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トベー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-
ドリ
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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Tsukawaki,S、,Okawara,M,Lao,K・-LandTada,
Ⅲ.(1994):Preliminarystudyofsedimentationin LakeTbnleSap,Cambodia、JGeOg7qp恥103,623-636.
唖ukawaki,S,Okuno,M・andNakamura,T・
(1997):Sedimentationratesinthenorthernpart ofLakelbnIeSap,Cambodia,duringthelast
6,000years・Sam・RBS・zLsmgAMSatMZgqya
U7zju.,8,125-133.(inJapanesewithEnglish abstract)
恥ukawaki,S、,Okuno,Ⅲ.,Okawara,M,Kato,M、
andNakamura,T,(1998):Undergroundstruc‐ turesoftheEastBarayWaterReservoirin Angkordistrict,centralCambodiaSzmz.R28.
zJsj7zgAMSat」VagOyQU)Lju.,9,272-280.(inJapanesewithEnglishabstract)
UnitedStateGeologicalSurvey(1971):GeoZogZcaZ MIpO/・Cam60djasbozumgLocatjo〃o/L肋oZQgjcSectjo"sjscaJeZ:1,000,000.UnitedStateDe‐
partmentofthelnterior,GeologicalSurveyB
(Received22Januaryj2001;Accepted mJanuaryう2002)
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「
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,イライトボ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
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