cummingtonite-bearing clay in the kanto plain: reconnaissance … · 2013. 6. 14. · with similar...

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: [email protected] 2 Saitama Cultural Deposits Research Corporation,

4–4–1 Funakidai, Kumagaya, Saitama, 369–0108 Japan E-mail: [email protected]

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-


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


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


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-


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


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.


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.


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.


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.


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.


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.


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


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


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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