Bull. Volcanol. Soc. Japan
Vol. /0 (,*++) No. . · /, pp. +0+ῌ+01

LetterRadiocarbon Dating of the Minamidake Debris-Avalanche Deposit,
Shiretoko-iozan Volcano, Eastern Hokkaido, Japan
Yoshihiko GOTOῌ, Nobuo GOUCHIῌῌ and Isao MATSUDAῌῌ
(Received March 1, ,*++ ; Accepted August ,0, ,*++)
Mt. Shiretoko-iozan is a Quaternary andesitic composite volcano located on the Shiretoko Peninsula, eastern
Hokkaido, Japan. An amphitheatre at the summit and a debris-avalanche deposit (Minamidake Debris-Avalanche
Deposit) at the western foot of the volcano suggest that sector collapse occurred during its eruptive history. Threetrenches (0.2, 1.-, and .., m deep) were dug at the western terminus of the Minamidake Debris-Avalanche Deposit
in order to date the sector collapse event. The stratigraphic section consists of (from lower to upper) : theSashiruidake Lower Lava, a debris-ﬂow deposit, a buried soil, a debris-ﬂow deposit, a buried soil, the Minamidake
Debris-Avalanche Deposit, a buried soil, the Ta-c tephra, and the surface soil. Radiocarbon dating of a buried soil
located immediately beneath the Minamidake Debris-Avalanche Deposit yields an age of -1.*ῌ.* years BP (.,-*ῌ.+3* and .+3*ῌ-32* years cal BP), suggesting that the sector collapse occurred at ca. . ka.
Key words : debris-avalanche deposit, radiocarbon age, sector collapse, Shiretoko-iozan Volcano, trench survey+. Introduction
Mt. Shiretoko-iozan is a Quaternary andesitic composite volcano located on the Shiretoko Peninsula, eastern Hokkaido, Japan (Fig. +). An amphitheatre at thesummit (the Minamidake Crater ; Katsui et al., +32,)
and a debris-avalanche deposit at the western foot of the
volcano (the Minamidake Debris-Avalanche Deposit ;Katsui et al., +32,) suggest that sector collapse occurred during its eruptive history, although the timing of
this event remains unknown. We conducted a trench
survey and performed radiocarbon dating of a buried
soil located immediately beneath the debris-avalanche
deposit to clarify the timing of the sector collapse event.
,. Shiretoko-iozan Volcano
Shiretoko-iozan Volcano (elevation of +/0- m above
sea level, base diameter +* km) is the largest Quaternarystratovolcano on the Shiretoko Peninsula (Fig. ,). The
volcanic ediﬁce consists mainly of andesitic lavas (Katsui
et al., +32,) that yield KῌAr ages of *.,.ῌ*.*-, *.+0ῌ*.*,, and *.*/ῌ*.*. Ma, suggesting stratovolcanobuilding activity began at ca. *.,. Ma and continued formore than ,**,*** years (Goto et al., ,***).
An amphitheatre (, km across) occurs just south of
the summit and opens to the west (Fig. ,). The
Minamidake Debris-Avalanche Deposit (Katsui et al.,+32,) extends from the amphitheatre to the western
῍College of Environmental Technology, Graduate Schoolof Engineering, Muroran Institute of Technology,
Mizumoto-cho ,1ῌ+, Muroran, Hokkaido */*ῌ2/2/, Japan.
῍῍Shiretoko Museum, Hon-machi .3, Shari-cho, Hokkaido

foot of the volcano, and details of the deposit aredescribed in section -. Katsui et al. (+32,) suggested
that a violent explosion triggered collapse of the northwestern part of the volcano summit and that the subsequent debris avalanche cascaded down the northwest
ﬂank.
The base of the amphitheatre is partly ﬁlled with the
Namakoyama Lava Dome and the Nanpo Lava. The
Namakoyama Lava Dome, located in the middle part of
the amphitheatre, is +*** m across, ,,* m thick, andgrades to the west into a lava ﬂow that extends for .***m (Namakoyama Lava, Fig. ,). The Nanpo Lava,
located on the northern rim of the amphitheatre, comprises three lava lobes extending to the northeast, west,
and south, respectively, over distances of 1**ῌ+/** m.
No geochronological data have been reported for the
Namakoyama Lava Dome or the Nanpo Lava.
The volcano is active, with recorded eruptions occurring in +2/1, +210, +223, and +3-/ (Katsui et al., +32,).
These historic eruptions took place at a parasitic crateron the northwestern ﬂank of the volcano (Fig. ,) andwere characterized by phreatic explosions with extrusions of large amounts of sulphur (Watanabe, +3.*).
-. Minamidake Debris-Avalanche Deposit
The Minamidake Debris-Avalanche Deposit is 0 km
long (trending EῌW), - km wide (NῌS), up to -* m*33ῌ.++-, Japan.
Corresponding author : Yoshihiko Gotoe-mail : ygoto@mmm.muroran-it.ac.jp

162

Yoshihiko GOTO, Nobuo GOUCHI and Isao MATSUDA

thick, and covers an area of ++.1 km, (Fig. ,). The
deposit retains its primary morphological features, including hummocky topography and a distal cli# (terminology following Ui et al., ,***). The hummocky topography is prominent in the western part of the deposit. The hummocks are +*ῌ+** m across with /ῌ,* m ofrelief. Several lakes are nested within the hummocky
terrain (Shiretoko-goko Lakes ; Fig. -). The distal
cli# is located at the terminus of a westerly lobe of thedebris-avalanche deposit (Fig. -). The well-preserved
cli# is +*ῌ,* m high and has a slope of -*ῌῌ.*ῌ. Theinterior of the deposit is exposed at a site located . km
west of the amphitheatre (Katsui et al., +32,), where
the deposit is +/ῌ,* m in exposed thickness, massive(non-stratiﬁed), matrix supported, very poorly sorted,and composed of fresh to altered andesite clasts up to ,m across in a matrix of lithic fragments. The clastsconsist of olivineῌhyperstheneῌaugite andesite and hyperstheneῌaugite andesite.

Fig. +. Location map of Shiretoko-iozan Volcano on
the Shiretoko Peninsula, eastern Hokkaido, Japan.

.. Trench surveys
Stratigraphic and geochronological studies of the
Minamidake Debris-Avalanche Deposit are hamperedby limited exposures due to thick vegetation cover. We
conducted a trench survey of the deposit to determine
the stratigraphic record of events and to collect a buried
soil sample from immediately beneath the debris-ava-

Fig. ,. Simpliﬁed geological map of Shiretoko-iozan Volcano (modiﬁed from Katsui et al., +32,).
Contour interval is +** m.

Radiocarbon Dating of the Minamidake Debris-Avalanche Deposit, Shiretoko-iozan Volcano, Eastern Hokkaido, Japan

163

Fig. -. Locations of trench sites at a distal cli# on thewestern edge of the Minamidake Debris-Avalanche
Deposit. The deposit occurs above the Quaternary
Sashiruidake Lower Lava.

lanche deposit, for radiocarbon dating. To minimize
the digging depth, a distal cli# on the western terminusof the deposit was selected for the trench sites (Fig. -).
Three trench sites on the slope of the cli# were selectedon the basis of aerial photographs and ﬁeld surveys(trenches +ῌ- ; Fig. -). At these sites, the Minamidake
Debris-Avalanche Deposit (+*ῌ,* m thick) occurs above
the Quaternary Sashiruidake Lower Lava (῍,** m thick ;Katsui et al., +32,) derived from Sashirui Volcano.
The trenches were excavated on +2 October ,**0,
using a ,.3-tonne backhoe (Komatsu PC-,**) with a*.1 m- bucket. Trench + was / m wide, +* m long, and
0.2 m deep (Fig. .) ; trench , was a /ῌ/ m square, 1.m deep ; and trench -, adjoining trench ,, was a /ῌ/ msquare, .., m deep. The observation faces of trenches+ and , were oriented NῌS, perpendicular to the ﬂow
direction of the deposit, whereas that of trench - wasoriented EῌW, parallel to the ﬂow direction.
/. Geological sections/ῌ+ Trench +The stratigraphic section in trench + consists of a
subhorizontal sequence comprising (from lower toupper) the Sashiruidake Lower Lava, a debris-ﬂow deposit, a buried soil, a debris-ﬂow deposit, a second
buried soil, the Minamidake Debris-Avalanche Deposit,
a third buried soil, a volcanic ash, and the surface soil
(Fig. /A).
The Sashiruidake Lower Lava (thickness῍-* cm)
consists of a massive interior and a blocky surface
composed of angular lava blocks (/*ῌ+** cm across)with curviplanar surfaces. The lava consists of grey
porphyritic andesite containing phenocrysts of plagioclase, augite and hypersthene.

Fig. .. Photograph of the Minamidake Debris-Avalanche
Deposit in trench +. The deposit occurs above a
debris-ﬂow deposit capped by a +*-cm-thick soil
layer. Sample GK-,, (collected for radiocarbon
dating) was taken from the uppermost + cm of thissoil layer. Scale bar is + m.

The debris-ﬂow deposit (thickness῍+3 cm) above thelava is brown, matrix supported, very poorly sorted, andcomposed of andesite boulders up to /* cm acrossembedded in a ﬁne-grained, clay-rich matrix. The debrisﬂow deposit grades upwards into a dark brown soil layer(+ cm thick).
The overlying debris-ﬂow deposit (thickness, .-* cm)
is yellowish brown, matrix supported, very poorly
sorted, and composed of andesite boulders up to +** cmacross embedded in a ﬁne-grained, clay-rich matrix.
The boulders show reverse grading, with large boulders
concentrated in the uppermost part of the deposit. The
matrix contains a high concentration of volcanic glass.
The debris-ﬂow deposit grades upwards into a darkbrown soil layer, +* cm thick (Fig. .).
The Minamidake Debris-Avalanche Deposit (thickness, +0* cm) sharply overlies the +*-cm-thick soil layer(Fig. .). The deposit is pale brown, massive (nonstratiﬁed), matrix supported, very poorly sorted, nongraded, and composed of angular to subangular andesite
clasts up to .* cm across in an inhomogeneous, ﬁne- tocoarse-grained matrix. The andesite clasts are com-

164

Yoshihiko GOTO, Nobuo GOUCHI and Isao MATSUDA

Fig. /. Stratigraphic cross-sections of trench + (A) and trench , (B), showing the locations ofsamples collected for radiocarbon dating (GK-,, and GK-2).

posed of fresh to altered olivineῌhyperstheneῌaugite
andesite and hyperstheneῌaugite andesite. No jigsaw
cracks (Ui et al., ,***) were observed in the clasts.
The matrix consists mainly of andesitic rock fragments
up to / mm across, and locally contains amoeboid
patches (/ῌ+* cm) of pale grey, clay-rich ash. The
debris-avalanche deposit is overlain by a dark brown soil
layer, ,1 cm thick (Fig. /A).
The volcanic ash (thickness, - cm) located immediately below the surface soil layer is reddish brown, ﬁnegrained, well sorted, and non-laminated. It consists offresh volcanic glass and crystals of plagioclase, hypersthene, augite, opaque minerals, and minor olivine. The
volcanic glass is mainly pumice-shaped (see Machida
and Arai, ,**-) and has a hydrated rim, +ῌ, mm thick(see Nakamura et al., ,**,). The refractive index of

the volcanic glass (hydrated rim) ranges from +..33+ to+./+,. (mean, +./*0 ; mode, +./*2 ; Kyoto Fission-Track
Co. Ltd.). The refractive index of hypersthene ranges
from +.1** to +.1+3 (mean, +.1++ ; mode, +.1++). The
mineral assemblage, refractive index, presence of
hydrated rim, and geographic location of the ash suggest it is the Tarumai-c tephra (Ta-c) (Ishikawa et al.,+31, ; Machida and Arai, ,**- ; Nakamura et al., ,**,)
derived from Tarumai Volcano, southwest Hokkaido.
The volcanic ash is overlain by dark brown surface soil,,* cm thick.
/ῌ, Trench ,The stratigraphic section in trench , consists of a
subhorizontal sequence comprising (from lower to upper)
the Sashiruidake Lower Lava, a debris-ﬂow deposit, aburied soil, the Minamidake Debris-Avalanche Deposit,

Radiocarbon Dating of the Minamidake Debris-Avalanche Deposit, Shiretoko-iozan Volcano, Eastern Hokkaido, Japanand the surface soil (Fig. /B). The sequence resembles that in trench +.
The Sashiruidake Lower Lava (thickness῍+* cm) is
grey and massive, with a smooth surface. The petrographical features of the lava are identical to that intrench +. The overlying debris-ﬂow deposit (thickness,-1* cm) is yellowish brown and composed of andesiteboulders (ῌ+** cm across) in a clay-rich matrix. The
boulders show reverse grading, with large boulders concentrated in the middle to upper parts of the deposit.
The deposit contains high concentrations of volcanic
glass in the matrix and corresponds to the .-*-cm-thickdebris-ﬂow deposit in trench +. It grades upwards intoa dark brown soil layer, / cm thick.
The Minamidake Debris-Avalanche Deposit (thickness, -** cm) sharply overlies the /-cm-thick soil layer
and consists of angular to subangular andesite clasts (upto +-* cm across) in a ﬁne- to coarse-grained matrix.
The texture and lithology of the deposit are identical tothose in trench +. The deposit is covered by darkbrown surface soil, /* cm thick. This section does notcontain the volcanic ash present in trench +.
/ῌ- Trench The stratigraphic section in trench - comprises atapered sequence thinning to the west (Fig. 0), consisting of a debris-ﬂow deposit, a buried soil, the Minamidake Debris-Avalanche Deposit, a clay-rich ash, and thesurface soil (Fig. 0B).
The basal debris-ﬂow deposit (thickness῍/* cm) is
yellowish brown and composed of andesite boulders up
to ,* cm across in a ﬁne-grained, glass-rich matrix.
This deposit corresponds to the uppermost part of the-1*-cm-thick debris-ﬂow deposit in trench ,. It grades
upwards into a dark brown buried soil, / cm thick.
The Minamidake Debris-Avalanche Deposit, which
sharply overlies the /-cm-thick soil layer, is wedgeshaped and decreases in thickness westward from -/* to* cm (Fig. 0B). The deposit consists of angular tosubangular andesite clasts (up to +-* cm across) in aﬁne- to coarse-grained matrix. The andesite clasts
show a westward decrease in diameter from +-* to -*cm. No jigsaw-ﬁt texture was observed in the andesite
clasts. A pale grey, clay-rich ash layer extends for ,./m westward from the terminus of the deposit (Fig. 0B).
The layer is wedge-shaped (its thickness decreases westward from /* to * cm), massive (non-stratiﬁed), nongraded, composed of intensely altered lithic fragments
up to + mm across, and resembles the clay-rich ash in
the matrix of the Minamidake Debris-Avalanche Deposit in trench +. X-ray di#raction analysis indicatesthe clay-rich ash contains smectite, kaolinite, quartz,
natroalunite, cristobalite, and talc. The Minamidake
Debris-Avalanche Deposit is overlain by dark brownsurface soil, /*ῌ+** cm thick.

165

Fig. 0. Photograph (A) and accompanying sketch(B) of a longitudinal cross-section through the
distal cli# of the Minamidake Debris-Avalanche
Deposit exposed in trench -. Scale bar is + m.
The debris-avalanche deposit is wedge-shapedand decreases in thickness westward from -/* to* cm.

0. Radiocarbon dating
Radiocarbon ages were determined for two soil
samples, both collected from trench + (samples GK-2
and GK-,, ; Fig. /A). Sample GK-2 is a dark brownburied soil (organic sediment, grain sizeῌ*./ mm) collected from the entire thickness of the +-cm-thick soil
layer located immediately below the .-*-cm-thick debrisﬂow deposit. The soil layer contains no modern plantroots, and shows no sign of disturbance (e.g., bioturbation, erosion) during or after deposition. Sample GK,, is a dark brown buried soil (organic sediment, grain
sizeῌ+ mm) collected from the uppermost + cm of the+*-cm-thick buried soil layer located immediately below
the Minamidake Debris-Avalanche Deposit. The soil
layer contains no modern plant-roots, and shows no signof disturbance during or after deposition.
Radiocarbon dating for both samples was performedby Beta Analytic (Miami, USA). Sample GK-2 was166

Yoshihiko GOTO, Nobuo GOUCHI and Isao MATSUDA
Table +.

Radiocarbon ages of buried soils in trench +.

The stratigraphic positions of the samples are shown in Fig. /A. *Based on Libby’s half-life (//02 years),
uncorrected by d +-C values. Ages are expressed in BP (years before AD +3/*) with an error range of +s. **Theconventional +.C age includes d +-C correction. Ages are expressed in BP with an error range of +s. ***Calibrated
calendar ages were calculated from the conventional +.C ages, using the program developed by Talma and Vogel(+33-), based on the IntCal*. calibration database (Reimer et al., ,**.). Ages are expressed in cal BP with an error
range of ,s (3/ῌ probability) and +s (02ῌ probability). AMS῍accelerator mass spectrometry.

acid washed, and the remaining carbon was analysedusing a standard radiometric method with extended
counting, yielding a conventional radiocarbon age of-33*ῌ0* years BP (error range +s, Table +). Sample
GK-,, was acid washed, and the remaining carbon was
analysed using accelerator mass spectrometry (AMS),
yielding a conventional radiocarbon age of -1.*ῌ.*
years BP (error range +s, Table +).
Calibrated calendar ages were calculated using the
program developed by Talma and Vogel (+33-) andbased on the IntCal*. calibration database (Reimer etal., ,**.). Calibrated calendar ages, calculated fromthe GK-2 conventional radiocarbon age (-33*ῌ0*years BP), are .12*῍.11* and ./2*῍.,3* years cal BP(,s, 3/ῌ probability), and ./,*῍..+* years cal BP (+s, 02ῌ probability) (Table +). The intercept of theconventional radiocarbon age with the calibration curveis ..-* years cal BP (,.2* years cal BC). Calibrated
calendar ages, calculated from the GK-,, conventional
radiocarbon age (-1.*ῌ.* years BP), are .,-*ῌ.+3*
and .+3*ῌ-32* years cal BP (,s, 3/ῌ probability), and.+/*῍.*2* and .*.*῍.*** years cal BP (+s, 02ῌ probability) (Table +). The intercept of the conventional
radiocarbon age with the calibration curve is .*3* years
cal BP (,+.* years cal BC).
1. Discussion
Previous geochronological studies (Orlova and Panychev,+33- ; Okuno et al., +331 ; Xu et al., ,**.) suggested
that the radiocarbon age of a buried soil located immediately below a mass-ﬂow or pyroclastic deposit represents the emplacement age of the deposit, in the casethat the deposit overlies the soil with no disturbance.
In the case of the .-*-cm-thick debris-ﬂow deposit intrench +, the soil shows no sign of disturbance, suggesting the debris ﬂow conformably overlies the pre-existing
ground surface without any disturbance. Therefore,
the radiocarbon age of the buried soil immediately
below the .-*-cm-thick debris-ﬂow deposit in trench +(sample GK-2 ; -33*ῌ0* years BP ; .12*ῌ.11* and./2*῍.,3* years cal BP) is inferred to represent theemplacement age of the deposit. Thus, the .-*-cmthick debris-ﬂow deposit is considered to have been
emplaced at ca. ../ ka.
The radiocarbon age of a buried soil located immediately below a debris-avalanche deposit may also represent the emplacement age of the deposit, in the case thatthe debris avalanche conformably overlies the soil withno disturbance. In the case of the buried soil locatedimmediately below the Minamidake Debris-Avalanche
Deposit, the soil shows no sign of disturbance or erosion, suggesting the debris avalanche conformably overlies the pre-existing ground surface without any disturbance or removal of material. This preservation of theunderlying soil is probably due to the location of thetrench (at the terminus of the debris-avalanche deposit). We therefore infer that the radiocarbon age of theburied soil located immediately below the Minamidake
Debris-Avalanche Deposit (sample GK-,, ; -1.*ῌ.*
years BP ; .,-*῍.+3* and .+3*ῌ-32* years cal BP)
represents the emplacement age of the deposit. Thus,
the Minamidake Debris-Avalanche Deposit is inferredto have been emplaced at ca. . ka.
The age of sample GK-2, which dates the .-*-cmthick debris-ﬂow deposit (ca. ../ ka), and the age ofsample GK-,,, which dates the Minamidake Debrisavalanche Deposit (ca. . ka), are consistent with thestratigraphic positions of these deposits (Fig. /A). The
ages are also consistent with the age and stratigraphic
position of the Ta-c tephra, which was probably
emplaced at ca. ,./ ka (Furukawa and Nanayama,,**0) (Fig. /A). We therefore conclude that thesector collapse of the Shiretoko-iozan Volcano occurredat ca. . ka.
Acknowledgements
This research was sponsored by the Shiretoko Museum
and was supported ﬁnancially by the Muroran Instituteof Technology. We thank H. Motomiya and K.
Ogasawara (Tatsumi-Kogyo Co., Ltd.) for operating

Radiocarbon Dating of the Minamidake Debris-Avalanche Deposit, Shiretoko-iozan Volcano, Eastern Hokkaido, Japanthe backhoe, T. Danahara (Kyoto Fission Track Co.,
Ltd.) for identiﬁcation of the Ta-c tephra, and the sta#of the Shiretoko Museum for assistance in the ﬁeld.
We also wish to thank the Ministry of the Environment,
Japan, for granting permission for the trench survey(permit number *0*3+-**.). We are grateful to S.
Togashi (AIST) and an anonymous referee for reviewing the manuscript. N. Hasebe (Kanazawa University)
is thanked for editing the manuscript.
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