHall,KanazawaUniversitybKakuma-machi,Kanazawa,920-1192Japan．

－３４１－

’１

咄

levelandareaduetoalternatingrainyand

dryseasons，characteristicsofthetropical

monsoonreglonHowever，onlyalittle

numberoflimnologicalandsedimentological

studieshasｂｅｅｎｃａｒｒｉｅｄｏｕｔｉｎｔｈｅｌａｋｅａｎｄ

ｔｈｅａdjacentriversystemsmCambodia（e､9.,

CarbonnelandGuiscafr６，１９６２－６３；Mitusio

etaJ.，１９７０；Lao，1992,1993；msukawakiet

aZ.,1994;IlsukawakiandLao,1995)．

Detailsofthesedimentarysequenceofthe

lakebottomwerefirstreportedbyCarbonnel

andGuiscafr6(1962-63)．Theyrecognized

theupperiiuaｓｅａｃｍｅ此ｌ１ａｎｄｔｈｅｌｏｗｅｒＩ１ｕａｓｅ

α"Cie"刀e1Iwithinthebottomsedimentsonthe

basisoflithologicalinvestigationofseveral

coredsedimentsobtainedfromthelake，and

thebaseoftheuppersequencewaｓｄａｔｅｄａｔ

５７２０±３００yearsBETheyalsonotedthat

illiteandmontmorillonitearetheprevailing

claymineralsintheuppersequenceinspiteoｆ

ｋａｏｌｉｎｉｔｅｉｓｃｏｍｍｏｎｉｎｔｈｅｌｏｗｅｒｓｅquence、

IlsukawakieZaZ．（1994）preliminarilyillus‐

tratedsedimenttransportinthewaｔｅｒ

ｓｙｓｔｅｍｉｎＬａｋｅＴｂｎｌｅＳａｐａｎｄｔｈｅａdjacent

riversystemsinCambodiabasedoncomposi-

tionalfbatureｓｏｆｂｏｔｈｓａｎｄｙｓｅｄｉｍｅｎｔｓａｎｄ

ｃｌａｙｍｍeralsinthesedimentsfromthelake

andtheadjacentrivers，andsurfacesoils

aroundthelake・Theyconcludedthatwithin

allsedimentsderivedfromtheMekongRiver

tothelakethroughthelbnleSapRiverby

backcurrentsduringtherainyseason，sandy

sedimentsaresettledrestrictedlyinthe

southeｒｎｍａｒｇｉｎａｌｐａｒｔｏｆｔｈｅｌａｋｅｂｕｔｃｌａｙ

mineralsreachtothenorthernmostｐａｒｔof

thelakeassuspendedsediments・

nkingtheabove-statedpreviousstudies

intoaccount,the11IbnleSap96ⅡprOjectwas

carriedoutasthefirststeptograspthe

presentsedimentaryprocessesofthelakeand

toreconstructenvironmentalhistoryofthe

lakeoveracertainperiodLithologicalfbaJ

turesandtheresultsofradiocarbondatingSOf

thecoredsedimentswerealreadyreportied (Mildenhall,l996MSjmsukawaki,1997；、U=l

kawakietaZ.，1997)．Onthebasesofth鰊L-e

analyticalresults，thepresentarticledeSsi

cribescompositionandprovenanceofclayl mineralsinthesurfacesedimentsfromthel

l---

northernpartofthelakeandillustratestheir；

temporalchanges．

ⅡEnvironmentalSettingsofLalKe

lbnleSap，

Figurelshowsthetopographicfbatuｵesof

鱗繍鱗鷺ltainswiththehighestpeakofl,744mabovel

sealevel,andtheDangrekEscarpment,７６５１

sedimentaryrocksoftheUpperlndosiniasl

cenelnage（Gubler，1933；Saurin，1935;i

ESCAPI1993a)fbrmthewestern,southeml

騨畿:鰯鵬謡:劉鰯鶴Jtropicalregion（UnitedStatesGeologicalI

Surveyb1971)．Severaltopographichighs

－３４２－

「lｌ６Ｑ

､４､

DnPZpyqe尼」｡‘配Z"

、■

’

Fig.１LocationofLakeTbnleSapandtopographicfbaturesof

CambodiaDottedlineindicatespresumablewaterareaatthe

highestwaterlevel（partlymodifiedfrommsukawakieオαZ・’1994)．

dryseasonThewaterareaisabout3,OOO

lkm②atthelowestwaterlevelswellsmore lthanfivefbldatthehighwaterleｖｅｌｉｎｔｈｅ

Ｉｅｎｄｏｆｒａｉｎｙｓｅａｓｏｎｔｈｕsthelakelacksfixed

lcoastallines・Atthelowestwaterlevel，ｔｈｅ

ｌｄｅｐｔｈｉｓｖｅｒｙｓｍａｌｌｌｅｓｓｔｈａｎｏｎemetre

lwhollyandthehighwaterlevelthedeepest

lp………伽､､(Mmi…,PublicWOrksandTransport，Kingdomof lCambodia,1999)．Forestsinthecoastalareas

lofthelakeareinundateddurmgthehigh lstandsofwaterMa…naSpaddyfieldS ’Thereisanetworkofnuxnerousriversand

lstreamsflowingdirectoryiｎｔｏｔｈｅｌａｋｅｆｉ･om

litssurroundingareas，buttheirflowis

ladequateonlyintherainyseasons,exceptfbr lacertainnumberofriverssuchastheSiem

ReapRiveroriginatingintheKulengMoun-

tainsthatareasalienttopographichigh

Smallriversandstreamsaroundthelake

partlynoodintherainyseason・Againinthe

rainyseason，waterthroughthisgroupof

riversthatcontainsalargeamountofmuddy

suspendedsedimentderivedmainlyfrom

surfacesoilsdistributedaroundthelakefloWs

intothelakeOntheotherhand,waterofthe

MekongRiver，Originatmginandbringing

muchmeltedsnowfromthenorthernslopesof

theHimalayasinthewesternpartofChina

togetherwiththeheavymonsoonrainalso

nowsintothelakeTherefbre，ｗａｔｅｒｏｆｔｈｅ

ｌｂｎｌｅＳａｐＲｉｖｅｒｆｌｏｗｓｂａｃｋｗａｒｄｓｔｏｔｈｅｌａｋｅ

intherainyseason，andwaterlevelofthe

riverincreasesmorethaneightmetresin

－３４３－

comparisonwithitinthedryseason・The

actualamountofsedimentsupplytothelake

througllthelbnleSapRiverhasnotbeenstill

known，butDelvert（1993）estimatedthat

wateroftherivercarriesabout1.7×１０７

grammesofsuspendedsedimentspersecond

tothelakeduringitshighestwaterleveLThe

inflowfromtheriversandstreamsaroundthe

lakedecreasesmarkedlyindryseasonand

waterflowsoutfromthesouthernpartofthe

laketotheMekongRiverthroughthelbnle

SapRiver・

Ｔｈｅｌａｋｅｗａｔｅｒｉｓｒｅｄｄｉｓｈｂｒｏｗｎｔｏ

ｙｅllowishbrownincolOurallyearrouｎｄｄｕｅ

ｔｏａｌａｒｇｅａｍｏｕｎｔｏｆｍｕｄｄｙｓｕｓｐended

sedimentscontainedinthewater，ａｎｄｅｘ‐

tremelylowintransparencyeveninthedry

season（MitusioemZ.，1970)．Thetempera‐

tureofsurfbLcewaterisextremelyｈｉｇｈ，２６－

３０℃，ａｌｌｙｅａｒｒｏｕｎｄａｎｄｔｈｅｐＨｉｓａｌｍｏｓｔ

ｎｅutraldVIitusioetaZ.,1970;Lao,1992,1993； IlsukawakiemJ.,1994)．

mentsderivedfromtheriverhavebeen

accumulatedinthebottomofthemovera

thousandyears・ThebottomsedimentsfroIn

thelbnｌｅＳａｐＲｉｖｅｒｉｎＰｈｎｏｍＰｅｎｈＣｉけ，

CollectedinMarchl993(Fig2,upperleft1 配ukawakietaZ.,1994;配ukawakiandLaql995)werereanalyzedinthepresentstudｙｔｏｌ

驚蝋難HlHji;labove-mentionedsamples・

蝋鱗iiiiiiiiilil1lf:識鶏笂鰯證蹴雛総Ithelake・Ｏｎｔｈｅｏｔｈｅｒｈａｎｄ，thesurftlcelsedimentsampleTS96-5werecollectedfroml

thesurfbuceemergedatthesamplingSurfacel

(TS96-9）wasScoopeddirectlybyasamplingl bottletograspclaymineralcompositionofthel

suspendedsedimentsinthewater・Surfacel

soilsamplessouthoftheWestBaraywaterl reservoir（TS96-7)，northoftheEastBarayl

sedimentsoftheSiemReapRiver(TS96-6andl

EB97-SR）wereobtainedbyscooping、TheibottomsampleoftheWestBarayreservolrl

(TS96-WB1）wasobtainedusingbythecorel

翔鰡：鶴二期:堪鯛鷲:Irecoveredbyasixmetresl・nghandaugeエｌＴｈｅｃｏｒｅＴＳ９６－１ｕｓｅｄｉｎｔｈｅｐｒｅｓｅｎｔstudy

蝿`,揺孟霊鰯;晋具Jilv鑪麓魁二brushedwellbyastainless-steelspatulaand

ln．SamplingMetllodandAnalytical Procedures

Ｔｈｅｓｅｄｉｍｅｎｔａｎｄｓｏｉｌｓａｍｐｌｅｓｕｓｅｄｆbrthe

presentstudywerecolleCtedinMayl996

fromLakeTbnleSap，ｔｈｅＳｉｅｍＲｅａｐＲｉｖｅｒ

ａｎｄｉｎａｎｄａｒｏｕｎｄtheWestBaraywater

reservoir,ａｎｄｉｎＡｕｇｕｓｔｌ９９７ｆｒｏｍｔｈｅＳｉｅｍ

ＲｅａｐＲｉｖｅｒａｎｄｉｎａｎｄａｒｏｕｎｄｔｈｅＥａｓｔＢａｒａy

waterreservoir(Fig.２)．TheSiemReapRiver

flowmgallyearroundisjudgedasoneofthe

importantsourceofsedimenttothenorthern

partofthelake，sincemostriverssupplying

sedimeｎｔｓｉｎｔｏｔｈｅｌａｋｅｆｌｏｗｏｎｌｙｉｎｔｈｅｒａｉny

seasonsTheWestandEastBaraywater

reservoirswereconstructednoｒｔｈｏｆｔｈｅｌａｋｅ

ｂｏｔｈａｔａｂｏｕｔｌ,OOOyeａｒｓＢＰｉｎｔｈｅＫｈｍｅｒ

ＤｙｎａｓｔｉｃｐｅｒｉｏｄＳincewateroftheSiem

ReapRiverwerｅｕｓｅｄｆｂｒｆｍｉｎｇｕｐｔｈｅ

－３４４－

「

Ｆｉｇ．２Bottomsurfacesedimentandlakewatersamplingsites

inthenorthernpartofLakelbnleSap，bottomsediment

samplingsitesintheSiemReapRiverandsampling

sitesofsurfacesoilsintheA11uvialplain，andbottom

sedimentsamplmgsitesiｎｔｈｅｌｂｎｌｅＳａｐＲｉｖｅｒｉｎ

ＰｈｎｏｍＰｅｎｈＣｉｔｙ（upperleft）

diffractionpatternsofthesampleswere

recordedwithaMacScience(MXP-3AHF)X

raypowderdiffractometerequippedwitha

graphitemonochromator,usingCu-Karadia‐

tion,ｔｕｂｅｖｏｌｔａｇｅ４０ｋＭｔｕｂｅｃｕｒｒｅｎｔ２０ｍＡ，

slitsystemofO5o-0.30mmandscanning

speedof200o／ｍin・X-rayanalysiswasper‐

fbrmedfbruntreatedspecimensandsuch

treatedspecimensafterethyleneglycol-solva‐

tion，Mg-saturationandglycerol-solvation，

HC1-treatment，K-saturation，andonehour

heatingat300℃ａｎｄ600℃．

bbservations、Aftervisualobservationsand

lcoredescriptionsweremade，acertaincen-

ltimetresthickslicesweresectionedfbrclay l●

lmlneralogicalinvestigationsandradiocarbon

ldatings lForclaymineralogy)sampleswerefirst

disaggregatedindistilledwaterfbrfbur

Ihours，andclayfractions，approximatelyless

than2/ｕｍｉｎｄｉａｍｅｔｅｒ,wereconcentratedby

acentrifnge・Ｔｈｅｎ，clayfYactionsweredried

atroomtemperatureandpreparedfbrX-ray

lanalysis・X-rayanalysiswascarriedouton

lorientedsamplesonglassslidesX-ray

－３４５－

‘il:(i鐘i:鴬!】菫ilii1HlIiilfjii:il1Hjl霊?鰯i蝋闘此澱搬菫lmoderatelybrownmudThemudcontentisl about80%mweightM・deratelybrownclaysl

am…｡fquart…dbiogenio…rialsl

３)Surfac…dimentSofSiemReapmⅧＩ

ｌＶＧｅｎｅｒａＩＦｅａｔｕｒｅｓｏｆＳａｍｐｌｅｓａｎｄ

ＳamplingSites

Generalfbaturesofthesamplesusedinthe

presentstudyareasfbllows・Thesurfncesoils

intheEastBaraywaterreservoirandthe

surfncesedimentsfromthelbnleSapRiverin

PhnomPenhCityaredescribedbasedon

msukawakiemJ．（1998）andmsukawakietaZ．

(1994),respectively

l）SurfacesedimentsofLakelbnleSap

FourbottomsurfをLcesedimentsTS96－１，２，３

and4wereobtainedfroｍｔｈｅｎｏｒｔｈｅｒｎｐａｒｔｏｆ

ＬａｋｅｌｂｎｌｅＳａｐａｔｗａｔｅｒｄｅｐｔｈｓｏｆａｌｌａｒｏｕｎｄ

0.9ｍ・OnesurfacesedimentsampleTS96-5

ｗａｓｃｏｌｌｅｃｔｅｄｆｒｏｍｔｈｅｌａｎｄａｒｅａｏｎｔｈｅｎｏｒｔｈ

ｏｆｔｈｅｌａｋｅ，ｂｕｔｔｈｅｓａｍｐｌｉｎｇｓｉｔｅｂｅｃｏｍｅｓａ

ｐａｒｔｏｆｔｈｅｌａｋｅｆｌｏｏｒｗｈｅｎthewaterlevelis

high

Thesedimentsfromthefbrmerfbursites

consistｏｆｌｉｇｈｔｏｌｉｖｅｇｒｅｙｏｒｒｅｄｄｉｓｈｂｒｏｗｎ

ｓｏｆｔｏｒｓｏｕｐｙｍｕｄ・Ｍｕｄｃｏｎｔｅｎｔｓｏｆｔｈｅｍａｒｅ

ｍｏｒｅｔｈａｎ９0％inweight、Yellowishbrown

clayspredominatemuddysedimentswith

smallamountsofquartzandbiogenicma-

terialssuchasspongespicules，pollensand

diatoms・Sandysedimentsarecomposed

mainlｙｏｆｖｅｒｙｆｉｎｅ‐tofIne-grainedquartz

withalittleamountoflateriticrockfra-

gments・Plantdebrisandmolluscanshell

fragmentsarecommonlycontainedinthe

sediments・Freshwaterostracodes，andsqua－

maeandteethoffishareoccasionallyre-

cognizedinthesandysediments・

Ｏｎｔｈｅｏｔｈｅｒｈａｎｄ,thelatteriscomposedof

paleorangesandymudSandysediments

consistｍａｉｎｌｙｏｆｖｅｒｙｆｉｎｅ‐ｔofine-grained

quartzwithalittleamountoflateriticrock

fragments・Muddysedimentsconsistmainly

ofyellowishbrownclayswithacertain

amountofquartz．

鱗蝋誓iiiiiiiijislessthan20％inweight・Sandysediments

in瀦濫晋就蝋鵜警鯛IcoloumMuddysedimentsarecomposedchiefly

SoilsamplefromtheAlluvialplainsouthOS

theWestBaraywaterreservoir（TS96-7）ｉｓ

fromthedeepestpartofthereservoiｒａｔｔｈｅｌ

ｗａｔｅｒｄｅｐｔｈｏｆ８０ｍ（TS96-WB1）consistI

照穏.:患鰯:｡鵜:毛灘藍鰯Ｉsandysedimentsinadditiontoasman l

amountoflateriticrockfi･agments．

－３４６－

￣

，ＴｈｅｓａｍｐｌｅｓｆｒｏｍｔｈｅＥａｓｔＢａｒａｙwater

，reservoirtakenatthreehorizonsoｆａｎａｕｇｅｒ

ｈｏｌｅ（EB97-S）consistofyellowishbrown

lclayeyｓａｎｄ（160-180ｃｍs)，lightgreysandy

clay（360-380ｃｍs）andyellowishgreyclayey

Sand（560-580ｃｍs)．Thesandysedimentsof

lthemarecomposedmainlyoffine‐ｔｏmedium‐

,ｇｒａｍｅｄｑｕａｒｔｚｗｉｔｈａｓｍａｌｌａｍｏｕｎｔｏｆ

lateriticrockfragments・Theradiocarbonagelofsedimentsfｒｏｍｌ６０－１８０ｃｍｓｉｓ２５４０±１００

１yearsBP（msukawakiemZ.，1998)．Ｔｈｅsur‐

Ifacesoilsampleonthenorthofthereservoir

l(EB97-AD）consistsofpaleorangeclaybA1-'mostnosandysedimentsarecontainｅｄｉｎｔｈｅ

ｓａｍｐｌｅ

ｌ５）CoredsedimentsofLakenmeSap lThecoredsedimentsTS96-1wasobtained

lfromtheflatlakeflooralmoStinthecentreof

IthenorthernmostpaｒｔｏｆＬａｋｅＴｂｎｌｅＳａｐ ａｂｏｕｔｌＯｋｍｓｏｕｔｈｗｅｓｔofTtherivermouthof

ltheSiemReapRiver、Ｔｈｅｗａｔｅｒｄｅｐｔｈａｔ

ｓａｍｐｌｉｎｇｗａｓＯ９ｍ、Sedimentsfromsixhori‐

ｚｏｎｓｏｆｔｈｅｃｏｒｅｗｅｒｅａｎａｌｙｚｅｄｉｎｔｈｅpresent

,studyLithologicalfbaturesoftheｃｏｒｅｂａｓｅｄ

ｏｎ恥ukawakietaL（1997）andlmkawaki

（1997）areasfbllows（Fig.３)．

ｌＴｈｅｃｏｒｅ，l25cminlength,iscomposedof lightolivegreysoupymudintheupper22cm

landhomogeneousorbioturbatedmudsinthe

llowｅｍｌｎｔｈｅｌｏｗｅｒｐａｒｔ，ａ２５ｃｍｔｈｉｃｋｌｉｇｈｔ 'olivegreyhomogeneousmudoverlieｓａ５５ｃｍ

ｔｈｉｃｋｄｕｓｋｙｙｅｌｌｏｗｉｓｈｇｒｅｅｎbioturbatedmud，

ｌｆｂｌｌｏｗｅｄｂｙａｎａｂｏｕｔ２５ｃｍｔｈｉｃｋｇｒｅyisholive lgreenhomogeneousmudThelowerbiotur‐

lbatedandhomogeneousmudsarerelatively

'compactratherthanoverlainhomogeneous

lmudParallellaminationsarerarelydevel-

lopedinpartofthebioturbatedmudThe

lsandiscomposedmainlyofveryflne-grained

lquartz，lateriticrockfragmentsandcharred

lwooddebrisThemudconsistsmainlyof

clayswithasmallamountofbiogenicmate-

rialssuchasdiatomsandspongespiculesA

molluscanshellandshellfragmentslayeris

intercalaｔｅｄａｔ２６ｃｍｂｅｌｏｗｔｈｅｌａｋｅＨｏｏｒ・

Theradiocarbonagesofsedimentsfrｏｍ４２－

４８，１００－１０６ａｎｄｌｌＯ－１１６ｃｍｓａｒｅ５０８１±86,

6233±８４ａｎｄ６５０５±８８yearsBErespec‐

tively（MildenhalL1996MS)８

Inaddition，thecoreTS96-2obtainedaboUt

２ｋmWNWofthesiteTS96-1islithologically

correlatablewiththecoreTS96-1．Aslight

butclearincreaseofsandysedimentcontents

fromOtol0％isrecogmzedatthehorizon

about55ｃｍｓｂｅｌｏｗｔｈｅｌａｋｅｆｌｏｏｒｉｎｔｈｅｃｏｒｅ・

TheradiocarbonagesofsedimentsfｉＰｏｍ４８－

５３ａｎｄｌＯ３－１０８ｃｍｓａｒｅ５６２０±１２０ａｎｄ６０７０

±９０yearsBRrespectively(咄UkawakiemZ.，1997)．

Ｖ･Results

Figures４，５，６and7showselectedX-ray

difYractionpatternsfbrtheuntreatedand

treatedspecimenofthesurfncesedimentB

fromLakeTbnleSap（TS96-3),thelbnleSaP

River(TSR93-2),theSiemReapRiver(TS96

6)，ａｎｄｔｈｅｓｕｒｆｎｃｅｓｏｉｌｓｓｏｕｔｈｏｆｔｈｅＷest

Baraywaterreservoir(TS96-7)．Thepatterns

fbrtheuntreatedspecimenofthecored

sedimentsfroｍｔｈｅｌａｋｅａｒｅａｌｓｏｓｈｏｗｎｉｎ

Ｆｉ９．８．

１）surfaceandsuSpenCledsedimentsof

thenorthernpartofLakelbnleSap

Figure4showstheX-raydiffractionpa‐

tternsfbruntreatedandtreatedspecimenof

thelcmthicksurfacesedimentsTS96-3from

thenorthernpartofLakelbnleSapThe

untreatedspecimenhasabroadreflexion

peakaroundl51A,relativelysharppeaksat

lO､0,7.1ａｎｄ３．３４Ａ、Ｔｈｅｓｈａｒｐｐｅａｋａｔ３､３４

Aisattributedtoquartz・Ｔｈｅｂｒｏａｄｐｅａｋａｔ

15.1Ａｓｈｉｆｔｓｔｏａｂｏｕｔｌ７０Ａｂｙｅｔｈｙｌｅｎｅ

－３４７－

ｉｉｒ

ｌｌ

■ Sandcontent TSO6-2 TS96-1 ０ 臣陪陰塵Ⅲ際一溌例熱Ｚ乞仇写ご〆々／で仏‐

』室一一一一一一讓一烈種鰯乃小出巧寓Ｃ立嗜肋置一口

ZigⅦ12o勘８９匂１

ｍ曙ＡＷｊｍは

Ｓ

ｍ

ｃ

〔凸・１つ

ＰＳ

Ⅱ■ⅡⅡ１

￣…巧層ラニミ

卿Zo腕`9超ｙ

ルｏｊ"０？qgF720zJT

m"⑳'"“

'１ ，１

にｉ

ｌｌ 133.40cm。

０ ５

（⑰日。）鞘。◎国⑭着一①昌彦。『⑪皇署巳：

5081±8６

5620±120

Ｓ

ｍ ｃ

７○

一Ｏ■

（Ｕ

》０

打ＩＩＪ

｡h`s勺j）eKlm"g"e〃６ヵｍ”αだ‘'”。

'８３．，…

100

蕪昌|藁謡：6070±90

０１０２０

（weightqO

Fig.３ SimplifiedlithologicalcolumnarsectionsofthecoredsedimentsTS96-1

ａｎｄ２ｆｒｏｍｔｈｅｎｏｒｔｈｅｒｎｐａｒｔｏｆＬａｋｅｌbnleSapwithradiocarbondatings (SimplifIedfiPomandadaptedtomsukawaki，1997)．Thicklinesbeside thecolumnindicatesamplinghorizonsfbrclaymineralogy

■■

０－‐’’９１－１１－－１－００－‐－－０

glycol-solvationanditshiftstoaboutl80A

byMg-saturationandglycerol-solvationThis

indicatesthatthebroadpeakisattributedto

smectiteHowever,smallpeaksaroundl45A

areobservedinuntreated，ethyleneglycole-

solvated，andMg-saturationandglycerol‐

solvatedspecimens，ａｎｄｔｈｅｐeakdoesnot

changebyK-saturationThesefactssuggest

thepresenceｏｆａｓｍａｌｌａｍｏｕｎｔｏｆｃｈｌｏｒｉｔｅｉｎ

ｔｈｅｓample・ThesharppeakatlOOAandits

highorderreflexiondonotchangeby glycolations，HOI-saturationand300℃heat‐

ingtests,indicatingthatthepeakatlO・OAis

attributedtoillite・Ｔｈｅｂａｓａｌｐｅａｋａｔ７､１A

thatisnotaffbctedbyHCl-treatｍｅｎｔｉｓ

attributedtokaolinmineralaTheX-ray

diffiPactionpatternsofuntreatedandtreated

specimenofotherfbursamples（TS96-1,2,'４

and5)fromthelakesurfaceandthoseofthel suspendedsedimentsmthelakewaterl

(TS96-9)showverysimilartothoseofTS96-3

Thus，claymineralsofthesurfbucesedimentsl

fromthenorthernpartofLakeTbnleSapand l

thesuspendedsedilnｅｎｔｓｉｎｔｈｅＩａｋｅｗａｔｅｒ， ‘I

arecomposedmainlyofslnectite，chlorite,l illiteandkaolinminerals，

２）SurfacesedimentsofIbnleSapRiver

Figure5showstheX-raydiffiactionpat-’

ternsfbruntreatedandtreatedspecimensofi21 I1

cmsthicksurfblcesedilnentsofsampleTSR93-I

－３４８－

＆

「〉

〉

Ｉ

ＴＳ,6-3

15.1Ａ 秘爬峻

ニコヨ6.0 ５０Ｋ ILO

⑱ＱＵ

一ｑ一

山山一一一一一一人人一》３．０Ｋ 」■

2.0Ｋ 岻一 地一一八一》一「』1.010 0.0

5.0１００15.0２０．０25.0３０．０

°２０(CuＫα）

Fig.４ X-raydiffractionpatternsfbrorientedaggregatesofuntreatedandtreatedsurface

sedimentsampleTＳ９６－３ｆｒｏｍｔｈｅｎｏｒｔｈｅｒｎｐａｒｔｏｆＬａｋｅＴｂｎｌｅＳａｐ ＵＴ：untreatedsample，ＥＧ：ethyleneglycolsolvatedsample，ＭＧ－Ｇ１ｙ：Mg-saturatedandglycelrol-solvatedsample，Ｋ：K-saturatedsample，ＨＣＬＨＣ１‐ treatedsample，300℃ａｎｄ600℃：thermaltreatedsamplesfbronehourat300℃ and600℃，respectively

２ｆｒｏｍｔｈｅＴｂｎｌｅＳａｐＲｉｖｅｒｉｎＰｈｎｏｍＰｅｎh

CityBTheuntreatedspecimenhasreflexion

peaksatl4､4,10.1,7.2ａｎｄ３．３４Ａ、Ｔｈｅｐｅａｋ

ａｔ３３４Ａｉｓａｔｔｒｉｂｕｔｅｄｔｏｑｕａrtz・After

lethyleneglycol-solvation,ｔｈｅｐｅａｋａｔｌ４４Ａ

ｓｅｐａｒａｔｅｓｉｎｔｏｔｗｏｗｅａｋｐｅａｋｓａｔｌ７０ａｎｄ

ｌ４０Ａ,ａｎｄｔｈｅｆｂｒｍｅｒｐｅａｋｓｈｉｆｔｓｔｏａｂｏｕｔ

ｌ8.ＯＡｂｙＭｇ－ｓaturationandglycerol-solva‐

tion，thusthediffUsepeakareattributeｄｔｏ

ｓｍｅｃｔｉｔｅＯｎｔｈｅｏｔｈｅｒｈａｎｄ，thelatterweak

peakataboutl4・OAincreasesitsintenseness

andisslightlycontractedinspaceafter

heatingat600℃whilehigherorderreflexion

disappearedlnaddition,thepeakat14.4Ａ

ｄｏｅｓｎotchangebyK-saturation，indicating

thatthepeakisattribuｔｅｄｔｏｃｈｌｏｒｉｔｅＴｈｅ

ｂａｓａｌｐｅａｋａｔ７２ＡｉｓｎｏｔａｆｆｅｃｔｅｄｂｙＨＣｌ‐

ltreatmentalthouglltheintensityweakened

slightlymherefbre,thebasalreflexionpeakat

７．２Aisattributedtochloriteoverlapping

withkaolinminerals，Ｔｈｅ10.1Areflexion

peakdoesnotchangebyHCl-treatmentand

3００℃heatingtests，indicatingthatthe

reflexionpeakisattributedtoilliteAccord-

ingly）claymineralsofthesurfacesediments

fiomthelbnleSapRiverinPhnomPenhCity

arecomposedmainlyofsmectite，chlorite，

illiteandkaolinminerals、

３）SurfacesedimentsoftheSiemReap River

Figure6showstheX-raydifIractionpat-

ternsfbruntreatedandtreatedspecimensOf

thesurfncesedimentsTS96-6fTomtheSiem

ReapRiverinSiemReapCityTheuntreated

specimenshowsabroadandweakreflexion

peakaroundl4.６Ａ,ａｗｅａｋｐｅａｋａｔｌＯＯＡ,ａ

ｓｈａｒｐａｎｄｓｔｒｏｎｇｐｅａｋａｔ７２Ａ，ａｎｄｓｈａｒＰ

ｐｅａｋｓａｔ３５８ａｎｄ３､３４ＡＴｈｅｓｈａｒｐｐｅａｋａｔ

ａ３４Ａｉｓａｔｔｒｉｂｕｔｅdtoquartz・Theweak

reflexionpeakaroundl46Adiffilsesabout

－３４９－

｛明ＱＵ》『

【Ⅱ】

-－－－－－－－

２０（ＣｕＫａ

Fig.５ X-raydiffractionpatternsfbrorientedaggregatesofuntreatedandtreatedsurface sedimentsampleTSR93-2fromthelbnleSapRiverinPhｎｏｍＰｅｎｈＣｉｔｙ８ ＵＴ：untreatedsample，ＥＧ：ethyleneglycolsolvatedsample，ＭＧ－Ｇ１ｙ計Ｍｇ‐saturatedandglycelrol-solvatedsample，Ｋ：K-saturatedsample，ＨＣＩ：ＨＣ1‐ treatedsample，300℃ａｎｄ600℃：thermaltreatedsamplesfbronehourat300℃ and600℃，respectivelyb

M１

的ｅＵ

】四

２０(CuＫα）

Fig.６X-raydiffractionpatternsfbrorientedaggregatesofuntreatedandtreatedsurfnce sedimentsampleTＳ９６－６ｆｒｏｍｔｈｅＳｉｅｍＲｅａｐＲｉｖｅｒｉｎＳｉｅｍＲｅａｐＣｉｔｙｂ ＵＴ：untreatedsample，ＥＧ：ethyleneglycolsolvatedsample，Ｋ：K-saturated sample,ＨＣ１：HCl-treatedsample，300℃ａｎｄ600℃：thermaltreatedsaｍｐｌｅｓｆｂｒ ｏｎｅｈｏｕｒａｔ３００℃ａｎｄ600℃，respectively

－３５０－

7.2ＡＴＳ９６－７

ｉﾄﾍﾞｰ１－－艇－人JLHU：358Ａ3.35Ａ

5． OＫ

ＢＧ 5．

●

△幻皿□

⑭ＱＵ

0Ｋ Ｋ

一人地3. OＫ

2． DＫ

～-｣I蝿一/L－－－蘂… 300℃ I． 0Ｋ

ｈへw勘 600'Ｃ 9.7Ａ 一凸庁P凸一一Ｗ介辿可…￣ｖ’0． 0Ｋ

５．０１０．０１５０２０．０２５．０３０．０

．２０(CuＫα）

Ｆｉｇ．７X-raydiffmctionpatternsfbrorientedaggregatesofuntreatedanｄｔｒｅａｔｅｄｓｏｉｌ

ｓａｍｐｌｅＴＳ９６－７ｆｒｏｍｔｈeA11uvialplainsouthoftheWestBaraywaterreservoir、

ＵＴ：untreatedsample，ＥＧ：ethyleneglycolsolvatedsample，Ｋ：K-saturated

sample,HCl：HC1-treatedsample，300℃ａｎｄ６00℃：thermaｌｔｒｅａｔｅｄｓａｍｐｌｅｓｆｂｒ ｏｎｅｈｏｕｒａｔ３00℃ａｎｄ600℃，respectively

'17.OAbyethyleneglycol-solvationandit

lshiftstoaroundl2､OAbyHC1-treatment．

'Thus，thisdiffnsepeakisattributedto smectiteTheweakpeakatlOOAisassiｇｎｅｄ

ｔｏａｖｅｌｙｓｍａｌｌａｍｏｕｎｔｏｆｉｌｌｉｔｅbyglycola‐

,tions，HC1-treatmentandheatingtests、The

Istrongpeakat72AdoesnotchaｎｇｅｂｙＨＣｌ－

Ｉtreatment，butcollapsesafterheatingat60０

，℃、Thisphenomenonimpliesthatthe7､２A

reflexionpeakanditshigherorderreflexion

l(e9.,3.58Ａ）aremainlyattributedtokaolin

lminerals・TheX-raydifIractionpatternsoftheuntreatedandtreatedspecimensｏｆｔｈｅ

ｓａｍｐｌｅＥＢ９７－ＳＲｆｒｏｍｔｈｅｒｉｖeｒｏｎｔｈｅｎｏｒｔｈ

'oftheEastBaraywaterreservoirarｅｖｅｒｙ

ＩｓｉｍｉｌａｒｔｏｔｈｏｓｅｏｆＴＳ９６－６・Therefbre，itis

lstatedthatclaymineralsofthesurmce

lsedimentsfromtheSiemReapRiverconsist

ImainlyOfkao1inmineralsandaverylittle

lamountofsmectiteandillite

４）AlluvialsonsinandaroumltheWest

andEastBaraywaterreservolrs ●

Figure7showstheX-raydiffractionpat-

ternsfbrtheuntreatedandtreatedspecimens

ofthesurfncesoilsampleTS96-7fromthe

A11uvialplａｉｎｏｎｔｈｅｓｏｕｔｈｏｆｔｈｅＷｅｓｔＢａｒａｙ

ｗａｔerreservoirwherethefloodedwateｒｏｆ

ＬａｋｅＴｂｎｌｅＳａｐdoesnotreachevenatthe

highestwaterlevel・Theuntreatedspecimen

showsabroaｄｒｅｆｌｅｘｉｏｎｐｅａｋａｔｌ５６Ａ,ａｎｄ

ｓｈａｒｐａｎｄｓｔｒｏｎｇｐｅａｋｓａｔ７､2,4.28,3.58ａｎｄ

3.35Ａ､Ｔｈｅｓｈａｒｐｐｅａｋｓａｔ４､２８and335A

areattributedtoquartz、Ｔｈｅｂｒｏａｄｐｅａｋａｔｏ､

15.6Ａｓｈｉｆｔｓｔｏａｂｏｕｔ17.2Abyethylene

glycol-solvationanditalsoshiftstol25Aby

HCl-treatment・Further，ｔｈｅｎｅｗｐｅａｋｓａｔ

ｌＯＯＡａｎｄ９,７Aappearafterheatingat300

℃ａｎｄ６００℃，respectivelybThus，thebroad

peakisattributedtosmecｔｉｔｅＴｈｅｓｔｒｏｎｇ

ｐｅａｋａｔ７２ＡｄＯｅｓｎｏｔｃhangebyHC1-

－３５１－

千・・』。一

蕊議繍蕊論調鮒淵鮒臘洲蝋伽附川岻ｉｌＩ１Ⅲ１１１腓ｌ‐‐ルー

（ｎＱＵ）】

)６－１８６０－（

9６

￣－－－－－－一一￣

DＨＩ 410

Fig.８ X-raydiffractionpatternsfbrorientedaggregatesofuntreated sedimentsamplesfromsixhorizonsofthecoredsedimentTS96-1from thenorthernpartofLakelbnleSap．

treatment，butcollapsesafterheatingat6００

℃、Thisindicatesthatthe72Areflexion

peakanditshigherorderreflexioｎ（e9., 3.58Ａ）aremainlyattributedtokaolinmi‐

nerals、TheX-raydiffractionpatternsofthe

untreatedandtreatedspecimenｓｏｆｔｈｅ

ｓａｍｐｌｅｓＥＢ９７－Ｓｆｒｏｍｔｈｅａｕｇｅｒｈｏｌｅｉｎｔｈｅ

ｓｏｕｔｈｏｆｔｈｅＥａｓｔＢａｒａｙwaterreservoirand

thoseofthesamｐｌｅＥＢ９７－ＡＤｏｎｔｈｅｎｏｒｔｈｏｆ

ｔｈｅｒｅｓｅｒvoir,ａｎｄｔｈｅｓａｍｐｌｅＴＳ９６－ＷＢ１ｆｒｏm

thebottomsurfnceoftheWestBaraywater reservoiraresimilartothoｓｅｏｆＴＳ９６－７・

Accordingly）claymineralsoftheA11uvialsoils

inandaroundWestandEastBaraywater

reservoirsarecomposedmainlyofsmectite andkaolinminerals、

５）CoredsedimentsofLakelbnleSap TheX-raydiffractionpatternsfbrthe

untreatedspecimensfromsixhorizonsofthe

coredsedimentｓＴＳ９６－１ａｒｅｓｈｏｗｎｉｎＦｉｇ・aTheuntreatedspecimensofupperｔｗｏｈｏ‐

rizonshavebroadreflexionpeaksaroundl51

Aandsharppeaksatl0.0,7.2,4.98,3.57ａｎｄ 3.34Ａ・Thesharppeakat334Aisattributed

toquartzThebroadpeakaround5０゜ｉｓ

ａｓｓｉｇｎｅｄｔｏｓｍｅｃｔｉｔｅａｎｄａｓｍａｌｌａｍｏuntof

chloritebyexpansiontestsusingethylene

glycol-solvation，andMg-saturationandgly‐ cerol-solvation,Ksaturationandheatingtests・

ＴｈｅｓｈａｒｐｐｅａｋａｔｌＯＯＡａｎｄｉｔｓｈｉｇｈｏｒder

reflexion（e9.,4.98Ａ）donotchangeby glycolationsand3００℃ｈeating，indicating thatthepeakatlO・OAisattributedtoillibe・

Ｔｈｅｂａｓａｌｐｅａｋａｔ７２Ａａｎｄｉｔｓｈｉｇｈｏｒｄer reflexion（e､９．，３５７Ａ）fhatarenotaffbcted

byHC1-treatmentisattributedtokaolin

minerals・Thus，claymineralsoftheuppertwohorizonsofthecoredsedimentsfi･omthe

northernpartofLakelbnleSapconsiSt

mainlyofsmectite，chlorite，illiteandkaolin minerals・

Ontheotherhand,ｔｈｅｐｅａｋａｔｌＯＯＡａｎｄ

ｉｔshighorderreflexionarenotdetectedinthe

X-raydiffractionpatternsoftheuntreated

specimensfiPomthelowerfburhorizonsofthe

coredsedimentssinceotherpeaksthat

recognizedinthosefromtheuppertWo

horizonsappear・Accordinglyぅclaymineralsof

thelowerfburhorizonsofthecoredsedimentjs

fromthenorthernpartofthｅｌａｋｅａｒｅ

－３５２－

Ｉ

nLblelResultsoftheX-raydiffractionanalysisfbrthesamplesofthesurftlcesedimentsfromLake'Ibnle Sap,theSiemReapandlbnleSapRivers,thesurfacesoilfromtheAlluvialplainsinandaroundthe EastandWestBaraywaterreservoirs,andthesuspendedsedimentinthelakewaten

SampleNo． Sediment

lyPe

Ａｒｅａ illitekaolin quartzchloritesmectite

minerals

１２３４５

』』』一』

６６６６６

９９９９９

ｓＳＳＳＳ

ＴＴＴＴＴ

ｅｅｅｅｅ

ｃｃｃｃｃ

色色ね色色

ｒｒｒｒｒ

ｕｕｕｕｕ

Ｓｓｓ８ｓ

ＬａｋｅｌｂｎｌｅＳａｐ

ＬａｋｅＴｂｎｌｅＳａｐ

ＬａｋｅＴｂnｌｅＳａｐ

ＬａｋｅｌｂｎｌｅＳａｐ

ＬａｋｅｌｂｎｌｅＳａｐ

＋＋＋

＋＋＋

＋＋＋

＋＋

＋＋＋

＋＋＋

＋＋＋

＋＋＋

＋＋

＋･＋＋

＋＋＋

＋＋＋

＋＋＋

＋＋

＋＋＋

＋＋＋

＋＋＋

＋＋＋

＋＋＋

十十

ＴＳ９６－６

ＥＢ９７－ＳＲ

surface

surface

SiemReapRiver

SiemReapRiver

＋＋

＋＋

＋＋＋

＋＋十

TＳ９６－７

ＴＳ９６－ＷＢ１

surface

surface

WestBaray

WestBaray

＋＋

＋＋

＋＋＋

＋＋＋

＋＋

＋

ＥＢ９７－ＡＤ

ＥＢ９７－Ｓ－１

ＥＢ９７－Ｓ－２

ＥＢ９７－Ｓ-３

ｃｃｃｃ

ｅｅｅｅ

地地池池

８８ｓｓ

ｕｕｕｕ

EastBaray

EastBaray

EastBaray

EasｔＢａｒａｙ

＋＋＋

＋＋

＋＋

＋＋＋

＋＋＋

＋＋＋

＋＋＋

＋＋＋

＋＋＋

＋＋＋

＋

(160-180）

(360-380）

(560-580）

ＴＳＲ93-2 surfacelbnleSapRiver ＋＋＋＋＋＋＋＋＋＋＋

ＴＳ９６－９ ＋＋＋＋＋＋ suspendedLakeTbnleSap＋＋＋ ＋

＋＋＋：abundant，＋＋：ｃｏｍｍｏｎ，＋：present，－：ｒａｒｅ

composedmainlyofsmectite，chloriteand

kaolinlninerals．

thesesamples

Kaolinmineralswerecommonlyobservedin

everysample,andwereparticularlyabundant

insuchsurfacesoilsamplesasTS96-7fTom

theA11uvialplainnorthofLakeＴｂｎｌｅＳａｐ

Ｓｍｅｃｔｉｔｅｗａｓａｌｓｏｃｏｍｍｏｎｉneverysample

withtheexceptionsoｆｔｈｅｓａｍｐｌｅｓｆｒｏｍｔｈｅ

ＳｉｅｍＲｅａｐａｎｄＴbnleSapRiversinwhicha

littleamountofsmectitewasdetectedIn

contrastofthem，illitewasfTequentlyde‐

tectedinallsurmcesedimentsamplesofLake

lbnleSapandthesuspendedsedimentsofthe

lake，andthesurfacesedimentsofthelbnle

SapRiverinPhnomPenhCityBFurthe刃a

littleamountofchloritewasobservedinthese

samplesthatillitewasdetected

llliteandchloritearenotgenerallyascribed

totheproductsfromtheotherclayminerals

bynaturalweatheringevenunderacid

conditionsmhumidtropicalregion(e､9.,SudO，

VIDiscussion

ｌ）Compositionandprovenanceofclay

mineralsinthenorthernpartofLake

TbnleSap

lldentifiedmineralsofsurfacesedimentsor

,soilsamplesfromthenorthernpartｏｆＬａｋｅ

ＩＩｂｎｌｅＳａｐ(TS96-1to-5),theSiemReapRiver

i(TS9MandEB97-SR),theAnuvialplamm 'ａｎｄａｒｏｕｎｄｔｈｅＷｅｓｔａｎｄＥａｓｔＢａｒａｙwater

'reservoirsonthenorthofthelake（TS96-7

1and-WB1,EB97-AD,andEB97-S-1to-3),the

nnleSapRiverinPhnomPenhCity（TSR93-

2）andthesuspendedsedimentsinthelake

Waterfromthenorthernpartoftｈｅｌａｋｅ

(TS96-9）aresummarizedandlistedinnLble

l・Kaolinminerals，illite，chloriteandsme-

lctiteweredeterminedasclaymineralsin

－３５３－

･･FO-｡:．」〆ﾛ‐ﾛ.

重'7卿iShiyama,1985)Therefbre,consider‐ｼmgsedimenttransportinpresentwaｔｅｒ ｒ,-1●_,p

systeminCambodia,itishighlyprobablethat

theilliteandchloriteinthenorthernpartof

thelakearederivedfromtheTbnleSapRiver・

Judgingfromthesamplingsiteoftheriver

wascloselylocatedtoitsconfluentpointwith theMekongRive己ｉｔｉｓｉｎｆｂｒｒｅｄｔｈａｔｔｈｅｒｅｉｓ

ｎｏｍａｒｋｅddiffbrenceinbottomsediment

compositionsbetweentheTbnleSapand

MekongRivers・Accordinglymtcanbestated

thatilliteandchloritedetectedinthesurfbLce

andsuspendedsedimentsfromthenorthern

partofLakeIIbnleSaparederivedfromthe

MekongRiver，andtheyaretransportedby backcurrentsfromtherivertothelake

throughthelbnleSapRiveｒｄｕｒｉｎｇｔｈｅｒａｉｎｙ ｓｅａｓｏｎ・

MatsuieｵαZ．（1993）notedthatthereisno

majordiffbrenceinchemicalandphysical

propertiesinwateramongLakelbnleSap，

ｔｈｅlbnleSapRiverandtheMekongRiver・

ＴｈｅｙconcludedthatthewaterintheMekong

RiverflowsintothelakethrouｇｈｔｈｅＴｂｎｌｅ

ＳａｐＲｉｖｅｒｉｎｔｈｅｒａｉｎｙseason・Ilsukawakiand

Lao（1995）suggestedthatsandysediments

oftｈｅＴｂｎｌｅＳａｐＲｉｖｅｒｉｎＰｈｎｏｍＰｅｎｈＣｉｔｙ

arederivedmainlyfrommetamorphicand

graniticrockbodiesdistributedprobablyin theMekongRiverbasｉｎｉｎＶｉｅｔＮａｍａｎｄＬａｏｓ

ｂａｓｅｄｏｎｔｈｅｉｒconlpositionalfbatures・Tl目luka-

wakietaZ．（1994）concludedthatsandy sedimentstransportedbybackcurrentsｏｆｔｈｅ

ｌｂｎｌｅＳａｐＲｉｖｅｒｉｎｔｈｅｒａｉｎｙｓｅasondonot

reachtothenorthernparｔｏｆｔｈｅｌａｋｅｉｎｓｐｉｔｅ

ｏｆｍｕｄｄｙｓｅｄｉｍｅntsreachthereassuspended sediments

Ontheotherhand,ｔｈｅｐｅａｋｓｏｆｑｕａｒｔｚａｎｄ

ｓｍｅｃｔｉｔｅｉｎｔheX-raydiffmctionpatternof

thelakesurfacesedimentsTS96-3arerather

conspicuousthanthoseofthesedimentsfrom

DｎｌｅＳａＤ冊

Ｄ２ｎｕｚｚ円Ii:「刊 」・ＬＬｅｒJｅｎｑｅＣ

」II1IU】【

JllsIron },':！

ゴ1ｍJｕｎｑｕｎｅｌａＫｅｌｎＳＰｌｔｅｃ

雪ａ、

ｓａｑＶｑｅＳＣｒｌＤｅＵ、ａｎｄｂＴ

LｍｌｓＩｒｏＩｒ ユｓｅｕｌｌｎｅｎｕｓｒｒｏｎ

sｍＲｅａＤＲｉｖｅｒｃｃ

［目ＯＴ Ｄｂｉ田

TｎＩｅＳａＤＨ

」ｅＳＣｒｌＤｅＵａＤＯｖｅＲｕＩｐｐＰ DＩｎＤＯＳ１Ｚ１ｍ

ＪＵＬＩｌＵＯＬＬ【 ユlｌｕｓｕＳＰｅｎＵｅＵＳｅｍＴｎｅｒ

」・ｒＬ

竪鬮:霊翻皇慨興謡呈鶴譽麓…lMmgab……edcomp…fM1yl 鰯基蝋霊:器腰患鰡:湖LakeTbnleSap,andthesurfacesedimentsofl

伽SiemR…nmnM印RW…ndthoIA11uvialsoilsnorthofthelakeintoconsidera-I

tion,itisconcludedthatclaymineralsofthel surfnceandsuspendedsedimeｎｔｓｉｎｔｈｅｌ

ｎｏｒｔｈｅｒｎｐａｒｔｏｆｔｈｅｌａｋｅarederivedbotｈｌ

船ｉｌｆ｡p藍鷺.ii:ilWi:蓬.驚麗MekongRiver（Fig.９)．C1aymineralsfroml thefbrmerconsistingmamlyofsmectiteandl kaolinmineralsaresuppliedintothelakel

thr･ughanetw･rkofriv…､｡…m…chl astheSiemReapRiverfromthesur…d-i ingsofthelakeduringmainlytherainyl season､Ｏｎｔｈｅｏｔｈｅｒｈａｎｄ,claymineralsfrom

thelattercharacterizedbyexistenceofilliteI

andchloriteareprovidedintothelakefroml

theMekongRiverthroughtheTbnleSapl

Riveronlyduringtherainyseasonbystrong， backcurrentsofthelbnleSapRiver．’

m3ukawakiemZ．（1994）recognizedthatCh-’

loritecharacteristicallyoccurreｄｏｎｌｙｉｎｔｈｅ

－３５４－

Ｆｉｇ．９ EstimatedtransportdirectionsofclaymineralsinwatersystemsaroundLakelbnle Sapin（１）ｄｒｙｓｅａｓｏｎａｎｄ（２）rainyseason（arrowsindicatethedirectionofwater flowandsupplyofprevailingclayminerals)．

1sedimentsamplesfromthelbnleSapRiver

landthenorthernpartofLakeTbnleSap,and ltheyledtheconclusionthatmuddysediments loftheMekongRiverreachtothenorthern partofLakelbnleSapassuspendedsedi-

,Inentsinspiteofthesandysedimentsare

lsettledinitssouthernpartｂａｓｅｄｏｎｔｈｅ

,comparisononbothsandyandnluddysedi-

mentcompositions．However，ｔｈｅｙcould

'analyzeonlyalimitedamountandnuｍｂｅｒｏｆ

ｓｅｄｉｍｅｎｔｓａｍｐｌｅｓｏｎｔｈｅｃｏｕｒseoftheirstudy

lsincetheCambodianpoliticalunrestper‐ Isistedatthattime・Theconclusionofthe

lpresentstudysupportsthepreviousreSult expressedbyllsukawakietaLinspiteofillite

wasalsodeterminedinthesedimentsfrom

thenorｔｈｅｒｎｐａｒｔｏｆＬａｋｅＴｂｎｌｅＳａｐａｓｔｈｅ

Ｃｈaracteristicclaymineralderivedｆｒｏｍｔｈｅ

ＭｅｋｏｎｇＲｉｖｅｍ

ｌ２）Verticalchangesofclaymineral

compositioninthecoredsediments

lTheidentifiedclaymineralsinallanalysed

samplesfromsixhorizonsofthecored

lsedimentsTS96－１０btainedfromthenorthern

'ｐａｒｔｏｆＬａｋｅｎｎｌｅＳａｐａｒｅｌｉｓｔｅｄｉｎＴａｂｌｅ２

Kaolinminerals，illite，smectiteandchlorite

aredeterminedasclaymineralsinthese

samples、Kaolinmineralsandsmectitewere

commonlyobservedineverysample・In

contrasttothis，illiteandalittleamountof

chloritewereobtainedonlyinthesamples

fromｔｈｅｕｐｐｅｒｔｗｏｈｏｒｉｚｏｎｓｏｆ５－１２ａｎｄ３３‐

４０ｃｍｓｂｅｌｏｗｔｈｅｌａｋｅｆｌｏｏｒ・Carbonneland

Guiscafre（1962-63）recognizedtheupper

lluaseacme此'１，２０ｔｏ３５０ｃｍｔｈｉｃｋ，ａｎｄｔｈｅ

ｌｏｗｅｒ１ｉｕａｓｅα"czemzei',morethan50cmthiCk，

withinthebottomsedimentsofLakelbnle

Saponthebasisoflithologicalobservationsof

severalcoredsedimentsfromthelakeThey

reportedｔｈａｔilliteandmontmorillomteare

theprevailingclaymineralsintheupper

sequenceinspiteofkaolmminerａｌｓｉｓ

ｄｏｍｉｎａｎｔｉｎｔｈｅｌｏｗｅｒｓｅｑｕｅｎｃe・A1though

theirstudywasshortofapreliminarygraSp

ononlycompositionaldescriptionsofthe

coredsediments，theirresultisinaccordance

withthepresentstudy

Anobviouslithologicalchangeisrecognized

inbothcoresTS96-1and2athorizonsabout

50cmbelowthelakefloor(Tsukawaki,1997)．

－３５５－

ﾄﾞリ

菫■Ⅲｊｑ，

印ｒ■■

□ロ日日■ロ日Ｊロ■

・日Ⅱ：：ｊｆ■日日□ロＬロ□叩已卍斫

■■Ｌ０日■‐ＩＬ】‐‐．．．．・１

nlble2ResultsoftheX-raydiffi･actionanalysisfbrthesamplesfroｍｔｈｅｓｉｘｈｏｌｉｚｏｎｓｏｆｔｈｅｃｏｒｅｄ ｓｅｄｉｍentsTS96-1fi･omthenorthernpartofLakelbnleSap、f･＿’

Horizon（cms）quartz illite kaolinmineralschlorite smectite

５－１２

３３－４０

６０－６７

８３－９０

１１５－１２０

１２０－１２５

＋＋

＋＋＋

＋＋＋

＋＋

＋＋＋

＋＋＋

＋＋

＋＋＋

＋＋

＋＋＋

＋＋＋

＋＋

＋＋＋

＋＋＋

＋＋＋

＋＋

＋＋

＋＋

＋

＋＋

＋＋＋：abundant，＋＋：ｃｏｍｍｏｎ，＋：present，－：ｒａｒｅ

Sedimentsabovethehorizonarecomposedof

lightolivegreyhomogeneoｕｓｓｏｆｔｏｒｓｏｕｐｙ ｓａｎｄｙｍｕｄｗｉｔｈａｎｉｎｔercalationofmolluscan

shelllayer,buttheybelowthehorizonconsist

ofgreyisholivegreenhomogeneousoｒｂｉ‐

oturbatedcompactmudTheradiocarbon

agesofsedimentsfrom42-48cｍｓｏｆｔｈｅｃｏｒｅ

ＴＳ９６－１ａｎｄ４８－５３ｃｍｓｏｆｔｈｅcoreTS96-2both

justbelowtheabove-mentionedhorizonsare

5081±８６ａｎｄ5620±120yearsBErespec‐

tively(Mildenhall,１９９６Ｍｓ；msukawakiemJ.， 1997)．

msukawakietaZ．（1997）recognizedthat

sedimentationratｅｏｆｔｈｅｎｏｒｔｈｅｒｎｐａｒｔｏｆ

ＬａｋｅｌｂｎｌｅＳapwasdecreasedmarkedlyfrom

abouｔｌＯｍｍ/yrtolessthanO､１ｍｍ/yratthe

horizonofthelithologicalchangeinboth

cores，andtheysuggestedthatanenviron‐

mentalchangeinand/oｒａｒｏｕｎｄＬａｋｅｌｂｎｌｅ

Ｓａｐａｔａbout5,OOOyearsBPwouldtriggerthe

suddendropofthesedimentationrate・

msukawaki（1997）illustratedthatthesedi-

mentsabovethehorizonofthelithological

changeinbothcoresaretheproductsunder

theactiveenvironmentofrepeatingerosion

anddepositioninspiteoftheybelowthat

fbrmedunderratherquietconditiononthe

basisofthesedimentologicalinterpretation

onsedimentaryfaciesandsedimentcomposi‐ tionsofthecoredsediments．Sincethe

horizonofthelithologicalchangeiscorre-

spondedwiththatofappearanceofilliteandl

chloriteinthecoreTS96-1,itcanbel

attributedtothattheappearanceoftheml resultedfromtheenvironmentalchangeinl and/oraroundthelakementionedbylmJ

，'ｌ

ｋａｗａｋｉｅｔａＪ・andllsukawaki，

Ｖｅｒｙｌｉｔｔｌｅｈａｓｂｅｅｎｋｎｏｗｎａｂｏｕｔｔｈｅ

moderntectonicmovementSinCambodia,bUtl itisinfbrredthatnomajortectonicmove-I

mentshavetakenplacemCambodiaduringl thelastlO,OOOyearsbecausenosuchl

movementshavebeenrecognizedinitsl

surroundingareassuchastheeasternand ’１

southernpartsofThailandandthesouthern

partofVietNamthroughtheperiod（e9.,1

ESCAE1993b;Salyapong…ｵαL,1997;I FentoneZaZ.，1997)．Thus，thesefacts’

supporttheideathattheenvironmental

changeinfbrredbymsukawaki（1997）andl msukawakiemJ．（1997)wouldnotbeascribedl

tosuchtectonicmovementsinandaroundl ’｜

LaMbnleS…wenasthelower…｡゜fltheMekongRiverBasin，

Asalreadydiscussedandstated，miteandl

chloritedetectedinbothsurfaceandsus-

pendedsedimentsfiPomthenorthernpartOf

LakelbnleSaparederivedfTomtheMekong

River、Theyweretransportedassuspended

sedimentsfromtherivertothelakethrouglll

theTbnleSapRiverbystrongbackcurrents， duringtherainyseasonsuggestedalsoby

－３５６－

Ｉ ；IlsukawakietaJ.(1994)．Apartfromthem,the X-raydifTractionpatternsoｆｔｈｅｓｅｄｉｍｅｎｔｓ

ｆｒｏｍｔｈｅｕｐｐｅｒｔｗｏｈｏrizonsareverysimilar

withthoseofthesurftlceandsuspended

sedimeｎｔｓｏｆｔｈｅｌａｋｅ・Ｏｎｔｈｅｏｔｈｅｒｈａｎｄ，illiteandchloritewerenotdetermｉｎｅｄｉｎｔｈｅ

ｓｅｄｉｍｅｎｔｓｆｒｏｍｔｈｅｌｏｗｅｒfburhorizons・Ｔｈｅ

）X-raydiffractionpａｔｔｅｒｎｏｆｔｈｅｓａｍｐｌｅｆｒｏｍ ｔｈｅｌｏｗｅｓｔhorizonofthecoredsedimentsis

1ｓｉｍｉｌａｒｗｉｔｈｔｈａｔｏｆｔｈｅｓｏｉｌｓａｍｐｌｅTS96-7

fromtheAlluvialplainnorthofthelake・

Consideringthemarkeddiffbrencesofclay

nlineralcompositions，depositionalsettings andsedimentationratesbetweensediments

abovｅａｎｄｂelowthehorizonofthecored

sediments，itwouldbeinfbrredthatthe

environmentalchangementionedbyTbuka‐

ｗａｋｉ（1997）ａｎｄ恥ukawakietaZ．（1997）ｉｓ

ａｓｃｒｉｂｅｄｔｏａｃｈａｎｇｅｏｆｓｅｄｉｍｅｎｔｔransportm

watersysteminCambodia､Accordingly)ｉｔｃａｎ

ｂｅｓｔａｔｅｄｔｈａｔｗａｔｅｒｆｒｏｍｔｈｅＭekongRiver

didnotflowintoLakeTbnleSapbefbreabout

５，OOOyearsBnandonlyriversandstreams

fromthesurroundingareasofthelakeflowed

andsuppliedsediｍｅｎｔｓｉｎｔｏｔｈｅｌａｋｅｌｎｆｌｏｗ

ｏｆｗａｔｅｒｗithsuspendedsedimentsfromthe

MekongRiverstartedtothelakeabout5,000

,yearsBPandthepresentsedimenttransport

systemintothelakehasappearedsinceｔｈａｔ

ｔｉｍｅ・Further,ｔｈｅｔｈｏｕｇｈｔｂｅｈｉｎｄｔｈｉｓｉｓｔｈａｔ

ｔｈｅｓtatedchangeofsedimentsupplyintｏｔｈｅ

ｌｌａｋｅａｌｓｏｍｅａｎｓｓｏｍｅｃｈａｎｇｅｏｆfluvialsystem

linthelowercourseoftheMekongRiver・Ｔｈｉｓ,pointshouldbefUrtherstudiedindetaiL

ｏｆｔｈｅＴｂｎｌｅＳａｐＲｉｖｅｒｉｎＰｈｎｏｍＰｅｎｈＣity arecomposedmainlyofkaolinminerals,illite，

smectiteandchloriteOntheotherhand,ｔｈｅ ｓｏｉｌｓａｍｐｌｅｓａｒｏｕｎｄｔｈｅｌａｋｅａｎｄthesurftlce

sedimentsoftheSiemReapRiverconsist mainlyofkaolinmineralsandsmectite

２．Judgingfromilliteandchloritearｅｎｏｔ

ｄｅｔｅｃｔｅｄｉｎｔｈｅｓｅｄｉｍｅｎｔａｎｄｓｏilsamples fromtheSiemReapRiverandtheAlluvial

plainsaroundthelakebutthebottom

sedimentsfromtheTbnleSapRiveryielda certainamouｎｔｏｆｔｈｅｍ，itisstatedthatillite

andchloritedistributedinthenorthernpart

ofthelakewouldbederivedfromtheMekong River、

３．Inspiteofacertainamountofillitｅａｎｄ

ａｌｉｔｔｌｅａｍｏｕｎｔｏｆｃｈｌｏｒｉｔｅａｒｅｄｅｔｅｃｔｅｄｉｎｔｈｅ

ｕｐｐｅｒｐａｒｔｏｆｔｈｅｃｏｒedsedimentsobtained

fromthenorthernpartofLakelbnleSap，

theyarenotrecognizedinthelowerpart・Sincetheyareconsideredtobederivedfrom

theMekongRiver,appearanceoftheminthe

upperpartofthecoreindicatesthatachange

ofsedimenttransportinwatersystemtook

placearoundthelakeatabout5,000yearsＢＰＬ

AcknowledgementS

ThisarticleisacontributiontotheI1IbnleSap96I1

prOjectfnndedbytheJapanSecuritiesScho1arship

FoundationGrantN０．８２３ｔｏＳｈｉｎｊｉｌもukawaki・

Theauthorsexpresstheirsinceregratitudetoall

membersoftheprOject,ProfbssorM・ＫａｔｏａｎｄＤｒ．

Ｔ､Kamiya，KanazawaUniversity)Ｄｒ.Ｄ・OMilden-

hall，NewZealandGeologicalandNuclearSciences，

Ltd.，ａｎｄＭｒ．Ｓ、ＳｉｅｎｇａｎｄＭｒ・SIbuch，General

DepartmentofMineralResources，Kingdomof

CambodiafbrtheirhelpfnlsuggestionsTheywiSh

tothankMr.Ａ、ＯｚａｗａａｎｄＭｒ・MSatofbrtheir

experimentalhelpduringthestudyTheyare

gratefnltoHisExcellencyVMollyvan，Authority

fbrtheProtectionandManagementofAngkorand

theRegionofＳｉｅｍＲｅａｐａｎｄＭｒ、X-SIbuch，

Ｖ11．ＣｏｎｃＵＩ】sioⅡ、

Thefbllowingconclusionsarereached

durmgthecourseofthepresentstudy： ，LClaymineralsofthesurfもlceand

lsuspendedsedimentsfromthenorthernpart ofLakelbnleSap，andthesurfacesediments

－３５７－

ｏｆCambodia（1999)：Ｈ)′dmgmp肱ＭａｓＭｂﾉｾongRjuerj〃Ｃａｍ６ｏｄｊａ，DCJ・３ＪTb"JeSCZpRjue派ａＭ

Ｚｂ"ＪｅＳｑｐＬａﾙｅｊ〃Cam6odjq

Mitusio，Ｈ・’０h,０，Ｎ、andMeas，S-A．（1970)ii

LimnologicalstudyoftheMekongwatersystem， Cambodia・Res・Rep.,Ｋｂｃｈｊ伽u,Ⅳａｔ・Scj.,16,59-68．

Nishiyama,Ｔ、（1985)：Claymineralsproducedlbyweatheringandtheirproperties・jVmdoKtZg[z伽（ｅｍＣＺｑｙＳｃＺ・ＳＯＣ．ｊ上りpα")，２５，１０１－１０６．（inJapanesewithEnglishabstract）

Salyapongse，Ｓ，Fontaine，Ｈ､，Putthapiban，Eand

Lamjuan，Ａ・（1997)：GeoZOgyo／ｔｈｅＥａｓｵe7〃Ｔ/DajJα"d,FjeZdEj[czLrsZo〃ＧⅡjde6oohRozz/ｅ〃o・Zb

theFirstlnternationalConfbrenceonStratigra今

GeneralDepartmentofMineralResources，King-

domofCambodia，andProfbssorYIshizawa，

SophiaUniversityfbrtheirvarioushelpduringthe

courseofthisstudy8TheyalsothankthePlant

ProtectionOffice，MinistryofAgriculture，Japan

fbrtakingmcilitiestoimportthesamplesinto

Japan．

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Dervert,』.（1993)：LeCam6odge、QueSais-Je?ｎ０．２０８０，PressesUniversitairesdeFrance，Paris．

（Japanesetranslationbylshizawa，mand Nakajima,Ｓ､,1996,Hakusui-sha,Ibkyo,146p,）

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andthePacific）（1993a):ＡｔＺａｓＯｆＭ"ｅｒａＪルーsozL7cesO/WieESCAPRegjo",DCJ・ＺＯＣａｍ６ｏｄｉａ、UnitedNations・

ESCAP（EconomicandSocialCommissionfbrAsia

andthePacific）（1993b)：QzJater"α'ySt7atig7α‐ ｐｈｙＯｆＡｓｊａａ几ｄ#ｈｅＰａｃ城ｃＩＧＣＰ２９６‐Regjo"αZCo77eJatjo",Bj6ZiOg7cZpﾉｶﾝｏ／ＥａｓｔａＭＳｏｚＬ仇ｅａｓｔ

Ａｓｊａ，Ｓｚｍｚｍａ７ｙＯ／F7e几cﾉｶＬｊｒｅ７ａｍ７ｅｏ〃オノｔｅ

ＱｚＬａｵe7"αアツ。／Ｃａｍ６ｏｄＺＱ，ＬａｏＨＯｐＺｅＩｓ比、o‐c7aticRepzJ6ZZcα"。ⅥetMzm・MineralResources

DevelopmentSeriesno､62,UnitedNations

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O/Ｔ"te7"α"o"αＺｏｏ〃たre"Ｃｅ。/BtratjgrcZp/toﾉα"ｄＴｂｃｵo"icEuo此tZo〃Ｃ／ＳｏｍｔｈｅａｓｔＡｓｊａａｎｄＳourh

Pac腕ｃｕｏＺ.】,BmzgAoﾙ,436-452．

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partmentofthelnterior,GeologicalSurveyB

(Received22Januaryj2001；Accepted mJanuaryう2002）

－３５８－

「

トンレサップ湖北部に分布する粘土鉱物の組成ならびに起源

大河原正文＊塚脇真二＊＊

湖北部周辺の水系や沖積層の堆積物にこれらがほ

とんど含まれないことから，同湖北部に分布する

イライトおよび緑泥石はメコン河本流を起源とす

るものと判断される。

一方，トンレサップ湖北部湖底から採集された

柱状試料では，その上部約５０ｃｍの堆積物からは

イライトならびに微量の緑泥石が検出されるもの

の，同試料下部にはこれがまったく認められない。

したがって，イライトおよび緑泥石がメコン河水

系に由来するという表層堆積物解析結果にもとづ

き，同湖における堆積物の供給源に大きな変化が

発生したことが推定され，その年代は今から約

5,000年前と考えられる。

東南アジア最大の湖であるトンレサップ湖は雨

季と乾季とでその水深・冠水面積が大きく変化し，

このような季節的変動に支配された特異な堆積作

用の発生が予期される。そこで同湖における堆積

|作用の解明へ向けて，同湖に堆積物を供給する関

,連水系や同湖周辺に分布する沖積層の堆積物の粘

土鉱物組成にもとづき，同湖北部湖底堆積物に含

まれる粘土鉱物の起源およびその時間的変化を調

べた。

ポトンレサップ湖北部の表層堆積物ならびに懸濁

|物からＭｵﾘﾝ鉱物,ｲﾗｲﾄﾎﾞｽﾒクタイトおよび緑泥石が検出される。同湖とメコン河と

,を連絡するトンレサップ川の堆積物からもイライトおよび緑泥石が検出されるにもかかわらず，同

キーワード：カンボジア，トンレサップ湖，メコン河，粘土鉱物，堆積作用

Ⅱ＊岩手大学工学部建設環境工学科'＊＊金沢大学自然計測応用研究センターエコテクノロジー研究部門

－３５９－