Physiographical and sedimentological characteristics of
submarine canyons developed upon an active forearc slope:
The Kushiro Submarine Canyon, northern Japan
Atsushi Noda
Taqumi TuZino
Ryuta Furukawa
Masato Joshima
Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Central 7, 1-1-1Higashi,
Tsukuba, Ibaraki 305-8567, Japan
Jun-ichi Uchida
Department of Earth Science, Faculty of Science, Kumamoto University, 39-1, Kurokami 2-chome, Kumamoto 860-8555, Japan
GSA Bulletin; May/June 2008; v. 120; no. 5/6; p. 750–767; doi: 10.1130/B26155.1; 15 figures; Data Repository item 2008047.
For permission to copy, contact [email protected]
© 2008 Geological Society of America
Physiographical and sedimentological characteristics of
submarine canyons developed upon an active forearc slope:
The Kushiro Submarine Canyon, northern Japan
Atsushi Noda†
Taqumi TuZino
Ryuta Furukawa
Masato Joshima
Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Central 7, 1-1-1Higashi,
Tsukuba, Ibaraki 305-8567, Japan
Jun-ichi Uchida
Department of Earth Science, Faculty of Science, Kumamoto University, 39-1, Kurokami 2-chome, Kumamoto 860-8555, Japan
ABSTRACT
Comprehensive geological surveys have
revealed the physiographical and sedimentological characteristics of the Kushiro Submarine Canyon, one of the largest submarine
canyons around Japan. The canyon indents
the outer shelf along a generally straight,
deeply excavated course of more than 230 km
in length upon the active forearc slope of the
Kuril Trench in the Northwest Pacific. The
forearc slope has a convex-upward geometry that can be divided into upper and
lower parts separated by an outer-arc high
(3200–3500 m water depth). The upper slope
consists of gently folded forearc sediments,
and the lower slope is underlain by sedimentary rocks deformed by subduction-related
processes. The upper reaches of the canyon
(~3250 m of thalweg water depth) are developed on the upper slope, showing a weakly
concave-upward longitudinal profile with
a gradual down-canyon increase in relief
between the thalweg and the canyon rim.
Although an infill of hemipelagic mud and
the absence of turbidite deposits indicates
that the upper part of the upper reaches of
the canyon (~900 m thalweg water depth) is
inactive, the lower part of the upper reaches
(900–3250 m thalweg water depth) is considered to be an active conduit to the lower
reaches, as determined from voluminous turbidites recovered in sediment cores (~76-yr
intervals) and rockfalls observed in the canyon bottom by deep-sea camera. A number
†
E-mail: [email protected]
of gullies developed upon the northern slope
of the lower part of the upper reaches might
well provide a frequent supply of turbidity currents, giving rise to a down-canyon
increase in the frequency of flow events. The
down-canyon increase in flow occurrence
is related to a gradual decrease in gradient, demonstrating an inverse power-law
relationship between slope and drainage
area. In contrast, the lower reaches of the
canyon (3250–7000 m thalweg water depth)
are characterized by a gradual decrease in
relief, a high gradient, and extremely low
sinuosity. The limited increase in drainage
area down-canyon of the confluence with
the Hiroo Submarine Channel, which is the
largest tributary of the main canyon, indicates that the erosional force of turbidity
currents decreases down-canyon. The gradient of the lower reaches largely reflects
the morphology of the forearc slope along
the canyon, which has been deformed by
subduction-related tectonics. The lack of
an inverse power-law relationship between
gradient and drainage area in the lower canyon supports the hypothesis that the topography of the lower reaches is dominated by
subduction-related tectonic deformation of
the substrate rather than canyon erosion.
Interrelationships between canyon erosion
by currents and tectonic processes along the
forearc slope are important in the development of the physiography of submarine canyons upon active forearc margins.
Keywords: Submarine canyon, forearc slope,
turbidite, erosion, Kuril Trench, Japan.
INTRODUCTION
For submarine canyons developed upon stable
continental slopes, erosion and sedimentation via
gravity flows are considered the primary mechanisms of the evolution of canyon physiography
(Daly, 1936; Heezen and Ewing, 1952; Menard,
1955; Shepard and Emery, 1941). Submarine
canyons and channels in such passive tectonic
regimes tend to exhibit concave-upward thalweg
profiles, which account for tectonic deformation
that is less effective than sediment fluxes, as
recorded in both aggradational channels such as
the Amazon Channel (Flood and Damuth, 1987;
Pirmez and Imran, 2003) and erosional canyons
such as the Zaire Canyon (Babonneau et al.,
2002; Droz et al., 1996) and Rhône Fan Valley
(Droz and Bellaiche, 1985; O’Connell et al.,
1991; Torres et al., 1997). This asymptotic form
of channel profile is considered to be a function
of the loss of flow competence as it erodes and
transports material, resulting in a progressive
decrease in downslope gradient (Carter, 1988;
Kneller, 2003; O’Connell et al., 1991; Pirmez
et al., 2000; Prather et al., 1998).
In addition to these exogenetic processes,
endogenetic processes (e.g., tectonically controlled) and the structural fabric of underlying
rocks are also important in terms of submarine
canyon development upon forearc margins
where subduction-related tectonics influences
slope morphology (e.g., Greene et al., 2002;
Hagen, 1996; Klaus and Taylor, 1991; Laursen
and Normark, 2002; Lewis et al., 1998; Soh
et al., 1990; Soh and Tokuyama, 2002). The
planforms of canyons developed upon the slopes
of active margins are typically erosional, and
GSA Bulletin; May/June 2008; v. 120; no. 5/6; p. 750–767; doi: 10.1130/B26155.1; 15 figures; Data Repository item 2008047.
750
For permission to copy, contact [email protected]
© 2008 Geological Society of America
Physiography and sedimentology of submarine canyons on a forearc slope
125°E
130°E
A
45°N
135°E
140°E
Okhotsk Plate
(North American Plate)
145°E
150°E
Sakhalin
Okhotsk Sea
rc
ch
N
n
Tre
ril
Ku
Northeastern
Japan Arc
Japan Tre
nch
Japan Sea
40°N
Eurasian Plate
Tokyo
35°N
Pacific
Plate
Pacific
Ocean
Philippine Sea
Plate
30°N
45°N
B
50 km
44°N
Hokkaido
N
rc
r
Ku
43°N
42°N
Ta
ns
tai
un
Mo
Us
Nemuro
Kushiro
ka
Sapporo
Hiroo
Ko
KSC
TFB
HSC
r
ute
h
nc
re
T
il
ur
8 cm/yr
41°N
139°E
h
hig
O
K
Fig. 2
Hokkaido is currently subjected to ongoing collisional tectonics at an arc-arc junction
(Fig. 1) where a forearc sliver of the Kuril Arc is
colliding with the Northeast Japan Arc (Kimura,
1996). Oblique subduction of the Pacific Plate
along the southern Kuril Trench may have given
rise to dextral movement of the Kuril Arc in
the late Miocene (Kimura, 1986), and may still
be occurring today (DeMets, 1992; Moriya,
1986). Islands along the southern Kuril forearc
(Fig. 1) and geological structures within eastern
Hokkaido have en echelon orientations that are
consistent with the dextral strike-slip motion of
the Kuril Arc (Fitch, 1972; Kimura, 1986).
Under the present tectonic regime, the Pacific
Plate is subducting beneath the Okhotsk (North
il A
Ma
PHYSICAL SETTING
Land Area
155°E
lA
ri
Ku
300 km
a
Hid
exhibit sharp bends or conspicuous knickpoints
that may be controlled by transverse accretionary
ridges, outer-arc basement highs and mounds,
or strike-slip faults (Hagen, 1996; Klaus and
Taylor, 1991; Lewis and Barnes, 1999; Laursen
and Normark, 2002; Soh and Tokuyama, 2002).
The morphology of submarine canyons developed upon forearc slopes is therefore considered to be primarily determined by interactions
between the relative rate of basement deformation by accretion or compression and the rate
of mass redistribution by surface processes.
Despite recent major advances in the seafloor
and subsurface imaging of submarine canyons,
our knowledge of the effects of active tectonics
on canyon morphology is insufficient to enable
quantitative studies of canyon development on
forearc slopes. Quantifying the geomorphological and sedimentological characteristics of submarine canyons on forearc slopes would therefore assist in improving our understanding of
canyon evolution along active plate margins.
The Kushiro Submarine Canyon cuts the
forearc slope along an active convergent margin
off eastern Hokkaido, along the Kuril Trench.
Although this is one of the largest submarine
canyons within Japanese waters, its geomorphological and geological features have yet to
be firmly established. The aims of this paper are
therefore to (1) quantitatively describe the morphological features of the Kushiro Submarine
Canyon along the active forearc margin, (2)
investigate recent erosional and depositional
processes within the canyon, and (3) quantify
the relative impacts of tectonics, sedimentation,
and erosion on the evolution of the canyon profile. The results and associated canyon model
are potentially applicable to other submarine
canyons developed upon active margin slopes.
140°E
141°E
142°E
143°E
144°E
145°E
146°E
147°E
Figure 1. Tectonic setting and location of the study area. KSC—Kushiro Submarine Canyon;
HSC—Hiroo Submarine Channel; TFB—Tokachi-oki forearc basin; Ko—Komagatake volcano; Us—Usu volcano; Ta—Tarumai volcano; Ma—Mashu volcano.
American) Plate in the direction of N62° W at a
rate of ~8 cm y−1 (DeMets et al., 1990). Recent
global positioning system (GPS) observations
suggest that eastern Hokkaido is moving in a
WSW direction at a rate of 2–3 cm y−1 (Ito et al.,
2000). Based on surveys of marine terraces, the
uplift rate of eastern Hokkaido over the past
0.1 Ma is estimated to be 0.2 mm y−1, with an
accumulated increase in elevation over this time
of 20 m (Okumura, 1996).
Coastal and Submarine Areas
The coastline between Nemuro and Kushiro
is complex, with several lagoons and rock cliffs
as high as 80 m above sea level (Fig. 2). In contrast, the coastline between Kushiro and Hiroo
is smooth and arcuate. The Tokachi River is one
of the largest rivers in Hokkaido that drains into
the Pacific Ocean, with the Kushiro River having a lesser discharge. The drainage area, length,
Geological Society of America Bulletin, May/June 2008
751
Noda et al.
143˚E
144˚E
145˚E
Kutcharo
Caldera
N
km
0
50
Mashu
Volcano
Nemuro
Kushiro
River
Kushiro
Swamp
Akan
River
43˚N
A
Kushiro
Fig. 4
Kushiro
Submarine
Canyon
Obihiro
00
−10
Rekifune
River
B
Hiroo
0
00
−2
Tokachi
River
es
lin
c
i
t
An
Hiroo
Submarine
Channel
0
−300
e
rac
ter
r
pe
h
Up
r
uote
Cape Erimo Sout
h
Hi
ro
oS
pu
Sp
ur
B´
−5
00
0
er
Low ace
r
ter
0
Fig. 55
A series of topographic highs
Submarine
canyon/channel
−1000
0
00
−2
0
−
0
30
0
00
0–500 m
−400
Rivers
Anticline
500–1000 m
O
r
> 2000 m
1000–2000 m
Fig. DR1
−50
Elevation above sealevel
Hi
ro
o
−4
00
0
42˚N
hig
Boundary for drainage areas
00
−6
0
00
−7
Kuril Trench
0
00
−7
A´
Figure 2. Bathymetric map of the drainage areas of the Kushiro Submarine Canyon and the Hiroo Submarine Channel. Structural data
were derived from TuZino and Noda (2007). Lines A–A′ and B–B′ indicate the locations of the cross-sectional bathymetric profiles shown in
Figure 3. Dashed rectangles indicate the areas covered by the detailed topographic maps shown in Figures 4, 5, and DR1.
and average flow rate of the Tokachi River are
9010 km2, 156 km, and 256 m3 s−1, respectively;
the equivalent values for the Kushiro River are
2510 km2, 154 km, and 60 m3 s−1. The shelf in
this area has a relatively narrow (10–30 km)
width, with the narrowest part coinciding with
752
the Kushiro Submarine Canyon. The uppermost
slope (at water depths of 200–1000 m) is more
than 5° (Fig. 3), decreasing to 2–3° in the upper
part (1000–3500 m depth). A series of anticlines
are observed at a water depth of 2000 m (Figs.
2, 3, and 4); these appear to be the extensions of
NE–SW trending anticlines observed off Hiroo
(Fig. 2), which may have begun to form in the
Pliocene (TuZino and Noda, 2007).
Outer-arc high (3200–3500 m water depth)
exists as an elongate topographic high oriented
parallel to the trench (Figs. 1 and 2) and is placed
Geological Society of America Bulletin, May/June 2008
Physiography and sedimentology of submarine canyons on a forearc slope
0
Water depth (km)
1
2
KSC
A–A′
B–B′
3
4
0°
1°
2°
5°
5
6
Anticline
Kuril Trench
Outer high
10°
7
0
50
100
150
200
Distance (km)
Figure 3. Cross-sectional bathymetric profiles of the forearc slope. The locations of lines
A–A′ and B–B′ are shown in Figure 2. The average slopes (m/m) are 0.048 for A–A′ from
the shelf break to the Kuril Trench, and 0.026 for B–B′ from the shelf break to the front
of the Kushiro Submarine Canyon.
144°20′E
144°30′E
144°40′E
144°50′E
between the forearc basin (upper slope) and
the accretionary prism (lower slope) (cf. Clift
et al., 1998; Dickinson and Seely, 1979;
Gulick et al., 2002; McNeill et al., 2000). The
high can be correlated with the middle-slope
boundary of a steep landward-dipping reflection described by Klaeschen et al. (1994). Two
terraces are recognized on the slope: the upper
terrace (3200 m water depth) is located on the
upper slope immediately behind the outer high,
and the lower is located on the trenchward side of
the outer high (5000–5500 m water depth) (Figs.
2 and 5). The upper terrace is underlain by wellstratified reflectors (Schnürle et al., 1995; TuZino
et al., 2006) and possibly formed via a combination of subsidence and regional tilting (Klaeschen
et al., 1994). TuZino et al. (2006) reported the
formation of half-grabens along the upper terrace
145°00′E
km
N
10A
Fig.
145°10′E
5
0
10
Seismic survey
42°50′N
Deep-sea camera
Fig.
−5
00
Terrace
10B
Core localities
Anticline
Segment A
42°40′N
Segment B
GH03–1033
Fig.
−500
10C
00
−10
Fig. 10E
00
−15
42°30′N
Fig. 8
Fig.
0
00
−1
0
−250
A
42°20′N
11
Fig.
10D
KR0504–PC07
−2000
11B
Fig.
−3
000
00
−2
−
0
Segment C
00
15
KR0504–
PC09
42°10′N
KR0504–PC10
Figure 4. Detailed bathymetry of the upper segments (A–B) of the Kushiro Submarine Canyon. Solid
lines indicate the locations of seismic profiles (Figs. 10 and 11), and dashed lines indicate the locations
of deep-sea camera survey lines. Solid circles denote the sampling localities of sediment cores (Fig. 12;
Table DR2).
Geological Society of America Bulletin, May/June 2008
753
Noda et al.
145˚00'E
42˚10'N
145˚10'E
A
145˚30'E
gh
r hi
e
t
Ou
Depth(km)
0
00
−3
−4000
HSC
Segment D
20
30
40
Right-side
−3.5
Left-side
−4.0
−5000
−4.5
Thalweg
−5.0
00
000
0
00
41˚50'N
−5
−5
−5000
−5
00
0
00
0
−6000
−6
41˚40'N
−6
00
0
Segment E
D
Segment D
00
00
0
0
−7
41˚30'N
N
r
floo
h
c
n
Tre
km
1
GSA Data Repository Item 2008047, submarine
canyons on an active forearc slope, is available at
www.geosociety.org/pubs/ft2008.htm. Requests may
also be sent to [email protected].
10
−40
0
Kushiro Submarine Canyon
The Kushiro Submarine Canyon, with an
8266 km2 drainage area below the shelf break and
a 233 km channel length, is a large shelf-indenting
canyon located off the eastern Hokkaido forearc
of northern Japan (GSA Data Repository Table
DR11; Sato, 1962; Shimamura, 1989; Yo, 1953).
The canyon head incises the forearc slope from
north to south; the canyon then changes course
to the east, and finally to the southeast or southsoutheast to the Kuril Trench at a water depth
of ~7000 m (Fig. 2). The canyon cuts through
two prominent topographic highs: an anticline
(2000 m water depth) and the outer high (3,000 m
water depth) (Figs. 2, 4, and 5). Two large tributaries empty into the canyon at water depths of
1700 m and 4800 m (Fig. 2); the deeper of these
tributaries is known as the Hiroo Submarine
0
−3.0
Segment C
42˚00'N
145˚40'E
Distance (km)
B
Overview of the Canyon and Channel
754
145˚20'E
−6
and proposed that displacement of the outer high
originally occurred along normal faults.
The slope between the upper and lower terraces exceeds 5° and is covered by a 600-mthick series of low-amplitude reflections oriented parallel to the slope and underlain by
weakly folded Neogene sediments (Schnürle
et al., 1995). Uplift of this slope might have
resulted from the underplating of sediment
within the subduction zone (Klaeschen et al.,
1994). The origin of the lower terrace has been
explained in terms of the upheaval of the ridges
located immediately trenchward of the terrace
(Ogawa et al., 1993). The slope below the lower
terrace is characterized by ridges and domes
(Ogawa et al., 1993). The toe of the Kuril margin consists of a sedimentary wedge (P-wave
velocity of <4.0 km s−1) interpreted to have been
strongly deformed during subduction-related
plate dynamics (Nakanishi et al., 2004).
The Tokachi forearc basin lies between the
Kushiro Submarine Canyon and the Hiroo Spur
(Figs. 1 and 2). The slope of the basin averages 1.5°, which is gentler than that off Kushiro
(Fig. 3). The Tokachi forearc basin beneath the
Hiroo Submarine Channel is considered to be an
area of longstanding subsidence, as suggested by
a negative gravity anomaly (Honza et al., 1978),
seismic data (TuZino et al., 2004), and the results
of drilling (Sasaki et al., 1985). The presence of
more than 4000 m of Neogene sediments in the
basin suggests an average sedimentation rate of
330 m/Ma during the Miocene and 160 m/Ma
during the Plio-Pleistocene (Sasaki et al., 1985).
5
0
00
−7
10
Figure 5. (A) Bathymetry of the lower segments (C–E) of the Kushiro Submarine Canyon.
(B) Longitudinal plots of the water depths of the canyon bottom (thalweg) and the tops of
the canyon walls. HSC—Hiroo Submarine Channel.
Channel. In addition, several small gullies join
the canyon on its northern side between water
depths of 1800 m and 2500 m (Fig. 4). At the
outer high, the canyon shows a right-lateral displacement of 1–2 km (Fig. 5). A submarine fan
(10 km long and 35 km wide) has developed
at the end of the Kushiro Submarine Canyon,
within the trench (Fig. 5) (Ogawa et al., 1993).
Hiroo Submarine Channel
With a drainage area of ~4000 km2 below the
shelf break and a channel length of 156 km, the
Hiroo Submarine Channel is the longest tributary
of the Kushiro Submarine Canyon (Fig. 2; Table
DR1 [see footnote 1]). The channel heads are situated at a water depth of 300 m and thus do not
indent the outer shelf (Fig. 2). The Hiroo Sub-
marine Channel runs eastward with low to moderate sinuosity until it merges with the Kushiro
Submarine Canyon at a water depth of 4800 m.
Several other tributaries from the northwest
converge at acute angles (65° ± 20°), forming a
pinnate drainage pattern. According to TuZino
and Noda (2007), the architectural development
of the channel occurred by aggradation, with the
progressive growth of levees along the channel.
DATA AND METHODS
Our study is primarily based on an analysis of
multibeam, swath-bathymetric data (SeaBeam
and Hydrosweep). These data were collected by
the Hydrographic and Oceanographic Department of the Japan Coastal Guard (Maritime
Geological Society of America Bulletin, May/June 2008
Physiography and sedimentology of submarine canyons on a forearc slope
D = α Lβ ,
of a linear, exponential, or Gaussian profile
(Adams and Schlager, 2000; Goff, 2001). We
divided the profile into segments based on channel curvature (Fig. 6). Longitudinal profiles of
channel water depths and lengths for each segment fit the following power function:
Channel water depth (km)
A
(1)
where L is channel length, D is channel water
depth, and α and β are constants (Komar, 1973).
The constants of α and β were calculated via an
implementation of the nonlinear least-squares
0
Right-hand canyon wall
Left-hand canyon wall
2
HSC
0º
0.5º
1º
4
5º
2º
A (D=0.086L0.71)
B (D=0.030L0.94)
C (D=0.0010L1.9)
6
D (D=0.013L0.65)
E (D=0.047L1.2)
0
50
100
150
200
Channel length (km)
B
Channel water depth (km)
Figure 6. Longitudinal depth
profiles along the Kushiro
Submarine Canyon (A) and
the Hiroo Submarine Channel
(B) with curves fitted by nonlinear least-squares estimates.
Arrows indicate points where
tributaries join the main canyon and channel. (C) Comparison of the Kushiro Submarine
Canyon with other submarine
canyons developed upon active
tectonic margins. Data for
Aoga Shima Canyon, Monterey
Canyon, and the San Antonio
Submarine Canyon are derived
from Klaus and Taylor (1991),
Greene et al. (2002), and Hagen
(1996), respectively.
0
2
a (D=0.014L0.49)
b (D=0.012L1.0)
c (D=0.0045L1.6)
4
d (D=0.056L0.73)
e (D=0.0034L2.1)
0
C
Channel water depth (km)
Safety Agency, 1998); cruises KH90–1 and
KH92–3 of the R/V Hakuho-Maru of the
Ocean Research Institute, University of Tokyo
(Kobayashi et al., 1998; Ogawa et al., 1993);
cruise KR0504 (April and May 2005) of R/V
Kairei, run by the Japan Agency for MarineEarth Science and Technology (JAMSTEC);
and cruises GH02 (August 2002) and GH03
(June 2003) of the Geological Survey of Japan,
the National Institute of Advanced Industrial
Science and Technology (AIST) conducted
using the R/V Hakurei-maru No. 2 of the Japan
Oil, Gas and Metals National Corporation
(JOGMEC). Quantitative parameters of canyon
water depth, relief, width, gradient, and sinuosity were measured at 5 km intervals along the
thalweg on bathymetric maps with 10 m contours compiled using 100–200 m grid data.
Seismic reflection profiles were collected
during cruises GH02 and GH03 using a GI gun
(generator 250 in3 and injector 105 in3 air gun)
with a six-channel streamer cable. The survey
speed was 8 knots (14.8 km/h), and the shooting
interval was 6 s. The survey lines were densely
meshed at intervals of 2 miles (3.7 km) E-W and
4.5 miles (8.3 km) N-S.
Gravity and piston cores were collected in
and around the Kushiro Submarine Canyon
(Fig. 4; Table DR2). Samples of volcanic ash
were collected for measurements of their glass
chemistries to enable comparisons with those
reported previously (Furukawa et al., 1997;
Furukawa and Nanayama, 2006; Katsui et al.,
1978; Shimada et al., 2000). Benthic foraminifers were extracted from sandy turbidites and
hemipelagic mud to determine their sources,
as deduced from the distributions of modern benthic foraminifers (Abe and Hasegawa,
2003; Matsuo et al., 2004). Halved cores were
measured by gamma-ray attenuation at 1 cm
intervals using a GEOTEK multisensor core
logger. Accelerator mass spectrometer (AMS)
14
C measurements of planktonic and benthic
foraminifers were carried out on five horizons
(seven samples) in cores PC07, PC09, and
PC10. We used a reservoir age of 386 ± 16 yr for
the 14C ages in this region (Yoneda et al., 2001).
The obtained conventional radiocarbon ages
were calibrated to calendar ages using CALIB
rev. 5.0.2 (Stuiver and Braziunas, 1993) and the
data set marine04.14c (Hughen et al., 2004).
50
100
Channel length (km)
0
150
Monterey
canyon
2
Kushiro
Submarine
Canyon
San Antonio
Submarine
Canyon
4
RESULTS
Aoga Shima
Canyon
6
Quantitative Morphological Parameters
Longitudinal Profile
The slope of the canyon and channel increases
downstream (Fig. 6), meaning that the entire
longitudinal profile cannot be described in terms
0
Geological Society of America Bulletin, May/June 2008
50
100
150
200
Channel length (km)
755
Noda et al.
method. For values of the exponent β of less
than 1 in equation 1, the profiles show concaveupward shapes.
The longitudinal profiles of the two studied
channels are similar (Figs. 6A and 6B), and
both are divided into five segments. As a whole,
the channels have convex-upward profiles, comparable with those of submarine canyons developed within other active margins (Fig. 6C).
Relief
Relief is defined here as the difference in
water depth between the thalweg and the canyon rim. Relief within the Kushiro Submarine
Canyon increases gradually in the upper segments (A–B), maintains high values within
Segment C, and then decreases toward the
trench (D–E) (Fig. 7A). The right-hand relief
(when looking down-canyon) is higher than
the left-hand relief (Table DR1), especially at
water depths of between 2800 and 5000 m. The
Anticline
A
Outer high
Relief (m)
1000
pattern of relief within the Hiroo Submarine
Channel is similar to that in the Kushiro Submarine Canyon, increasing with channel water
depth to 2500 m before gradually decreasing to
the channel end (Fig. 7A).
Gradient
The gradient of the Kushiro Submarine Canyon shows a gradual decrease, although with
some fluctuations, over Segments A and B
(Fig. 7C); the gradient is approximately constant where the canyon traverses the anticline.
The gradient increases markedly across Segment C where the canyon crosses the outer high
and then decreases with widening of the canyon
and reducing relief. At the end of the canyon the
gradient increases again, reaching a maximum
(0.16) at a point outside of the area shown in
Figure 7C. The gradient of the Hiroo Submarine
Channel shows a similar trend to that for the
Kushiro Submarine Canyon (Fig. 7C).
KSC (left)
KSC (right)
HSC (left)
HSC (right)
800
600
400
200
0
Width (km)
B
2
KSC
HSC
1.5
1
0.5
0
Gradient (m/m)
C
Sinuosity
D
Drainage area (103 km2)
E
Figure 7. Morphological profiles
along the Kushiro Submarine
Canyon (KSC) and the Hiroo
Submarine Channel (HSC). (A)
Channel relief with respect to
thalwegs. (B) Channel width. (C)
Channel gradient. (D) Sinuosity.
(E) Drainage area.
KSC
HSC
0.1
0.08
0.06
0.04
0.02
0
2
KSC
HSC
1.5
1
8
6
4
KSC
HSC
2
0
0
1
2
3
4
5
6
7
Channel depth (km)
Segments
KSC
HSC
756
Sinuosity
Along almost all reaches of the canyon, the
sinuosity is low (0.01–0.03) or approximately
straight. The highest sinuosity recorded within
the Kushiro Submarine Canyon occurs in Segment B, where the canyon crosses the upstream
side of the anticline and the gradient remains
relatively low (0.01–0.03) (Figs. 4, 7D, and 8).
Meandering bends in the canyon show asymmetric profiles, with steep cutbanks on the concave sides of bends and relatively gentle slip-off
slopes on the convex sides (Fig. 8). The progressive downstream decrease in the radius of curvature and meander wavelength (Fig. 8) leads
to their classification as deformed ingrowth
meanders (e.g., Schumm, 1977).
High-sinuosity sections of canyon are also
recognized in the Hiroo Submarine Channel
(Fig. 7D). The first sinuous section (~1000 m
water depth) corresponds to transection of the
Hiroo Spur (Fig. 2), while the second corresponds to increased gradient and relief (Figs.
7 and DR1). The gradual decrease in meander
wavelength and the radius of curvature and
degree of asymmetry of cross-sectional profiles
suggest that meanders within the Hiroo Submarine Channel are also deformed ingrowth
meanders (Fig. DR1).
A
a
B
b
c
d
C
e
D
E
Drainage Area
The topography of bedrock rivers typically
exhibits a scaling between the local channel gradient (S) and the contributing upstream drainage
area (A) (Flint, 1974; Hack, 1957; Howard and
Kerby, 1983). By analogy with subaerial rivers,
the erosion rates (E) of submarine canyons can
be written as E = KAmSn, where K is a dimensional coefficient of erosion, and m and n are
positive constants that depend on basin hydrology, channel hydraulic geometry, and erosion
process (Howard, 1994; Whipple and Tucker,
1999). If the bedrock uplift rate U is constant,
a topography that has evolved to a steady state
has balanced erosion and uplift such that U =
KAmSn. Hence, there arises an inverse power-law
relationship between S and A (a proxy for discharge), with S = kA−θ, where k is the steepness
index and θ is the concavity index (e.g., Whipple,
2004). The concavity index derived from river
catchment data generally varies between 0.1
and 1.1, although values generally range from
0.3 to 0.6 for onshore areas (Snyder et al., 2000;
Tucker and Whipple, 2002) and from 0.1 to 0.3
for offshore upper slope areas (Mitchell, 2004;
Ramsey et al., 2006). The inverse power-law
relationship is interpreted as arising from the
fact that the erosional effect of a down-canyon
increase in discharge (frequency of erosive
flows) is balanced by a reduction in gradient
to achieve a spatially equilibrated erosion rate.
Geological Society of America Bulletin, May/June 2008
Physiography and sedimentology of submarine canyons on a forearc slope
14
0′E
4°5
14
A
5′E
Depth (km)
4 °5
42
′N
°30
00
km
−15
A′
−2
0
00
−2.5
0
00
B
B′
e
lin
tic
An
A
B′
−2.5
−2.0
C′
−2.5
0
B
−25
00
−200
C
−2.0
C
−2500
Depth (km)
5′E
0
−250
4°4
Depth (km)
C′
14
−20
00
5 km
5
0
−2
′N
°30
42
A′
−2.0
Figure 8. Bathymetry and channel planform of the meandering part of the Kushiro Submarine Canyon. Solid lines indicate the locations of
cross-channel profiles. Two-stepped terraces are recognized in Sections B–B′ and C–C′.
with a homogeneous cover of fine-grained
sediment that contains a number of burrows
(Figs. DR2C and DR2D).
The bottom of the canyon is covered with
heavily burrowed, hemipelagic mud. We
observed highly turbid water (the nepheloid
layer) within the lowermost 5–10 m of the canyon bottom, and slowly drifting water downcanyon. We also observed angular to subangular
cobble- and boulder-sized gravels mantled by
hemipelagic mud in the thalweg at the base of
the canyon wall (Figs. DR2E and DR2F). No
ripples or current lineations were observed.
Deep-Tow Camera Observations
Seismic Profiles
The canyon walls, terraces, and thalweg in
Segment B were observed using a deep-tow
camera system (Fig. 4). Observations of the
seafloor indicate that the canyon walls are
characterized by steep or subvertical slopes
of semiconsolidated mudstone and sandstone
(Fig. DR2A). These sedimentary rocks have
been sampled previously using a grab sampler, and dated as Late Pliocene on the basis
of radiolaria (Motoyama, 2004) and diatom
biostratigraphy (Watanabe, 2004). The rocks
are widely burrowed by epibenthic organisms or fishes (Figs. DR2A and DR2B).
The canyon walls and terraces are draped
Seismic reflection profiles were collected
across the upper segments (A and B) of the
Kushiro Submarine Canyon. Analysis of the seismic profiles reveals that the canyon floor along
Segment A is covered with acoustically transparent or weakly stratified facies (Figs. 10A
and 10B); former canyon valleys are recognized
under the transparent facies. The canyon walls
have gentle slopes, forming U-shaped, crosssectional profiles. In contrast, the canyon along
Segment B has a V-shaped profile, devoid of fill
facies along the canyon axis (Figs. 10C–10E).
High-resolution bathymetry, combined with
the seismic profiles, shows the presence of ter-
100
Local slope (m/m)
The Kushiro Submarine Canyon and Hiroo
Submarine Channel have drainage areas of
similar size (Fig. 7E), although they differ in
other morphological parameters. The drainage area of the Kushiro Submarine Canyon
shows a gradual increase in the upper segments (A–B) but only a minor increase in
Segments D–E below the confluence with the
Hiroo Submarine Channel. The slope-area
plot (Fig. 9) shows an inverse power-law relationship between decreasing local slope and
increasing drainage area in the upper segments
(A–B; θ = 0.14).
Segments A–B
S = 0.42 A–0.14
Segments C–E
10−1
10−2
HSC
OH
10−3
107
108
109
1010
Drainage area (m2)
Figure 9. Slope-area plot for the Kushiro
Submarine Canyon. In the upper segments
(A–B), the local slope decreases gradually
with increasing drainage area. In contrast,
no clear relationship is recognized between
slope and drainage area in the lower segments (C–E). OH—outer high; HSC—Hiroo
Submarine Channel.
races in Segments A and B (Figs. 4 and 10). The
terraces have generally flat upper surfaces and
are up to 200 m in width; their elevations above
the axis of the canyon vary between 100 and
400 m. The observed acoustic reflectors generally continue through the terraces without any
evidence of structural discontinuities.
Geological Society of America Bulletin, May/June 2008
757
Noda et al.
ENE
A
5 km
WSW
0.5km
WSW
ENE
The canyon crosses a trench-parallel anticline
in the middle part of Segment B (Figs. 2 and
4). The anticline is accompanied by synclinal
subsidence on its landward side, which is filled
by accumulated sediments that form a narrow
zone of gentle slope (Fig. 11A). The surface
sediments are thick in the syncline and very thin
upon the anticline, suggesting that anticlinal
deformation is ongoing.
In the lower part of Segment B, the canyon
cuts down through eastward-tilting strata (Fig.
11B). The forearc slope subsides in the eastern (left-hand) side and is uplifted in the western (right-hand) side, thereby explaining the
observed difference in wall heights (Fig. 7B).
Late Quaternary Sedimentation
in the Canyon
B
ENE
WSW
C
NNW
SSE
D
ENE
WSW
PC07
E
758
Core GH03–1033, obtained from Segment
A (Fig. 4; Table DR2), consists of olive black
(7.5Y3/2) clayey silt (Fig. 12), layers of volcanic ash, and several thin, silty turbidites. The
volcanic ashes are correlated with tephras of the
Komagatake and Tarumai volcanoes (Figs. 1
and 12). No turbidite deposits are recognized
above the tephras in Core GH03–1033 (Noda
et al., 2004). A high sedimentation rate of
~400 cm ky−1 is estimated based on tephra
chronology.
Core KR0504–PC07, collected from the upper
part of Segment B (Fig. 4; Table DR2), consists of
olive black diatomaceous clayey silt intercalated
with more than 40 turbidites (Fig. 12). Turbidites
in the lower part of the core are generally thicker
and coarser (up to granule-sized grains) than
those in the upper part. Some of the turbidites
are amalgamated, with parallel-laminated silt
or sand layers at the base and cross-laminated
silt-sand at the top (Figs. 13A–13C). Relatively
coarse turbidites are occasionally associated
with chaotic muddy sediments beneath crosslaminated, medium-grained sand (Fig. 13D).
Turbidite layers also show both graded bedding and parallel laminations (Fig. 13E).
Calibrated14Cagesdeterminedusingmixedplanktonic foraminifers (mainly Neogloboquadrina
Figure 10. Interpretations of seismic profiles across Segments A and B of the Kushiro
Submarine Canyon. Transparent or weakly
stratified acoustic facies bury the former
canyon bottom (A–B). Reflectors on the tops
of the terraces are continuous in the sediments, with no evidence of faults or slides
(C–E). The locations of the profiles are
shown in Figure 4. Flow directions within
the canyon are from back to front.
Geological Society of America Bulletin, May/June 2008
Physiography and sedimentology of submarine canyons on a forearc slope
SSW
3000
NNW
Shot number 2800
Two-way travel time in seconds
2
2600
2400
Modeling of Canyon Profiles
Synclinal
subsidence
3
Uplift
VE = 7.2
5 km
km
Shot number
200
400
600
800
VE = 7.2
3
Two-way travel time in seconds
A
ENE
WSW
0
DISCUSSION
2200
1000
5 km
subsidence
4
The obtained seismic records indicate that
the Kushiro Submarine Canyon shows erosional features and did not develop levee systems in the recent past. Furthermore, the lack of
debris and presence of clast-supported gravels
upon the canyon floor suggest that the eroded
material was largely removed to the deep by
turbidity currents. In such cases, it is appropriate to use a detachment-limited erosion model
(e.g., Howard and Kerby, 1983; Howard, 1994)
to estimate the development of the longitudinal
profiles of channels and canyons. Such models have been widely applied to bedrock rivers
within active orogens (e.g., Whipple, 2004) and
submarine canyons (e.g., Mitchell, 2004, 2005).
A transport-limited type of erosion (Rosenbloom and Anderson, 1994; Whipple and
Tucker, 2002), which leads to the diffusion of
knickpoints (Mitchell, 2006), also partly controls canyon profiles. Mitchell (2006) proposed
a simple equation for such models of submarine
canyon topography:
z( x, t) = H0 ( x ) + U ( x, t ) − E ( x, t ) ,
Uplift
B
Figure 11. Interpretations of seismic profiles across the Kushiro Submarine Canyon. The
locations of the profiles are indicated in Figure 4. Flow direction within the canyon is from
back to front. (A) Canyon and the anticline within Segment B. (B) Difference in elevation
between the eastern (left-hand) and western (right-hand) canyon walls. This discrepancy
can be explained by tilting of the basement, involving subsidence of the east wall and uplift
of the west. VE—vertical exaggeration.
pachyderma) indicate an average sedimentation rate of 147 cm ky−1 (Table DR3). The
average interval of turbidite deposition over
the past 2343 yr is estimated to be ~76 yr.
Benthic foraminifers observed in two turbidite layers within Core PC07 (BF1 and BF3) are
dominated by Elphidium batialis and Uvigerina
akitaensis, with lesser Bolivina spissa and Epistominella pacifica (Fig. 12; Table DR4). These
dominant taxa are similar to those in the hemipelagic mud (BF2) that occurs between the two
turbidites (Table DR4), which is indicative of a
deep-water (>1000 m water depth) environment
(Abe and Hasegawa, 2003; Matsuo et al., 2004;
Uchida, 2006). Few shelf or upper slope benthic
foraminifers are recognized in the turbidites.
Core PC09, obtained from the landward side
of the terrace on the outer high (Fig. 4), consists
of olive black (7.5Y3/2–10Y3/2) diatomaceous
clayey silt and several turbidites of sandy silt
or very fine sand (Fig. 12). Our age model suggests that the average sedimentation rates and
depositional intervals between turbidites within
this core are 22–57 cm ky−1 and 371–1136 yr,
respectively (Fig. 12; Table DR3).
Core PC10 was obtained from a point bar
located ~20 m above the thalweg within Segment C (Fig. 4). The core consists of diatomaceous clayey silt with layers of sandy silt to very
fine sand (Fig. 12). Granule- to pebble-sized
gravel (up to 3 cm across) is observed near the
bottom of the core (Fig. DR3). The gravel layers
show no evidence of internal structures and lack
silt and clay but are normally graded at their
tops (Fig. DR3). A calibrated 14C age (Fig. 12;
Table DR3) suggests an average sedimentation
rate for this core of 32 cm ky−1 and a turbidite
recurrence interval of 311 yr.
(2)
with
E ( x, t ) =
⎛ ∂z
∫0 ⎜⎝K a ∂x
t
23
⎛ ∂2 z ⎞ ⎞
+ K d ⎜ 2 ⎟ ⎟ dt , (3)
⎝ ∂x ⎠ ⎠
where z(x,t) is the elevation over time, t; H0(x) is
the initial elevation; U(x,t) is the height of tectonic uplift or subsidence; E(x,t) is the depth of
erosion or height of sedimentation by currents;
Ka and Kd are constants; ∂z/∂x is the local canyon gradient; and ∂2z/∂x2 is curvature.
We developed a finite-difference model to
predict the forms of the canyon thalweg and wall
profiles using equations 2 and 3 (cf. Mitchell,
2006; Riihimaki et al., 2007; Seidl et al., 1994).
We tested two types of simulation; one was a
backward modeling from the present thalweg profile to an initial slope profile (Test A;
Fig. 14A), and the other was a forward modeling from inferred initial slope profiles to present
canyon thalweg and wall profiles (Tests B and
C; Figs. 14B and 14C). In testing the models
(Fig. 14), hemipelagic deposition was assumed
to occur only outside of the canyon. The seismic
profiles suggest that Quaternary deposits are
thicker in the upper slope (1000–3000 m water
depth) than in the uppermost slope (~1000 m),
being ~0.2 s (two-way travel time) off Nemuro
(Noda and TuZino, 2007; TuZino et al., 2004).
The finding indicates that the sedimentation rate
during Quaternary was greater than 7.5 cm ky−1,
Geological Society of America Bulletin, May/June 2008
759
Noda et al.
1033
(268 cm)
Density (g/cm3)
(m) 1 1.5 2 2.5
0
PC07
(492 cm)
Density (g/cm3)
(m) 1 1.5 2 2.5
0
PC10
(330 cm)
PC09
(730 cm)
v
v
Ta-a
v
v
v
Ta-a
(AD1739)
A
v
v
1
1
1
B
~
~
~
v
Ta-b
Us-b
(AD1663)
~
v
~~
~
v
Us-b
~
v
~
v
v
v
v
v
v
v
~~
~
~
~
v
~
v
Ko-c1
(AD1856)
~
~
v
Ta-a
Ko-c2
~ ~ ~ ~ ~~
~
~
~ ~ ~ ~ ~~
v
v v
Density (g/cm3)
(m) 1 1.5 2 2.5
0
Ta-b
(AD1694)
v
~ ~
~
~ ~
Ko-c2 C
(AD1667)
~
~
~~
v
v
2
v
v
2
2
v
Ta-c
(2.5 ka)
D
Figure 12. Logs of the sediment cores collected from the
Kushiro Submarine Canyon
(Fig. 4). The sources and ages
of the volcanic ashes in core
GH03–1033 are from Noda
et al. (2004).
3
v
v
9584
cal yr BP
3
3
BF1
2343
cal yr BP
BF2
BF3
~
~
E
4
4
7631
cal yr BP
~
~
~~
v
Ma-ghi
(8.5 ka)
5
1m
Olive black (7.5Y3/2)
clayey silt
Olive black (10Y3/2–
10Y3/1) clayey silt
Dark olive gray (2.5GY3/1)
clayey silt
~
~
~
vv
6
Bioturbation
Normal grading
Reverse grading
Horizon for benthic
foraminifers analysis
Soft-X radiographs
in Fig. 13
7
Photograph
in Fig. DR3
provided that the acoustic velocity is 1.5 km s−1
or more. Drilling data from the Tokachi forearc
basin indicate a sedimentation rate of 16 cm
ky−1 since the Pliocene (Sasaki et al., 1985). The
Holocene sedimentation rate on the slope is estimated to be 22–57 cm ky−1, based on Core PC09
760
10595
cal yr BP
Gravel
Very coarse sand
Coarse sand
Medium sand
Fine sand
Sandy silt–
very fine sand
Clayey silt
0
Tephra
Gravel
Very coarse sand
Coarse sand
Medium sand
Fine sand
Sandy silt–
very fine sand
obtained from outside the canyon (Fig. 12). In
contrast, onshore tectonic uplift has been estimated to be 20 cm ky−1 for the period since
125 ka (Okumura, 1996); this value is similar to
the sedimentation rate calculated for the slope.
Although accurate values of uplift, subsidence,
19585
cal yr BP
and sedimentation have yet to be determined, in
Tests A and C we assumed that relative uplift of
the outer-arc high and subsidence of the forearc
basin on the landward side of the high were
approximately balanced by hemipelagic sedimentation on the slope (Figs. 14A4 and 14C4).
Geological Society of America Bulletin, May/June 2008
Physiography and sedimentology of submarine canyons on a forearc slope
MGS μm
Grayscale
A
200
100
0
0
50 100 150
Grayscale
D
200
100
Grayscale
0
E
200
100
0
B
Section
boundary
C
5 cm
Figure 13. Soft X-radiographs of turbidites recovered from Core PC07. (A)–(C) Discontinuous turbidites. Lower sand layers are fine-grained
and thin. Upper sand layers are coarser grained and thick, generally with parallel or cross laminations. Sharp basal boundaries are common
in the upper sand layers. (D) The turbidite consists of deformed muddy sediment at the base, overlain by cross-laminated, medium-sized sand.
(E) Parallel-laminated and normal-graded turbidites. The horizons depicted in these sections are shown in Figure 12. MGS—mean grain size.
In Test A (backward modeling), we restored
the initial slope profile (H0) using the approximated curves fitted by equation 1 (Fig. 6) of the
present thalweg profile (Hp). The initial profile
(H0) was calculated by subtracting tectonic
deformation (U) and recovering canyon erosion (E) from the present thalweg profile (Hp):
H0 = Hp − U + E (Fig. 14A1). We then predicted
the present height of the canyon wall (Hwp) by
restoring tectonic deformation (U) and adding
hemipelagic sedimentation (S): Hwp = Hp + U + S
(Fig. 14A2). The total values of U and S were
assumed as in Figure 14A4, and the total erosion and sedimentation by currents in the canyon were calculated according to equation 3.
The model produced an example of the simulated canyon wall profile, which shows a good
correlation with the actual heights of the canyon
wall (Fig. 14A3). The modeled relief showed a
gradual increase toward the outer high, followed
by a decreasing trend to the trench. This result
shows that a large amount of erosion can be
expected at the canyon head and the outer high
(which forms a knickpoint), with a high uplift
rate (Fig. 14A4). Minor erosion is expected on
the landward side of the outer high (Segment B)
and in Segment D, where the thalweg profiles
are concave-upward.
In Tests B and C (forward modeling), we
simulated present longitudinal profiles using
inferred initial slopes. We adopted convex profiles as initial slopes, which indicate uplift on
land and subsidence in the trench. Test B represents a passive margin regime in which tec-
tonic deformation of the substrate is negligible
or spatially uniform, and in which hemipelagic
sedimentation is concentrated on the upper slope
(Biscaye and Anderson, 1994; Pirmez et al.,
1998; Sanford et al., 1990). Test B produced
smooth profiles of the thalweg and canyon wall,
without any knickpoints (Figs. 14B1–14B3).
A predicted gradual decrease in down-canyon
relief was discordant with the actual profiles.
Test C was performed to assess the parameters of tectonic deformation and hemipelagic
sedimentation employed in Test A, representing
an active forearc margin regime (Fig. 14C4).
The resultant profiles of the thalweg and canyon wall are similar to those produced in Test
A using actual canyon profiles (Fig. 14C3).
Canyon erosion is predicted to be small on the
Geological Society of America Bulletin, May/June 2008
761
Noda et al.
−100
−50
0
−200
A1
H0 = Hp – U + E
6
Ka = 0.03
5
Kd = 1
−150
Distance (km)
−100
−50
0
−200
B1
−150
−100
−50
0
C1
7
Ka = 0.03
Ka = 0.03
6
Kd = 1
Kd = 1
5
Hp = H0 + U – E
Hp = H0 + U – E
4
4
3
3
2
1
2
Simulated thalweg (H0)
Simulated thalweg (Hp)
Simulated thalweg (Hp)
Present thalweg (Hp)
H0 (x) = 0.51 √− x + 14 − 2
H0 (x) = 0.51 √− x + 14 − 2
1
0
0
B2
Hwp = H0 + U + S
C2
Hwp = H0 + U + S
7
6
5
5
4
4
3
3
Simulated canyon
wall (Hwp)
H0 (x) = 0.51 √− x + 14 − 2
Simulated canyon
wall (Hwp)
2
1
Simulated thalweg (H0)
Simulated canyon
wall (Hwp)
H0 (x) = 0.51 √− x + 14 − 2
2
1
0
0
Simulated canyon wall
(Hwp)
7
Height (km)
6
3
2
B3
C3
Simulated canyon wall
(Hwp)
7
Simulated canyon wall
(Hwp)
Simulated thalweg (Hp)
Simulated thalweg (H0)
5
3
Simulated thalweg (Hp)
2
Present right-hand wall
0
1.0
6
4
Present thalweg
(Hp)
1
Height (km)
A3
5
4
Height (km)
A2
Hwp = H0 + U + S
6
Height (km)
Height (km)
7
Erosion (–Ee)
Sedimentation (Es)
1
Present thalweg
Present thalweg
0
1.0
A4
B4
C4
0.5
0.5
0.0
0.0
−0.5
Height (km)
Height (km)
7
−150
Height (km)
Distance (km)
Distance (km)
−200
−0.5
Hemipelagic deposition (S)
Tectonic deformation (U)
−1.0
−1.0
−200
−150
−100
Distance (km)
−50
0
−200
−150
−100
−50
0
−200
Distance (km)
−150
−100
−50
0
Distance (km)
Figure 14. Results of numerical modeling of canyon longitudinal profiles. (A1) Initial slope profile (H0) calculated using the approximated curves fitted by equation 1 for the present thalweg profiles (Fig. 6). (A2) Present-day height of the canyon wall (Hwp) calculated
using the simulated initial slope profile in A1. (A3) Comparison among the simulated and present-day profiles. (A4) Cumulative values
of hemipelagic deposition (S), tectonic deformation (U), and canyon erosion (−Ee) and sedimentation (Es). Total erosion (E) is calculated
by Ee − Es. (B) Simulated canyon profiles based on an inferred initial slope with stable tectonics and linear regression of hemipelagic
sedimentation, as representative of passive plate margins. (C) Simulated canyon profiles based on the parameters used in Test A, as
representative of active plate margins.
762
Geological Society of America Bulletin, May/June 2008
Physiography and sedimentology of submarine canyons on a forearc slope
landward side of the outer high (Segments B
and C) but large where the canyon crosses the
uplifted outer high. These erosional patterns
are similar to those predicted from backward
modeling in Test A. The differences in erosion
in the area of the canyon head predicted by
Tests A and C can be attributed to the forms of
the initial slope profiles: the real canyon head
has an exponential profile.
Late Quaternary Sedimentology
Kushiro
Tokachi River
River
Uplifted
marine terraces
Mud filled
(broadly U-shaped)
Inactive
Sediment gravity flows
Present canyon head
heads
Shelf break
Remnant
terraces
Incised
meanders
Turbidity currents
Anticline
Hiroo Spur
Segment A is a mud-filled channel, devoid of
turbidites above the seventeenth-century tephra
layers. The accumulation of muddy sediments
(transparent acoustic facies) in the former narrow and V-shaped canyon floor generated broad
and U-shaped morphology of the floor during
the Holocene highstand. Although terrigenous
detritus could have been supplied directly into
the canyon from the Kushiro River under lowstand sea levels, our observations suggest that
little terrigenous sand is carried to the canyon
head, and suspended material fills the canyon at
a high sedimentation rate (~400 cm ky−1) under
the modern highstand; the upper canyon is considered to be inactive (Fig. 15).
The canyon bottom along Segment B is not
buried by fill facies and contains many turbidites (Figs. 10 and 12). The predominance of
deep-sea taxa among benthic foraminifers in the
turbidites suggests that the turbidites may have
been derived from the upper slope rather than
the river mouth or canyon head (water depth
greater than 1000 m) (Fig. 15). The occurrence
of numerous gullies on the northern (left-hand
side) slope of Segment B (Fig. 4) indicates a
local sediment source, either via repeated slope
failure or sediment gravity flows (cf. Field et al.,
1999; Pratson and Coakley, 1996). The average turbidite recurrence interval of 76 yr determined for Segment B (Core PC07) over the past
2300 yr is more frequent than that determined
for Segment A (Core GH03–1033) and the area
outside of the canyon (Core PC09), thereby indicating that turbidity currents are concentrated in
the middle part of the canyon. Mass movements
or sediment gravity flows upon a deep-sea slope
may be triggered by a reduction in the shear
strength of the sediments in association with
strong and frequent seismic shaking (Hampton
et al., 1996; Lee and Edward, 1986; Normark
and Piper, 1991). Rapid subduction (8 cm yr−1)
of the Pacific Plate beneath Hokkaido likely
generates earthquakes in this area with recurrence intervals of 50–100 yr (Hirata et al., 2003;
Kanamori, 1970; Kikuchi and Fukao, 1987; Shimazaki, 1974; Yamanaka and Kikuchi, 2003).
The recurrence interval of turbidite deposition
may well correspond to that of earthquakes.
Active
Increasing relief and discharge
Decreasing gradient
Erosion ≈ Tectonics
Terrace
Outer high
Knickpoint retreat
(narrow width, high relief,
and steep gradient)
Erosion < Tectonics
Decreasing relief and width
Increasing gradient
Kuril Trench
Figure 15. Schematic representation of the geomorphological and sedimentological features
of the Kushiro Submarine Canyon and the Hiroo Submarine Channel.
Earthquake-induced turbidity currents probably
play an important role in the morphological evolution of the canyon.
A number of the analyzed turbidites are discontinuous in terms of texture and structure,
being composed of a finer part that is thinly planar laminated and an overlying cross-laminated
coarser part (Figs. 12 and 13). This type of amalgamation has been recognized as a “grain-size
break” by Nakajima and Kanai (2000), thought
to represent sediments deposited by more than
one current associated with one or more simultaneous sediment gravity flows. Strong earthquakes are unlikely to induce sediment gravity
flows from just a single source. It is also possible
that a turbidity current associated with sediment
failure might have occurred upon local canyon
walls close to the coring site, with the main turbidity current passing through the site.
The occurrence in Segment B of active vertical
or horizontal incision by sediment gravity flows is
expected based on the narrow, V-shaped nature of
the valley, the lack of muddy fill material, and the
steepness of the walls composed of horizontally
stacked reflections (Fig. 10). Axial downcutting
by canyon-confined mass flows may undercut
and destabilize the canyon walls. Angular boulders of semiconsolidated rocks along the foot of
the canyon wall (Figs. DR2E and DR2F) imply
sidewall collapse (cf. Stubblefield et al., 1982).
Intense burrowing and scouring by epifauna and
some demersal fishes (Fig. DR2A and DR2B) are
also important factors in the erosion of canyon
walls (Dillon and Zimmerman, 1970; Palmer,
1976; Valentine et al., 1980; Warme et al., 1978),
as burrowing weakens local areas of the canyon
walls and promotes small-scale slumping and the
generation of debris. The burrowed rocks become
decomposed, making them susceptible to sliding
and down-canyon transport.
The clasts in pebble-sized gravels within
Core PC10 are similar to those in gravels found
around the shelf edge as relict sediments that
were not buried by younger sand and mud,
and those in gravels on the upper slope (water
depths of 500–1500 m) deposited as diamictite
(Noda and TuZino, 2007). Gravels might have
been transported from the shelf edge to the slope
Geological Society of America Bulletin, May/June 2008
763
Noda et al.
during the lowstand and early stages of transgression, subsequently being derived from the
slope via gravity events. The low silt and clay
contents of the gravel beds suggest effective
hydraulic sorting during current flows or the
selective removal of fine-grained material from
the beds. Fining-upward sedimentary structures
within the top parts of the gravel beds indicate
that the gravels might also have been deposited
by hyperconcentrated density flows (cf. Lowe,
1982; Mulder and Alexander, 2001). Parker
(1982) argued that an ignitive flow with a mean
velocity of 6 m s−1 is able to move cobbles up
to 8 cm across. Such catastrophic currents may
have transported and deposited pebble-sized
gravels during the last deglaciation.
Physiographical Implications
The gradual decrease in gradient in the upper
segments of the canyon (Fig. 9), which overall
defines an exponential profile, indicates that an
inverse power-law relationship can be adopted
between the gradient and the drainage area, similar to graded rivers in subaerial environments
(Flint, 1974; Howard and Kerby, 1983; Whipple,
2004) and submarine canyons at passive margins (Mitchell, 2004, 2005). Such a relationship
suggests that the effect of increasing frequency
of flow events with drainage area is balanced
by decreasing canyon gradient (Fig. 15). A
relatively high frequency of flow events might
be expected along Segment B, judging from
the numerous gullies located on the left-hand
(northern) slope of the canyon (Fig. 4).
Erosion of the canyon with uplift of the upper
slope is demonstrated by the presence of terraces
along Segments A and B (Fig. 4). Continuous
internal reflections beneath the terrace, without
any evidence of normal faulting (Fig. 10), argue
against the possibility that the terraces originated
by deposition with lateral migration of the thalweg (Damuth et al., 1988; Nakajima et al., 1998)
or slumping of the channel walls (Carlson and
Karl, 1988; Cramez and Jackson, 2000; Kenyon
et al., 1995; Liu et al., 1993). Flume experiments
demonstrated that with increasing slope gradient of substrate, channels incised into cohesive
substrate were characterized by progressively
lower ratio of width to depth and had a more
pronounced erosional thalweg than remaining
parts of the previously scoured channel floor
(Shepherd and Schumm, 1974; Wohl and Ikeda,
1997). Therefore, the terraces in the Kushiro
Submarine Canyon are considered as remnants
of a paleocanyon floor resulting from progressive erosion with uplift of the upper slope.
The narrowing canyon width in Segment
C with increasing gradient and relief suggests
that the outer high acts as a knickpoint (Figs. 5
764
and 7). Currents entering regions with high
uplift rates have the potential to narrow channels in areas where the slope steepens (Duvall
et al., 2004; Finnegan et al., 2005). The high
relief in the middle of the canyon (Fig. 7A) can
be explained by high uplift rate (high erosion
rate) of the outer high (Fig. 14). A zone of deep
incision where canyon crosses outer arc high
has also been recognized in other submarine
canyons developed along subduction-related
active margins (Hagen, 1996; Klaus and Taylor,
1991). In other cases in which uplift rate of an
outer high exceeds the erosion rate by currents,
a canyon may be unable to cut down through
the structural high, leading to the ponding of
sediment behind the high, as observed with
the Bonin forearc (Taylor and Smoot, 1984);
alternatively, the canyon may be deflected by
the high, as observed with the San Antonio
Submarine Canyon (Hagen et al., 1994). Therefore, the erosional potential of the Kushiro Submarine Canyon at the outer high must be equal
to or greater than the tectonic uplift (Fig. 15).
The lower segments (C–E) do not show a
clear relationship between slope and drainage
area (Fig. 9). Uplift of the outer high probably
interrupts any process capable of establishing a
near-constant relationship throughout the entire
canyon. The alternating convex-upward and
concave-upward profiles observed in the lower
segments are possibly controlled by the morphology of the slope close to the canyon (Fig. 3).
We suggest that the erosion rate decreases downcanyon, based on a gradual decrease in relief
and constant catchment area (i.e., constant discharge) (Figs. 7A and 7E). If erosion by turbidity currents is proportional to the slope gradient
(equation 3), relief will be higher in Segment E
(Figs. 14C3 and 14C4). Ridge and dome structures (Ogawa et al., 1993) developed upon the
lower slope, which consists of sedimentary rocks
deformed by subduction (Nakanishi et al., 2004;
Schnürle et al., 1995), can be assumed to have
a greater shear strength than hemipelagic sediments of the upper slope (e.g., Hempel, 1995);
therefore, the substrate of the lower slope is
potentially more resistant to erosion by turbidity
currents than sediments of the upper slope. We
consider that tectonic controls on canyon profile
might be stronger than canyon erosion along the
lower segments (Fig. 15).
In terms of the Hiroo Submarine Channel, longstanding subsidence in the Tokachi
forearc basin has led to the development of
aggradational rather than erosional channels
(TuZino and Noda, 2007). This in turn has led
to the development of narrower channel systems
with lower relief than those of the Kushiro Submarine Canyon. If the two channels had experienced sedimentary gravity flows of similar scale
and frequency, they would have met at the same
water depth (i.e., Fairplay’s law). The large difference in water depth of the two features at the
site where the channel joins the main canyon
suggests contrasting activity (erosive potential)
between them. In addition, the absence of any
change in the gradient of the main canyon at the
confluence with the Hiroo Submarine Channel
indicates the inferior contribution of the tributary in terms of discharge and erosive power.
CONCLUSIONS
The Kushiro Submarine Canyon is the main
conduit for sediments transported between the
forearc slope and the Kuril Trench in the active
forearc margin along the southwestern Kuril
Trench. The upper segments of the canyon (A–B;
~3250 m thalweg water depth) incise into the
upper slope along a weakly exponential, longitudinal profile, even where the canyon traverses
an anticline. The relief between the thalweg and
the canyon rim shows a gradual increase with
increasing drainage area. Remnants of a paleocanyon floor, evident as terraces, and ingrowth
meanders indicate progressive erosion of the
canyon with uplift of the upper slope. Frequent
turbidity currents within Segment B, with a
recurrence interval of less than 100 yr, were
triggered by earthquakes related to subduction of the Pacific Plate. Based on the occurrence of buried facies in the uppermost segment
(A) and benthic foraminifers in turbidites, the
present source of turbidity currents is regarded
to be the northern slope of the canyon rather than
the canyon head. An inverse power-law relationship between the canyon gradient and drainage
area indicates that the effects of a down-canyon
increase in the frequency of flow events is balanced by a decrease in canyon gradient.
The longitudinal profile of the lower segments
(C–E; 3250–7000 m) of the canyon largely
reflects the profiles of the forearc slope near the
canyon. The narrow width and high relief of
Segment C suggest that the outer high has acted
as a knickpoint: it breaks the continuity of erosional processes whose actions lead to an equilibrium state. The lowermost segments (D–E)
are characterized by gradual decreases in relief
and width and an increase in gradient; there is
no clear relationship between local slope and
size of the drainage area. This finding suggests
that the lower segments are unable to achieve a
steady-state condition between downcutting erosion and topographic deformation. These results
demonstrate the importance of canyon erosion
in the upper slope and tectonic deformation of
substrate in the lower slope in terms of the evolution of the physiography of submarine canyons
developed upon the slopes along active margins.
Geological Society of America Bulletin, May/June 2008
Physiography and sedimentology of submarine canyons on a forearc slope
ACKNOWLEDGMENTS
We are greatly indebted to the officers, crew, and
research staff of cruises GH02, GH03, KY0407, and
KR0504 for the collection of data. We also thank
Yukinobu Okamura, Kenji Satake, Ken Ikehara,
Tomoyuki Sasaki, and Kohsaku Arai for data obtained
on cruise KR0504. We are grateful to Kyohiko
Mitsuzawa and Hiroyuki Matsumoto for conducting the deep-sea camera survey, Azusa Nishizawa
for SeaBeam data, Kazuhiro Miyazaki for the finitedifference model, and Ken’ichi Ohkushi for picking the foraminifers. Neil C. Mitchell and Takeshi
Nakajima provided helpful comments on an early version of the manuscript. We acknowledge Ivano Aiello,
an anonymous reviewer, and Associate Editor Akira
Ishiwatari, whose constructive comments significantly
helped us to improve the manuscript. This study was
part of the “Marine Geological Mapping Project of the
Continental Shelves around Japan” program supported
by the Geological Survey of Japan, The National Institute of Advanced Industrial Science and Technology
(AIST). Financial support for this research was also
provided by the Japan Nuclear Energy Safety Organization (JNES).
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MANUSCRIPT RECEIVED 22 NOVEMBER 2006
REVISED MANUSCRIPT RECEIVED 4 OCTOBER 2007
MANUSCRIPT ACCEPTED 17 OCTOBER 2007
Printed in the USA
Geological Society of America Bulletin, May/June 2008
767
Table DR1: Comparison of the Kushiro Submarine Canyon (KSC) and the Hiroo
Submarine Channel (HSC).
KSC HSC
Length (km)
233
156
2
Drainage area (km )
8226 3968
Shelf indent
Yes
No
Levee
No
Yes
Water depth of canyon head (m)
70
300
Average relief (m, right)
499
119
Average relief (m, left)
357
98
Maximum relief (m, right)
910
330
Maximum relief (m, left)
595
280
Average width (km)
0.80 0.22
Maximum width (km)
1.96 0.41
Average gradient (m/m)
0.031 0.034
Maximum gradient (m/m)
0.16 0.28
Average sinuosity
1.10 1.16
Maximum sinuosity
2.03 1.90
Table DR2: Localities of core samples.
Date
7/June/2003
27/April/2005
28/April/2005
28/April/2005
Cruise
GH03
KR0504
KR0504
KR0504
No
1033
PC07
PC09
PC10
Longitude
144◦ 19.793’E
144◦ 42.964’E
144◦ 12.3349’E
144◦ 05.537’E
Latitude
42◦ 40.006’N
42◦ 29.038’N
42◦ 14.6466’N
42◦ 13.548’N
Water depth (m)
964
2089
3140
3308
Core length (cm)
268
492
730
330
Table DR3: Radiocarbon ages of foraminifers in hemipelagic muds. Calibrated
ages were applied for a local reservoir correction of 386±16 years (Yoneda et al.,
2001).
Core
Depth
Sample type
Conventional
C age (yr BP)
3,040±40
3,860±40
7,550±50
10,130±40
17,200±130
9,310±50
10,180±40
14
PC07
PC07
PC09
PC09
PC09
PC10
PC10
351–355
351–355
430–434
552–556
726–730
300–304
300–304
Planktonic Foram.
Benthic Foram.
Planktonic Foram.
Planktonic Foram.
Benthic Foram.
Planktonic Foram.
Benthic Foram.
δ13C
(permil)
−3.1
−2.8
−0.6
−2.8
−2.5
−1.0
−3.7
Calibrated age
(1σ) (cal yr BP)
2,293–2,403
3,309–3,414
7,575–7,673
10,545–10,638
19,449–19,601
9,515–9,643
10,577–10,693
Calibrated age
(2σ) (cal yr BP)
2,205–2,480
3,241–3,458
7,535–7,753
10,510–10,707
19,307–19,869
9,469–9,739
10,539–10,823
Median
probability
2,343
3,357
7,631
10,595
19,585
9,584
10,646
Table DR4: Occurrences (%) of benthic foraminifers in turbidites (BF1 and BF3)
and hemipelagic mud (BF2) within Core PC07. UMS, uppermost slope; US,
upper slope; WD, water depth.
Samples
Depth (cmbsf)
AGGLUTINATED BENTHIC FORAM.
Eggerelloides advenum
CALCAREOUS BENTHIC FORAM.
Angulogerina ikebei
Bolivina decussata
Brizalina pacifica
Bolivina spissa
Buccella spp.
Bulimina aculeata
Bulimina striata
Cassidulina norvangi
Cibicides lobatulus
Cibicides spp.
Cibicidoides sp.
Eilohedra nipponica
Elphidium batialis
Elphidium spp.
Epistominella pacifica
Fursenkoina cf. rotundata
Fursenkoina sp.
Globobulimina auricurata
Globobulimina spp.
Islandiella norcrossi
Nonionella globosa
Nonionellina labradorica
Oolina melo
Takayanagia delicata
Uvigerina akitaensis
Uvigerina senticosa
Valvulineria spp.
Other calcareous benthic foram.
Total benthic foram. number
Total planktonic foram. number
P/T ratio
Shelf (100–500 m WD)
Uppermost slope (500–1,000 m WD)
Uppermost–upper slope (1,000–2,000 m WD)
Upper slope (2,000–3,000 m WD)
Others
BF1
320–322
BF2
355–356
BF3
362–364
0.8
0.4
1.5
2.7
2.8
4.2
0.8
0.4
53.7
0.4
0.4
6.3
1.3
1.3
2.5
53.5
15.5
0.8
20.5
0.4
2.3
0.8
1.5
50.0
1.3
5.0
5.0
26.8
0.4
0.4
1.5
1.2
0.4
0.8
0.4
71
0
0.0
0.0
0.0
31.0
69.0
0.0
2.7
1.8
257
53
17.1
0.8
1.9
4.6
23.6
68.3
1.4
26.3
1.3
80
26
24.5
0.0
0.0
41.3
55.0
3.8
UMS
UMS
UMS–US
UMS–US
Others
Others
UMS–US
Others
Shelf
Shelf
UMS–US
US
US
Others
US
Others
Others
UMS–US
Others
UMS–US
Others
Others
Others
UMS–US
UMS–US
Others
Others
Others
C’
B’
Depth (km)
−2100
0
A’
−200
(A)
144˚50'E
0
90
−1
144˚40'E
42˚08'N
km
5
0
20
0
70
−2
Depth (km)
−2600
−25
−24
00
00
0
30
−2
Slope
Channel
5
10
B’
−2.0
−2.2
−2.0
C
C’
−2.2
−2.0
D
−2.2
−2.4
0
5
−2.4
Depth (km)
Depth (km)
(C)
−2.0
−2.5
Depth (km)
50
0
0
60
−2
B
−2
0
40
−2
−2
30
0
−2
D
0
−2.4
−2200
C
−2100
−20
00
−19
00
B
(B)
km
−2.4
A
42˚05'N
A’
A
−2.2
0
D’
−2.0
15
20
Distance (km)
Figure DR1: (A) Bathymetry and channel planform of the Hiroo Submarine
Channel. Dashed lines denote the locations of cross-channel and along-channel
profiles. (B) Cross-sectional channel profiles. (C) Along-channel depth profiles.
Channel length is adjusted to the slope length.
D’
A
B
C
D
E
F
Figure DR2: Photographs taken by deep-sea camera. The length between yellow
markers on the chain is 1 m. (A) Steep canyon wall of horizontally stratified
semi-consolidated mudstone, partly bored by benthic organisms. (B) Close-up
of the boreholes in the canyon wall. (C) Heavily burrowed hemipelagic muddy
sediments upon a terrace. (D) Canyon wall mantled by hemipelagic mud. (E)
Angular semi-consolidated mudstone clasts (up to 50 cm long) at the base of the
canyon wall. (F) Boulders along the thalweg (over 1 m in diameter) draped with
a thin coating of hemipelagic mud.
0
MGS (ø)
–1 –2
–3
310
Core top
cmbsf
320
330
Figure DR3: Photograph of pebbly gravel from the bottom of Core PC10 and its
mean grain-size distribution. The horizon of the extracted sections is shown in
Fig. 12. MGS, mean grain size.