ELSEVIER Journal of Volcanology and OTM Research 78 (1997) 5 l-64

Identification of a pair of co-ignimbrite ash and underlying distal plinian ash in the Early Pleistocene widespread tephra in Japan

Hiroki Kamata a, * , Akira Hayashida b, Tohru Danhara ’

a Osaka Cenrer, GeofagicuI Survey of Japan, Govt. Bldg. no.2, Bekkan, 4-l-67, Otemae, Chua-ku, Osaka 540, Japan b Science and Engineering Research Insritute, Doshisha University, Tanabe, Kyoto 610-03, Japan

’ Kyoto Fission-Track Co. Ltd., Omiya, Kita-ku, Kyoto 603, Japan

Received 4 October 1996; revised 10 January 1997; accepted 10 January 1997

Abstract

The Early Pleistocene Azuki and the Ku6C tephras in Japan have been correlated with a compositionally zoned ignimbrite, the Imaichi pyroclastic-flow deposit, using ratios of accurately determined refractive indices of volcanic glass shards and phenocrysts and paleomagnetic directions. Thin, tine-grained, white ash layers at the base of the Azuki and the Ku6C tephras have been correlated with the Imaichi plinian-fall deposit, which underlies the Imaichi pyrocladc-flow deposit near the source Shishimuta caldera on central Kyushu. This indicates that the Azuki and the Ku6C tephras comprise a pair of compositionally zoned co-ignimbrite ash and underlying distal plinian ash. Thus, the initial sequence of physical and chemical changes at the source of the pyroclastic eruption has been retained in the distal widespread tephras, even though the ashes were blown downwind by westerlies as much as 900 km. Identification of a distal plinian ash at the base of a co-ignimbrite ash ensures the stratigraphic correlation of the distal tephra with the source ignimbrite.

Keywords: widespread tephra; co-ignimbrite ash; Plinian fall; pyroclastic flow; Pleistocene; lmaichi pyroclastic flow; Azuki tephra; Ku6C

tephra; Shishimuta caldera

1. Introduction

During explosive ignimbrite-forming eruptions much of the pyroclastic materials may fail to be included in the resulting ignimbrite for total eruption volumes. In particular, fine-grained ash materials can be elutriated from the eruption column, dispersed mainly by high-altitude wind, and deposited as a co-ignimbrite ash in distal areas away from source caldera (e.g., Sparks and Walker, 1977). Machida

* Corresponding author. Fax: + 81-6-941-5378; e-mail: ka-

mata@gsj.go.jp

and Arai (1983) reported five widespread Quatemary tephras that were produced by gigantic caldera-for- rning eruptions in Japan and Korea/China. These tephras are of great significance in volcanology as well as in Quatemary stratigraphic research because they have very large volumes and are distributed as far as 1500 km from the source vents (Machida, 1984). Many of the other widespread tephras in the world were produced by large caldera volcanoes (e.g., Machida and Arai, 1992). Such tephras have been classified as co-ignimbrite ash and distal plinian ash according to the difference in their modes of eruption and transportation.

0377-0273/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved.

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52 H. Kamata et al./ Journal of Volcanology and Geothermal Research 78 (1997) 51-64

The term ‘co-ignimbrite ash’ is used to designate an ash fall deposit of the fine-grained material that occupies the upper part of convecting eruption col- umn during ejection of a huge pyroclastic flow (e.g., Machida and Arai, 1976; Sparks and Walker, 1977). It is characterized by abundant fine-grained glass shards and by widespread distribution more than 1000 km from the source of deposits that are typi- cally less than a few tens of centimeters in thickness. Calculated volumes of co-ignimbrite ash deposits are often equal or exceed those of the associated pyro- elastic-flow deposits that were emplaced around the source caldera (e.g., Sparks and Walker, 1977; Sparks and Huang, 1980; Machida, 1984). The term ‘distal plinian ash’ is used to designate fine-grained pumice-fall deposits that show a gradual decrease in grain size and thickness with distance from the plinian eruption vent (e.g., Kobayashi et al., 1968). These deposits are commonly distributed in a lobate shape on the downwind side of the source volcano. Fier- stein and Hildreth (1992) called this type of ash deposit ‘co-plinian ash’ in analogy with the co- ignimbrite ash. We prefer to use the term ‘distal plinian ash’ because its emplacement mechanism is considered to be the same as that of plinian-fall deposit even though it comprises much finer ash material which is carried farther away compared to a proximal plinian deposit.

Most widespread tephras studied previously (e.g., Machida and Arai, 1976, 1983; Sparks and Walker, 1977) have been considered to represent co- ignimbrite ash deposits. Few evidence has been pre- sented for the identification of the distal plinian ash associated with a co-ignimbrite ash (e.g., Sparks and Huang, 1980). There are two reasons for this: The first is that the volume of a caldera-forming pyro- elastic-flow deposit is usually about one order of magnitude larger than that of the underlying plinian- fall deposit. As a result, the distribution area of a co-ignimbrite ash is far greater than the lobate distri- bution of its distal plinian ash. In addition, the chemical composition of the essential material in a pyroclastic-flow deposit generally shows no signifi- cant difference from that of a plinian-fall deposit.

We have identified separate distal plinian ash and co-ignimbrite ash deposits in the Early Pleistocene Azuki and Ku6C tephras in Japan. To correlate these tephras with the source ignimbrite, Kamata et al.

(1994b) examined compositional zonation by the ratios of refractive indices of volcanic glass shards and phenocrysts. Hayashida et al. (1996a) showed that the Azuki and the Ku6C have paleomagnetic directions consistent with the Imaichi pyroclastic- flow deposit. In the course of these tephrostrati- graphic works, a distal plinian ash has been identi- fied at the base of the Azuki and the Ku6C tephras and was correlated with the source Imaichi plinian- fall deposit. Likewise, the overlying co-ignimbrite ash in the Azuki and the Ku6C tephras has been correlated with the compositionally zoned Imaichi pyroclastic-flow deposit that overlies the Imaichi plinian-fall deposit. This paper demonstrates that the pair of distal plinian ash and overlying co-ignimbrite ash identified in the widespread tephras was dis- persed as far as 900 km away from the source caldera and deposited in a shallow-marine environ-

Fig. 1. Location of exposures of the Azuki tephra and its correla-

tive volcanic ashes (closed circles). I = Azuki tephra of the Osaka

Group (this study); 2 = Kisen tephra of the Kobiwako Group

(Hayashi, 1974); 3 = Isobe tephra of the Sakishima Formation

(Machida et al., 1980); 4 = Ku6C tephra of the Kazusa Group

(this study). Circle with a dot = Shishimuta caldera (Kamata,

1989a); 0 = major cities; dotted line shows the approximate

distribution of the Imaichi pyroclastic-flow deposit from Kamata

et al. (1994a).

H. Kamata et al. /Journal of Volcanology and Geothemd Research 78 (1997) 51-44 53

2. Stratigraphic background

The Azuki tephra is one of the key widely dis- tributed volcanic ash beds in central Japan (l-3 in Fig. 1). It is intercalated in alternating shallow-marine clayey sediments and fluvio-lacustrine deposits of the Early Pleistocene Osaka Group (Huzita et al., 1951; Ishida et al., 1969). The Aztiki tephra is a stratified fine-grained volcanic ash with light brown

lmaichl I Kagonodail

- - - _ _ _ _ _ _ _ _

weakly e6

welded - - - - - _ _ _ _ _

04 non-

welded i

or light purple color (Takaya, 1963; Yokoyama, 1969; Itihara et al., 1975; Yoshikawa, 1984). It is 30-50 cm thick near Osaka and can be subdivided into five different lithological units (A-E in Fig. 2): unit A is a white volcanic ash of fine-sand size (l/8-1/4 mm) with a thickness of 1.5 cm; unit B consists of gray volcanic ash and pumice of medium-sand size (l/4-1/2 mm) with a thickness of 0.8 cm; unit C is a white volcanic ash of fine-sand

II marine clay

0 sample point

Azuki

llbukinol KUGC

gray ash

Cl3 5

D gray ash

f

[Okobol

white fine ash

pale-gray fine ash

od

._--_

gray OC fine ash

.____

I pale-pink

@b fine ash

Fig. 2. Standard stratigraphic columns, occurrence and sampling positions of the Imaichi pyroclastic-flow and plinian-fall deposits, the

Azuki tephra and the Ku6C teplsra. Location of the Imaichi deposits is shown as B in Fig. 5. MP = average of maximum three largest

pumice; ML = average of maximum three largest lithic clasts.

54 H. Kamata et al./Joumal of Volcanology and Geothemzal Research 78 (1997) 51-64

size with a thickness of 1.5 cm; unit D consists of gray or light purple volcanic ash and pumice of medium-sand size with a thickness of 9 cm; and unit E consists of gray volcanic ash and pumice of medium-sand size with a thickness of 35 cm. The Azuki tephra is composed of volcanic glass with

lmaichi

9M

8M

7

6

5

4

3P

2P

IP

Azu ki

E

D

C

B

A

KUGC

d

C

b

a

Ratios of whole components

subordinate amounts of plagioclase, orthopyroxene, clinopyroxene, apatite, opaque minerals and very small amount of green hornblende (Figs. 3 and 4).

Machida et al. (1980) suggested that the Azuki tephra is correlative with the Ku6C tephra of the Kazusa Group in eastern Japan (4 in Fig. 1) based on

n = 200

% 0 10 20 30 40 50 60 70 80 90 100

q plagioclase 1 heavy minerals

Fig. 3. Ratios (%) of glass, plagioclase and all heavy minerals in the Imaichi pyroclastic-flow and plinian-fall deposits, the Azuki tephra and the Ku6C tephra. Grain sizes of analyzed samples are l/16-1/8 mm. Numerals and/or alphabets represent sampling positions on the

stratigraphic columns as shown in Fig. 2.

H. Kamnta et al./ Journal of Volcanology and Geothermal Research 78 (1997) 51-64 55

mineral composition and the refractive index of glass shards. The Ku6C tephra is a white to light gray fine ash about 20 cm thick. The Ku6C tephra shows four different lithological units (a-d in Fig. 2): unit a is a white volcanic ash of very-fine-sand size (1 / 16- l/8

mm) with a thickness of 1 cm; unit b is a pale pink volcanic ash of very-fine-sand size with a thickness of 3 cm; unit c is a light gray volcanic ash of very-fine-sand size with a thickness of 4 cm; and unit d is a pale gray volcanic ash of very-fine-sand

Ratios of heavy minerals n q 200

Fig. 4. Ratios (%I of orthopyroxene (opx), clinopyroxene (cpx), green hornblende (gr-hb), brown hornblende (br-hb), biotite (bi), apatite

cap) and opaque minerals (opq) in the In&hi pyroclastic-flow and plinian-fall deposits, the Azuki tephra and the Ku6C tephra. Grain sizes

of analyzed samples are l/16-1/8 nun. Numerals and/or alphabets represent sampling positions on the stratigraphic columns as shown in Fig. 2.

56 ff. Kamata et al. /Journal of Volcanology and Geothermal Research 78 (1997) 51-44

‘-- ‘~180 --Y----___+~o A-

‘O& @D _--CA

\ 1’ &

.-/ / _--I \ /--.-_.

T------X. 1’ ~~~,__-.-’

f’ /’ Yb

‘i 0380

I

‘\ !’ En ’ y Beppu Bay

\ _ -.- _3o_u

Agt

”

” i? i ” .---__.A&

,,/ -1W”Bougutr anomaly contours in mgal

~-_JS~elevation in meters

ET .zi,{iii distribution of Imaichl p.f.

(A-J: sample locations)

o drill core of lmaichi p.f.

L Fig. 5. Distribution and surface elevation of the Imaichi pyroclastic-flow deposit. A-K = sampling locations. Dashed line shows the source

vent of the Imaichi pyroclastic-flow deposit (Shishimuta caldera; Kamata, 1989a). Distribution of the Imaichi is from Kamata et al. (1994a).

Bouguer gravity anomaly is in mgal and assumed density p is 2.3 g/cm3 (Komazawa and Kamata, 1985). Circle with a dot = drill holes

where the Imaichi pyroclastic-flow deposit (welded tuff) is present; attached numbers show elevations in meters. Triangle = volcanoes

younger than Middle Pleistocene. Am = Amagoidake; As = Aso-Nakadake; Ha = Haneyama; Kj = Kujusan; Wa = Waitazan; Yf =

Yufudake; Ag = Amagase; An = Asono; Bm = Bungomori; Bp = Beppu; He = Heikezan; Im = Imaichi; Km = Kaguma; KS =

Kasbinomure; Me = Miemachi; Mi = Miyanoharu; Mz = Mizuwake-toge; Ny = Nagayu; Oi = Oita; Ry = Ryuzan; Ss = Shishimuta; To =

Toyooka; Tr = Taketa; Lrnz = Uchinomaki; I% = Yabakei.

H. Kamata et al./ Journal of Volcanology and Geothemd Research 78 (1997) 51-64 51

size with a thickness of 8 cm. The Ku6C tephra is composed of volcanic glass with subordinate amounts of plagioclase, orthopyroxene, clinopyroxene, opaque minerals, and green hornblende (Figs. 3 and 4).

The Imaichi pyroclastic-flow deposit is one of the key ignimbrites on Kyushu Island in western Japan (Fig. 1; Kamata, 1989b). The Imaichi pyroclastic- flow deposit occurs mostly as a densely welded tuff (One, 1963; Hoshizumi, 19931, although in some places non-welded tuffs occur at the base and top of the deposit (Fig. 2). Essential obsidian lenses in the Imaichi are pyroxene dacite-andesite and show chemical compositional zonation. The Imaichi con- tains phenocrysts of plagioclase, orthopyroxene, clinopyroxene, opaque minerals, and a very small amount of green hornblende and apatite (Figs. 3 and 4). Obsidian lenses give ages of 0.85 + 0.03 Ma by the K-Ar method (NEDO, 1989). The source vent of the Imaichi pyroclastic flow has been inferred to be

in the buried Shishimuta caldera (Figs. 1 and 5) based on the distribution and lithology of the Imaichi welded t&f, drill-hole data, and a negative gravity anomaly (Kamata, 1989a; Kamata et al., 1994al. The Imaichi is distributed over an area 60 km in diameter and has a thickness of 5-40 m on the surface and > 150 m in drill holes; the original distribution area of the Imaichi has been estimated as 3000 km2 and its original bulk volume is thought to be 90 km3 (Kamata et al., 1994a). Given the size of the Imaichi pyroclatic-flow deposit, it is likely that the Imaichi eruption produced extensive co-ignimbrite ash in a downwind direction extending as far as eastern Japan.

The Imaichi pyroclastic-flow deposit is underlain by a plinian-fall deposit of about 2-5 m thickness 20-30 km east of the caldera (e.g., B in Fig. 5; Hoshizumi et al., 1992). No evidence of a significant time interval, such as a humic soil, has been found between the plinian-fall deposit and the overlying

Ku&

,AZUki

Fig. 6. (a> Site mean paleomagnetic directions and circles of 95% confidence calculated for the Imaichi pyroclastic-flow deposit.

A-J = sampling locations shown in Fig. 5. (b) Mean paleomagnetic directions and circles of 95% confidence calculated for the Imaichi

pyroclastic-flow deposit, the Azuki tephra and the Ku6C tephra, plotted on the lower hemisphere of equal-area projection. Antipodal

directions of the present geomagnetic field (squares) and the geocentric axial dipole field (crosses) in western Japan (Central Kyushu), central Japan (Kinki), and eastern Japan (Bose) are shown in the diagram.

58 H. Kamata et al. /Journal of Volcanology and Geothermal Research 78 (1997) 51-64

pyroclastic-flow deposit (Fig. 2). Mineral assem- blage and chemical composition of the pumice in the plinian-fall deposit are the same as those in the basal part of the Imaichi pyroclastic-flow deposit. There- fore, we interpret the plinian-fall deposit as originat- ing from a high eruption column just prior to the effusion of the Imaichi pyroclastic flow (e.g., Walker, 1981b).

Magnetostratigraphic studies on the Osaka and Kazusa groups (Ishida et al., 1969; Niitsuma, 1976; Maenaka et al., 1977) showed that the Azuki and the Ku6C tephras are both assigned with the late Matuyama Chron between the Jaramillo Subcron and the Brunhes/Matuyama boundary (0.99-0.78 Ma; Shackleton et al., 1990). The reversed magnetic po- larity (One, 1963) and the K-Ar age of the Imaichi pyroclastic-flow deposit are consistent with its em- placement within the same time interval. More de- tailed magnetic measurements (Kamata et al., 1994a) revealed that the thermoremanent magnetization of the Imaichi pyroclastic-flow deposit is characterized by a reversed magnetic direction with declinations deflected slightly to the east (Fig. 6a). Hayashida et al. (1996a) showed the directions of depositional remanent magnetization of the Azuki and the Ku6C tephras are consistent with the paleomagnetic data of the Imaichi pyroclastic-flow deposit (Fig. 6b).

3. Compositional zonation

The Imaichi pyroclastic-flow deposit shows a dis- tinctive compositional zonation throughout the col- umn, and the correlative widespread tephras reflect the same zonation pattern. Two parameters, refrac- tive indices of glass and orthopyroxene, are the most critical for correlation. Fig. 7 shows a histogram of refractive index of glass in the Imaichi pyroclastic- flow and plinian-fall deposits, the Azuki tephra, and the Ku6C tephra. The indices of refraction of glass in the Imaichi plinian-fall deposit (lP-3P) resembles those in the lower part of the Imaichi pyroclastic-flow deposit (4-7). In the upper part of the Imaichi, however, glass with a higher index is admixed with a predominance of glass with a lower index (8M, 9M and 9s). In case of the Azuki tephra, glass with a lower index of refraction predominates throughout the column (A-E), whereas glass with a higher

index occurs sporadically in the upper part (D and El. The Ku6C tephra shows the similar pattern. A felsic magma is generally characterized by glass of a lower index refraction and a mafic magma by a higher index (e.g., Kittleman, 1963; Deer et al., 1978). The vertical changes of glass in the strati- graphic columns suggest that a felsic magma pre- dominated in the Imaichi plinian-fall deposit and in the lower half of the Imaichi pyroclastic-flow de- posit. By contrast, in the upper horizon of the Imaichi pyroclastic-flow deposit, a mafic magma was mixed with the felsic magma that is supposed to have overlied the mafic magma in the pre-eruptive magma chamber. The same pattern is reproduced in both of the Azuki and the Ku6C tephras.

The refractive index of ortbopyroxene provides another means for correlation (Fig. 8). Basically, orthopyroxenes with higher indices predominate throughout the column. The Imaichi plinian-fall de- posit shows the higher index only (lP-3P). In the lower part of the Imaichi pyroclastic-flow deposit (4-61, small numbers of lower index orthopyroxene are added to the predominant group of higher index. At the top of the Imaichi pyroclastic-flow deposit (9S1, the population of lower index surpasses that of the higher index. In case of the Azuki tephra, the higher index also predominates throughout the col- umn, but lower index orthopyroxene is detected in the upper part of the column (C-E). The Ku6C tephra shows a similar pattern. A higher index or- thopyroxene generally represents a felsic magma and a lower index a matic magma (e.g., Deer et al., 1978). Therefore, the vertical changes in the index of orthopyroxene suggest that a felsic magma predomi- nates in the Imaichi plinian-fall deposit. In the lower half of the Imaichi pyroclastic-flow deposit, small amounts of mafic magma were added to the felsic magma. In the uppermost horizon of the Imaichi pyroclastic-flow deposit, mafic magma exceeds the felsic magma. A similar pattern is observed in both the Azuki and the Ku6C tephras. In contrast to the refractive indices of glass, however, the orthopyrox- ene data show that the mafic magma is already detectable in the lowermost part of the Imaichi pyro- elastic-flow deposit (4 in Fig. 8). This difference probably reflects the fact that the glass was more sensitive and easily subject to compositional change of the liquid melt than orthopyroxene.

H. Kamta et al. / Joumal of Volcanology ana’ Geothermal Research 78 (1997) 51-64 59

4. Co-ignbnbrite ash and distal plinian ash

Refractive indices of glass and orthopyroxene suggest that the Imaichi plinian-fall deposit should be correlated with the fine-grained, white ash layer at the base of the Azuki tephra (A and B in Fig. 2) and the Ku6C tephra (a in Fig. 2). The overlying Imaichi pyroclastic-flow deposit should be correlated to the upper units (C-E and b-d). Thus, both the co- ignimbrite ash and distal plinian ash can be identi-

volcanic glass

lmalchi

-n-l

1.51 1.52 1.53 1.54

n=50

n=50

1=50

1=50

7=2

fled in the Azuki and the Ku6C tephras. There are other lines of evidence that support our differentia- tion of the distal plinian ash from the co-ignimbrite ash.

The ratios of heavy minerals indicate that the Imaichi plinian-fall deposit (lP-3P in Fig. 4) con- tains more opaque minerals than the Imaichi pyro- elastic-flow deposit (4-9M). The A and B units of the Azuki tephra contain more opaque minarals than the overlying C, D and E units. Similarly, the basal

AZ&i 1.51 1.52 1.5

E

D

C

B

A L Ku6C

d

C

b

li

L a

l- 1 .5

;3 1.54

_ n=50

.__ _ _ .n=50

n=50

n=50

n=50

I

ty-ii--l:~~ 1 1.52 1.53 1.54

Fig. 7. Histogram of refractive index of glass in the Imaichi pyroclastic-flow and plinian-fall deposits, the Azuki tephra and the Ku6C tephra. Numerals and/or alphabets on the left side of column represent sampling positions on the stratigraphic columns as shown in Fig. 2. Measurement method for refractive index is described in Danhara et al. (1992) and Kamata et al. (1994b). IP-3P = Imaichi plinian-fall deposit; 4-9s = Imaicm pyroclastic-flow deposit; S = Scotia; M = matrix. The ‘n’ on the right side shows the number of measured grams.

unit a of the KuBC tephra is more abundant in erals in the source plinian-fall deposit is reproduced opaque minerals than the overlying b, c and d units. in the correlative units of the distal plinian ash. We infer that the greater abundance of opaque min- The presence of accessory obsidian fragments

Imaichi or thopyroxene

9s

QM

85

8M

7

6

5

4

3P

2P

1P L k

1.70 1.71

r-30

l-30 77 1’ *IL

72

n=30

n=30

n=30

1-30

1=30

n-30

7-30

I=30

1=25

Aruki 1.70

n n=l

1*71 1.72

E

D

c

B

A D

KU6C

d

c

b

a ”

n=30

n=30

r-l= 30

I-l=31

n=30

m 0 1.71 ;

n-30

n-30

rl=30

n=30 72

Fig. 8. Histogram of refractive index of oxthopyroxene ( y) in the Imaich pyroclastic-flow and plinian-fall deposits, rhe Azuki teplm and the

K&C tephra. Numerals and/or alphabets on the left side of column represent sampling positions on the sualigraphic columns as shown in

Fig. 2. Measurement method for refractive index is described in Danhara er al. (1992) and Kamtia et al. (1994b). JP-3’ = Imaichi

plinian-f&U deposit; 4-9s = hnaicbi pyroclastic-flow deposit; S = scoria; M = matrix. The ‘n’ on the right side shows the number of

mea5ured grains.

H. Kamuia et al. /Journal of Volcanology and Geothemd Research 78 (1997) 51-64 61

provides additional evidence (Fig. 9). Accessory ob- sidian has been interpreted as the product of magma which erupted just prior to the climactic eruption of the plinian fall and pyroclastic flow, as exemplified at Long Valley caldera and Crater Lake caldera (e.g., Hildreth and Mahood, 1986; Suzuki-Kamata et al., 1993). The Imaichi plinian-fall deposit contains more obsidian fragments than the overlying pyroclastic- flow deposit (Fig. 9). Likewise, the base of the Azuki tephra (A and B) contains more obsidian than the overlying units (C-E). The Ku6C results show no disagreement with the above results.

The difference in ratios of two types in morphol- ogy of volcanic glass represents the third type of

evidence. The basal two units of the Azuki tephra contain abundant micropumice-type glass and only small amounts of bubble-wall-type glass (A in Fig. 10). In contrast, the upper three units of the Azuki tephra contain almost equivalent amount of micro- pumice-type glass and bubble-wall-type glass (e.g., C in Fig. 10). The same pattern is observed between units a and b of the Ku6C tephra (Fig. 10).

Vertical changes in the sizes of grains are the forth type of evidence. The Imaichi plinian-fall de- posit shows reverse grading based on the average sizes of the maximum three largest pumice and maximum three largest lithic clasts (lP-3P in Fig. 2). In many cases grain sizes of proximal plinian-fall

Azuki

lmaichi / F

24m proportion of obsidian in whole components (%I

m3P stratigraphic posltion

L&B In Fig.5 Lo%.A distal plkan ash distance to vent (km) 25 40 450 900

glass (%) 79.0 86.4 94.4 plagioclase 15.3 10.2 4.6 heavy min. 5.7 3.4 1.0

Fig. 9. Correlation of the Imaichi pyroclastic-flow and plinian-fall deposits to the Azuki tephra and the Ku6C tephra, and their component change with distance from source vent. Numerals on the left side represent proportions (96) of obsidian in whole components. Numerals and/or alphabets on the right side of column represent sampling positions on the stratigraphic columns as shown in Fig. 2. F = felsic magma; M = matic magma; m = trace of mat3 magma.

62 H. Kamata et al./Joumal of Volcanology and Geothermal Research 78 (1997151-64

deposits are reported to be reversely graded (e.g., Aramaki, 1956; Lirer et al., 1973; Walker, 1981a; Kobayashi et al., 1983). Hayakawa (1983) reported that the distal Osumi plinian-fall deposit emplaced 240-500 km from the source retains the same re- verse grading observed in the proximal deposit. Likewise, the base of the Azuki tephra (A in Fig. 2) is composed of finer ash than that in the upper part (B). This may be correlative with the reverse grading of the Imaichi plinian-fall deposit.

The difference in intensity of remanent magneti- zation is the fifth type of evidence. The remanent magnetization of units A and B of the Azuki tephra is weak and unstable compared to units C, D and E. Likewise, the magnetization of unit a in the Ku6C tephra is weaker and less stable than units b, c and d.

These differences may reflect differences in content and grain size of the magnetic minerals between the plinian-fall deposit and the pyroclastic-flow deposit (Hayashida et al., 1996b).

These results lead to a detailed correlation of the individual units in the Imaichi, the Azuki, and the Ku6C (Fig. 9). The Imaichi plinian-fall deposit is correlated with the basal two units A and B of the Azuki tephra and the basal one unit a of the Ku6C tephra. These units are derived from a felsic magma only. The lower part of the Imaichi pyroclastic-flow deposit (4-7) is correlated with the unit C of the Azuki and the unit b of the Ku6C. These units are derived from a felsic magma with a trace of mafic magma. The upper part of the Imaichi pyroclastic- flow deposit (S-9) is correlated with the two units of

Fig. 10. Photomicrographs of constituent grain minerals enclosed in ultraviolet cure adhesives on a slide glass. Stratigraphic position of

samples: a = unit a of the Ku6C tephra; b = unit b of the Ku6C tephra; A = unit A of the Azuki tephra; C = unit C of the Azuki tephra. Grains: b = bubblewall-type volcanic glass; m = micropumice-type volcanic glass; pl = plagioclase; opx = orthopyroxene; cpx = clinopyroxene: ob = obsidian.

H. Kamata et al./ Journal of Volcanology and Geothermal Research 78 (1997) 51-64 63

the Azuki (D and E) and the Ku6C (c and d). These units are derived from a felsic magma with a subor- dinate or equivalent amount of mtic magma.

5. Correlation of widespread tephra with source ignimbrite

Component analyses of the Azuki and the Ku6C tephras demonstrate that the two stages of source eruption, the plinian-fall and pyroclastic-flow stages, are reproduced in the distal tephras even though they were blown downwind by westerlies as much as 900 km. The bulk proportion of components in the tephra has changed systematically with distance from the source Shishimuta caldera (Fig. 9). The proportion of glass increases with distance, whereas those of pla- gioclase and heavy minerals decrease. We interpret them as the result of density segregation as well as grain size segregation during aerial dispersion by high-altitude wind such as westerlies. Although the Azuki and the Ku6C tephras are intercalated in shallow-marine clayey sediments, the proportion of components seems to have been less subject to sub- aqueous segregation that might have disordered ini- tial proportion of tephra. Instead, the emplacement in relatively quiet or shallow water may have given the advantage of preserving small amounts of underlying distal plinian ash and the initial sequence of compo- sitional zonation in the overlying co-ignimbrite ash. Even in a deep-sea environment, for example, the bimodality in a primary volcanic feature distorted only slightly by redistribution of the ashes by marine processes (Sparks and Huang, 1980). Identification of a pair of a co-ignimbrite ash and a distal plinian ash in a single widespread tephra ensures the strati- graphic correlation of the distal tephra with the source ignimbrite.

6. Conclusions

(1) The Early Pleistocene Azuki tephra in central Japan and the Ku6C tephra in eastern Japan are correlated with the compositionally zoned Imaichi pyroclastic-flow deposit which erupted from Shishimuta caldera in western Japan.

(2) The Imaichi plinian-fall deposit, which under- lies the Imaichi pyroclastic-flow deposit, is similarly

correlated with the thin, fine-grained, white ash lay- ers at the base of the Azuki and the Ku6C tephras. This indicates that the Azuki and the Ku6C tephras comprise a pair of co-ignimbrite ash and underlying distal plinian ash.

(3) Systematic and detailed examination of a widespread tephra, including particularly its base, can clarify the initial sequence of physical and chem- ical changes at the source of the pyroclastic eruption, even though the tephra has been blown downwind by high-altitude wind as much as about 1000 km. Iden- tification of a distal plinian ash at the base of a co-ignimbrite ash ensures the stratigraphic correla- tion of the distal widespread tephra with the source ignimbrite.

Acknowledgements

We were grateful to Tohru Yamashita, Hideo Hoshizumi and Keiji Takemura for their support during field works and measurements. A part of this work was financially supported by the organizing committee of the 29th International Geological Congress and the grant-in-Aid for Scientific Re- search (07640610) from the Japanese Ministry of Education, Science and Culture. The manuscript was greatly improved by review by Kenneth L. Verosub and Richard V. Fisher.

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