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In the case that the individual tephra formations of frequent

small scale eruptions is obscure in the distal area, the

deposits are regarded as volcanic ash soils.

The thickness of the volcanic ash soils exceeds four

meters on the east foot of Yotei volcano (Fig. 14). Then it

decreases with increasing distance from the volcano, with the

maximum distribution axis of an east-northeast direction. This

distribution pattern coincides with that of Yotei and Shikotsu

tephra layers. Beyond the Hidaka mountain range to the east,

the amount of volcanic ash soils thickness increases once

again to over 150 cm, and then it gradually decreases eastward

again. This variation seems to suggest that the volcanic ash

soils in the western area of the Hidaka mountain range are

largely composed of the particles derived from the small scale

eruptions of Yotei and Shikotsu volcanoes. Whereas the

volcanic ash soils in the eastern area of the Hidaka mountain

range are suggested to be largely contributed to the

non-volcanic particles derived from the bare ground in the

summit area of the Hidaka mountain range and the river floor

from the range.

Nakasatsunai in Tokachi plain (Fig. 11-6) provides the

type locality where samples are obtained from section.

Similarly with Ishikari plain, the analyses of primary mineral

and clay mineral composition are analyzed on the samples, and

33

the results of the analyses are compared with those

of

Ishikari plain (Fig. 15, 16) . The amount of quartz and

biotite, which don't commonly occur in the tephra formatio

ns

in southwestern Hokkaido, is larger in the volcanic ash soil

s

in Tokachi plain than those of Ishikari plain

. The X-ray

diffraction patterns of volcanic ash soils in Tokachi plai

n

show larger peaks than those of Ishikari plain

, which indicate

the presence of quartz, plagioclase and crystalline clay

minerals. Particularly, the peak at ca . 14A of smectite, the peak at ca. 10A of illite, and the peak at ca . 7A of chlorite

is obviously recognized . The reason for the smaller peaks of

Ishikari plain is supposed to be the effect of abundant fi

ne

volcanic glass particles in the clay fraction . Thus, the

composition of constituent particles of volcanic ash soils in

Ishikari and Tokachi plain showed a clear difference

. The

volcanic ash soils in Ishikari plain include a large

proportion of volcanic particles, and those in Tokachi plain

include a large amounts of nonvolcanic materials

. This implies

that the source of the constituent particles of those volcanic

ash soils is different .

It is difficult to draw isopach maps of older deposits

,

owing to their limited distribution and small number of

observation points. The thickness of volcanic ash soils

between several marker tephra formations was measured at some

34

Table 7 Estimated Hokkaido.

ages

of major

tephra layers in southern

Tephra

Estimated age Dating Methods Adopted ages

in Figure 30 Ta-c

Ta-d En-a Spfa 1

Kt-1 Z-M Kt-Tk Kt-3 Ssfa Kt-6 Aso 4

Toya

2.5-3 ka*

8-9 ka*

15-17 ka*

31-39 ka 38-39 ka**

35-45 ka***

44-48 ka**

47-51 ka**

47-53 ka**

z49 ka*

70-90 ka*

90-120 ka*

C, A C,A C C C C C C C C

ST, TL, FT, E, U, KA FT, ST, TL

3 ka 8.5 ka 16 ka 38 ka 39ka 40ka 45ka 47 ka

50ka 70 ka 87 ka 105 ka

A: archeology, C: radiocarbon, FT: fission track, TL: thermo-luminescence, E: ESR, U: uranium-series, KA: potassium-argon, ST: stratigraphic relation with dated tephras.

*: Machida and Arai (1992)

, **: Kato et al. (1995), ***: Yamagata et al. (1989)

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ε6五五〇璽oト⊂ 6五焉y一̀辺(邸図)0980⑩o卜o⑩8響oの︒Noア(⑩図)09880卜o⑩oの響oのoNor

一

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a

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VVJ

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• vv v V/

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• 61,40 enail e c A A G Ail An AAn 4.1 enden .4 l y57 ~ ~1

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614111

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0 20 40

I{Oc{) 20 40 1 1 1

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0

20

20 40 611

1

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0

00 1__L- 1

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20 40 60 00

> 20 m

0 20 40 G0

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---1 1

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20 40 00

00 1

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0 20 40 60

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. 1

I]

- 1

till ( `.' o )

00 0

—1 1 1 _1_1-1-1_1—L

-->63/cm (%O ) 20 40 6000

,v ) 2 ^ene do4

31_

^{4 t}

---Fig. 18 Grain size variation of volcanic ash soils as a functi

on

of depth. Grain size data is expressed as percentage of

total sample. 1: pumice fall deposits

, 2: scoria fall deposits, 3: ash fall deposits

, 4-6: volcanic ash soils (4:

black, 5: brown, 6: dark brown) .

good out-crops in Ishikari and Tokachi plain. In Fig. 17, cumulative thicknesses of the volcanic ash soils are plotted

against the estimated ages of underlying tephra layers. The

tentative ages are shown in Table 7. The medians of ages are adopted in this figure. Each line is shifted at the interval

of one meter equivalent at the left end. The inclination of

each line indicates the rates of sedimentation.

The lines are not straight, indicating fluctuation of the

depositional rates with time. Each line shows different

inclination between localities, even in the same stratigraphic

position. After the Kt-6 eruption, the depositional rates

increased similarly in Ishikari and Tokachi plains. This

tendency is more pronounced in Ishikari plain than Tokachi

plain, though the depositional rates of Ishikari plain exceed Tokachi plain in this unit.

The vertical changes of grain size distribution of the

volcanic ash soils were also examined at Genbu and

Nakasatsunai. The weight percentage of SROCO, clay to silt

(<20 micron), sand (>20 micron), and coarse sand (>63 micron)

for whole samples is plotted against each stratigraphic

position (Fig. 18). The proportion of each class demonstrates a marked fluctuation. The samples of Ishikari are generally more abundant in sand than those of Tokachi. In Ishikari, the units between Kt-1 and Kt-6, and above En-a are abundant in

35

13-4

aQ a) -r;

(~O

20

10

(;O

100

1 2(1

1 1O

(5 1 8() /u I 1'.U.13

O(( )

ISIItI)pc

stage ephI a

3

a

SI)

"(

5d Sc

f)

ra11

SI(a 1 Kt 1—

Z M Kt3 Ss(a

Kt :3

Aso 1

fova (I

20

In

(i(l

All

1U(1

1 2(1

1 I(1

Fig.

19 Oxygen isotope levels of major

curve tephra

(Shackleton, layers.

1987) and stratigraphic

sand. In Tokachi plain, the units above Kt-6 are more abundant

in sand than the lower units. Particularly the units above

Spfa 1 are abundant in coarse sand.

In this manner, the depositional rate and grain size of

the volcanic ash soils showed fluctuation with age. Such

fluctuation of depositional rate and grain size are supposed

to be concerned with such environmental changes in the source

region as volcanic activity and climatic change. As mentioned

later, the volcanic activities in southwestern Hokkaido have

changed as following: a period of relatively dormancy from the

marker tephra Toya to Kt-6; a period of frequent large scale

explosive eruptions of Kuttara and Shikotsu volcanoes from

Kt-6 to Spfa 1; a period of frequent small scale eruptions of

Yotei and Shikotsu volcanoes from Spfa 1 to Ta-d; a period of

large scale explosive eruptions of Tarumai, Usu and Komagatake

volcanoes from Ta-d to historical age.

Figure 19 shows the oxygen isotope curve (Shackleton,

1987) and the ages of some marker tephra formations. The warm

climate of the last interglacial ended about the age of the

Kt-6 deposition. The climatic change after the last

interglacial in Hokkaido was examined by Sakaguchi and Katoh

(1993) using pollen analysis. Particularly, the pollen

assemblage suggesting the glacial maximum was recognized at

two levels: between Kt-6 and Ssfa and nearly below the En-a.

36

The relation between environmental change and variation of

characteristics of volcanic soils was examined.

•A period from Toya to Kt-6 eruptions

During the inter-eruption age between Toya and Kt-6

tephras, the generation rates of volcanic ash soils were slow

and sandy fraction were small both in Ishikari and Tokachi

plain, suggesting that the volcanoes were inactive. A few

large and very large scale eruptions occurred (once on Toya

volcano and twice on Kuttara volcano) but no frequent small

scale eruptions were recognized. At that time the warm climate

of the last interglacial maximum turned a little colder (MIS

stage 5d-5a), but vegetation was supposed to be still rich.

Thus, the amount of the particles derived from the bare ground

around the volcanoes and the alpine region were considered to

be small.

•A period from Kt-6 to Spfa 1 eruptions

Volcanic ash soils above Kt-6 occurred at higher rates and

were composed of a larger proportion of sand than below Kt-6

both in Ishikari and Tokachi. During the inter-eruption period

between Kt-6 and Spfa 1 tephra, large and very large scale

eruptions occurred frequently on Shikotsu and Kuttara

volcanoes, and large amounts of coarse pumice were supplied to

those plains. Abundant pumice crusts and sand size euhedral

minerals are found in volcanic ash soils. Active movement of

37

particles was likely caused by the destruction of the

vegetation due to the deposition of thick pumice falls

. This

period was also coincident with the early cold epoch of the

Last Glacial age (MIS stage 4), when the glaciers on the

Hidaka mountain range advanced most extensively (Porosiri

Stadial: Ono and Hirakawa, 197 5) .

• A period after Spfa 1 er

uption

Volcanic ash soils between Spfa 1 and Ta-d were found with

high generation rates in Ishikari and Tokachi . The proportion

of sand fractions in volcanic ash soils are larger than the

earlier soil layers in Tokachi plain . This period coincides

with the later cold epoch of the Last Glacial age (MIS stage

2), and the glacier on the Hidaka mountain range advanced

again (Tottabetsu Stadial: Ono and Hirakawa

, 1975). It is

supposed that bare grounds extended over the summit area

, and

supplied more eolian dust . During this period, a number of

sand dunes were constructed of reworked En-a and Spfa 1 pumice

fall deposits. During this period , Tokachi plain was likely

covered with poor vegetation under cold and arid condition

(Kimura et al., 1970; Kimura et al., 1972; Ono and Hirakawa ,

1975), which is favorable for producing dust . The depositional

rate of volcanic ash soils , however, was lower than that of

the earlier period, possibly due to strong wind erosion on the

poor vegetation terrain. In Ishikari plain, the proportion of

38

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3

7

sandy fraction increases above En-a. This is probably caused

by the frequent supply of coarse pumice from the Tarumai

volcano.

In this manner, the depositional rate and grain size

distribution of the volcanic ash soils fluctuated in

accordance with the Pleistocene environmental change including

volcanic activity and surface condition owing to climate and

vegetation. The difference of the fluctuation pattern between

Ishikari and Tokachi suggest that each volcanic ash soil in

those plains has a different source of constituent particles .

In summary, volcanic ash soils are supposed to be formed

by eolian particles and residual material of the underlying

tephra formations. The eolian particles were derived from

several different sources. The particles from each source have

a specific grain size (Fig. 20). In Ishikari plain near the

volcanoes, volcanic ash soils are mainly constructed of the

residual material of the underlying tephra formations

, primary

ash falls, and eolian particles derived from bareground around

the volcanoes. Thus, the volcanic ash soils were formed

rapidly during a period of frequent volcanic activity . In

Tokachi plain distant from volcanoes, volcanic ash soils

mainly consists of eolian particles derived from bareground in

the alpine area and river floors. Thus, the volcanic ash soil

was formed rapidly, when the bareground extended under the

39

glacial cold climate. Vegetation, however, became

the colder climate, so that the effect of erosion

deposition of particles

ash soils decrease.

and the generation rate

sparse under

exceeded the

of volcanic

4-3 Tephra stratigraphy of southwestern Hokkaido

4-3-1 Unit of tephra formation and eruptive activity

The tephra stratigraphy of southwestern Hokkai.do is

examined associated with the recognition of volcanic ash

soils. Identification of the tephra formation is carried out

at localities in the proximal area, where many thick and

coarse grained tephra layers are observed. Furthermore, the

absence of volcanic ash soils in each level is confirmed at

many localities in the broad area from proximal to distal.

Plant opal contents, primary mineral composition, and clay

mineral composition are analyzed on several fine-grained

layers which are difficult to be identified by facies whether

they are tephra formation or volcanic ash soils.

Each tephra formations are subdivided into "member" and

"bed" according to the change of litho-facies . There is no

time interval between members and beds as shown in the lack of

soil between them. Each tephra member has a different

distribution lobe, so that it is difficult to observe all the

members at one section. Thus broad areas from proximal to

40

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