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,**.  0  -  ,**.  2  -* Pyroclastic Deposits of Unknown Source, Discovered in the Tamagawa Welded Tu#s,
West of Hachiman-Tai Volcano, NE Japan
Kazuhiko KANO῍ and Takeshi OHGUCHI῍῍
Tamagawa Welded Tu#s comprise the late Pliocene Kurasawayama Welded Tu# and the early Pleistocene
Bunamori-bokujo Welded Tu#, which represent the Plio-Pleistocene regional felsic magmatism in NE Japan Arc.
They are thought to have issued from the same caldera buried below Akita Yake-Yama volcano, though themultiple drillings into the caldera have recovered no or little materials of the two welded tu#s and a time gap of
ca. + million years is present between the two welded tu#s. This paper describes the Taihei-Ko and Haruyama
pyroclastic deposits that individually occur between the two welded tu#s at a road cut on the northern side of
Taihei-Ko (Lake) and at a cutting wall of Haruyama on the eastern side of Tazawa-Ko (Lake). The deposits atthe two locations are -* km apart but contain the same crystal fragments of quartz, plagioclase, orthopyroxene,
hornblende, magnetite, and extremely minor biotite. The Taihei-Ko pyroclastic ﬂow deposit and the Haruyama
pyroclastic deposit are +..*., Ma and +.1*., Ma, respectively in ﬁssion-track age for zircon. They are likely
to have derived from the same source at the same time or shortly after another and may represent a missing activity
in the source area of the Kurasawayama Welded Tu#, to which they are similar in constituents. The Taihei-Ko
pyroclastic deposit comprises a pyroclastic surge deposit and overlying pyroclastic ﬂow deposit. The Taihei-Ko
pyroclastic surge deposit is +. m thick and well stratiﬁed with rip-ups of the underlying rhyolitic Kurasawayama
Welded Tu#. The Taihei-Ko pyroclastic ﬂow deposit is over 0 m thick and entirely welded. Stratiﬁcation andgrading are not obvious. The Haruyama pyroclastic deposit ﬁlls an incised valley of the Kurasawayama Welded
Tu#, and is vaguely stratiﬁed across the thickness of over ,* m. Compared with the Taihei-Ko deposit, theHaruyama deposit is, however, relatively poor in ﬁnes and contains many lithic fragments, so is likely to be more
proximal to the source. The stratiﬁcation of the Haruyama deposit inclines outward from Tazawa-Ko. The
ﬂow direction of the Taihei-Ko pyroclastic surge is from the south to the north and -* km to the south, there is
Tazawa-Ko. In addition, a deposit potentially correlative to the Haruyama deposit occurs between Tazawa-Ko
and Taihei-Ko. These facts collectively suggest that an alternative source of the Taihei-Ko and Haruyama
deposits is Tazawa-Ko. Because of the dimension /.2 km across and .,- m deep and absence of conﬁrmed
deposits, it has been open to question for a long time whether the lake is a caldera or an impact crater.
Key words : Tamagawa Welded Tu#s, Tazawa-Ko, caldera, pyroclastic ﬂow, pyroclastic surge, Early Pleistocene*$,-./012345678 (Fig. +). 9:'+ῌ ῐ ῎ ῑ ῏

!"#$%&!' ()*ῌ+)G-*/ῌ2/01 B4HIJ + KL +ῌ+ B4HMN 1
OPQRSTUVW%.XYUVZ[
Institute of Geoscience and Geoinformation, Geological Survey of Japan, AIST, Tsukuba Central 1,+ῌ+, Higashi +-chome, Tsukuba -*/ῌ2/01, Japan.
G*+*ῌ2/*, \I]^_`a +ῌ+\ _b_cd_ZefghcdijklUVmn

1;<=>$?@8AB$=CDE@F(TableResearch Institute of Materials and Resources,
Faculty of Engineering and Resource Science, Akita
University, +ῌ+, Tegatagakuen-cho, Akita *+*ῌ2/*,,
Japan.
Corresponding author : Kazuhiko Kanoe-mail : kazu.kano@aist.go.jp

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mX_X`wde <mX_`de x(E <¡un X < / D(IBDoyz= pq rs  de{|}~ DE «< (? (¬­(? n( srNO( ® ¯ 121°±+(²³´µ¦? 

¶·<h (+321 a ; +321 b)

Fig. +. Distribution of Tamagawa Welded Tu#s and related rocks (modiﬁed from Sudo, +321 b).
Tamagawa and Kowase Rivers.

 Bhv

J¸Junction ofIJKL> MNO $PQRST1UVWGX

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?@AB C D() +32/E FGH9AB

Table +.

Correlation of Tamagawa Welded Tu#s and related rocks with the rock units ever proposed.

Table ,. Stratigraphy of Tamagawa Welded Tu#s and related rocks (Data adopted
from Sudo, +321 a, b, +33,).

8

º/01312»¼

286  

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 789:; <= >?5@A BF)HC)D!E G I&'JKLM NO(

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 JlA$ ( G 0sH tuQ/lmI )# v=lm5w

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u%x(p yz{ !p |%x(p}z{JlAQ/0(~)$`(%1234 +31- +32* ; 123 +311 ;  +321b `E +-* km- /* km- (P/0 (Table ,).
 ( G I

K-Ar t  +.1,./ Ma, *.1+./ Ma (% (Table ,).
Q!Q   =gJ $(  | , Ma, + Ma (P  +321 b

G Iῌ

S# I

_w

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Fig. ,. Locality of the outcrop of the Taihei-Ko
pyroclastic surge and pyroclastic ﬂow deposits.
Topograhic map adopted from + : ,/,*** quadrangle map Taihei-Ko (Geographical Survey
Institute of Japan, +321).

1234 +31- ;  +321 b b +** !  %#O+(`(%- -ῌ+"@ "@#$%I& '
0($ (Fig. ,, Fig. -). ` ( J[)Q/ * +. m

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

V¡

¢O "@ V¡(£F %`¤)  N¥)¦§JH*/  0 m ¢¥  ¢O "@(£F %¤) (Fig. .).
`"@

(¨#!EBCQ/

Fig. -. Sketch map of the outcrop of the Taihei-Ko
pyroclastic surge and pyroclastic ﬂow deposits.

~ ¸ 678¹N4º»bA5B¼

+..*., Ma 

 (Table -). Danhara et al. (+33+) 







287

X( cm 8/F<(efg/ ABCD'j/9:kFh*/i  , Galbraith

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chute and poolstructure rFig. /A sbt umnDDEv; rFig. /BwxDybt zFC.GHI{ rFig. 0A |}/-ῌ, 
-ῌ,ῌ+$q3!+94Y0F?" ~Puq.=t G.R34 Schmincke

et al. (+31-), chute and pool structure FJAB

Fig. - ?

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4<?ZG[ \]G ^[.R34./ (Fig. 1).  /< _`ab
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678ij"kZlmTWng/W , Danhara et al. (+33,)`/0YZ o"oUS/ 34pq (n,)+.02,+.021 r¡ +.02.t

(Table .). A5 / 34pq (n)r +..3/ +..32 r¡ +..31t 0+..3.

+./** r¡ +..31t  ¢ Y4G@/pq rr0G¡ +..3*t F?"A5 
Wdeo£" (Table .). #¤ _`ab ¤ D`cabA5 f¥" KS o/GW¦Rd./Rpq /./
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2)

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Fig. .. Geologic column at the outcrop of the
Taihei-Ko pyroclastic surge and pyroclastic ﬂow
deposits.

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Md´ 0tµ s´ FU43=>| rInman |t ¶

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288
Table -.

Fission-track ages for zircon of the Taihei-Ko and Haruyama pyroclastic deposits.

 Mdf

        s  +.2 z{|}~dy (Fs) .0 ῌῌ : NO +312T <

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289



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 ! "#$%&'( )$*HIJKLM./" NOPQ ,

Fig. /. Dune bed-forms and low-angle cross-stratiﬁcation of the Taihei-Ko pyroclastic surge deposit. Flow
direction is from right to left. Cross-laminations inclined to the right constitute chute and pool structure(lower right in photo A). Overlying beds locally truncate dune bed-forms (center in photo B). A hammer is-* cm long.

290

opqrs+tuv.ῌ, ῎῏ῌῑῐ῔῍ΐῒ

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hn&Fig. 0. Plastically deformed stratiﬁcation and sparse fragments of quartz-phyric rhyolite welded tu# in the
Taihei-Ko pyroclastic surge deposit. A lens cap is 0 cm across.

 

291

Fig. 1. A low-magniﬁcation microscopic view of the *.+,/ῌ*.*3* mm fraction of the Taihei-Ko pyroclastic surge
deposit. The angular, transparent fragment in the center is a glass. Many blocky, transparent fragments arequartz and plagioclase. Milky white, variably rounded particles are vitroclastic aggregates, perhaps broken
matrix of rhyolitic welded tu#.

Table .. Refractive indices of orthopyroxene, hornblende, and volcanic glass from the
Taihei-Ko and Haruyama pyroclastic deposits.

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Fig. 2. Cumulative curves for grain-size frequency distribution of samples *-+*+-*,, *-*2,1*-A, and *-*2,1*-B
from the Taihei-Ko pyroclastic surge deposit. Curves for grain-size frequency distribution of the fraction ﬁner
than .f of samples TAZ+ and TAZ, from the surge deposit at Haruyama, Tazawa-Ko are also shown forcomparison.

Fig. 3. Mdf-sf plots of the Taihei-Ko pyroclastic surge deposit and the Haruyama pyroclastic surge deposit.
Boundaries for pyroclastic ﬂow, ﬁnes-depleted pyroclastic ﬂow and pyroclastic surge are adopted from Walker(+32-). Boundary for pyroclastic fall is adopted from Walker (+31+).

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cdNd1N῍etfBP uWghiKa'(q( vrwjῌ ῑ ῐ ῍Allen, J. R. L. (+32.) Sedimentary structures : Their character and physical basis, Volume II. Elsevier, 00- p.
Oe P (+33-) xklyzWIJmn opq{|
N}r# q{|s"tn , lmuvt
n cd1Nwx} p. +.3ῌ+/2.
Danhara, T., Kasuya, M., Iwano, H. and Yamashita, T.
(+33+) Fission-track age calibration using internal andexternal surfaces of zircon. J. Geol. Soc. Japan, 31, 311ῌ
32/.
Danhara, T., Yamashita, T., Iwano, H. and Kasuya, M.
(+33,) An improved system for measuring refractive
index using thermal immersion method. Quaternary International, +-/+., 23ῌ3+.
Galbraith, R. F. (+32+) On statistical models for ﬁssion
track counts. Mathematical Geology, +-, .1+ῌ.22.
~ y (+30+) Cz<<3=KV a;c>Dῌ*{e῎ (+3/3) +, g4|}
9~  . 0+ῌ10.
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 34\: <3=K -0 p.
Kozu, S. and Yoshiki, B. (+3,1) The dissociation temperature of brown hornblende and its rapid expansion at thistemperature. Sci. Rep. Tohoku Imp. Univ., Ser. III, ,,+*1ῌ++1.
Kozu, S., Yoshiki, B. and Kani, K. (+3,1) Note on the
study of the transformation of common hornblende into
basaltic hornblende at 1/*. Sci. Rep. Tohoku Imp.
Univ., Ser. III, ,, +.-ῌ+/3.
1*f (+311) > >9;~<3 aWq* Wquc >1<,lmV no. .. 01ῌ2+.
1**Z (+32*) |}<<3 >O}*NS=Kr#
 |}<vNS=KV +ῌ+1.
1*¡*~ y (+31-) 5)~f9 a(c op<3N}cW}V no. . ++.
1 ῌ* ¢ (+3/1) /  + <3 34\: <3=K ., p.
£ZB *D¤¥*¡¦¢£*§¨ (+312) 5)*6789lm aq , Vc ¤῏ /+ Kk][©ª]lm¥«V ¬¦W<§,¨5­lm p. +.+ῌ+/,.
£Z ®*Z%¯*~w©ª*«ῐ¬f*­Z®*¯°:*±~²ª*9³ (+32+) +, <5)*6789´ <3° 21 ,01ῌ,1/.
Sato, H. and Taniguchi, H. (+331) Relationship between
crater size and ejecta volume of recent magmatic andphreatomagmatic eruptions ; Implications for energy
partitioning. Geophys. Res. Lett., ,., ,*/ῌ,*2.
Schmincke, H.-U., Fisher, R. V. and Water, A. C. (+31-)
Antidune and chute and pool structures in base surge
deposits of the Lachester See area, Germany. Sedimentology, ,*, //-ῌ/1..
µ9<§<<3r#H±² (+32*) +*  + µ
9<§<<334\: ¶<3 ,+ῌ, <3=K <3 + °³,- p.
YZ [ (+32/) µ9<§<W-./´?./B4E(' ´ 9µ<·¶ K-Ar K
L <3=K·V -0 /+-ῌ/--.
YZ [ (+320) >¸<¸¹  -++.3 a¹º(cYZ [ (+321 a) µ9<§<º»W<3¸¹ <3=KV no. ,00 .-ῌ10.
YZ [ (+321 b) µ9<§<A31@2´5)*6789 µ5)*6789´ <3=
KV no. ,00 11ῌ+.,.
YZ [ (+33,) µ9<º»W<§<3 ¶<3
 ,+ῌ/ <3=K /  + <3 + °34: 1- p.
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Sci. Univ. Kyoto, Ser. B, -,, .1ῌ21.
¼ (+30/) >Wq» ¹¸¹¼
4E(' >1<,lmV no. -, +ῌ1+.
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Yagi, K. (+300) Experimental study on pumice and obsidian. Bull. Volcanol., ,2, //1ῌ/1,.
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