10th Asian Regional Conference of IAEG (2015)

Geomorphological and geological characteristics of deep-seated gravitationalslope deformation in the Kamikochi area, central Japan
Satoru KOJIMA(1), Yoshiyuki HASEGAWA(1, 4), Naoya IWAMOTO(1, 5) ,Keitaro KOMURA(2) and Hidehisa NAGATA(3)(1) Department of Civil Engineering, Gifu University, Japan
E-mail: skojima@gifu-u.ac.jp(2) Department of Earth Sciences, Chiba University, Japan, (3) Fusuido Co. Ltd., Japan,
Present address: (4) Dai Nippon Construction, Japan, (5) Sanyu Consultants Inc., Japan
Abstract
Geomorphic features related to the deep-seated gravitational slope deformations(DSGSDs) such as double and multiple ridges and uphill-facing scarps are ubiquitous on
the Nagakabe Ridge in the Kamikochi area, Northern Japan Alps. We carried out
hand-auger drilling at two linear-depressions between double ridges on the Nagakabe Ridge,
and got nine cores about 80 to 160 cm long. The lithology of the nine cores are more or lesssimilar, and the sediments are composed of carbonaceous black mud, dark or light gray mud,and yellowish brown silt, from top to bottom. Refractive index analyses on bubble typevolcanic glasses included in the sediments near the gray mud horizons suggest that they arederived from the Kikai Akahoya tephra erupted about 7.3 ka about 1,000 km southwest
from this area. The sediment accumulation rate is calculated as about 0.1 mm/year on thebasis of this age. The formation of these DSGSDs might be related to the climate change
from the dry and cold condition in the Last Glacial Maximum to the wet and humid climate
in the post-glacial period about 10 ka.
Keywords: deep-seated gravitational slope deformation, landslide, Kamikochi, Japan
1.

Introduction

Tsuenomine area, Kii Peninsula (Kojima et al., 2015).

Recent development of LiDAR (Light Detectionand Ranging or Laser Imaging Detection andRanging) technique makes it possible to map thelandform in detail, and the topography of mountains
which is difficult to access because of poor road
network system or thick vegetations can be visualized
clearly and be analyzed precisely. By using theLiDAR maps, many deep-seated gravitational slope
deformations (DSGSDs; Crosta et al., 2013) havebeen recognized in mountain ranges of Japan (e.g.,Kojima et al., 2015). The study of DSGSDs is very
important, because they are considered to beprecursors of deep-seated catastrophic landslides (e.g.,Chigira et al., 2013). The geologic characteristics anddevelopment history of the DSGSDs, however, arepoorly understood in Japan, except a few pioneering
works (e.g., Mokudai and Chigira, 2004). This study
aims at revealing the geomorphological andgeological characteristics of the DSGSDs on theNagakabe Ridge in the Kamikochi area, central Japan(Fig. 1), following our similar researches in theEtsumi Mountain Range, central Japan, and the Mt.

Fig. 1 Topographic map and index map (inset) of theNagakabe Ridge in Kamikochi area, central Japan.
Red dotted line: linear depression, blue line withhatched line: landslide scarp, star: hand-auger drillingsite. The topographic map is prepared on the basis of
the DEM data provided by the Matsumoto Sabo
Office, Hokuriku Regional Development Bureau,
Ministry of Land, Infrastructure, Transport andTourism, Japan, and the contour interval is 2 m.

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10th Asian Regional Conference of IAEG (2015)2.

Geomorphological and geological setting

3.

The study area in the southern part of theNorthern Japan Alps is about 2,000 m above sea level,
and is located about 500 m below the tree line of thisarea.
Rocks in and around the study area consist mostlyof sandstone and mudstone with minor amounts ofchert; they are constituents of the Mino terranecomposed of Jurassic accretionary complexes(Harayama, 1990). The bedding planes of the rocks
are trending NE-SW parallel to the direction of theNagakabe Ridge, and are dipping to NW about
40-60°. Many small-scale landslides occur on thesteeper SE-facing slopes of the ridge, whereas
DSGSDs are common on the NW-facing slopes.
Many double and multiple ridges and uphill-facing
scarps, which are considered to be the DSGSDs, arepresent along the Nagakabe Ridge (Fig. 1).

Lithology of sediments accumulated in theridge-top depressions

Two sites are selected for detail studies (Figs. 1and 2), and hand-auger drilling was carried out atthese sites. We obtained nine cores, GUNK-1 to 5
and GUNK-7 to 10, from these two sites, respectively(Fig. 1). Length of the cores ranges from 80 to 160cm.
The lithology of the sediments is more or lesssimilar through the nine cores; black carbonaceous
mud is the topmost sediments, dark or light gray mudat the middle, and yellowish brown silt at the bottom(Figs. 3 and 4). The carbonaceous mud includes richplant fragments derived from the surrounding forests.
The deposition of the gray mud without plant remains
suggests an environmental change at this horizon ofthese sites. The silt formations occasionally including
granule- to pebble-size clasts of volcanic andsedimentary rocks at their lower horizons areinterpreted to be loam or aeolian deposits.

Fig. 2 Photographs showing the flat surfaces of thedrilling sites. A: drilling site of GUNK-7 to 10, B:drilling site of GUNK-1 to 5.

Fig. 4 Geologic columnar sections of therepresentative cores drilled on the Nagakabe Ridge,
Kamikochi area, central Japan. a: carbonaceous mud,b: carbonaceous mud with sand grains, c: dark graymud, d: light gray mud, e: silt, f: silt with granule- orpebble-sized clasts.

Fig. 3 Photographs of the two examples of cores,
GUNK-2 and GUNK-7, drilled on the Nagakabe
Ridge, Kamikochi area, central Japan.

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4.

Clast composition and age of the sediments

the Nagakabe Ridge, Kamikochi area, central Japan.
Randomly extracted 200 grains were counted for eachhorizon. a-c: volcanic glass a: bubble type, b:
pumice-fibrous type, c: pumice-sponge type, d:crystal grain, e: plant fragment, f: charcoal, g:
unknown.

4.1 Composition of the sediments
In order to estimate the age and depositional
environment of the sediments, composition of clastic
grains between 62 and 200 micrometers in diameter(250 and 100 mesh) was analyzed by using thepolarizing optical microscope. The grains include
volcanic glass, crystal grains, plant fragments,
charcoal, and unknown grains (Figs. 5 and 6). The
plant fragments are rich in the upper horizons, and it
is consistent with the lithology of the sediments. The
volcanic glasses are of three kinds: bubble,pumice-fibrous, and pumice-sponge types. The
drilling sites are within 15 km from the volcanoes,
such as Mt. Yakedake and Akandana active during
the last 30,000 years (Harayama, 1990; Oikawa,
2002).
4.2 Origin of the volcanic glass
Local small volcanic eruptions usually ejectpumice type volcanic glasses, whereas extremely
large volcanic eruptions typically disperse bubble
type glasses over a wide area (Machida and Arai,2003). Tephras derived from such large eruptions
have been well identified mainly on the basis of therefractive index of the bubble type volcanic glasses
and provide key evidence on the age of the
Quaternary sediments in Japan (Machida and Arai,2003). The refractive index of the bubble typevolcanic glasses extracted from their peak horizons(62-72 cm in depth of GUNK-2 and 87-92 cm ofGUNK-7) were concentrated between 1.508 and1.516 (Fig. 7), and coincide with that of Kikai
Akahoya tephra (K-Ah) erupted about 6.3 ka (7.3 calka) about 1,000 km southwest of the study area
(Machida and Arai, 2003).

Fig. 6 Composition of grains between 62 and 200
micrometers in diameter in the sediments
accumulated in the linear depression, GUNK-7, on
the Nagakabe Ridge, Kamikochi area, central Japan.
See Fig. 5 for explanation of the abbreviations.

Fig. 7 Refractive index of bubble type volcanic
glasses in the sediments accumulated in the linear
depressions, GUNK-2 (62-72 cm in depth) andGUNK-7 (87-92 cm in depth), on the Nagakabe
Ridge, Kamikochi area, central Japan.

Fig. 5 Composition of grains between 62 and 200
micrometers in diameter in the sediments
accumulated in the linear depression, GUNK-2, on310th Asian Regional Conference of IAEG (2015)

5.

Discussion

DEM data were provided by the Matsumoto Sabo
Office, Hokuriku Regional Development Bureau,
Ministry of Land, Infrastructure, Transport andTourism, Japan. The measurements of refractive
index of the volcanic glasses were performed at theDepartment of Earth Sciences, Chiba University
under the guidance by Associate Professor Heitaro
Kaneda. Part of this study was financially supported
by Grants-in-Aid for Scientific Research, Japan
Society for the Promotion of Science, 23540531 and26400487.

Sedimentary environmental change and/or
climate change, from the poor to rich vegetation
environment, can be estimated at the horizons of graymud. Although the tree line is about 500 m higher inelevation than the drilling sites now, it must be lower
than the sites before the age of gray mud horizon.
Peak concentration of volcanic glasses of bubble type
and their refractive index suggest that the change
occurred around 7.3 cal ka. After the Last Glacial
Maximum about 25-20 ka, the climate was becoming
warmer, milder, and wet. The sedimentary
environmental change detected in the drill cores
treated in this study might be related to this global
climate change.
Sediment accumulation rate of the carbonaceousmud (and gray mud) calculated by using the age ofthe K-Ah tephra is about 0.1 mm/year. If we couldextend the rate to the bottom of the sediments, theDSGSDs are estimated to have formed about 15 ka.
This might be also related to the climate change after
the Last Glacial Maximum.
6.

References
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Conclusions

The sediment cores drilled by hand-auger
equipments at the linear depressions on the Nagakabe
Ridge in the Kamikochi area, central Japan, are
80-160 cm in length, and are composed ofcarbonaceous black mud, dark-light gray mud, andbrown silt, from top to bottom. Refractive index
analyses on bubble type volcanic glasses included
near the gray mud horizons suggest that they arederived from the Kikai Akahoya tephra erupted about
7.3 cal ka. The sediment accumulation rate iscalculated to be about 0.1 mm/year on the basis of
this age. The formation of these DSGSDs might berelated to the climate change between the Last Glacial
Maximum and the present-day warm and humid
condition.
Acknowledgements
The authors thank to the Agency for Cultural
Affairs of Japan, the Ministry of the Environment
Government of Japan, and the Ministry of Agriculture,Forestry and Fisheries of Japan for the permission ofsample collection by the hand-auger drilling. The4