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 /  ,*3ῌ,,+ ῕ ῔

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"   "",**2 E +, F / GHI ,**3 E 3 F +* GHJ

Intracaldera Debris Avalanche Deposit Associated with Osuzuyama
Volcano-plutonic Complex in Kyushu, Southwest Japan
Koji MIMURA῍ and Toshimitsu IWAYA῍῍
Miocene Osuzuyama volcano-plutonic complex -* km across in diameter around +/ Ma reveals intracaldera
debris, newly named Kanaiso debris avalanche deposit. The solidiﬁed deposit involves irregular blocks reaching/* m across as a megablock and displays sedimentary characteristics clearly instead of the lost topography, suchas blocks showing peculiar plastic deformation, disjunction and displacement along the joints as well as
jigsaw-puzzle cracks and patchwork texture with the matrix. Imbrication and ﬂame-shaped tail of the blocks
develop near the boundary with the basement, indicating the ﬂow direction of avalanche not from the innercaldera. Also, a concentration in orientation of the long axis develops.
The avalanche deposit consists of debris derived from the basement Shimanto supergroup of Cretaceous to
Early Miocene, Iorigawa conglomerate and Welded tu# + of rhyolite in the lower units of the complex. But nodebris derived from Welded tu# , of dacite in the upper unit of the complex appears in the deposit, except a few
obsidian blocks in the same chemical composition of the Welded tu# ,. Neither pumice nor volcanic ash exists
in the deposit obviously.
The avalanche deposit occupies two horizons in detail. The lower one overlies the basement and thesolidiﬁed Welded tu# + and is overlain by the Welded tu# ,. Another one intercalates in the lower part of theWelded tu# ,.
These facts suggest that the Kanaiso debris avalanche deposit derived from the unstable caldera wall mostprobably soon after the caldera collapse of the complex. No debris avalanche appeared during the ﬁrst pumice
eruption to issue the Welded tu# +. But putting a cooling lag after the ﬁrst pumice eruption, avalanches occurred
before the main phase in the second pumice eruption producing the Welded tu# ,.
Key words : intracaldera debris avalanche deposit, plastic deformation, imbrication, orientation of blocks, Miocene Osuzuyama volcano-plutonic complex.

+ῌ ῐ ῎ ῑ ῏Ratté, +30/) KE LMNOPQRSTU VWXYZ

ter (Lipman, ,***)  caldera-collapse landslide deposit,!

 San Juan caldera clus-[\] ^R_`a`b ! cde /* Ma f

landslide breccia ghd caldera-collapse breccia i bd Bennett lake cauldron subsidence complex (Lambert,

megabreccia  (Lipman, +310) \jkVWXl [f+31.)   ,1 Ma (Lanphere, ,***)

mfSTU VWXnopb qrsi !

ftu San Juan Mountain, Creede caldera (Steven and" #-*/ῌ**-. $%&'() -0,ῌ.
,-./0123456789:;<=>89?@
ABC89D
Former chief stu# of Institute of Geology and Geoinformation, National Institute of Advanced Industrial
Science and Technology.
Present address is Onozaki -0,ῌ., Tsukuba -*/ῌ**-.,
Japan.

Gvd wxGvyz{|}~fLMN
"" #-*/ῌ2/01 $%&* +ῌ+ῌ+ + 1,-./0123456789:;<=>89?@
Institute of Geology and Geoinformation, National
Institute of Advanced Industrial Science and Technology, Higashi +ῌ+ῌ+, Tsukuba -*/ῌ2/01, Japan.
Corresponding author :RS

210

Fig. +. Locality map based on Iwaya and Mimura (+33,). Miocene volcano-plutonic complex around +/ Ma
along the Outer zone : Osuzuyama (O), Okueyama (S), Ishizuchiyama (I), and Kumano (K). Circle :outcrop of the debris avalanche deposit and the locality number in the text. The debris avalanche deposit
contains blocks abundantly of the basement and the Iorigawa conglomerate (solid circle), abundantly of
the Welded tu# + (open circle) or apparently even of them (half solid circle). The maximum size of theobserved blocks is shown in parentheses.
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211

Fig. ,. Sequence of the stratigraphy (left column) and caldera-related events (right column) at theOsuzuyama volcano-plutonic complex modiﬁed from Iwaya and Mimura (+33,). See text for details.
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<P´ Yµ¶3¬ L3Q· (Fig. +) - 

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+32+) (¼· (Fig. 0) ½    LM D +.-212

cdefghij

Fig. -. Debris avalanche deposit which covers the Welded tu# + and underlies the Welded tu# ,. a. Debris
avalanche deposit (D) on the Welded tu# + (W+) at locality 1 north of Yamanota, western southwest of
Hyuga city. B : outstanding blocks of the Welded tu# +, F : ﬂow boundary shown with a wavy etchingline, p : step under construction, t : road, v : vegetation, j : a jeep-type car, c : concrete-covered sidewall of
the Welded tu# +. The present crop is covered. b-d. Debris avalanche deposit (D) crops under the
Welded tu# , (W,). b. North coast of the type locality -. c. The type locality. d. View from thedirection of C in ﬁgure c. s : sea side.
 -  +,      a   D  !"#"$%$&'( )* 1 B : +,-+ W+
+ $./01 F : 2345$60789:;5<=>?@ p : ABCDE0F t : GH
v : IJ j : KLFM$G c : + $N81OLPQR S23TUVN81OLPWRX 
, W, Y$  D b Z[))* - $'\] c Z[)d  c ^$ C (_`a? s : \b

TUVWXYXZ[\] ^(_3`ab 

213

Fig. .. Debris avalanche deposit intercalated in the Welded tu# ,. a. Distribution map around the northern sea shoreof Oguragahama, locality 0. b. Contact between the debris avalanche deposit (D) and the lower Welded tu# , (W,). s : sea side. Scale is + m. c. Irregular contact cuts the foliation of the lower Welded tu# , (W,) on ahorizontal sea-side crop. See the direction of photo in a. Scale is ,* cm. d. Debris avalanche deposit (D) cuts thefoliation in the block of the lower Welded tu# , (W,) near their boundary. s : sea side. Scale is ,* cm.
 ,     a  . 
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214

Fig. /. Debris avalanche deposit including various blocks. a. View of the outcrop with weathered surface inlight color at locality +, western northwest of Iorigawa. Several persons are standing in front of the centerof the crop. b. Sketch of the cross section of debris avalanche deposit. Included blocks are mudstone(m), silt stone (t), sandstone (s) and green stone (g) from the basement. Others are the Iorigawa
conglomerate (i) and the Welded tu# + (w). The matrix (M) is composed with mixed fragments of theblocks. p : steps under construction. The crop is covered with concrete now.
 / 
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1JVW XYCZ[0

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215

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 .  / +,-.+/  1 +0%+RSTUVK ^hijW8 V5
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9:;<0=>?678  . +@4 <0o RSTU=8* RSTU+,+ !5i:;tuvYQ (Fig. ++)9:;<0 Fig. 2b, c +,-.+/
RSTUV5J0 ]+Pw&-  x. + m yzRSTU+-.+/{[+ |}~'M6/Q_&012+3_R
STUs643 M5  46
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locality / along Kashi seashore. wt : Welded tu#+, nw : non-welded part of the Welded tu# +, M :matrix. s : sea side, v : vegetation. +m scale.
0  
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6789:;<216

Fig. 1. Chips mixed in the matrix of the debris avalanche deposit shown on a stone wall at locality ,. Chips
bear the peculiar behaviors as same as those of blocks. ῌ : in area. F : foliation of welded tu#s.
 1 
 
' () ,* +,-. ῌ/0 F : 123

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IJKLM,MNOPQR:S7TUVWXY)Z[\

Fig. 2. Modiﬁed blocks including irregular plastic
deformation in the avalanche deposit. a.
Meandering block of sandstone at locality /.
The left rim of the photo is about +/ cm long.
b. Thin mudstone (ms) attaches a modiﬁed
block of sandstone (ss) which has a ﬂameshaped tail at the left end at locality .. Scaleis ,* cm. c. Jointed block of sandstone getting
untied at the lower right end showing a ﬂame
like tail at locality /. Scale is ,* cm. d. Pullapart mudstone block along an open crack at
locality -. e. Displacement along a joint in asandstone block at locality /. Scale is ,* cm.
 2


 a
  +/ cm.  /  b
!ss" #$% !ms" &'()* +&,-./0123
4567 ,* cm.  . c 8)9:;
 <=+
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  - e 8)9&FG
H
 4567 ,* cm.  /  217 

218

Fig. +*. Imbrications along the boundary with the basement at locality -. a. Imbrications at the bottom of
the debris avalanche deposit in the upper ,/- of thephoto on the mudstone of the basement in the lower+/-. Scale is ,* cm. Imbrications suggests a ﬂow
direction from the left, northeast to the right, southwest. A larger block on the right hand has an open
crack showing the same ﬂow direction of theimbrications. b. Marking sketch of the long axes ofblocks revealing imbrication. D : debris avalanche
deposit. B : basement. Scale is ,* cm.

Fig. 3. Sketches of the blocks in the avalanche
deposit. a. A block of the Welded tu# + (wt)coming loose along jigsaw cracks at locality +. b.
A conglutinated block of mudstone (ms) and
Welded tu# + (wt) at locality +. A ribbon-shaped
block of mudstone appears in the upper left. c.
Patch worked blocks of mudstone (ms), sandstone(ss) and Welded tu# + (wt) with rare matrix (M)at locality ,.
2345`a*+4^ a b[c,TU;de<;fghij& + #wt' + b , kl #ms : )_ wt : ghij&+' mn?S2345  + 9 opq
r?SstZu)2345 c v4^w,
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Fig. ++. Flame-shaped tail of a block indicates the ﬂow
direction of the debris. Left end of the imbricate
block of mudstone (m) over a block of sandstone(s) has a tail getting untied like a ﬂame at locality.. Scale is ,* cm. M : matrix. Remember similar
blocks shown in ﬁgs. 2b and 2c.
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Fig. +,. Concentration in orientation of the blocks is observed crossing diagonally for the direction of thedebris avalanche supposed by imbrications (Fig. +*) on the ﬂat seashore at locality -. Near the cropped
basement, the orientation tends parallel to the ﬂow direction. ῌ in area. A : the vertex of the most far-o#
distance, B : center of gravity, C : the intersection of the extension line of the line segment AB and a blockoutside.
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- @A# ,/3p.
Glicken, H. (+332) Rockslide-debris avalanche of May +2,+32*, Mount St. Helens Volcano, Washington. Bull.
Geol. Surv. Japan, .3, //῍+*0.
Hasebe, N. and Hoshino, H. (,**-) Igneous rocks emplacement and exhumation of sedimentary basement : ﬁssion
track age determination on the Osuzuyama volcanoplutonic complex and surrounding rocks, Miyazaki prefecture, southwest Japan. Geochem. Jour., -1, /-1῍/.-.
Henry, C.D., Kunk, M. J., Muehlberger, W.R. and McIntosh, W.C. (+331) Igneous evolution of a complex
laccolith-caldera, the Solitario, Trans-Pecos Texas : implications for calderas and subjacent plutons. Geol. Soc.
Am. Bull., +*3, +*-0῍+*/..
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YZX¢ £[TU C TU1¤=¥@# 3.῍30.
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,-. .2ῌ1-.
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  K-Ar ?@ABCDE $C FG ,M  .- /0/ῌ/1,.
Kawakami, Y., Hoshi, H. and Yamaguchi, Y. (,**1)
Mechanism of caldera collapse and resurgence : observations from the northern part of the Kumano Acidic
Rocks, Kii peninsula, southwest Japan. J. Volcanol.
Geotherm. Res., +01, ,0-ῌ,2+.
2i1234 (+33+) ῌ jk 2il/0  1234  cdmn  cdef  opqr [ [st u/  + v ,-. .ῌ3.
wxy0Oz (,**/) {|}"~ s  no. /- 1+ῌ2*.
Lambert, M.B. (+31.) The Bennett Lake cauldron subsidence complex, British Columbia and Yukon Territory.
Geol. Surv. Canada Bull., ,,1, +13 p.
Lanphere, M.A. (,***) Duration of sedimentation of theCreede Formation from .3Ar/-3Ar ages, In Ancient
Lake Creede : its volcano-tectonic setting, history of sedimentation, and relation to mineralization in the Creedemining district. (Bethke, P.M. and Hay, R.L. eds.) Geol.
Soc. Am. Spec. Pap., -.0, 1+ῌ10.
Lipman, P.W. (+310) Caldera-collapse breccias in the western San Juan Mountains, Colorado. Geol. Soc. Am. Bull.,
21, +-31ῌ+.+*.
Lipman, P.W. (+331) Subsidence of ash-ﬂow calderas :relation to caldera size and magma-chamber geometry.
Bull. Volcanol., /3, +32ῌ,+2.
Lipman, P.W. (,***) Central San Juan caldera cluster :regional volcanic framework. In Ancient Lake Creede : its
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Pap., -.0, 3ῌ03.
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1234 (,**+) *x7¬­®YK+,¯°±²-&'()*+ +.C FG ,-st u /, -*-ῌ-*1.
Mimura, K. and Kawachi, S. (+32+) Nirasaki debris avalanche, a catastrophic event at the Yatsugatake volcanic
chain, central Japan. Abstracts +32+ IAVCEI symposium221 Arc volcanism , ,-1.
123489:; (+321) <=>&'()*+ H IJKL I +*. KH L I
22  N 3.
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TUV (,***) WXY Z[ \]!"#$=^_`ab S +*0 ,*/ῌ,,,.
cdef (+33+) ῍  gh
 2il/01234cdmncdefopqr [ [st u /  + v ,-.
1.ῌ13.
Shreve, R.L. (+302) The Blackhawk landslide. Geol. Soc.
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2.,.
Smith, R.L. and Bailey, R.A. (+302) Resurgent cauldrons.
In Studies in volcanology. (Coats, R.R., Hay, R.L. andAnderson, C.A. eds.) Geol. Soc. Am. Memoir, ++0, 0+-ῌ
00,.
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!"#$a  &'()*+ H I ++, FTI! 01.
Steven, T.A. and Ratté, J.C. (+30/) Geology and structuralcontrol of ore deposition in the Creede district, San Juan
Mountains, Colorado. U.S. Geol. Surv. Prof. Pap., .21, 3*p.
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Williams, H. (+3.+) Calderas and their origin. California
Univ. Pubs. Geol. Sci., ,/, ,-3ῌ-.0.
Yamamoto, T. (,**-) Lithofacies and eruption ages of late
Cretaceous caldera volcanoes in the Himeji-Yamasaki
district, southwest Japan : implications for ancient largescale felsic arc volcanism. Island Arc, +,, ,3.ῌ-*3.
"#V3 (+323) $!"#$%!"#$£¤J(¥¦ s -0 +3+ῌ,*2.
Yoshida, T. (+32.) Tertiary Ishizuchi cauldron, southwestern Japan arc : formation by ring fracture subsidence. J.
Geophys. Res., 23, 2/*,ῌ2/+*.
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