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Introduction
The Kanto Plain is widely covered by volcanicash derived successively from its western andnorthern sides. The ash layer is more or lessweathered and is usually brownish in color. Suchweathered ash is collectively known as “KantoLoam” and is divided into four stages: fromlower to upper, Tama, Shimosueyoshi, Musa-shino and Tachikawa. Whitish to grayish clay hasbeen considered to belong to the ShimosueyoshiLoam stage of the Late Pleistocene. It occursnear the surface on terraces in the Kanto Plain.
The clay contains sandy- to silty-size mineralsparentally derived from volcanic eruption. In thispaper, we studied modal proportions and chemi-
cal compositions of heavy minerals in the sam-ples in or around the clay layers in the KantoPlain. Modal proportion and chemical composi-tion of heavy minerals provide us with importantinformation for provenance or correlation study(Yokoyama et al., 1990; Morton, 1991). Lightminerals, such as quartz and feldspar, are less di-agnostic for the correlation. This method is es-sentially the same as that of the petrologicalstudy of early Pleistocene tuffs in the BosoPeninsula (Yokoyama et al., 1997 & 1998).
The clay studied here is thought by some in-vestigators to form the body paste of some an-cient pottery found at a local archeological site.Cummingtonite was recognized from both thepottery and the clay from the Kounan Terrace
Cummingtonite-bearing Clay in the Kanto Plain: Reconnaissance Study for Body Paste of Ancient Pottery
Kazumi Yokoyama1 and Michinori Oya2
1 Department of Geology, National Museum of Nature and Science, 3–23–1, Hyakunin, Shinjuku, Tokyo, 169–0073, Japan
E-mail: yokoyama@kahaku.go.jp 2 Saitama Cultural Deposits Research Corporation,
4–4–1 Funakidai, Kumagaya, Saitama, 369–0108 Japan E-mail: ooya@saimaibun.or.jp
Abstract Cummingtonite is a rare mineral found in volcanic ashes from the Japanese Islands. Itis frequently present in whitish or bluish-gray clays collected near the surface at several localitieson terraces in the Kanto Plain. The clays are a member of the Shimosueyoshi Stage of the LatePleistocene and contain sandy- to silty-size minerals. About 70 samples from in and around theclay layers were collected on the Kounan and Sakado terraces, Saitama Prefecture, and ShimousaTerrace, Chiba Prefecture. Modal proportions of heavy minerals and chemical compositions of or-thopyroxene and cummingtonite were measured. Even though modal proportions of the heavy min-erals are different from sample to sample and also from terrace to terrace, most of the clay samplesare characterized by presence of cummingtonite. Cummingtonite in the clays on the Kounan Ter-race are relatively common. The clays from the Sakado Terrace are characterized by abundant epi-dote and only rarely cummingtonite, whereas the clays from the Shimousa Terrace are character-ized by enrichment of calcic amphibole. Cummingtonite in the Kounan Terrace has chemical com-position with an XMg of 62–64. In the other terraces, XMg values of 66–68 or around 54 are recog-nized in addition to that of 62–64. The characteristic mineral assemblage and chemical composi-tion furnish an indicator between the clay and the body paste of ancient pottery found at a local ar-chaeological site.Key words :
Bull. Natl. Mus. Nat. Sci., Ser. C, 35, pp. 19–37, December 22, 2009
(Oya et al., 2006). However, the clay has not pre-viously been petrologically studied in detail andno correlation has been attempted between thepaste of the pottery and the clay. This paper pro-vides the basic data for such a correlation.
Sampling Site
The clay layers occur near surface of terracesin the Kanto Plain. Most of the samples are col-lected at the archeological sites on the KounanTerrace (Fig. 1). The sites for the KM and KNseries samples have been reported in detail by theSaitama Cultural Deposits Research Corporation(SCDRC, 1993) and Kounan Town Board of Ed-ucation (KTBE, 2005), respectively. The KH se-ries samples from a small cliff on the KounanTerrace are the same as those described byMachida et al. (2002). The other two sites at theSaitama Prefecture are from the archeologicalsites at Yorii (SCDRC, 2006) and Sakado(SCDRC. 2008). In the Shimousa Terrace, ChibaPrefecture, the clay samples and associatedloams were collected from small cliffs at Otakeand Matsumushi, Inba Village (Fig. 1), wheremany archeological sites occur. In addition tothese clay samples on the Kanto Plain, volcanicashes were collected at Shinano Town, NaganoPrefecture, 150 km northwest of the Kounan Ter-race as a possible source of the cummingtonite inthe Kanto Plain.
Simplified columnar sections at the samplingsites are shown in Fig. 2. On the Kounan Terrace,clay sample is white to bluish-gray, occasionallygrayish-brown or brown in color. Although theclay bed usually overlies a gravel or sand bed, itis sometimes difficult to distinguish it from theoverlying brownish loam. In the Sakado Terrace,clays are yellowish-gray or yellowish-brown incolor. They also overly gravel or sand. On theShimousa Terrace, Chiba Prefecture, the clay isbluish or white, locally yellowish and overlies asand bed.
At Shinano Town, volcanic ashes and interca-lating sediments were collected at the same out-crop as that studied by Furukawa and Oba
(2003). Ashes termed by them as KT-a, KT-b,KT-c and NY correspond to IB-21, IB-19, IB-14and IB-8 in the current study, respectively (Fig.2).
Analytical Procedures
The procedures used for separation and identi-fication of heavy minerals in the samples weredescribed in detail by Yokoyama et al. (1990 &1997). All the samples are loosely packed. Theywere washed in running tap water to remove fineparticles. Subsequently, they were dried andsieved. The fractions used for analyses were lessthan 250 mm in diameter. The specific gravity ofmethylene iodide was reduced to 2.82 to recovercomposite grains and aggregates of the heavyand light minerals. Carbonates, micaceous min-erals and authigenic pyrite were not further ex-amined.
Most of the minerals were identified from anX-ray profile done with an Energy DispersiveSpectrometer (EDS). Many grains were com-posed of a single mineral species (Fig. 3), butcomposite grains and aggregates were also com-
20 Kazumi Yokoyama and Michinori Oya
Fig. 1. Locality map of the collected samples.Grayish zone is hill or mountain with heightmore than 100 m.
Cummingtonite-bearing Clay in the Kanto Plain: Reconnaissance Study for Body Paste of Ancient Pottery 21
Fig. 2. Simplified columnar sections for the collected samples.
mon. For the latter, we measured the major con-stituent. A total of about 200 grains were identi-fied from the heavy fraction. In the X-ray pro-files, minerals with the same chemical composi-tions were described as polymorphs, e.g. TiO2polymorphs for rutile, anatase and brookite. Epi-dote-group minerals sometimes compromisedone of the major heavy mineral suites. Theircompositions varied from zoisite to epidote, andare tentatively and collectively denoted as “epi-dote” here. As orthopyroxene cannot be simplydistinguished from cummingtonite by the X-rayprofile, they were calculated based on chemicalcompositions obtained by an electron-micro-probe analyzer (EPMA, JEOL 8800). The modalproportions of the heavy minerals in the clay,loam and sand are listed in Tables 1, 2 and 3.
Heavy Minerals and Modal Proportions
As listed in Tables 1–3, more than fifteen min-eral species were observed in the samples. In theKanto Plain, magnetite and ilmenite usuallydominated as did sediments. Other major miner-als are orthopyroxene, cummingtonite, clinopy-roxene, calcic amphibole and epidote. Zircon,olivine and TiO2 polymorphs are mostly rareminerals, but occasionally observed as importantconstituents. Whatever the samples were volcanicash or reworked sediment, the modal proportionsof heavy minerals were highly affected duringtransportation or volcanic eruption by the densityof the minerals. The relative proportions areshown in Figs. 4 to 9 by the selected mineralswith similar density: orthopyroxene, cumming-tonite, clinopyroxene, calcic amphibole and epi-dote. These minerals have density from 3.1 to 3.5and are expected to behave in similar way duringgravitational settling.
All the samples are essentially volcanic in ori-gin. Orthopyroxene is well preserved in the loamat the top of the sequences (Fig. 3A). However, itis highly dissolved in the clay layer (Fig. 3B). Al-though discussion of the persistence of a mineralafter deposition is inevitable for the provenancestudy (Pettijohn, 1941; Morton, 1991), the differ-
ence of dissolution may be due to deposition onland, i.e. terrestrial deposition, in the former,whereas the latter is aqueous or marine deposit.Similar dissolution occurs in the other pyroxeneand amphiboles. Cummingtonite is relativelywell preserved in some clay layers (Fig. 3C), butmostly highly dissolved in the layers (Fig. 3E &F). Cummingtonite-bearing ash from the ShinanoTown, Nagano Prefecture, was deposited on landat a similar time as the clay as discussed latter.Cummingtonite are well preserved (Fig. 3D).
The KM, HK, KH and KN series sampleswere collected on the Kounan Terrace. Amongthem, the KH series section has a clear loamlayer at the top. The loam samples are composedmainly of orthopyroxene with a subordinateamount of clinopyroxene and without cumming-tonite. Cummingtonite appears in the clay layers.In two samples, KH-8 and KH-9, cummingtoniteis predominant. Although modal proportions ofpyroxenes, amphiboles and epidote are variablein the other samples, it is noted that the clay sam-ples are characterized by the consistent presenceof cummingtonite. In the other localities on theKounan Terrace, the samples consist mainly oforthopyroxene and calcic amphibole with a sub-ordinate or small amount of cummingtonite. Onesample, KN-8c, similar to those of KH-8 andKH-9, occurring at the bottom of the KN se-quence contains abundant cummingtonite. Claysamples in the Yorii site are essentially similar tothose from the Kounan Terrace, having a subor-dinate or trace amount of cummingtonite.
At the Sakado archaeological site, orthopyrox-ene is abundant in the loam samples occurring atthe top of the G6 sequence. In the other samples,pyroxenes and cummingtonite are small inamount due to strong dissolutions of the miner-als. Instead of these minerals, the clay samplesare composed mainly of epidote with a subordi-nate amount of calcic amphibole, probably due torelatively resistance against dissolution.
In the Shimousa Terrace, Chiba Prefecture,loams at the top of the MT and OT sequences arecomposed mainly of orthopyroxene with subordi-nate amounts of clinopyroxene and calcic amphi-
22 Kazumi Yokoyama and Michinori Oya
Cummingtonite-bearing Clay in the Kanto Plain: Reconnaissance Study for Body Paste of Ancient Pottery 23Ta
ble
1a.
Hea
vy m
iner
als
of t
he s
ampl
es f
rom
var
ious
loc
alit
ies
in t
he S
aita
ma
Pre
fect
ure.
Num
eric
al v
alue
sho
ws
a nu
mbe
r of
cou
nted
gra
in o
f ea
ch m
iner
al.
Loc
ali-
ties
: ar
chae
olog
ical
sit
e- K
M,
grav
eyar
d on
the
Kou
nan
Terr
ace-
HK
, sm
all
clif
f at
the
Kou
nan
Terr
ace-
KH
. M
iner
als
abbr
evia
tion
s; o
px:
orth
opyr
oxen
e, c
um:
cum
-m
ingt
onit
e, c
px:
clin
opyr
oxen
e, h
b: c
alci
c am
phib
ole,
oli
: ol
ivin
e, i
lm:
ilm
enit
e, m
t: m
agne
tite
, ep
i: e
pido
te,
tit:
tit
anit
e, C
ag:
Ca-
rich
gar
net,
gar:
Ca-
poor
gar
net,
apa:
apa
tite
, all
: all
anit
e, s
pi: s
pine
l, zi
r: z
irco
n, T
iO: T
iO2
poly
mor
phs,
tou:
tour
mal
ine.
KM
-1U
KM
-1M
KM
-1L
HK
-UH
K-M
HM
-LK
H-1
KH
-2K
H-3
KH
-4K
H-5
KH
-6K
H-7
KH
-8K
H-9
KH
-10
KH
-11
KH
-12
opx
8480
4792
8916
100
100
100
100
1824
41
12
2
cum
1620
371
137
626
8399
712
6
cpx
129
53
71
2623
3224
54
11
11
hb30
7110
016
2127
21
32
5040
4711
1712
27
oli
11
ilm
1052
2836
2245
1215
23
151
9321
474
4328
6127
mt
620
412
056
2255
3112
2943
102
2423
3112
010
014
6
epi
812
253
415
17
1239
1213
1218
12
tit
Cag
12
51
23
11
1
gar
11
12
1
apa
1
all
1
spi
21
12
1
zir
12
15
21
1
TiO
21
31
11
33
14
16
23
tou
1
24 Kazumi Yokoyama and Michinori Oya
Tabl
e1b
.T
he s
ampl
es f
rom
the
Yor
ii a
rcha
eolo
gica
l sit
e (Y
I) a
nd K
ouna
n ar
chae
olog
ical
sit
e (K
N).
YI-
1Y
I-2
YI-
3UY
I-3L
YI-
4K
N-1
KN
-2K
N-3
KN
-4K
N-5
KN
-6K
N-7
aUK
N-7
aLK
N-7
bK
N-7
cK
N-7
dK
N-8
aK
N-8
bK
N-8
c
opx
9294
4564
480
9586
9745
343
168
820
2233
cum
86
57
1820
514
35
634
2116
3626
194
100
cpx
4449
24
2618
1145
22
hb1
3155
361
3420
4050
5050
100
100
100
18
oli
12
ilm
448
1111
84
36
384
125
222
127
199
136
121
2418
68
Mgt
1025
7432
705
48
866
667
310
81
5
epi
31
181
210
28
53
35
tit
Cag
21
21
21
1
gar
12
11
apa
all
1
spi
11
1
zir
13
41
12
TiO
17
12
2
tou
2
Cummingtonite-bearing Clay in the Kanto Plain: Reconnaissance Study for Body Paste of Ancient Pottery 25
Tabl
e1c
.T
he s
ampl
es f
rom
the
Sak
ado
arch
aeol
ogic
al s
ite
(D9
and
G6)
.
D9-
2D
9-3
D9-
4D
9-5L
D9-
6UD
9-6M
D9-
6LG
6-1U
G6-
1MG
6-1L
G6-
2G
6-3
G6-
4G
6-5
G6-
6UG
6-6L
G6-
7
opx
61
22
136
3369
7361
147
43
54
24
cum
22
11
12
11
12
21
cpx
23
43
72
32
hb13
923
2122
1135
414
1420
414
1614
2646
oli
11
1
ilm
8584
4494
8282
329
1423
3324
2763
6034
39
Mgt
2315
1716
4173
3776
9182
169
175
120
109
8650
74
epi
8093
8374
8224
813
56
621
5053
9779
tit
11
55
24
22
23
Cag
11
3
gar
21
11
13
1
apa
all
spi
45
43
25
31
11
33
33
zir
64
85
41
11
11
61
1
TiO
49
49
62
31
11
13
12
tou
bole as found in loams from the Kounan Terrace.In the clay layer below the loam, calcic amphi-bole is predominant with small or trace amountsof orthopyroxene and cummingtonite. Clinopy-roxene is usually scarce or absent. Both orthopy-roxene and cummingtonite show strong dissolu-tion texture (Fig. 3 F).
The samples from the Shinano Town, NaganoPrefecture, are volcanic ashes or sediments de-posited on land. All the minerals are well pre-served (Fig. 3D). The volcanic ashes at the topand bottom of the sequence are composed of am-phibole and orthopyroxene. Most of the othersamples are composed of cummingtonite and cal-cic amphibole. Epidote and clinopyroxene are
usually absent or present in trace amount.
Chemical Compositions of Minerals
The chemical compositions of orthopyroxeneand cummingtonite were analyzed. Their compo-sitional variations are shown in Figs. 4 to 9 withXMg, 100�Mg/(Mg�Fe) atomic ratio. As the dif-ference of XMg in orthopyroxene between thecore and the rim is mostly less than 2 % as XMg(Yokoyama et al., 1997), we chose to analyze thecentral part of the grain.
In a simple volcanic eruption, the XMg value ofminerals in the ash layer should have a strongpeak in the diagram shown in Figs. 4 to 9. How-
26 Kazumi Yokoyama and Michinori Oya
Table 2. Heavy minerals of the samples from two cliffs at the Matsumushi (MT) and Otake (OT) on the Shi-mousa Terrace, Chiba Prefecture. Numerical values shows a number of counted grains of each mineral. Miner-al abbreviations are the same as those in Table 1.
MT-1 MT-2 MT-3 MT-4 MT-5 MT-6 MT-7 MT-8 MT-9 OT-1 OT-2 OT-3 OT4 OT-5
opx 50 9 2 2 2 8 100 1 2 1
cum 3 6 10 3 8 5 2 4 6 11 19
cpx 9 1 13
amp 16 30 32 61 100 50 62 60 51 18 45 43 75 87
oli 13
ilm 39 123 248 190 164 137 70 70 161 18 136 103 56 58
mt 157 222 75 12 10 24 25 20 6 79 27 11 5 9
epi 1 3 4 4 3 14 6 24 7 1 20 21 29 20
tit
Cag 1
gar 2 3 1
apa
all 1 1 2 1
spi 1 1 1 1 2
zir 9 11 4 3 1 1 1 6 5 6 6
TiO 2 1 4 5 4 2 2 2 3 1
tou 1 1 1 1 1
Cummingtonite-bearing Clay in the Kanto Plain: Reconnaissance Study for Body Paste of Ancient Pottery 27Ta
ble
3.H
eavy
min
eral
s of
the
sam
ples
fro
m t
he S
hina
no T
own
(IB
) , N
agan
o P
refe
ctur
e N
umer
ical
val
ue s
how
s a
num
ber
of c
ount
ed g
rain
s of
eac
h m
iner
al .
Min
eral
abbr
evia
tion
s ar
e th
e sa
me
as th
ose
in T
able
1.
IB-1
IB-2
IB-3
IB-4
IB-5
IB-8
LIB
-8U
IB-9
LIB
-9M
IB-1
1IB
-12
IB-1
3IB
-14
IB-1
6IB
-17U
IB-1
9LIB
-20
IB-2
1LIB
-22
opx
5016
95
2442
293
254
5515
cum
760
6010
010
010
060
143
6118
3117
51
cpx
212
433
23
1
amp
2861
306
965
1828
7448
767
8693
100
100
100
100
100
oli
105
ilm
712
2516
125
623
811
226
26
1010
34
mt
226
179
202
123
8650
4115
911
726
210
619
372
7760
5657
2452
epi
24
51
1
tit
Cag
51
gar
apa
8
all
spi
1
zir
33
11
TiO
tou
11
ever, the ratio is more or less variable even in aneruption and bimodal or trimodal distribution issometimes recognized. These characteristics aremainly due to mixing of essential, accessory andaccidental fragments at the eruption. In ShinanoTown where most of the layers are composed of
volcanic ash, orthopyroxene and cummingtonitehave a strong peak at XMg�62-64 (Fig. 9). Suchstrong peaks show that the minerals are essentialones formed in a magma chamber. The XMg ratioof orthopyroxene in two samples, IB-12 & IB-1,varies from 62 to 72. It is probable that they are
28 Kazumi Yokoyama and Michinori Oya
Fig. 3. Back-scattered electron image of orthopyroxene (opx) and cummingtonite (cum). A: fresh orthopyrox-ene partly surrounded by volcanic glass, sample KH-1. B: highly dissolved orthopyroxene, sample KN-8b. C;relatively fresh cummingtonite, sample KH-8. D; fresh cummingtonite, sample IB-4. E; highly dissolvedcummingtonite, sample KN-8c. F; highly dissolved cummingtonite, sample MT-4.
Cummingtonite-bearing Clay in the Kanto Plain: Reconnaissance Study for Body Paste of Ancient Pottery 29
Fig.
4.H
isto
gram
s of
rep
rese
ntat
ive
heav
y m
iner
als
and
com
posi
tion
al v
aria
tion
s of
ort
hopy
roxe
ne a
nd c
umm
ingt
onit
e in
th
e sa
mpl
es f
rom
the
Kou
nan
Terr
ace
(KM
& H
K)
and
Yor
ii s
ite
(YI)
. Ver
tica
l axi
s, N
, is
a nu
mbe
r of
gra
ins
coun
ted
or a
naly
zed.
30 Kazumi Yokoyama and Michinori Oya
Fig.
5.H
isto
gram
s of
rep
rese
ntat
ive
heav
y m
iner
als
and
com
posi
tion
al v
aria
tion
s of
ort
hopy
roxe
ne a
nd c
umm
ingt
onit
e in
the
sam
ples
fro
m a
sm
all
clif
f, t
he K
ouna
nTe
rrac
e (K
H).
Ver
tica
l axi
s, N
, is
a nu
mbe
r of
gra
ins
coun
ted
or a
naly
zed.
Cummingtonite-bearing Clay in the Kanto Plain: Reconnaissance Study for Body Paste of Ancient Pottery 31
Fig.
6.H
isto
gram
s of
rep
rese
ntat
ive
heav
y m
iner
als
and
com
posi
tion
al v
aria
tion
s of
ort
hopy
roxe
ne a
nd c
umm
ingt
onit
e in
th
e sa
mpl
es f
rom
arc
haeo
logi
cal
site
,K
ouna
n Te
rrac
e (K
N).
Ver
tica
l axi
s, N
, is
a nu
mbe
r of
gra
ins
coun
ted
or a
naly
zed.
32 Kazumi Yokoyama and Michinori Oya
Fig.
7.H
isto
gram
s of
rep
rese
ntat
ive
heav
y m
iner
als
and
com
posi
tion
al v
aria
tion
s of
ort
hopy
roxe
ne a
nd c
umm
ingt
onit
e in
the
sam
ples
fro
m th
e S
akad
o ar
chae
olog
ical
site
(D
9 &
G6)
. Ver
tica
l axi
s, N
, is
a nu
mbe
r of
gra
ins
coun
ted
or a
naly
zed.
Cummingtonite-bearing Clay in the Kanto Plain: Reconnaissance Study for Body Paste of Ancient Pottery 33
Fig.
8.H
isto
gram
s of
rep
rese
ntat
ive
heav
y m
iner
als
and
com
posi
tion
al v
aria
tion
s of
ort
hopy
roxe
ne a
nd c
umm
ingt
onit
e in
th
e sa
mpl
es f
rom
the
Mat
sum
ushi
(M
T)
and
Ota
ke (
OT
) on
the
Shi
mou
sa T
erra
ce, C
hiba
Pre
fect
ure.
Ver
tica
l axi
s, N
, is
a nu
mbe
r of
gra
ins
coun
ted
or a
naly
zed.
34 Kazumi Yokoyama and Michinori Oya
Fig.
9.H
isto
gram
s of
rep
rese
ntat
ive
heav
y m
iner
als
and
com
posi
tion
al v
aria
tion
s of
ort
hopy
roxe
ne a
nd c
umm
ingt
onit
e in
the
sam
ples
fro
m t
he S
hina
no T
own
(IB
),N
agan
o P
refe
ctur
e. V
erti
cal
axis
, N, i
s a
num
ber
of g
rain
s co
unte
d or
ana
lyze
d. T
he s
ampl
e G
OP
is
a cu
mm
ingt
onit
e-be
arin
g as
h fr
om t
he M
t. Y
atsu
gata
ke, w
hich
erup
ted
at a
bout
300
ka
ago.
mixture of essential, accessory and accidentalfragments or sediments carried from a drainagebasin.
In the Kounan Terrace, orthopyroxene shows awide compositional variation of XMg from 54 to74, and shows no strong peak (Figs. 4 to 6). Evenwithin a single sample, a wide compositionalvariation is recognized. Cummingtonite alsovaries from 52 to 68, but usually shows a strongpeak at 62 or 64. Another sharp peak is recog-nized in one sample, KH-5: 68 as XMg. A sharppeak in cummingtonite shows that it was derivedfrom a single volcano, whereas wide variation oforthopyroxene shows that the samples are mix-ture, i.e. sediments derived from a drainage basinor products of various volcanic eruptions. If theclay was an aqueous sediment carried by a river,the relatively high proportion of epidote in theclays on the Kounan Terrace could be explainedas a sediment derived from the Kanto and Ashiomountains where epidote is the main heavy min-eral.
The XMg ratio of orthopyroxene from the Saka-do Terrace also shows a wide variation like thosefrom the Kounan Terrace (Fig. 7). Cumming-tonite is mostly strongly dissolved and then it isusually small in amount. As the number of cum-mingtonite analyzed was restricted, a strong peakis not recognized except in one sample, D9-3,which has a XMg peak at 68. Cummingtonite withXMg from 62 to 64 is sporadically recognized inmost of the samples.
Orthopyroxene in the Shimousa Terrace, Chibaprefecture, has a wide compositional variation(Fig. 8). Cummingtonite is also variable. In twosamples, MT-3 & MT-4, s sharp peak is recog-nized at XMg�66. Bimodal or trimodal peaks arerecognized in the some samples; peaks at 68 and62–64 and a weak peak around 54. These data re-flect a mixture of various types of volcanic prod-ucts.
Discussion
The volcanic ash layer has been used as a keybed in the Middle to Late Pleistocene sequence
in the Japanese Islands where successive vol-canic eruptions occurred. More than 400 asheshave been described as a key bed (Machida andArai, 1992). Among them only nine ashes con-tain cummingtonite. Numerous tuffs were de-scribed in the Early Pleistocene sequence fromthe Boso Peninsula (Yokoyama, et al. 1997 &1998). Cummingtonite is rare mineral in thepeninsula. Two cummingtonite ashes of Middleto Late Pleistocene age have been reportedaround the Kanto Plain. They are the ashes de-rived from the Iizuna volcano and a GOP ashfrom the Yatsugatake Volcano. The former asheswere collected from the Shinano Town for thisstudy as a possible source of cummingtonite withan XMg of 62–64. The GOP ash is composed ofcalcic amphibole and cummingtonite (Fig. 9). Asthe GOP ash was formed about 300 ka ago(Machida and Arai, 1992) and cummingtonitehas a peak at 70 as XMg, its source is clearly dif-ferent to that of the minerals in the clays studiedhere.
Modal proportions and chemical compositionsof heavy minerals are important indicators for thecorrelations of ash, tuff and sediment. Even whenwe used selected minerals with similar density,the clay samples from the terraces in the KantoPlain were variable in the modal proportion ofheavy minerals. This is probably due to mixingof essential, accidental and accessory fragment atthe time of eruption, in addition to the supply ofminerals from various drainage sources.
Clays occurring on the terraces in the KantoPlain are locally known as the Jousou Clay orItabashi Clay. It has been suggested that the claywas used for the paste of the pottery found in thearchaeological site. This paper petrologically an-alyzed the samples in and around the clay layersto provide basic data to examine whether or notthe clay was used in pottery manufacture. Theclays are variable in modal proportion. It is, how-ever, clear that the clay layers in the Kounan,Sakado and Shimousa terraces contain more orless cummingtonite which is a specific mineral inthe Japanese Islands. In the Kounan Terrace,cummingtonite is abundant in some samples and
Cummingtonite-bearing Clay in the Kanto Plain: Reconnaissance Study for Body Paste of Ancient Pottery 35
shows strong peak at 62–64 as XMg ratio. In theSakado Terrace, epidote is the most commonminerals, and pyroxene and amphibole are smallin amount, but it is important that the cumming-tonite with 62–64 was more or less recognized.In the Simousa Terrace, Matumushi and Otake,calcic amphibole is predominant and the cum-mingtonite content is small. Cummingtonite with62–64 is confirmed in the Shimousa Terrace.Other peaks at around 68, 66 and 54 are also pre-sent. So far there is no candidate for the sourcerock for the various cummingtonite occurrences.Despite the uncertainty about the source, thesharp composition or bimodal and trimodal na-tures may act as a correlation marker of clay. Thepresence of cummingtonite and chemical compo-sitions will be useful for the correlation betweenthe clays and body paste of the ancient pottery atthe archaeological site.
Nakazato and Nakazawa (2007) concluded thatthe cummingtonite ash in the Kounan Terracewas correlated with NY ash in the ShinanoTown. The ash described by them will be similar
to the samples, KH8, KH-9 and KN-8c. Theythought that the other clay layers were eolian de-posit formed around 180 ka ago. On the otherhand, Suzuki (2001) concluded that the KT-a ashin the Shinano Town was formed at 125–150 ka.Pm-1 ash derived from the Ontake volcano hasbeen recognized in the clay layers from theKounan and Shimousa terraces. The age of Pm-1is around 100 ka. The clays from both theKounan and Shimousa terraces contain spongespicules (Fig. 10), clearly showing sedimentationunder aqueous conditions. The wide composi-tional variation of orthopyroxenes shows thatthey were derived from various sources and indi-cate that the sediments were aqueous. Further-more, moderate amounts of heavy minerals,magnetite, ilmenite and zircon, do not support aneolian sediment deposited on land. If all the claylayers from the three terraces were formed bysimilar process, the abundance of epidote fromthe Sakado Terrace would also supports an aque-ous origin. Even though depositional age of theclay layers is uncertain, most of the clays were
36 Kazumi Yokoyama and Michinori Oya
Fig. 10. Sponge spicules in the clay sample from the Kounan Terrace (A & B) ans Shimousa Terrace (C & D)
essentially formed under lacustrine or marineconditions.
Acknowledgements
The authors are very grateful to Mrs. M. Shi-geoka for her help in modal and chemical analy-ses and heavy mineral separations throughoutthis study.
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