Journal of Earth Science and Engineering 2 (2012) 317-327
Relationship between Landslides, Geologic Structures,
and Hydrothermal Alteration Zones in the
Ohekisawa-Shikerebembetsugawa Landslide Area,
Hokkaido, Japan
Hiroyuki Maeda1, Takashi Sasaki2, Kazuyuki Furuta3, Katsuhiro Takashima4, Akihiro Umemura5 and Masanori
Kohno6
1. Sapporo Technology Professional Training College, Sapporo 007-0895, Japan
2. Earth Consultant UEYAMA Co., Ltd., Sapporo 060-0032, Japan
3. Shin-Nikken Consultant Co., Ltd., Tokyo 133-0057, Japan
4. Takashima Land and Buildings Investigator Office, Nagano 399-8102, Japan
5. Japan Foundation Engineering Co., Ltd., Osaka 530-0037, Japan
6. Graduate School of Engineering, Tottori University, Tottori 680-8552, Japan
Received: April 27, 2012 / Accepted: May 14, 2012 / Published: June 20, 2012.
Abstract: This paper elucidates the relationship between landslides, geologic structures, and hydrothermal alteration zones based primarily on X-ray powder diffraction and uniaxial compressive strength tests on weakly weathered and hydrothermally altered rocks from the Ohekisawa-Shikerebembetsugawa landslide area in Teshikaga Town, Hokkaido, Japan. The OHS (Ohekisawa slide) occurred on a dip slope of sedimentary rocks from the Upper Miocene Shikerepe Formation within a homocline, and also on weathered and hydrothermally altered rocks within the boundary area between the hydrothermal smectite zone and smectite-bearing mordenite zone. The SHS (Shikerebembetsugawa slide) occurred on a dip slope of sedimentary rocks from the Upper Miocene Hanakushibe Formation within wavy folds and was also controlled by a cap rock of Teshikaga Volcano Somma Lava. The SHS occurred also on weathered and hydrothermally altered rocks within the boundary area between the hydrothermal smectite zone and smectite-bearing laumontite zone. The mechanical properties of smectite, smectite-bearing mordenite, and smectite-bearing laumontite zone weakly weathered rocks indicate that they are very weak, soft rocks. These landslides are regarded as HAZLs (hydrothermal alteration zone landslides). The hydrothermal alteration yielding smectite is thus closely related to these two ancient landslides, suggesting that the potential for HAZLs within a hydrothermal area can be assessed based on the swelling clay mineral-bearing hydrothermal alteration types, dip slope, and cap rock. Key words: HAZL (hydrothermal alteration zone landslide), swelling clay mineral-bearing hydrothermal alteration zone, weathered and hydrothermally altered soft rock, dip slope, cap rock.
1. Introduction
The existence of swelling clay minerals in slip
surface clays in landslides is one of the main causes of
reactivation-type landslides [1]. Although DZLs
Corresponding author: Hiroyuki Maeda, doctor, emeritus
professor, main research field: landslide science. E-mail: [email protected], [email protected].
(diagenetic zone landslides) and HAZLs
(hydrothermal alteration zone landslides), based on
geological landslide classification, have soft or hard
bedrock, and are induced by earthquakes, they operate
by the swelling of clay minerals in slip surface clays,
as well as a rise in underground water pressure during
heavy rains, long spells of rainy weather or the spring
DAVID PUBLISHING
D
Relationship between Landslides, Geologic Structures, and Hydrothermal Alteration Zones in the Ohekisawa-Shikerebembetsugawa Landslide Area, Hokkaido, Japan
318
thaw. HAZLs are defined as landslides that occur
within hydrothermal alteration zones formed by
volcanic-hydrothermal systems over geologic time
and historic period [2-5]. For instance, HAZLs in
Japan include those at the Asamushi Hot Spring and
Hakkoda in Aomori Prefecture, Onikobe and Naruko
Hot Springs in Miyagi Prefecture, Manza and
Kumaike in Gunma Prefecture, Mt. Sounzan and
Owakudani in Kanagawa Prefecture, Myoban in Oita
Prefecture, etc. [6-8]. These HAZLs occurred mainly
within acid hydrothermal alteration zones formed by
fumarolic gases derived from active acid hydrothermal
systems. Heavy rains accompanying the approach of a
typhoon led to the occurrence of the 2001 Manza
Slide and Kumaike Failure-Flow within neutral and
acid hydrothermal alteration zones formed by the
hydrothermal systems of the active Kusatsu-Shirane
volcano [8]. In contrast, HAZLs occurring on
weathered and hydrothermally altered rocks, which
were formed mostly by ancient neutral hydrothermal
systems of the Upper Miocene Ikutawara and Yahagi
Formations of the Ikutahara area in Engaru Town, and
the Kanehana area in Rubeshibe Town, Kitami City,
northeastern Hokkaido, Japan, from the Late Miocene
to the Early Pliocene, are closely related in space to
interstratified illite/smectite mineral and smectite zones
that are characterized by swelling clay minerals, and
swelling clay mineral-bearing zeolite zones, as well as
kaolin mineral, illite, and K-feldspar zones [2, 9-13]. In
addition, HAZLs took place within hydrothermal
smectite zone or smectite-bearing mixed-layer
mimeral zone along the Median Tectonic Line in
Shikoku, Japan [14, 15].
The strength of fresh and altered rocks, including
hydrothermally altered or weathered rocks, is
generally evaluated based on their uniaxial
compressive strengths. They are divided into hard
rocks with uniaxial compressive strengths greater than
25 MPa and soft rocks that can bear less than 25 MPa
in a forced-dry state. The uniaxial compressive
strength test, in particular, for weakly weathered and
hydrothermally altered rocks taken from outcrops or
floats in a hydrothermal field is effective to estimate
mechanical properties of landslide bedrocks and
bodies in weathered and hydrothermally altered
rockslides.
Teshikaga Town is situated in eastern Hokkaido,
Japan (Fig. 1). A number of studies have examined
landslides in this town, including papers by Maeda et
al. [5, 16-18]. From the Middle Miocene to the Early
Pleistocene, the Sattomonai-Okushunbetsu area in the
western part of the town (Fig. 1) was subject to intense
terrestrial volcanic-hydrothermal activity [19, 20]. Two
ancient landslides have been identified, based on
topography, in the southern part of the Okushunbetsu
area (Figs. 1 and 2). These ancient landslides have
been tentatively named the OHS (Ohekisawa Slide)
and SHS (Shikerebembetsugawa Slide) (Fig. 2). The
OHS is 350 m wide and 410 m long, whereas the SHS
is 175-450 m wide and 595 m long (Table 1).
The purpose of this study is to clarify the
relationship between HAZLs, geologic structures, and
hydrothermal alteration zones in the
Ohekisawa-Shikerebembetsugawa landslide area.
2. Methods and Equipment
The relationship between HAZLs, geologic
structures, and hydrothermal alteration zones was
examined based on an aerial photograph interpretation,
a 1:5,000-scale topographical and geological mapping,
and an XRD (X-ray powder diffraction) and uniaxial
compressive strength tests on weakly weathered and
hydrothermally altered rocks from the southern part of
the Okushunbetsu area.
Weakly weathered and hydrothermally altered
rocks were collected from the ground surface. The
modes of occurrence of these altered rocks were
examined in the field, and the hydrothermal alteration
minerals in the rocks were determined primarily by
the XRD test. Opal-CT was determined based on the
XRD patterns obtained by Jones et al. [21]. Clay
minerals in the rocks were identified from the
Relationship between Landslides, Geologic Structures, and Hydrothermal Alteration Zones in the Ohekisawa-Shikerebembetsugawa Landslide Area, Hokkaido, Japan
319
Nakajim a
Lake Kutcharo
Biruw a
Teshikaga
M t. Pekereyam a732.3
581
573
M t. Shikerepeyam a
M t. Nichieizan
Shibecha Tow n
M t. Ikurushibeyam a729
Fig. 2
0 4 km
N
Sattom onai
O kushunbetsu
Tow n
M ashu
Hokkaido
Sea of O khotsk
JAPAN
145˚E140˚E
130˚E
40˚N
35˚N
40˚N
35˚N
135˚E
45˚N
RUSSIA
C HINA
NO RTH
KO REA
SO UTH
KO REA
45˚N
25˚N125˚E200 km0
Teshikaga Tow n
Fig. 1 Location map of the Sattomonai-Okushunbetsu area in Teshikaga Town, eastern Hokkaido, Japan.
・513
・476
・328
・432
・425
573・
・377
・485
・348
・351
・390
・375
・581
・408
・305
・246
・294
・345
479・
347・
M t. Om onaiyam a
Route 241
OHS
1 km0
L E G E N D
O HS:O hekisaw a Slide
SHS:Shikerebem betsugaw a Slide
SHS
M t. Shikerepeyam a
M t. Nichieizan
N
Fig. 2 Two ancient landslide areas in the southern part of the Okushunbetsu area. The topographical map is part of a 1:25,000 map of Pekereyama from the Geographical Survey Institute of Japan.
Relationship between Landslides, Geologic Structures, and Hydrothermal Alteration Zones in the Ohekisawa-Shikerebembetsugawa Landslide Area, Hokkaido, Japan
320
Table 1 Analysis of ancient landslides in the southern part of the Okushunbetsu area.
Items OHS SHS
Total length (m) 410 595
Width (m) 350 175 (foot)-450 (main body)
Angle of slope 10-20° 10-25°
Bedrock geology Shikerepe formation Coarse tuff and tuffaceous medium sandstone
Hanakushibe formation Fine tuff, mudstone, and lapilli tuff
Geological structures Dip slope Dip slope on the whole and cap rock of lava
Hydrothermal alteration zone Smectite and smectite-bearing mordenite zones Smectite and smectite-bearing laumontite zones
diffraction patterns of untreated and ethylene
glycol-treated samples. XRD was performed using a
Rigaku RAD-3R diffractometer (30 kV, 20 mA)
equipped with a Cu tube, an Ni filter, a 0.3-mm
receiving slit, and 1° divergence and scattering slits.
Rock samples for uniaxial compressive strength
tests were taken using a drilling machine. The
specimens were 50 mm in diameter and cut to 100
mm lengths using a diamond cutter. The uniaxial
compressive strength tests were performed on core
specimens in a forced-dry state, after drying in an
electric oven at a temperature of 60 ± 3 °C for 4 days
or more to achieve a constant mass [13].
3. Geologic Setting
The Teshikaga district is within the inner belt of the
Kurile arc [22]. The geology of this district was
described mainly by Maeda et al. [20, 23-25]. The
geology consists mainly of Neogene and Quaternary
Systems, as well as Neogene and Quaternary intrusive
rocks. The Neogene System can be divided, in order
of ascending stratigraphy, into the Middle Miocene
Ikurushibe Formation, the Upper Miocene
Oteshikaushinai, Hanakushibe, and Shikerepe
Formations, and the Upper Pliocene Ikurushibeyama
Lava and Shikerepeyama Lava [23-26]. The
Quaternary System can be subdivided, in ascending
stratigraphic order, into the Shikerepempetsu
Formation, Pekereyama Lava, Teshikaga Volcano
Somma Lava, higher fluvial terrace deposits, lower
fluvial terrace deposits, talus deposits, landslide
deposits, and alluvial river deposits (Table 2).
Neogene and Quaternary intrusive rocks are chiefly
composed of andesite dikes, with lesser basalt and
rhyolite dikes [24]. These dikes are generally affected
by hydrothermal alteration and mineralization. K-Ar
ages of 2.43 ± 0.45 and 2.3 ± 0.8 Ma for the andesite
dikes [24] are believed to indicate the time of the
hydrothermal alteration and mineralization. The
Neogene formations and the Lower Pleistocene
Shikerepempetsu Formation underwent extensive
hydrothermal alteration and mineralization during the
Pliocene to Early Pleistocene epoch [19, 20]. The rock
facies of the Neogene and Lower Pleistocene
formations related to landslides in the
Sattomonai-Okushunbetsu area are as follows:
The Oteshikaushinai Formation is chiefly
composed of alternating andesitic and dacitic lapilli
tuff, tuff breccia, and tuff, with basal tuffaceous
conglomerate, sandstone, and mudstone;
The Hanakushibe Formation conformably
overlies the Oteshikaushinai Formation and is chiefly
composed of mudstone, and alternating fine sandstone
and tuff with andesitic tuff breccia and lapilli tuff;
The Shikerepe Formation conformably overlies
the Hanakushibe Formation and is chiefly composed
of andesitic and dacitic volcaniclastic rocks,
tuffaceous sandstone and mudstone, and dacitic
welded tuff;
The Ikurushibeyama Lava unconformably overlies
the Shikerepe Formation and is chiefly composed of
andesitic lava, volcanic breccia, and tuff breccia.
Pekereyama Lava is chiefly composed of basaltic
lava and has not suffered hydrothermal alteration [24].
Teshikaga Volcano Somma Lava is chiefly
composed of basaltic and andesitic lavas and volcanic
breccia and has also not suffered hydrothermal
alteration.
Relationship between Landslides, Geologic Structures, and Hydrothermal Alteration Zones in the Ohekisawa-Shikerebembetsugawa Landslide Area, Hokkaido, Japan
321
Table 2 Stratigraphy and volcanic-hydrothermal activity in the Sattomonai-Okushunbetsu area. Modified from Ref. [20].
Geologic time
Stratigraphy Rock facies of clastic rocks and extrusive rocks and extrusion ages
Hydrothermal alteration and mineralization ages
Holocene Pleistocene
Alluvial river deposits Gravel, sand, silt, and clay
Landslide deposits Rubble, sand, silt, and clay
Talus deposits Rubble, sand, silt, and clay
Lower fluvial terrace deposits Gravel, sand, silt, and clay
Higher fluvial terrace deposits Gravel, sand, silt, and clay Teshikaga Volcano Somma Lava
Basaltic and andesitic lavas, and tuff breccia 2.2 Ma Taiho (Kanehana) illitization 2.3 Ma Shikerepeyama natroalunitization 2.4 Ma Tobetsu Au and Ag mineralization
Pekereyama Lava Basaltic lavas (1.84 Ma)
Shikerepempetsu Formation Conglomerate, sandstone, mudstone, and dacitic volcaniclastic rocks (2.3 Ma)
Pliocene Shikerepeyama Lava Dacitic lavas (rhyolite: 2.63 & 2.81 Ma)
3.2-3.8 Ma Akan Au and Ag mineralization
Ikurushibeyama Lava Andesitic lavas and volcaniclastic rocks (2.61 & 2.73 Ma)
Late Miocene
Shikerepe Formation Andesitic and dacitic volcaniclastic rocks, sandstone, and mudstone
Hanakushibe Formation Mudstone and andesitic volcaniclastic rocks
Oteshikaushinai Formation Conglomerate and andesitic and dacitic volcaniclastic rocks (7.9-8.2 Ma)
Middle Miocene
Ikurushibe Formation Andesitic and dacitic lavas and volcaniclastic rocks (13.0 Ma)
The geologic structures of the Neogene formations
in the study area (Fig. 3) are as follows:
An NW-SE trending anticline can be mapped in
the lower reaches of Oteshikaushinaisawa Creek and
the Hanakushibegawa River in the northwestern part
of the study area (Fig. 3). An ENE-WSW trending
syncline leads to folds in the rocks in the middle
reaches of the Hanakushibegawa River in the western
part of the study area (Fig. 3), and an NNW-SSE
trending anticline leads to folds in the rocks in the
middle reaches of the Shikerebembetsugawa River in
the southeastern part of the study area (Fig. 3). Folds
with wavelengths of several hundred meters and
ENE-WSW fold axes are common in the middle
reaches of the Shikerebembetsugawa River in the
southern part of the study area (Fig. 3). Comparable
folds with NNE-SSW and ENE-WSW axes are
mapped in the upper reaches of the
Shikerebembetsugawa River in the southwestern part
of the study area (Fig. 3);
The Shikerepe Formation in the northeastern part
of the study area is characterized by an NNW-SSE to
NW-SE striking homocline and with dips less than
10° toward the NE (Fig. 3);
The faults are aligned in NE-SW, NNE-SSW,
and NW-SE directions in the study area (Fig. 3).
Hydrothermal alteration minerals identified in the
study area include quartz, opal-CT, K-feldspar, albite,
chlorite, illite, interstratified illite/smectite minerals,
interstratified chlorite/smectite minerals, smectite,
nacrite, dickite, kaolinite, analcite, laumontite,
heulandite-clinoptilolite series minerals, mordenite,
chabazite, stilbite, calcite, alunite, minamiite,
natroalunite, and pyrite.
Hydrothermal alteration can be zoned according to the
occurrence of characteristic minerals such as silica
minerals, feldspars, clay minerals, zeolites, and alunite.
The hydrothermal alteration zones have been
identified in the study area, based on mineral
assemblage, propylitic, laumontite, heulandite,
analcite, mordenite, clinoptilolite, stilbite, smectite,
interstratified illite/smectite mineral, illite, K-feldspar,
and alunite-quartz zones (Table 3). These alteration
zones are oblique to bedding planes within the
formations described previously.
The distribution of landslide-related hydrothermal
Relationship between Landslides, Geologic Structures, and Hydrothermal Alteration Zones in the Ohekisawa-Shikerebembetsugawa Landslide Area, Hokkaido, Japan
322
Pleistocene- H olocene Series
L E G E N D
Alluvial river deposits
Landslide deposits
Talus deposits
Low er fluvial terrace deposits
Higher fluvial terrace deposits
Teshikaga Volcano Som m a Lava
D ikes
U pper M iocene Series
Shikerepe Form ation
Hanakushibe Form ation
Pliocene Series
Shikerepem petsu Form ation
Shikerepeyam a Lava
O teshikaushinaiForm ation
G ravel, sand, silt, and clay
Rubble, sand, silt, and clay
Rubble, sand, silt, and clay
G ravel, sand, silt, and clay
G ravel, sand, silt, and clay
Basaltic and andesitic lavas
M udstone and andesitic
C onglom erate, sandsone,
Dacitic lavas
Rhyolite
Andesite
Basalt
volcaniclastic rocks,
m udstone, and dacitic
dacitic volcaniclastic rocks
Strike and dip of strata
Anticlinalaxis
Synclinal axis
Faults
O H S:O hekisaw a Slide
SH S:Shikerebem betsugaw a Slide
▲ ▲
V V
・∟
V・
V・
∟ ∟
Andesitic and dacitic
C onglom erate and andesitic and
and tuff breccia
volcaniclastic rocks
sandstone, and m udstone
volcaniclastic rocks
Al
La
Lr
Hr
Td
Te
Sh
Ha
Sm
Sy
O t
V
V
・
・
30
10
20
20
10
5
5
8
910
∟ ∟ ∟ ∟ ∟ ∟
∟ ∟ ∟ ∟ ∟ ∟ ∟
∟ ∟ ∟ ∟ ∟ ∟ ∟ ∟
∟ ∟ ∟ ∟ ∟ ∟ ∟ ∟ ∟
∟ ∟ ∟ ∟ ∟ ∟ ∟ ∟
∟ ∟ ∟ ∟ ∟ ∟
∟ ∟ ∟ ∟
∟
∟
∟ ∟
▲ ▲ ▲ ▲ ▲
▲
▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲
▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲
▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲
▲ ▲
▲ ▲ ▲ ▲
▲ ▲ ▲ ▲
▲ ▲ ▲
▲ ▲
▲
▲
▲ ▲
▲ ▲ ▲ ▲
▲ ▲ ▲
▲ ▲
▲
▲
▲
V・
V・
V・
V・
V・
V・
∟∟
∟
∟
∟∟
∟
・・
・
・
・・
・
V V
V
V
V V V V
V
V V V V V V
V V V V V
V V V V
V V V
V V V
V V V
V V V
V V V
N
0 1 km
M t. Shikerepeyam a
M t. Nichieizan
La
Al
Hr
Hr
Hr
Lr
Hr
Td
Sy
Sy
Sh
Ha
Sm
Ha
Sh
Al
5
SHS
O HS
O t
Hr
18
19
7
Te
La
Ha
Sh
Ha
O t
Sm
Sh
Sh
Fig. 3 Geological map of the southern part of the Okushunbetsu area. Modified from Refs. [24, 27-29].
alteration zones, and the mineral composition and
uniaxial compressive strengths of their alteration zone
rocks documented in the study area are as follows:
The laumontite zone is widely distributed in the
western part of the study area (Fig. 4) and its rocks
consist mainly of laumontite with lesser amounts of
smectite, quartz, albite, illite, chlorite, calcite, and
pyrite (Table 3). The uniaxial compressive strengths
of laumontite zone weakly weathered tuffaceous
conglomerates from the Ikurushibe Formation in a
forced-dry state range from 84.07 to 91.10 MPa, with
an average of 87.59 MPa, whereas the strengths of
smectite-bearing laumontite zone weakly weathered
lapilli tuff and fine tuff from the Hanakushibe
Formation are 6.18 and 12.91 MPa, respectively
(Table 4).
The mordenite and clinoptilolite zones are widely
distributed in the eastern and southern parts of the study
area and occur locally in the western part (Fig. 4). The
mordenite zone rocks consist mainly of mordenite
with lesser amounts of opal-CT, quartz, smectite,
clinoptilolite, and pyrite (Table 3). The uniaxial
compressive strengths of smectite-lacking mordenite
zone weakly weathered fine tuffs from the Shikerepe
Formation in a forced-dry state range from 17.95 to
44.64 MPa, with an average of 27.37 MPa, whereas
smectite-lacking mordenite zone weakly weathered
pumice tuffs from the Shikerepe Formation have
strengths ranging from 7.24 to 10.62 MPa, with an
average of 8.93 MPa. The uniaxial compressive
strengths of smectite-bearing mordenite zone weakly
weathered pumice tuffs from the Shikerepe Formation
range from 14.00 to 19.69 MPa, with an average of
17.15 MPa (Table 4).
The smectite zone occurs mainly in the northeastern
and southwestern parts of the study area (Fig. 4), and
the rocks consist mainly of smectite with lesser
amounts of opal-CT, quartz, and pyrite (Table 3). The
uniaxial compressive strengths of smectite zone
weakly weathered fine tuffs from the Hanakushibe
Formation in a forced-dry state range from 6.28 to
7.94 MPa, with an average of 7.11 MPa, and smectite
zone weakly weathered fine tuff from the Shikerepe
Formation has a strength of 10.67 MPa (Table 4).
4. Results and Considerations
The bedrocks of the OHS in the southern part of the
Relationship between Landslides, Geologic Structures, and Hydrothermal Alteration Zones in the Ohekisawa-Shikerebembetsugawa Landslide Area, Hokkaido, Japan
323
Table 3 Mineral assemblages of hydrothermal alteration zones.
Alteration zone
Mineral
Sme zone
Stb zone
Cpt zone
Mor zone
Hul Zone
Anl zone
Lmt zone
Prop zone
Ilt/Smezone
Ilt zone
Kfs zone
Alu-Qz zone
Quartz
Opal-CT
K-feldspar
Albite
Illite
Chlorite
Int. Ilt/Sme
Smectite
Stilbite
Clinoptilolite
Mordenite
Heulandite
Chabazite
Analcite
Laumontite
Calcite
Alunite
Natroalunite
Minamiite
Pyrite
Major occurrence
Minor occurrence
Int. Ilt/Sme: interstratified illite/smectite minerals; Sme zone: smectite zone; Stb zone: stilbite zone; Cpt zone: clinoptilolite zone;Mor zone: mordenite zone; Hul zone: heulandite zone; Anl zone: analcite zone; Lmt zone: laumontite zone; Prop zone: propylitic zone; Ilt/Sme zone: interstratified illite/smectite mineral zone; Ilt zone: illite zone; Kfs zone: K-feldspar zone; Alu-Qz zone: alunite-quartz zone.
Relationship between Landslides, Geologic Structures, and Hydrothermal Alteration Zones in the Ohekisawa-Shikerebembetsugawa Landslide Area, Hokkaido, Japan
324
L E G E N D
Faults
Illite zone
Least altered zone
Analcite zone
Propylitic zone
O utline of ancient landslide
Alunite-quartz zone
Sm ectite zone
M ordenite and clinoptilolite zones
Laum ontite zone
Int. Ilt/Sm e zone
Int. Ilt/Sm e zone:Interstratified illite/sm ectite m ineral zone
Heulandite and stilbite zones
O HS:O hekisaw a Slide
SHS:Shikerebem betsugaw a Slide
M t. Shikerepeyam a
M t. Nichieizan
N
1 km0
O HS
SHS
Fig. 4 Hydrothermal alteration map of the southern part of the Okushunbetsu area.
Okushunbetsu area are weathered and hydrothermally
altered and are composed of coarse tuff and tuffaceous
medium sandstone of the Upper Miocene Shikerepe
Formation. The landslide has a dip slope structure
with an NNW-SSE strike and a dip direction of 7°E
(Fig. 3 and Table 1). The scarp of the OHS consists of
an N-S striking andesite dike that is 125 m wide and
325 m long (Fig. 3). The bedrock under the upper part
of the landslide body is composed of smectite-bearing
mordenite zone rocks, whereas that under the middle
and lower parts of the landslide body is composed of
smectite zone rocks (Fig. 4 and Table 1). As described
previously, the uniaxial compressive strengths of
smectite-bearing mordenite zone weakly weathered
pumice tuffs from the Shikerepe Formation range
from 14.00 to 19.69 MPa, with an average of 17.15
MPa and smectite zone weakly weathered fine tuff
from the Shikerepe Formation has a strength of 10.67
MPa (Table 4). These bedrocks, therefore, are
classified as soft rocks.
The SHS bedrock also consists of weathered and
hydrothermally altered rocks, mainly fine tuff,
mudstone, and lapilli tuff of the Upper Miocene
Hanakushibe Formation. The landslide has a dip slope
structure as a whole because the bedrocks have a
synclinal structure on an ENE-WSW axis; the
bedrocks under the upper and middle parts of the
landslide body dip 10° toward the SE, whereas those
of the lower part dip 0-5° toward the NW (Fig. 3). The
landslide scarp consists of a cap rock of Teshikaga
Volcano Somma Lava. The bedrocks under the upper
part of the landslide body are composed of smectite
zone rocks, whereas those under the middle and lower
parts are composed of smectite-bearing laumontite
zone rocks (Fig. 4 and Table 1). As described
previously, the uniaxial compressive strengths of
smectite-bearing laumontite zone weakly weathered
lapilli tuff and fine tuff from the Hanakushibe
Formation in a forced-dry state are 6.18 and 12.91
MPa, respectively, and those of smectite zone weakly
weathered fine tuffs from the Hanakushibe Formation
range from 6.28 to 7.94 MPa, with an average of 7.11
MPa. These, therefore, are also classified as soft
rocks.
Both of the landslides, therefore, occurred
mechanically on very weak rocks consisting of
smectite-bearing weathered and hydrothermally
altered soft rocks such as smectite, smectite-bearing
mordenite, and smectite-bearing laumontite zone
rocks.
The OHS occurred on weathered and
hydrothermally altered rocks within the boundary area
between the smectite zone and the smectite-bearing
mordenite zone, whereas the SHS occurred on
weathered and hydrothermally altered rocks within the
boundary area between the smectite and
smectite-bearing laumontite zones. This circumstance
has been reported in only a few other cases such as the
Relationship between Landslides, Geologic Structures, and Hydrothermal Alteration Zones in the Ohekisawa-Shikerebembetsugawa Landslide Area, Hokkaido, Japan
325
Table 4 Uniaxial compressive strength of hydrothermal alteration zone rocks in a forced dry-state, dried in an electric oven at 60 ± 3°C, in the southern part of the Okushunbetsu area.
Hydrothermal alteration zone Formation Rock facies N qu (MPa)
Smectite-lacking laumontite zone Ikurushibe Fm. Weakly weathered tuffaceous conglomerate 1 84.07 Smectite-bearing laumontite zone Smectite-bearing laumontite zone Smectite-bearing laumontite zone
Ikurushibe Fm. Hanakushibe Fm. Hanakushibe Fm.
Weakly weathered tuffaceous conglomerate Weakly weathered fine tuff Weakly weathered lapilli tuff
1 1 1
91.10 12.91 6.18
Smectite-lacking mordenite zone Smectite-lacking mordenite zone
Shikerepe Fm. Shikerepe Fm.
Weakly weathered fine tuff Weakly weathered pumice tuff
35 2
17.95-44.64 (Avg. 27.37) 7.24-10.62 (Avg. 8.93)
Smectite-bearing mordenite zone Shikerepe Fm. Weakly weathered pumice tuff 20 14.00-19.69 (Avg. 17.15) Smectite zone Smectite zone
Shikerepe Fm. Hanakushibe Fm.
Weakly weathered fine tuff Weakly weathered fine tuff
1 2
10.67 6.28-7.94 (Avg. 7.11)
N: number of specimens; qu: uniaxial compressive strength; Avg.: average.
Yasukuni landslide area [10] and the Kanehana
landslide area [12] in northeastern Hokkaido, Japan.
Three ancient landslides are known to have
occurred in the Sattomonai-Okushunbetsu area in the
western part of Teshikaga Town [18]. Two in the
Sattomonai area and one in the northern part of the
Okushunbetsu area occurred within the hydrothermal
interstratified illite/smectite mineral zone and the
smectite zone, respectively [18]. These landslides are
classified as HAZLs based on their bedrock geology.
According to Ref. [18], the bedrocks of the landslide
in the northern part of the Okushunbetsu area are
weathered and hydrothermally altered rocks composed
mainly of fine tuff of the Upper Miocene Hanakushibe
Formation. This fine bedrock tuff belongs to a
smectite-bearing mordenite zone having a
hydrothermal alteration mineral assemblage of
mordenite-smectite-quartz whereas a debris of fine
tuff in the landslide body belongs to a
smectite-lacking mordenite zone having an
assemblage of mordenite-quartz. The landslide
occurred on a dip slope of fine tuff from the
Hanakushibe Formation. The scarp of the landslide
consists mainly of basaltic lava of Pekereyama Lava,
which is a cap rock. Therefore, the landslide is
classified as a weathered and hydrothermally altered
rockslide based on its landslide body [7, 30, 31] and
as an HAZL on the basis of bedrock geology. In
contrast, the bedrocks of the two landslides in the
Sattomonai area, according to Ref. [18], are also
weathered and hydrothermally altered rocks and are
mainly composed of lapilli tuff of the Upper Miocene
Oteshikaushinai Formation and Lower Pliocene
Ikurushibeyama Lava. Debris of lapilli tuff in the
landslide body belongs to the interstratified
illite/smectite mineral zone having a hydrothermal
alteration mineral assemblage of quartz-interstratified
illite/smectite minerals-calcite. The scarps of these
landslides consist mainly of andesitic lava from the
Ikurushibeyama Lava, which is a cap rock. Therefore,
the landslides are classified as weathered and
hydrothermally altered rockslides based on their
landslide body and as HAZLs based on their bedrock
geology.
In the Sattomonai-Okushunbetsu area, five ancient
landslides, including the OHS and the SHS occurred
within the hydrothermal smectite, interstratified
illite/smectite mineral, smectite-bearing mordenite,
and smectite-bearing laumontite zones. All these
landslides are classified as HAZLs on the basis of
their bedrock geology, and weathered and
hydrothermally altered rockslides on the basis of their
landslide body types. All these landslides also
occurred within weathered and hydrothermally altered
soft rocks. This indicates that HAZLs occur within
weathered and hydrothermally altered, very weak, soft
rocks of the smectite zone, interstratified
illite/smectite mineral zone, and smectite-bearing
zeolite zone such as the smectite-bearing mordenite
zone, smectite-bearing laumontite zone, etc.. The
HAZL potential within a hydrothermal area can,
therefore, be assessed by the swelling clay
Relationship between Landslides, Geologic Structures, and Hydrothermal Alteration Zones in the Ohekisawa-Shikerebembetsugawa Landslide Area, Hokkaido, Japan
326
mineral-bearing hydrothermal alteration types, dip
slope, and cap rock.
5. Conclusions
The relationship between two ancient landslides,
the OHS and SHS, and their geological characteristics
is as follows:
Both of the landslides occurred within the
Pliocene to Early Pleistocene hydrothermal alteration
zones in the tuffaceous clastic rocks and the
volcaniclastic rocks of the Upper Miocene
Hanakushibe and Shikerepe Formations. These
landslides are classified as HAZLs based on bedrock
geology;
Both of the landslides are classified as weathered
and hydrothermally altered rockslides based on their
landslide bodies.
The OHS occurred within the boundary area
between the hydrothermal smectite zone and
smectite-bearing mordenite zone.
The SHS occurred within the boundary area
between the hydrothermal smectite zone and
smectite-bearing laumontite zone.
The scarp of the OHS consists of a dike, whereas
that of the SHS consists of a cap rock of lava.
The HAZLs in the Sattomonai-Okushunbetsu area
of western Teshikaga Town, Hokkaido, Japan,
occurred within weathered and hydrothermally altered,
very weak, soft rocks such as those of the smectite,
interstratified illite/smectite mineral, smectite-bearing
mordenite, and smectite-bearing laumontite zones.
The HAZL potential within a hydrothermal field
can be assessed based on the swelling clay
mineral-bearing hydrothermal alteration types, dip
slope, and cap rock.
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