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島根大学地球資源環境学研究報告 22,111~120ページ(2003年12月)
Geoscience Rept.Shimane Univ.,22,p.111~120(2003)
團 Majorandtraceelementabundancesinthe<180μmfractionsofstreamsediments fromthe Kando River,ShimanePrefbcture,JaOan
Edwin Ortiz*and Ba皿y Roser*
Aわstract
Sieved fractions(<180μm)of86stream sediment samples ffom active channels of the Kando River and its tributaries
in east Shimane Prefecture were analyzed by X-ray fluorescence for major elements and l4trace elements.Histograms of
alldatashowthatdistributions aregenerallynomalorareright-skewedtoisolatedhighervalues,asistypicalforriver
sediments.Several elements(e.g.K20,Rb)have clearly bimodal distributions that are related to source.There is no
evidence of anthropogenic impacts for elements which could be affected,such as P205,Cr,Ni or Pb.The results show
clearcontrasts in the composition ofsediments collected upstream and downstream ofthe site ofthe Shitsumi dam.Those
upstream are more aluminous,andtendto have greaterabundances ofK20,P205,MnO,Na20,Ba,Ce,Nb,Rb,Th,Y and
Zr than those downstream.Conversely,Fe203T,TiO2,Sc and V are generally more abundant in samples from below the
dam site.These contrasts are related to source geology,with the catchment above the Shitsumi dam site being dominated
by granitoids and felsic volcanic rocks,and that below by mixed volcanic and sedimentary rocks ofthe Hata,Kawai-Kuri
and Omori Formations.The data clearly suggest that isolation ofthe upstream granitoid and felsic volcanic source terrane
by clos皿e of the Shitsumi dam will ultimately lead to more mafic compositions in stream sediments in the downstream
reaches of the Kando River.
Key words:Geochemistry,stream sediments,<180μm fねction,Kando River,Shimane
lntroduction
Studies of chemical compositions of river and stream
sediments provide key baseline data that can be utilized in a
variety of geological and geoenvironmental studies.This
report contains X-ray fluorescence(XRF)analyses of the
<180μm fraction f士om 86 stream sediment samples
collected from the catchment of the Kando River,east
ShimanePrefecture.TheKandocatchmentislocatedSWofIzumo City,and has an area of about448km2(Fig.1).
Study of the Kando catchment is of particular relevance
because a high dam is currently being constructed at
Shitsumi.This structure will eventually isolate part of the
present catchment from the downstream areas,thus
modifying the volume and characteristics of sediments
reaching the Sea of Japan.
The purpose of this paper is to present the data obtained
by XRF analysis,and to describe broad elemental
abundances and variations of the fine fraction of sediments
from the Kando River.Further and more detailed discussion
of the results will be published elsewhere.The dataset
reported here is intended to be used as comparative baseline
data for similar studies of other river systems in the central
San-in district,in addition to acting as a benchmark for
meas皿ing future change in the composition ofthe sediment
in the Kando Riveraftercompletion ofthe Shitsumi dam.
*Dept.ofGeoscience,Shimane University,Matsue690-8504,Japan
GeologicalOutline
Although the geology of the Kando River catchment is
locally complex,at a regioml scale it is relatively simple.
Bedrock geology to the south(upstream)of the Shitsumi
dam site is domimted by largely Paleogene to Cretaceous
granitoids and felsic volcanic rocks(Fig.1).The granitoids
include granites, granites porphyries, granodiorites,
ga.bbrOS,qUartZ diOriteS,and tOnaliteS,WhereaS the VOICaniC
rocks consist of rhyolite to dacite lava and pyroclastic rocks,
and andesites(EBGMSP,1997).In contrast,the down-
stream area to the north is a more complex assemblage of
mainly Miocene rocks,p血cipally of the Kawai-Kuri,
Omori,Hata,Matsue and Fujina Formations(Fig.1).These
lithostratigraphic units and their relations have been the
subject of numerous studies with diverse foci(e.g.
Takayasu,1986;Kano αα1.1989;1991;1994;1997;
Morita&Nakayama,1999;Roserαα1.,2001).Lithotypes
described in that area are varied,comprising basaltic to
rhyolitic lavas and pyroclastic rocks,mafic intrusives,and
locally derived sedimentary rocks,including sandstones,
conglomerates and shales.
Several tributaries of the Kando River drain catchments
consisting of dacite lavas and dacite pyroclastics from
Mount Sanbe,located in the westem central pa貰of the
catchment area.Other minor lithotypes including alkali
basalt lava,dolerite,porPhyrite,and Sangun Group basic
and psammitic schist are scattered across the area.
A significant contrast in source rock compositions thus
exists between the areas above and below the dam,in terms
111
112
Majorandtraceelementabundances inthe<180μmfractions ofstream sediments
from the Kando River,Shimane Prefecture,Japan
STUDYAREA
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しEGENDStream
Samplelocation
Fault
Drainage divide
Quatemary deposits
M.Sanbe products
Alkali basalt lava
Matsue Formation
FujinaFormation
Dolerite,Basalt
Rhyolite-dacite
Omori Fonnation
Kawai-Kuri Fm
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Fig.1.Map showing thegeneralizeddistribution oflithotypes inthecatchmentofthe Kando Riverandlocation ofsample sites.
Geology based on the1:200,000geological map of Shimane Prefecture(Editorial Board of the Geological map of Shimane
Prefecture,1997).
Edwin Ortiz and Barry Roser 113
ofboth lithotype and chemistry.It is possible,therefore,that
intermption of sediment flow down the Kando River by the
Shitsumi dam will cause a change in the composition of
both suspended sediment(very fine sand to mud)and
coarser bedload(coarse to fine sand)in the downstream
areas.
Samplingan(l Sample Preparation
Fi614Sα脚ling
Sampling was conducted on eight days during autumn
2002(Sept.25,0ct.1,9,23,25,29,Nov.8,12).On all
except one day weather was fine,and streams were clear
and low.Rain fell on NOv.8th(samples K65-77),stream
levels were raised by10-20cm,and waters were slightly
turbid.Samples were collected with a plastic water scoop,
from active channels.Sub-samples taken over a channel
lengthof20-50mwerecombinedassinglesamplesandstoredinplastic zip-topbags.Wherepossible,both sides of
streams were sampled.
Samples sites were selected using a1:50,000topographic
base map,to which geology had been added from the1:
200,000geological map of Shimane Prefecture(EBGMSP,
1997).Site selection was based on areal coverage,and with
the aim of sampling representative drainages containing the
range of lithotypes seen in the area.A number of samples
were also collected from the main channe1,downstream to
near the mouth of the river.Sample locations are given in
Figure1,numbered in order ofcollection.
Sampling was not possible at some intended sites due to
lack of sand or silt-sized materia1,particularly in upland
areas.At a number of localities,samples could not been
taken exactly where desired due to lack of access,as many
smaller channels were fully-confined by steep concrete
walls.In some cases channel floors were also concreted,and
no samples could be collected.Lack of fines in channels
between successive dams or weirs at some sites also meant
that samples had to be taken from standing water behind
these structures,rather than from freely flowing channels.A
total of86samples were collected,representing a sampling
density of one sample/5.2km2.This density is considered
acceptable for the nature of the study.
S岬ρ1ερ即α醜ion
Bulk samples collected the day before were placed in
stainless trays,and dried in a drying cabinet at80-90℃.
Each sample was then passed through a stainless steel mesh
sieve to remove granules and pebbles coarser than2mm.
That fraction was weighed and retained.The<2mm
fraction was then weighed.Total sample weights were
generally between1000and1500g,averaging1235g.The<2mm fraction was split into quarters or eighths using a
simple alumina chute.Depending on size and silt content,a
quarter or eighth split(half for2samples)was then hand
sieved through an83mesh stainless steel sieve to separate
sufficient<180μmmateria1(c.5-20g)forXRF analysis.
Onaverage,the<180μmffactioncomprised13.4%ofthe
materia1<2mm.Only in nine samples did the<180μm
fraction comprise30%ormore.
The<180μmf士actionswerethengroundinanautomatic
agate pestle and mortar for15mimtes,and a7-8g sub-
sample placed in glass vials.These sub-samples were then
dried at110℃foratleast24hpriorto detemination ofloss
on ignition(LOI).
AmlyticalMethoαs
Gravimetric LOI deteminations were made by weighing
the dried samples into ceramic crucibles,followed by
ignition in amuffle fumace at1000℃for atleasttwo hours.
LOI was then calculated from the net weight loss.The
ignited material was then manually disaggregated or re-
crushed in an agate pestle and mortar,and retumed to a110
℃oven for at least24hours.This ignited material was used
for preparation of the fusion beads for X-ray fluorescence
analysis(anhydrous basis).
Analyses of m句or elements and l4trace elements(Ba,
Ce,Cr,Ga,Nb,Ni,Pb,Rb,Sc,Sr,Th,V,Y,Zr)were made
using a Rigaku RIX-2000XRF at Shimane University.All
analyses were pe㎡ormed on glass beads prepared in an
automatic bead sampler(fusion240seconds,agitation360
seconds),using an alkali flux comprising80%lithium
tetraborate and20%1ithium metaborate,with a sample to
flux ratio of 1:2.Analytical methods,instrumental
conditions and cahbration follow those described by
Kimura and Yamada(1996).Analyses were monitored by
repeat analyses of seven GSJ and USGS standards,from
new beads not included in the original calibration.The
results were verified by cross-calibration against the
recommended values for those seven standards.Additional
descriptions ofthe general methods used are given in Roser
8∫α1.(2000,2001).
Results and Discussion
The<180μmfractionXRFdataarelistedonahydrousbasis in Table 1.LOI contents were generally low to
moderate(<10%),but sporadic high values>10%were
observed,due to the presence of significant quantities of
organic matter in some samples.Many ofthe samples with
elevated LOI values were collected from sites behind small
dams or weirs.Organic matter may be especially significant
in these samples.This will be verified by CHNS analysis.
However,some high values were also recorded from open
channel sites(Table1).
Elemental abundances show considerable ranges,even
after nomalization to100%anhydrous to eliminate the
effect of varying LOI(Table2).The average composition
over the entire suite is similar to that of average Upper
Continental Crust,except for depletion in Nb and slight
SaNr TYPE
Kl
K2K3K4K5K6★
K7K8K9★
K10Kl l
K12K13☆
K14K15☆
K16K17K18K19K20
K21K22K23K24K25★
K26K27K28K29☆
K30
K31★
K32K33K34K35K36K37K38K39K40
K41K42K43K44K45★
K46K47K48K49K50
MCBMCBMCBAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAABBBBBBBBBB
Table1.M司orand traceelementanalyses ofthe<180μmfractions,Kando River(hydrous basis).M司orelementvalues weightpercent,trace elementvaluesp.p.m.
Sio2 TiO、 Al、0。Fe、0、T MnO MgO CaO Na、O K、O P、0。 LOl TotaI Ba Ce Cr Ga Nb Ni Pb Rb Sc Sr Th
62.93
61.74
63.40
57.59
56,97
55,81
56.03
60.68
53.09
58.03
57.41
61,03
57,39
59,37
54.61
58.04
55.33
59.28
60.53
57.77
57.70
53.29
50.98
62,80
50,56
56.31
47.15
59.29
54.75
54,00
57.12
56,90
56.80
60.20
63,19
64.02
60,11
60.70
59,04
61,88
60.30
59.75
58.60
58.88
56.76
59.21
61.87
60,61
59,97
62.91
1.08
1.06
0,67
1,07
1.54
0.87
1.43
0,66
0,62
0.64
0,84
0,86
0.44
0.82
0.53
1.07
0.51
0.83
0,46
0.93
0,52
0.58
0.40
0,70
0,39
0.55
2.06
0,60
0.66
0.57
0.59
0,53
0.58
0.46
0.38
0.40
0.43
0.93
0.69
0,65
1.00
0.83
0.79
0,81
0.63
0.57
0.95
1.12
1,29
1,33
14.32
15,21
15,77
16,20
14,46
15.37
14.88
16.26
17.86
17.10
16.60
16.39
17.10
15.66
15.20
16.08
20,08
15.86
16.30
15.99
16.22
16.11
15.90
16.42
17.40
18.68
14,28
16.32
16,73
18.06
17.39
16.62
17.60
16.77
17.33
17.25
16,25
15.59
15.65
16,26
16,65
16.15
18.44
17.50
18.70
18.50
15.21
15,61
15.40
13.87
8.33
8.98
5.53
10。58
11,34
6.93
12.11
6.86
7,08
6.74
9.49
7.30
5.26
6.11
4.28
9.03
4,88
7.70
4.51
8.72
5.77
6.93
5.70
5.67
4.85
6.30
21.05
7,02
6.28
4.68
6.21
7.74
6.15
5.14
3.84
4.05
4.53
8.13
5.97
5.78
8,28
7.77
6.50
7.77
5.13
5.70
8.28
8,59
10.05
9.82
0.18
0.19
0.14
0.27
0.25
0.20
0,32
0,33
0,14
0,45
0,35
0.25
0.26
0.16
0.12
0.26
0.19
0.23
0.24
0.34
0.48
0.64
0.51
0.23
0.18
0.30
0.43
0.36
0.24
0.17
0.23
0.34
0.17
0.23
0.13
0.14
0.20
0.25
0.18
0.22
0.22
0.23
0.18
0,32
0.16
0.26
0,20
0,20
0,19
0.20
1.18
1,29
1,14
1,03
1,41
1.06
0,97
0,88
0,99
1.01
1.12
1.15
0.75
1.04
0.84
1.23
1.13
1.25
0.98
1,30
0.96
0.99
0.84
1.14
0.95
1.24
1.28
1.05
1,28
1,12
1.18
0.99
1.22
1.41
1,28
1.28
1.25
1.25
0.99
1,15
1.53
1,25
1,09
1.42
1,12
1,03
1.35
1.22
1,43
1.06
3,04
3,21
3,26
2.17
2.21
1,82
1.82
1.82
1.63
2.15
2.45
2.53
1.70
2,52
2.02
2,64
2.97
2.79
2.60
2。94
2.30
2.39
2.35
2.98
2,52
2,81
2.15
2.67
2.72
2.26
2,57
2.90
3.14
4,78
4,63
4,32
4.43
3.04
2,47
2,81
3.20
3,06
2.22
2.85
2,60
2,57
2.47
1.53
2,87
1.66
2.83
3.21
3,08
2.52
1.58
1.28
1.95
1,99
1.54
2.10
2.73
2.58
2.21
2.19
1.79
2.52
3.09
2.98
2.62
2.80
2.40
2.20
1.84
3.57
2.37
3.23
2.63
3.37
2,43
2.23
2.66
3.13
3.35
3.62
3.87
3.85
3.55
2.99
2.18
3.08
2.52
1,98
1.68
2,32
1,99
3,00
2.37
1.86
2.23
2.12
1.95
2,11
2,35
2,58
2,40
2,44
2,46
2.89
2,23
2,36
2.33
2.95
3.14
2,39
2.44
2.44
2.10
2.24
2.52
2.24
2.34
2.14
2,08
2.46
2.08
2.35
1.93
2.54
2,11
2.13
2.40
2.18
2.32
1.56
1.71
1,81
1.71
2.27
2.06
2.36
1,62
1,42
1.35
1.76
1,30
2.69
1.65
1.09
1.46
1.44
0.11
0.14
0.13
0.17
0.15
0.21
0.19
0.19
0,35
0.27
0.15
0.15
0.18
0,16
0.19
0.20
0.16
0,15
0,13
0.17
0.21
0,24
0,25
0.12
0.23
0.20
0.18
0.21
0.23
0,36
0.23
0.25
0.15
0.29
0,18
0.21
0.23
0,12
0.12
0.14
0,12
0,14
0,12
0.14
0.23
0,10
0.14
0.16
0.12
0,11
3.57
2.85
3.97
6.09
7.23
13.97
7.49
7.62
14.54
9.34
6.25
4.77
11.46
9.82
17.53
6.51
9.28
6.57
8,48
6.68
10,71
14.54
18.71
3.81
18.47
8,38
5,66
6.66
12,43
14,64
9.32
8,73
8,39
5,30
2,24
2.45
5,71
4,58
10.62
5.63
4,65
7,39
9.21
6.32
11,52
6,63
5.35
8.15
4.90
5.55
99,52
99,99
99,43
100.25
99.53
99.97
99,65
100,18
100,08
100,19
99.72
99.97
99.91
100.23
99.55
100.03
99.72
99,87
99.36
99.88
99.62
100,04
99.55
99.90
100,00
100,35
98,79
100.08
99.85
100.20
99,90
100,30
99.88
99.76
98.79
99,77
98.41
99.86
99.96
99,94
100.08
99,98
100.18
100,10
100.12
100.24
99.84
100.12
99.91
100.08
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Notes: SaNr=Sample number.TYPE=Catchment type;A-above dam site;B-secondary drainages belowdam site;MCB-main channel belowdam site.
LOI=loss on ignition.Astered samples were collected from behind weirs.
団O∈ぎ○昌一N帥昌O切”塁力OooO【
=い
116
M勾orandtraceelementabundancesinthe<180μmffactions ofstream sediments
from the Kando River,Shimane Prefecture,Japan
Table2.Summary statistics for all<180μm fraction samples
(anhydrous normalized data).Nニ86.
Element Mean Min Max SDp 10
M司りrθ,θmeηfS碑%ノ
Sio、
Tio、
Al,O,
Fe、O、T
MnOMgOCaONa、O
K,O
P、O,
64.23
0,92
17,43
8.04
0,24
1.30
2,83
2.70
2.14
0,18
πaoe e,θmθηfsρρmク
50.63
0.39
14.67
3.98
0.10
0.85
1.62
1.49
1,18
0,11
324.5
33.0
10.8
15.1
5.2
7.9
5.0
34.4
6.7
182.9
2,4
49,9
11.6
136.3
69.89
2.21
22,20
22,60
0.75
2.06
5,06
4,01
3.55
0.42
614.2
132,0
141.3
23.5
23,9
63.6
42,7
129.1
23.7
828,2
30.7
470.5
46,8
1762.7
2.46
0.35
1,58
2.53
0.11
0.20
0.68
0,59
0,50
0.06
61,1
20,4
23.9
1.3
2.7
12,8
7.3
23,9
4,3
142.2
5.1
83,4
6,4
196,1
1
0.1
0.01
Kando Riverく180岬average/UCC
aerabibbCrh r
BCCGNNPRSSTVYZ
422.7
60,9
45.1
19,7
9,1
23.1
21.3
75,7
14,7
349.6
10.1
172.1
23.6
275.7
MinimumMaximumPopulation standard deviation
Nb Pb U K Ca Zr Th Na Sr Rb M◎La Ce Si Al Ba Sc Y FもTi Ni Cr V
Fig.2.Multi-element plot showing the average composition
(anhydrous normalized)of the<180μm fractions from the
Kando River(data from Table2)normalized against the Upper
Continental Crust(UCC)averageofTaylor&McLennan(1985).
Elements are arranged from left to right in order of increasing
normalized abundancein average Mesozoic-Cenozoic greywacke
(Condie1993)relative to UCC,following the methodology of
Dinelliαα1.(1999).The m句or elements are normalized as
oxides.
Min
MaxSDp
enrichment in Zr(Fig.2).The largest departure from UCC
composition is seen among the elements linked with mafic
minerals phases or mafic igneous detritus,which increase
fromleftto rightin the segment Sc-V.
Histograms of anhydrous-no㎜alized major element
abundances show the data for a number of elements(SiO2
(Fig.3a),Fe203T,MnO,P205)resemble normal distribu-
tions,but all show some anomalous values.One sample
(K27)has very low SiO2(50.63%),which is caused by
marked enrichment in Fe203T(22.60%),TiO2(2.21%),and
MnO(0.46%),presumably through Fe-Ti-Mn oxide heavy
mineral concentration or authigenic crust materia1.MnO
(Fig.3b)and TiO2are strongly skewed to higher values,as
is P205.The distribution ofK20is clearly bimoda1(Fig.3c);
this may also be the case forMgO and CaO.A1203(Fig.3d)
and Na20 contents show considerable variation,with
polymodal distributions and skew to highervalues.
Among the trace elements,two elements(Ba(Fig.4a),
Pb)show relatively normal distributions,with little
evidence of skewing or anomalous values.The remainder
show moderate or marked skew to higher values.This is
mostpronounced forCe,Cr,Ni,Th,V,and Zr(Fig.4b),for
which a small number of samples have contents more than
two standard deviations from the mean.The most
anomalous values forZr(1762ppm),V(471ppm),Ce(132
ppm)are seen in sample K27,which also has the extreme
values ofFe,Ti andMn,as noted above.The distribution of
the Rb data is clearly bimoda1(Fig4c),with the two modes
split almost equally about the mean.Weak tendency for
polymodality is also seen in the Ga(Fig.4d),Nb,and Sr
distributions.
The results of this study provide baseline data for the
<180μm fraction of stream sediments from the Kando
River.The generally norma1,right-skewed distributions are
typical offfactions taken from river sediments.Most ofthe
elements analyzed to date are not prone to m句or
disturbances from anthropogenic so皿ces,with the possible
exception of P205(from fertilizer),and metallic elements
such as Cr,Ni,and Pb.The abundances of all four of these
elements are unexceptiona1,and hence there is no clear
evidence of influence from human activity. The
compositions observed thus primarily reflect that of the
source rock types,and of the soils developed on them.
Skewed distributions are predictable,as are occasional
anomalous values exceeding two standard deviations from
the mean.In this case,anomalous values ofFe,Mn,Ti,Ce,
Cr,Ni,Th,V,Y and Zr are most likely due to
concentrations ofhigh-density accessory minerals.Probable
phases involved include the Fe-Ti oxides which formed the
basis ofthe historic iron industry in the area(Fe,Ti,Cr,Ni,
V),fragments of authigenic or pedogenic Fe-Mn oxide
coatings,and heavy minerals such as zircon and apatite(Ce,
Th,Y,Zr)that occur in the granitoids.High values for each
ofthese groups are commonly contained in single samples。
Heavy mineral concentration cannot,however,explain
Edwin Ortiz and Ban甲Roser 117
25
20 『「) 0
イー -
のΦ五E田も」2EコZ
5
0
(a)Sio2
Mean64.23
Min50.63
Max69.89
Std Dev2.46
2sd
x
50 52 54 56 58 60 62 64 66 68 70
25
20
15
10
5
x
2sd
(b)MnO
Mean O.24
Min O.10
Max O.75
Std Dev O.11
0.04 0.12 0.20 0.28 0.36 0.44 0,52 0.60 0.68 0.76
0
25
20
5 0
1 1
のΦ五∈霧ち」2∈コZ 5
0
0
(c)K20
Mean2.14
Min1.18
Max3.55
Std De》0.50
x
2sd
.6 1,0 1.4 1,8 2.2 2.6 3.0 3,4 3.8
wt%oxide
25
20
15
10
5
x
(d)A1203
Mean17.43
Min14.67
Max22.2
Std Dev1.58
2sd
14 15 16 17 18 19 20 21 22 23
wt%oxide
O
Fig.3.Examples of histograms of m勾or element abundances(anhydrous normalized data)in the<180
μm fraction,Kando River.Min=minimum;Max=maximum;Std Dev=standard deviation;vertical
bar x=mean;horizontal bar(2sd)ニ±2standard deviations from the mean.(a)SiO2-no㎜al
distribution;(b)MnO-skewed to higher values;(c)K20-bimoda1;(d)A1203-considerable variation
with polymodality and skew to higher values.
the tendency forbimodality seen for some elements,such as
Mg,K,Ga,and Rb,which are nomally concentrated in
clays or low-density silicate phases.Such elements are more
likely to be controlled by the composition of so皿ce
lithotypes.The Kando catchment is almost equally divided
between Paleogene to Cretaceous granitoids and felsic
volcanics in the south,and a mixed assemblage of Hata,
Kawai-Kuri and Omori Formation volcanics and sediments
in the north,along with small amounts of mafic rocks.
Consequently,the watershed above the new dam site is
primarily felsic,and that below somewhat more mafic.As a
first step in examining the influence of so皿ce rock
lithotype,we have divided the sample sites into three
categories:
(1).Sites above the dam site(all sites upstream of Site3),
representing the total catchment forthe new dam.
(2).Mainchannel sitesbelowthedam site(3,63,64,2,66,
65,68,1,79,82,83,84-86),which now carry
granitoid-derived material originating from above site
3,and material from secondary catchments below the
dam site.
(3).Secondary drainages below the dam site.These
catchments will form the main source of sediment after
completion ofthe dam.
Simple element-A1203 variation diagrams reveal
significant differences in the chemistry of the samples in
each category.The first feat皿e ofthe m勾or elements is that
the<180μm fractions above the dam site are generally
more aluminous(>17wt%)than those in the main channel
or in the secondary drainages below the dam site(Fig.5).
Althoughthere is littledifferenceinCaO(Fig.5a)andMgO
between the three categories,some contrasts are evident for
the remaining major elements,although overlap is
considerable.Fe203T(Fig.5b),TiO2,and to a lesser extent
SiO2are generally more abundant in samples from below
the dam site,especially when A1203contents are low(<
17%).Main channel samples below site3tend to have
intermediate values.In contrast,K20,P205(Fig.5c;d),
MnO and to some extent Na20show greater values above
thedam site.Mainchannel samples are againintermediate.
Trace element abundances also show some clear contrasts
between the groups.Th,Rb(Fig.6a;b),Ba,Ce,Nb,Y and
Zr show clear enrichment in the fractions collected above
118
M勾orandtraceelementabundancesinthe<180μmfractionsofstreamsediments
丘om the Kando River,Shimane Prefecture,Japan
25
20
5 0
1 1
のΦ五E⑩29$∈コZ
5
0
(a)Ba
Mean423
Min325Max614
Std De〉61
x
2sd
450 500 550 600 650
30
25
20
15
10
5
0
x
1 (b)Zr
Mean276
Min136Max1763
Std Dev196
25d
250 300 350 400 0 200 400 600 800 1000 1200 1400 1600 1800
25
20
5 0
イー -
ωΦ五∈0222∈コZ
5
(c)Rb
Mean76
Min34Max129
Std Dev24
x
2sd
25
20
15
10
5
x
(d)Ga
Mean19.7
Min15.1
Max23,5
Std Dev1.3
2sd
2030405060708090100110120130 1415161718192021222324 ppm ppm
Fig.4.Examples ofhistograms oftrace element abundances(anhydrous normalized data)in the<180,αm
fraction,Kando River.Min=minimum;Max=maximum;Std Dev=standard deviation;vertical barx
=mean;horizontal bar(2sd)=±2standard deviations from the mean.(a)Ba&nd(b)Zr-relatively
nomaldistributionswithskewtohighervalues;(c)Rb-bimodal;(d)Ga-polymodaL
o O
6
」冊 【∠
Φ窟×O雷峯
140
(a)CaO
●▲
●●●
● aboveO MC beIow▲ beIow
ム ム ム
羅鰍:…皇二3●●
▲ ○ ●L ●▲ ● ●▲▲ ▲ ▲ ▲
16 18 20 22 24
16
12
8
4
140
(b)Fe203T ▲ ●
● ▲
ム
▲8 あ 鬼 o▲▲幽8
卜難い㍗. ▲●●
16 18 20 22 24
4
2
Φロ×Oま崖
140
(c)K20
、よ.:纏3
▲免』鬼籔 ▲ ▲ ▲▲
●
●
●
、
●
▲●,
3
●
▲
▲ ▲
16 り
wt%AI20322 24
0.4
0.2
0.0
(d〉P205♂
●
●
▲
●
▲
● ▲
も●●
8 ▲
▲
●
●● ▲
.確蒙
@副畢
●O▲
● ▲
▲
Oo
▲▲
14 16 ま
wt%A120322 24
Fig.5.Examples ofmajor element-A1203variations in the<180μm fraction(anhydrous no㎝alized data),
Kando River,according to position w童th respect to the dam site.MC below=main channel below the
dam site.(a)CaO-little difference between groups;(b)Fe203T-greater abundances in samples fヤom
below the dam site;(c)K20and(d)P205-greater abundances in samples from above the dam site.
Edwin Ortiz and Barry Roser 119
40
30
∈Ω.20α
10
140
(a)Th
●
● aboveO MC bebw▲ beIow
●
● ●
● ● ● ひ
。懲=㌻●・▲く~垂尾鱒、 ▲▲
▲
16 18 20 22 24
120
80
40
140
●
●
● ▲
●
● ▲
▲●
● ●0
● ●▲
● ▲
●● ▲
%%電.。翫滞へ
象。庵
b● ∬▲▲亀
R● o ▲
)
● ▲
b ▲ も ▲
( ▲
▲
40 30
16
30
∈Ω.20α
10
18 20 22 24
(c)Sc
▲集 ▲ へ▲ ▲
▲▲塗 ▲
全輝凝.3:.‘.・
ひ馨●▲・8
20
(d)Ga
● ▲ 2
、δ、:蕊齢2・・.
q ▲ ●
●
●
て ね て て り
wt%A1203 wt%A1203
Fig.6.Examplesoftraceelement-A1203vari&tionsinthe<180μmffaction(anhydrousnormalizeddata),
Kando River,according to position with respect to the dam site.MC below=main chamel below the
dam site.(a)Th and(b)Rb-greater abundances in samples ffom above the dam site;(c)Sc-greater
abundances in samples ffom below the dam site;(d)Ga-little difference between groups.
10 0
the dam site.This association of elements is compatible
with the granitoid and felsic volcanic source at these sites,
and the greater concentration of K20in the samples of this
group.Contents of Cr,Ni,and Sr also tend to be a little
greater in that group,but the distinction is not as clear-cut.
In the secondary drainages below the dam site,Sc(Fig.6c)
and V abundances are clearly greater than those above the
dam site or in the main channe1,consistent with their
enrichment in Fe203T and TiO2.Only Ga(Fig.6d)and Pb
show little contrast between the three groups.
In all the variation diagrams,samples from the main
channel fall between the distributions of those from above
and below the dam site.This reflects mixing of detritus
derived from the lithologically varied catchments in the
lower area with granitoid-felsic volcanic detritus derived
from upstream.This and the clear associations of elements
with each group suggest that source lithotype is the main
determinant ofbulk composition.
Conclusions
Bulk chemical compositions of<180μm fractions in
sediments from the Kando River and its tributaries reflect
thenatureoftheirsourcelithotypes.Aclearcontrastin
composition exists between sediments derived from
granitoids and felsic volcanics in the south(upstream of the
Shitsumi dam site)with those from more varied and more
mafic lithotypes(Hata,Kawai-Kuri and Omori Formations)
in the north.Isolation of the felsic catchments in the south
by completion ofthe Shitsumi dam will inevitably lead to a
shift in the composition of sediments in the Kando River
below the dam site,toward a more mafic character.
Acknowle“gements
This work was part of a collaborative project with The
Tokuoka Laboratory for Study of Brackish Water
Environments,Matsue.Our thanks to Takao Tokuoka for
suggesting the project and for facilitating it,and to Masami
Saito (Tokuoka Laboratory) for her help with
administration.We are also grateful to Miwa Akashi and
Minoru Oono of Takeshita Engineering Consultant Co.for
their invaluable assistance during field work.
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(Received:Oct.31,2003,Accepted:Dec.12,2003)
(要 旨)
エドウィン・オルティス、パリー・P・ロザー、2003、島根県神戸川における<180μm画分河川堆
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神戸川とその支流から86の堆積物試料を採取し,180μm画分を篩い分けてXRF分析を行った.
全体の組成は河川堆積物に特徴的な元素分布を示すが,K,0やRbについてはバイモーダルな分布
を示した.組成は志津見ダムの上流と下流で顕著な差を示す.下流の組成はより塩基性成分が卓越
することを示している.このダムの上流には酸性の珪長質岩が分布し,下流には波多・川合一久利,
大森層の中性火山岩が露出する.志津見ダムによって上流の酸性火山岩フラックスが下流で妨げら
れていることが,下流域の塩基性成分の富化の原因であると結論づけられる.