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Slope-stability change from late Pleistocene to Holocene in the Ulleung Basin, East Sea (Japan Sea)
SEDIMENTARY GEOLOGY ELSEVIER
Sedimentary Geology 104 ( 1996) 39-5 I
Slope-stability change from late Pleistocene to Holocene in the Ulleung Basin, East Sea (Japan Sea) H.J. Lee a, SK. Chough b, S.H. Yoon b,1 it Marine Geology Laboratory, Korea Ocean Research and Development Institute, Ansan, P.O. Box 29, Seoul 425-600, South Korea b Department of Oceanography. College of Natural Sciences, Seoul National University, Seoul 151-742, South Korea
Received 28 July 1993; revised version accepted 3 November 1994
Abstract A wide variety of mass-movement deposits prevail along the entire margin of Ulleung Basin, overlain by a hemipelagic mud unit (1-2 m thick). The boundary between the lower turbidite sequence and the upper hemipelagic mud on the basin plain is distinct, and approximately coincides with the well-known, basin-wide ash layer (Ulleung ash) of early Holocene age. In the absence of significant terrigenous sediment input, the lithologic change from the turbidite to hemipelagic mud may reflect physical effects of glacio-marine eustatic sea-level change. Sea-level lowering during the glacial period may have generated excess pore pressure within the sediments caused by reducing hydrostatic confining pressure, and facilitated triggering of the deep-water slope failure by earthquakes. Storm waves may also have affected the uppermost slope area to induce small-scale slope instabilities during the lowered sea-level stand. These glacial factors destabilizing the slope appear to have enhanced large-scale slope failures during the late Pleistocene. In contrast, the topmost hemipelagic mud attests to a stable slope phase associated with raised sea-level in the Holocene, consistent with the infinite slope stability analyses. Calculations of the sedimentation rates based on C- 14 dates of several ash layers indicate that the pre-Holocene turbidites on the basin plain had accumulated at an average rate of 40 cm/1000 years with a minimum recurrence interval of 50 years during the late Pleistocene. Mass-flux estimations between the slumps and turbidites suggest a predominance of the nondisintegrative mode of slope failures in the Ulleung Basin. In each event of slope failure, most of the failed mass rested on the slope as slide/slump blocks, and only its meager portions were deposited on the basin plain as turbidite layers.
1. Introduction The Ulleung Basin, part of the East Sea (Sea of Japan), reveals a suite of mass-movement deposits down the slope (Fig. 1). Slumping generally occurred in the upper slope region, and detached slump masses and blocks rotated and were displaced downslope onto the middle and lower slope (Figs. 2
’Present address: Dept. of Oceanography, Cheju National University, Cheju 690-756, South Korea.
and 3). In addition, varying portions of the slump masses were further liquefied and transported basinward as debris flows and turbidity currents. As a result, the Ulleung Basin margin exhibits a contourparallel zonal distribution of mass-failure deposits with a thick accumulation of fine-grained turbidites on the basin floor (Fig. 1). The failures on the Ulleung Basin slope occurred frequently during the last glacial period. Chough and Bahk (1984-1985) reported a centenary cyclicity of turbidity currents in the late Pleistocene, based on radiocarbon dates of ash layers (Arai et al., 1981)
0037-0738/%/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0037-0738(95)00119-O
40
H.J. Lee et al./Sedimentary
Geology
and facies analyses of the turbiditic mud intervals. The subsequent quiescence in the Holocene is indicated by basin-wide hemipelagic sediments, forming a 1 to 2 m thick layer on top of the turbidite mud (Bahk and Chough, 1983; Chough et al., 1984; Chough and Bahk, 1984- 1985). Geotechnical stability analyses of surface sediment indicate that at present the slope is exceptionally stable even under dynamic loading by earthquakes (Chough et al., 1992; Lee et al., 1991, 1993). Glacial-time slope activities and absence of slumping in the Holocene
104 (1996) 39-51
can also be observed along the eastern continental margin of North America and in the Mediterranean Sea, where various types of slope instabilities, mostly of late Pleistocene age, mar the otherwise flat seafloor (Embley and Jacobi, 1977; McGregor and Bennett, 1977, 1981; Almagor, 1982; Baraza et al., 1990). The Ulleung Basin margin is not moulded by significant deltas or submarine fans, an ideal site for deciphering regional variations (or cyclicity) of slope stability associated with climatic change. The present study focuses on the sharp change from turbidite to
38
37
36
(Sea
DEBRIS-FLOW
of
Jaoan)
DEPOSIT
132" Fig. 1. Map showing bathymetry, major physiographic features, location of cores (dots), and distribution of mass-failure deposits in the Ulleung Basin. Contours in meters. Distribution of mass-failure deposits modified from Chough et al. (1985, 1992) and Lee et al. (1991, 1993). Seismic line segments of airgun and high-resolution (3.5 kHz) profiles are indicated with their respective figure numbers. Inset shows location of the study area in the East Sea.
HJ. Lee et al./Sedimentaty
Geology IO4 (1996) 39-51
41
Fig. 2. Airgun profile (A) and line drawing (B) from the western margin of the Ulleung Basin (for location, see Fig. 1). Slide/slump deposits with hyperbolic reflectors (HS) are located immediately downslope of slide scar (XS). The slide mass grades at the base of slope into a stratified wedge of turbidites and debris-flow deposits.
hemipelagic facies at the transition from the Pleistocene to the Holocene, and intends to address the factors which favored slumping during the last glacial environments. For this purpose, we studied the sedimentary characteristics of both turbidite and hemipelagic muds in some detail and further used descriptions previously made by Chough et al. (19841, Chough and Bahk (1984-1985) and Bahk and Chough (1983).
2. Physiography The Ulleung Basin is semi-enclosed by continental margins and prominent submarine ridges (Fig. 1).
To the west and south the continental slopes of the Korean Peninsula and Japanese Arc bound the basin, whereas to the east and north the basin is bounded by the Oki Bank and Korea Plateau, which are punctuated by volcanogenic islands (Fig. 1). Between these islands, a gap (Ulleung Interplain Gap), approximately 3000 m deep and 70 km wide, runs northeastward connecting the Ulleung Basin to the Japan Basin. The continental shelf of the eastern Korean Peninsula is narrow, less than 25 km, and flanked by a steep slope (slope gradient 4-6”); slopes as steep as lo” occur locally around topographic highs such as the Korea Plateau and the Hupo Bank (Fig. 1). In the south and east, the basin is bordered by a rather gentle slope (l-2”) and broad shelf. As a
42
H..J. Lee et al./Sedimentary
Geology 104 (1996) 39-51
Fig. 2 (continued).
whole, the basin deepens progressively toward the northeast until the 2000-m isobath marks the rim of the flat basin floor (Fig. I>. The western margin of the Ulleung Basin is characterised by a complex series of ridges and rises such as the Korea Plateau and the Hupo Bank (Fig. 1). The Korea Plateau, a vast ridge which rises up to about 500 m above the basin floor, consists of a large terrace extending about 200 km eastward. It is probably the extension of a granite massif on land which intruded in Jurassic time into the Precambrian gneiss. A thin sediment sequence infills troughs in the plateau (Chough, 1983). The sequence is acoustically either transparent or strongly stratified, suggesting ponded turbidites in the troughs. Further south, the shelf is rather smooth except where the broad mound of the Hupo Bank rises more than 100 m
above the surrounding seafloor. The Hupo Bank is a result of fault movements uplifting thick sedimentary sequences and basement (Fig. I). Due to the absence of substantial terrigenous input via large rivers, the slope of the Ulleung Basin lacks distinctive submarine canyons and fans. Only a few small-scale valleys can be seen on seismic profiles (Lee et al., 1991).
3. Mass-failure
features
A number of slide/slump deposits occur on the upper to lower slope areas along the entire margin at water depths of 300 to 1500 m (Fig. I>. This facies is well defined by both rugged surface morphology and disorganized or remoulded internal structures. On
H.J. Lee et al./Sedimenrary
Geology 104 (1996) 39-51
43
Table 1 Summary of slide scar characteristics in the western margin of the Ulleung Basin (modified after Chough et al., 1992) Slope gradient (degr.)
- I .o- 10.0
Water depth Cm)
300-700
Scar depth Cm)
20-70
Scar area (km*)
45-275
Removed sediment (slide/slump) volume
bulkdensity
mass
(km31
(Mg/m3)
(kg X lo’*)
2-8
1.4-1.6 a
0.8-4.8
a Estimated with the ODP data for the upper 100 m of silty clay from the Japan and Yamato basins (Nobes et al., 1992). as well as measured values of surficial sediment in this study.
airgun profiles, these features are characterized by hummocky or blocky, hyperbolic surface reflectors, often accompanied by partially transparent, hyperbolic or mingled subsurface reflectors (Figs. 2 and
3). With a relief on the order of 5-10 m, individual slide sheets reach up to 50 m in thickness and 10 km in length. At their base discontinuous glide planes are often traceable. On some seismic profiles, the
Fig. 3. Airgun profile (A) and line drawing (B) from the western margin of the Ulleung Basin (for location, see Fig. 1). The mid-slope sediments were dislocated sliding.
downslope
leaving irregular
slide scar (XT). Step-like appearance
of the slide/slump
(HS) suggests retrogressive
HJ. Lee et al./ Sedimentary Geology 104 (1996) 39-51
44
failed mass has a step-like appearance, strongly suggesting retrogressive sliding (Fig. 2). In plan view, the scars upslope from the slide masses are crescent-shaped, measuring up to 275 km* in area and 70 m in depth (Table 1). In general, the eastern and southern slopes of the Ulleung Basin are more heavily moulded by mass failure than the western slope (Figs. 2 through 4). At the base of slope, the slide masses rest on the stratified basin-floor turbidite sequence (Figs. 2 and 3) and are occasionally fringed with transparent debris-flow deposits (Fig. 4). Debris-flow deposits at water depths of about 1100-2100 m form a transitional facies between the slide/slump facies on slopes and turbidites of the basin plain (Figs. 1 and 4). They are acoustically
transparent, lens-shaped masses and commonly overlie previously deposited slumps and older debris-flow deposits at the base of slope. On the basin floor, turbidites form an extensive layered sequence (Figs. 1 through 3).
4. Mud facies associated with slope stability Variations in slope stability directly affect depositional processes on the basin floor and hence its type of dominant mud deposit, turbidite or hemipelagic sediment. A turbidite sequence is generally found at water depths greater than 2000 m, although episodic turbidite intervals also occur at water depth of about 1000 m (Lee et al., 1991). According to the dating of
B
Fig. 3 (continued).
H.J. Lee et nl./Sedirnenrary
several distinct ash layers (Arai et al., 1981), the turbidite deposition started at least 75,000 years B.P. (Fig. 5). These turbidites are always overlain by uniform, l-2 m thick, hemipelagic sediments (Fig. 5). The sharp boundary between the turbidite and hemipelagic mud approximately coincides with the beginning of the Holocene (9300 years B.P.). 4.1. Late Pleistocene turbidites-unstable
slope
Numerous thin turbidite units form a finely laminated section (Fig. 5). Parallel-laminated mud, designated E, following Piper’s scheme (19781, is the dominant unit in the Ulleung Basin. It consists of olive grey and grey olive (5Y5/2-5Y3/2) mud. The unit repeats itself, or it is commonly followed by
Geology IO4 (1996) 39-51
45
homogeneous mud (Es) or laminated silt CT,) of the subsequent turbidite. The individual units usually have sharp lower boundaries and rarely exceed 1 cm in thickness. Some laminated muds contain foraminiferal tests or tuffaceous fragments; they are concentrated near the base as discrete lenses and aggregates, and elongate materials are oriented parallel to the bedding plane. The homogeneous mud largely contains materials finer than 30 pm, consisting of terrigenous as well as biogenic ooze components. Some homogeneous muds, however, are slightly bioturbated with abundant pyrite filaments indicating pelagic settling under suboxic conditions. Grading is evident in many parallel laminated layers and in some homogeneous muds. A core from the lower slope (core 25) shows a mixture of mud with semi119
-800
Fig. 4. High-resolution (3.5 kHz) profile from the southern margin of the Ulleung Basin (for location, see Fig. I). The hyperbolic (KY) and blocky slump (BS) occur mainly on the middle slope and debris-flow deposits (D) at the foot of the blocky slump.
slump
46
H.J. Lee et al./Sedimenrary
consolidated mud-clasts, floating in the mud matrix. Mud clasts are up to 6 cm in diameter and poorly sorted; some clasts retain original laminae.
0,
100
PI03
PI05
Geology 104 (1996) 39-51
Sedimentation rates and recurrence interval of turbidity-current events can be precisely determined by using well correlatable ash layers including the
PI06
-25
-24
1
FI
-, 200
1.
93ooYBP
- Fl El .--_
200 El,2
FI .__.
E3 2 300
I
E2
L
300 FI
400
_. 500
500
3 600
Td -Td
E3 El
600 Td E3
\_
7oc
700
ecti
800
Fig. 5. Description of sediment cores from the Ulleung Basin floor. For location of cores, see Fig. 1. The uppermost ash layer is called the Ulleung ash (Oki ash) erupted from the Ulleung Island at about 9300 years B.P. The second and third layers are dated at about 21,OOO-22,ooO years B.P. and 25,00%35,COO years B.P. or 75,000 years B.P., respectively (Arai et al., 1981). Note a sharp lithological boundary, correlatable among cores, between upper hemipelagic clay and lower turbidite sequences, located roughly at the Ulleung ash layer horizon. Modified after Chough et al. (1984) and Chough and B&k (1984-85).
HJ. Lee et al./ Sedimentary Geology 104 (I996) 39-51
Ulleung (Oki) ash (9300 years B.P.), Aira-Tn ash (22,000 years B.P.), and Aso- ash (75,000 years B.P.) (Arai et al., 198 1) (Fig. 5). This yields an average sedimentation rate of 40 cm/1000 years and a minimum recurrence interval of 50 years for the mud turbidites (Bahk and Chough, 1983; Chough and Bahk, 1984- 1985). The acoustically identifiable zonal distribution of mass-failure deposits in the basin (Fig. 1) suggests that slide/slumps on the slope are the precursors of turbidity currents. The thickness of individual layers of turbidite muds tends to decrease toward the basin centre (Fig. 51, due to the thinning of individual flows. Assuming that one turbidite bed originated from one slumping event, we can roughly evaluate their mass flux. The area of turbidite deposition can be approximated by the 2000-m isobath, amounting to 5000 km2. With an average layer thickness of 2 cm and wet bulk density of 1.5 g/cm3 (Table I), one turbidity current event is estimated to deposit a muddy sediment mass on the order of 0.15 X 10 I2 kg on the basin floor. The estimation of failed slope masses is much more complicated because of the uncertainties in dimension and geometry of the slumps across the entire slope. In addition, smallscale slumps and scars remain undetected, because they are below the limit of seismic resolution. Using the estimated values for the slumps on the western margin of the Ulleung Basin (Chough et al., 1992) may only yield part of the full range of individual slope-failure masses (0.8-4.8 X lOI kg, Table 1).
47
Nonetheless, this initial estimation clearly suggests that most of the failed slope masses were merely displaced as slides or slumps to the lower slope, that is, in a nondisintegrative mode. Less than about 10% of the mass only was transported through turbidity currents to the basin floor. 4.2. Holocene hemipelagic sediments-stable
slope
Hemipelagic sediments are rich in diatoms and fine-grained terrigenous material and contain minor amounts of silicoflagellates, spicules, and volcanic glass. Foraminiferal sand grains are rare, partly due to the dissolution of carbonates by highly oxygenated bottom water activated since the postglacial sea-level rise (Chough et al., 1984). The sediments are poorly sorted, olive to olive grey (5Y4/3 or 5Y5/2) muds, and intensely bioturbated largely by benthic deposit feeders. Chondrites and rind burrows are identified occasionally, but the muds are usually completely mottled and yield a strong smell of biogenie gases. The hemipelagic sediments on the western and southern margins have been extensively analysed geotechnically as well as sedimentologically (Chough and Lee, 1987; Lee et al., 1991, 1993; Chough et al., 1992) (Table 2). They are highly plastic and watery, and on the southern slope the muds are slightly overconsolidated and highly compressible. According to the infinite slope stability analysis (Lee et al., 1991, 19931, the entire slope would seldom experi-
Table 2 Geotechnical properties of topmost sediments on the western and southern margin of the Ulleung Basin (modified after Lee et al., 1991, 1993) Province
Texture sand (%I
Shelf (<3OOm) upper slope (300-700 m) Middle slope (700- 1400 m) Lower slope (> 1400 m)
CaCO, silt (%)
clay (o/o)
OM (o/o)
G
Atterberg limits (o/o) 1,
W,
20-100
2-50
<5
I .5-6.5
25-75
5-15
5-15
2-5
2.65-2.70
O-40
O-20
25-70
20-75
6.0-9.5
50-200
2-10
5-10
5-10
2.55-2.60
40-95
60- 130
15-50
45-90
8.0-10.0
140-220
2-5
2-10
5-10
2.60-2.65
60-100
E-140
20-40
55-80
8.0-9.5
140-210
l-5
o-5
5-15
2.55-2.60
55-85
85- 135
- 0 o-5
O-65
W = water content; SS = shear strength, OM = organic matter; G = grain specific gravity; I, = plasticity index; W, = liquid limit. a Averaged for the uppermost 1 m interval.
48
H.J. Lee et al./Sedimentary
Table 3 Results of infinite slope stability analysis for sediments s
Y/Y 0.22-0.23
from the southern margin of the Ulleung Basin (after Lee et al., 1993) 9’ (degr.)
OCR at 1.5 m
F
TfO- ’ cm*/s)
k, (fraction of gravity)
18-23
2-4
X=-1
0.08-0.28
C,
Pdegr.) <4
Geology 104 (1996) 39-51
0.64- 1.28
1.11-1.86
a = slope angle; ‘y/y = ratio of submerged to total bulk densities; S = ratio of undrained shear strength to consolidation stress; C, = compression index; c v = coefficient of consolidation; $J’= effective angle of internal friction; OCR = overburden consolidation ratio; F = factor of safety; k, = critical pseudostatic horizontal earthquake acceleration at failure.
ence any slope instabilities caused by overburden of sediment itself because of low slope gradients ( < 4”) and high shear strength (S > 0.6) (Table 3). The triaxial test results of the sediment from the southern margin also indicate that the effective frictional angle (4’) of sediment, equivalent to the maximum stable slope, ranges from 18 and 23” (Lee et al., 1993). The calculated values of the critical pseudostatic horizontal earthquake acceleration at failure (k,) amount to 0.08-0.28 (Table 3). Assuming that the Yangsan Fault on land (Fig. 1) is the nearest probable seismic centre for the surrounding margin of Ulleung Basin, Chough and Lee (1987) and Lee et al. (1993) have converted the k, values to the actual magnitudes of earthquake intensity greater than 7, which have never occurred during the 1905-1982 seismic measurements in the area. Consequently, the infinite slope stability analyses suggest that the surficial sediment of the Ulleung Basin slope is presently stable. Such a stability must have maintained
I 0
I
20
I
40
1
60 Thousand
throughout the Holocene so that the entire seafloor is covered with only the thickness of hemipelagic sediments (Fig. 5).
5. Sea-level fall as a possible cause of intensifying slope instabilities Since the end of the last interglacial (at 123,000 years B.P.) when sea-level was approximately 6 m higher than at present, the eustatic sea-level tended to lower to about - 120 m at the peak glacial maximum (at 18,000 years B.P.), superimposed by a number of minor fluctuations (Chappell and Veeh, 1978; Dawson, 1992). From the curve of Shackleton (1987), the individual sea-level fluctuations, or relative glacio-marine sea-level falls, can be estimated to average 30 m in magnitude and 10 ka in time duration, except for the marked drops of approximately 50 m between 123,000 and 115,000 years
I
80
I
100
I
120
I 140
years 6.P
Fig. 6. Global sea-level curve of Late Quatemary glacial period, based on planktonic and benthonic “0 data (modified from Shackleton, 1987). Note that since 123,000 years B.P. when the sea-level stood about 6 m higher than at present, the sea-level continuously lowered to about - 120 m until the last glacial maximum (18,000 years B.P.) with a number of minor fluctuations.
H-I. Lee et al./Sedimentary
B.P. and 90 m between 30,000 and 18,000 years B.P. (Fig. 6). Therefore, the representative rate of sea-level fall appears to range from 3 to 10 m per 1000 years in the Late Pleistocene. Such a glaciomarine sea-level lowering directly reduced the hydrostatic pressure on the near-surface sediments. As a result, there was generation within the sediment of some excess pore pressure, which may have either dissipated effectively or been sustained for some time, depending largely on the permeability of the sediments. Another probable cause for generating remarkable excess pore pressure is associated with the formation of gas hydrates, which occur well in submarine, near-surface, fine-grained sediments below water depths greater than about 300400 m (Shipley et al., 1979; Field and Kvenvolden, 1985; Kvenvolden and McDonald, 1985). Gas hydrates are in the solid hydrate form, clathrates (MacLeod, 1982; McIver, 1982). As confining pressure, i.e. hydrostatic pressure, decreases sufficiently to disturb their equilibrium conditions, some of the solid gas hydrates at the base of the zone begin to decompose into free gases and water, further yielding excess pore pressure. Once generated at the gas hydrate base, increased excess pore pressure may well reduce the stability of the overlying sediment mass, and sometimes trigger slope failure under dynamic loading by earthquakes and waves &unmerhayes et al., 1979; Jansen et al., 1987). It is interesting to note that the massive slope failures in the Ulleung Basin occur consistently below a water depth of 300 m (Fig. 11, the shallow limit for gas-hydrate formation. The heavy discharge of biogenic gases in a number of deep-core samples also suggests the likelihood of its presence. The dominance of slope instabilities in the Ulleung Basin, in turn, warrants studies concerning gas hydrates in this region. In concert with the maximum sea-level fall, glacial sea weather would have also produced storm waves that were sufficiently intense to influence the upperslope sediments, although present-day offshore stability investigations normally eliminate wave-control problems. In the subarctic Gulf of Alaska, Lee and Edwards (19861, using a wave-loading equation of Seed and Rahman (19781, have estimated a maximum water depth of 81 m, to which the maximum probable storm waves in this area (37 m in height;
Geology 104 (1996) 39-51
49
300 m in wavelength) would cause nearly flat-lying, clayey-silt surficial sediment to fail. If we assume that modem subarctic sea conditions are more or less comparable to those of the glacial period in the East Sea, adding the maximum sea-level fall (120 m> to the Lee and Edwards’ estimated value of 81 m yields a ‘maximum probable wave-influenced water depth’ over 200 m, corresponding to the region of shelfbreak or uppermost slope. This rough evaluation suggests that the storm waves could have played a significant role in agitating sediments on the upper slope during the glacial period. In the Ulleung Basin, voluminous slope failures (Fig. 1) occur at water depth greater than 300 m. This depth exceeds by more than 100 m the ‘maximum probable wave-influenced water depth’. This means that most of the seismically detectable slope failures on the Ulleung Basin margin may have been triggered exclusively by seismic shaking. Nevertheless, it is still plausible that small-scale slope instabilities might have been initiated near the uppermost slope by cyclic loading exerted by infrequent giant storm waves during sea-level low stand.
6. Conclusions The lithologic boundary between homogeneous hemipelagic clay and the laminated turbidite sequence that occurs l-2 m below the seafloor of Ulleung Basin approximately coincides with the Holocene-Pleistocene boundary at about 10,000 years B.P. The underlying pre-Holocene turbidites consist mostly of parallel-laminated, foraminiferal/ tuffaceous mud. Their sedimentation rate averages 40 cm/1000 years with a minimum recurrence time of 50 years during the Late Pleistocene. A rough estimation of the coupled slide-turbidite mass flux indicates that most of the mass failures in the basin were nondisintegrative, contributing only meager portions of failed sediment to the basin floor. In contrast, the Holocene hemipelagic sediments are intensely bioturbated, carbonate-deficient, diatomaceous muds. These watery silts and clays are highly plastic and compressible. The variation in mud facies on the Ulleung basin floor most likely was caused by glacio-marine eustatic sea-level lowering. Excess pore pressures, as a
50
HJ. Lee et al./ Sedimentary Geology 104 (1996) 39-51
result of differing hydrostatic pressures, may have degraded the regional slope stability, facilitating earthquake-triggering of seabottom sediments. The culmination of sea-level fall might have allowed storm waves to affect the uppermost slope area, as a potential mechanism for inducing small-scale slope instabilities. On such an increasingly destabilized slope, mass failures occurred frequently, yielding a bulk of late Pleistocene turbidites in the basin centre. However, subsequent Holocene sea-level rise has excluded the glacial adverse effects to slope stability. The uniform deposition of hemipelagic mud over the entire basin area reflects a stable slope phase, consistent with the results of infinite slope stability analyses.
Acknowledgements The research was supported through grants to SKC by the Basic Science Research Institute Program (19961, Ministry of Education. Seismic profiles were provided by Dr. C.M. Kim (Korea Institute of Geology, Materials and Mining). We thank the Geological Survey of Japan and the Ocean Research Institute for allowing us access to the cored samples. The manuscript was reviewed by Profs. G. Einsele and T. Shiki, and an anonymous reviewer. All this help is gratefully acknowledged.
References Almagor, G., 1982. Marine geotechnical studies at continental margins: a review-Part II. Appl. Ocean Res., 4: 130-150. Arai, F., Oba, T., Kitazato. H., Horibe, Y. and Machida, H., 1981. Late Quatemary tephmchronology and palaeo-oceanography of the sediments of the Japan Sea. Quat. Res., 20: 209-230. Bahk, KS. and Chough, SK., 1983. Provenance of turbidites in the Ulleung (Tsushima) back-arc basin, East Sea (Sea of Japan). J. Sediment. Petrol., 53: 1331-1336. Baraza, J., Lee, H.J., Kayen, R.E. and Hampton, M.A., 1990. Geotechnical characteristics and slope stability on the Ebro margin, western Mediterranean. Mar. Geol., 95: 379-393. Boggs, S., Jr., 1984. Quatemary sedimentation in the Japan arc-trench system. Geol. Sot. Am. Bull., 95: 669-685. Chappell, J.M.A. and Veeh, H.H., 1978. Late Quatemary tectonic movements and sea level changes at Timor and Atauro Island. Geol. Sot. Am. Bull., 89: 356-368 Chough, S.K., 1983. Marine Geology of Korean Seas. Int. Hum. Resour. Dev. Corp., Boston, Mass., 157 pp.
Chough, SK. and Bahk, K.S., 1984-1985. Deposition of muds in the Ulleung marginal basin. Geo-Mar. Len., 4: 235-241. Chough, SK. and Lee, H.J., 1987. Stability of sediments on the Ulleung Basin slope. Mar. Geotechnol., 7: 123- 132. Chough, S.K., Lee, G.H., Park, B.K. and Kim, SW., 1984. Fine structures of turbidite and associated muds in the Ulleung (Tsushima) Basin, East Sea (Sea of Japan). J. Sediment. Petrol., 54: 1212-1220. Chough, SK., Jeong, KS. and Honza, E., 1985. Zoned facies of mass-flow deposits in the Ulleung (Tsushima) basin, East Sea (Sea of Japan). Mar. Geol., 65: 113- 125. Chough, S.K., Yoon, S.H. and Lee, H.J., 1992. Submarine slides in the eastern continental margin, Korea. Mar. Geotechnol., 10: 71-82. Dawson, A.G., 1992. Ice Age Earth: Late Quatemary Geology and Climate. Routledge, New York, N.Y., 293 pp. Embley, R.W. and Jacobi, R.D., 1977. Distribution and morphology of large submarine sediment slides and slumps on Atlantic continental margins. Mar. Geotechnol., 2: 205-227. Field, M.E. and Kvenvolden, K.A., 1985. Gas hydrates on the northern California continental margin. Geology, 13: 5 17-520. Jansen, E., Befring, S., Bugge, T., Holtedahl, H. and Sejrup, H.P., 1987. Large submarine slides on the Norwegian continental margin: sediments, transport and timing. Mar. Geol., 78: 77107. Kvenvolden, K.A. and McDonald, T.J., 1985. Gas hydrates in Middle American trench and slope-DSDP/IPOD Leg 84. Init. Rep. DSDP, 84: 667-682. Lee, H.J. and Edwards, B.D., 1986. Regional method to assess offshore slope stability. J. Geotech. Eng., 112: 489-509. Lee, H.J., Chough, S.K., Chun, S.S. and Han, S.J., 1991. Sediment failure on the Korea Plateau slope, East Sea (Sea of Japan). Mar. Geol., 97: 363-377 Lee, H.J., Chun, S.S., Yoon, S.H. and Kim, S.R., 1993. Slope stability and geotechnical properties of sediment of the southem margin of Ulleung Basin, East Sea (Sea of Japan). Mar. Geol., 110: 31-45. MacLeod, M.K., 1982. Gas hydrates in ocean bottom sediments. Am. Assoc. Pet. Geol., 66: 2649-2662. McGregor, B.A. and Bennett, R.H., 1977. Continental slope sediment instability northeast of Wilmington Canyon. Am. Assoc. Pet. Geol. Bull., 61: 918-928. McGregor, B.A. and Bennett, R.H., 1981. Sediment failure and sedimentary framework of the Wilmington geotechnical corridor, U.S. Atlantic continental margin. Sediment. Geol., 30: 213-234. Mclver, R.D., 1982. Role of naturally occurring gas hydrates in sediment transport. Am. Assoc. Pet. Geol., 66: 789-792. Nobes, D.C., Langseth, M.G., Kuramoto, S., Holler, P. and Hirata, N., 1992. Comparison and correlation of physical-prop erty results from Japan Sea basin and rise sites, Legs 127 and 128. Proc. ODP, Sci. Results, 127/128: 1275-1296. Piper, D.J.W., 1978. Turbidite muds and silts on deep sea fans and abyssal plains. In: D.J. Stanley and G. Kelling (Editors), Sedimentation in Submarine Canyons, Fans, and Trenches. Dowden, Hutchinson and Ross, Stroudsburg, Pemr., pp. 163176.
NJ. Lee et al./Sedimentary
Seed, H.B. and Rahman, M.S., 1978. Wave-induced pore pressure in relation to seafloor stability of cohesionless slopes. Mar. Geotechnol., 3: 123-150. Shackleton, N.J., 1987. Oxygen isotopes, ice volume and sea level. Quat. Sci. Rev., 6: 183- 190. Shipley, T.H., Houston, M.H., Buffler, R.T., Shaub, F.J., McMillen, K.J., Ladd, J.W. and Worzel, J.L., 1979. Seismic
Geology IO4 (1996) 39-51
51
evidence for widespread possible gas hydrate horizons on continental slopes and rises. Am. Assoc. Petrol. Geol. Bull., 63: 2204-2213. Summerhayes, C.P., Bornhold, B.D. and Embley, R.W., 1979. Surficial slides and slumps on the continental slope and rise of South West Africa: a reconnaissance study. Mar. Geol., 31: 265-277.
Sedimentary Geology 104 ( 1996) 39-5 I
Slope-stability change from late Pleistocene to Holocene in the Ulleung Basin, East Sea (Japan Sea) H.J. Lee a, SK. Chough b, S.H. Yoon b,1 it Marine Geology Laboratory, Korea Ocean Research and Development Institute, Ansan, P.O. Box 29, Seoul 425-600, South Korea b Department of Oceanography. College of Natural Sciences, Seoul National University, Seoul 151-742, South Korea
Received 28 July 1993; revised version accepted 3 November 1994
Abstract A wide variety of mass-movement deposits prevail along the entire margin of Ulleung Basin, overlain by a hemipelagic mud unit (1-2 m thick). The boundary between the lower turbidite sequence and the upper hemipelagic mud on the basin plain is distinct, and approximately coincides with the well-known, basin-wide ash layer (Ulleung ash) of early Holocene age. In the absence of significant terrigenous sediment input, the lithologic change from the turbidite to hemipelagic mud may reflect physical effects of glacio-marine eustatic sea-level change. Sea-level lowering during the glacial period may have generated excess pore pressure within the sediments caused by reducing hydrostatic confining pressure, and facilitated triggering of the deep-water slope failure by earthquakes. Storm waves may also have affected the uppermost slope area to induce small-scale slope instabilities during the lowered sea-level stand. These glacial factors destabilizing the slope appear to have enhanced large-scale slope failures during the late Pleistocene. In contrast, the topmost hemipelagic mud attests to a stable slope phase associated with raised sea-level in the Holocene, consistent with the infinite slope stability analyses. Calculations of the sedimentation rates based on C- 14 dates of several ash layers indicate that the pre-Holocene turbidites on the basin plain had accumulated at an average rate of 40 cm/1000 years with a minimum recurrence interval of 50 years during the late Pleistocene. Mass-flux estimations between the slumps and turbidites suggest a predominance of the nondisintegrative mode of slope failures in the Ulleung Basin. In each event of slope failure, most of the failed mass rested on the slope as slide/slump blocks, and only its meager portions were deposited on the basin plain as turbidite layers.
1. Introduction The Ulleung Basin, part of the East Sea (Sea of Japan), reveals a suite of mass-movement deposits down the slope (Fig. 1). Slumping generally occurred in the upper slope region, and detached slump masses and blocks rotated and were displaced downslope onto the middle and lower slope (Figs. 2
’Present address: Dept. of Oceanography, Cheju National University, Cheju 690-756, South Korea.
and 3). In addition, varying portions of the slump masses were further liquefied and transported basinward as debris flows and turbidity currents. As a result, the Ulleung Basin margin exhibits a contourparallel zonal distribution of mass-failure deposits with a thick accumulation of fine-grained turbidites on the basin floor (Fig. 1). The failures on the Ulleung Basin slope occurred frequently during the last glacial period. Chough and Bahk (1984-1985) reported a centenary cyclicity of turbidity currents in the late Pleistocene, based on radiocarbon dates of ash layers (Arai et al., 1981)
0037-0738/%/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0037-0738(95)00119-O
40
H.J. Lee et al./Sedimentary
Geology
and facies analyses of the turbiditic mud intervals. The subsequent quiescence in the Holocene is indicated by basin-wide hemipelagic sediments, forming a 1 to 2 m thick layer on top of the turbidite mud (Bahk and Chough, 1983; Chough et al., 1984; Chough and Bahk, 1984- 1985). Geotechnical stability analyses of surface sediment indicate that at present the slope is exceptionally stable even under dynamic loading by earthquakes (Chough et al., 1992; Lee et al., 1991, 1993). Glacial-time slope activities and absence of slumping in the Holocene
104 (1996) 39-51
can also be observed along the eastern continental margin of North America and in the Mediterranean Sea, where various types of slope instabilities, mostly of late Pleistocene age, mar the otherwise flat seafloor (Embley and Jacobi, 1977; McGregor and Bennett, 1977, 1981; Almagor, 1982; Baraza et al., 1990). The Ulleung Basin margin is not moulded by significant deltas or submarine fans, an ideal site for deciphering regional variations (or cyclicity) of slope stability associated with climatic change. The present study focuses on the sharp change from turbidite to
38
37
36
(Sea
DEBRIS-FLOW
of
Jaoan)
DEPOSIT
132" Fig. 1. Map showing bathymetry, major physiographic features, location of cores (dots), and distribution of mass-failure deposits in the Ulleung Basin. Contours in meters. Distribution of mass-failure deposits modified from Chough et al. (1985, 1992) and Lee et al. (1991, 1993). Seismic line segments of airgun and high-resolution (3.5 kHz) profiles are indicated with their respective figure numbers. Inset shows location of the study area in the East Sea.
HJ. Lee et al./Sedimentaty
Geology IO4 (1996) 39-51
41
Fig. 2. Airgun profile (A) and line drawing (B) from the western margin of the Ulleung Basin (for location, see Fig. 1). Slide/slump deposits with hyperbolic reflectors (HS) are located immediately downslope of slide scar (XS). The slide mass grades at the base of slope into a stratified wedge of turbidites and debris-flow deposits.
hemipelagic facies at the transition from the Pleistocene to the Holocene, and intends to address the factors which favored slumping during the last glacial environments. For this purpose, we studied the sedimentary characteristics of both turbidite and hemipelagic muds in some detail and further used descriptions previously made by Chough et al. (19841, Chough and Bahk (1984-1985) and Bahk and Chough (1983).
2. Physiography The Ulleung Basin is semi-enclosed by continental margins and prominent submarine ridges (Fig. 1).
To the west and south the continental slopes of the Korean Peninsula and Japanese Arc bound the basin, whereas to the east and north the basin is bounded by the Oki Bank and Korea Plateau, which are punctuated by volcanogenic islands (Fig. 1). Between these islands, a gap (Ulleung Interplain Gap), approximately 3000 m deep and 70 km wide, runs northeastward connecting the Ulleung Basin to the Japan Basin. The continental shelf of the eastern Korean Peninsula is narrow, less than 25 km, and flanked by a steep slope (slope gradient 4-6”); slopes as steep as lo” occur locally around topographic highs such as the Korea Plateau and the Hupo Bank (Fig. 1). In the south and east, the basin is bordered by a rather gentle slope (l-2”) and broad shelf. As a
42
H..J. Lee et al./Sedimentary
Geology 104 (1996) 39-51
Fig. 2 (continued).
whole, the basin deepens progressively toward the northeast until the 2000-m isobath marks the rim of the flat basin floor (Fig. I>. The western margin of the Ulleung Basin is characterised by a complex series of ridges and rises such as the Korea Plateau and the Hupo Bank (Fig. 1). The Korea Plateau, a vast ridge which rises up to about 500 m above the basin floor, consists of a large terrace extending about 200 km eastward. It is probably the extension of a granite massif on land which intruded in Jurassic time into the Precambrian gneiss. A thin sediment sequence infills troughs in the plateau (Chough, 1983). The sequence is acoustically either transparent or strongly stratified, suggesting ponded turbidites in the troughs. Further south, the shelf is rather smooth except where the broad mound of the Hupo Bank rises more than 100 m
above the surrounding seafloor. The Hupo Bank is a result of fault movements uplifting thick sedimentary sequences and basement (Fig. I). Due to the absence of substantial terrigenous input via large rivers, the slope of the Ulleung Basin lacks distinctive submarine canyons and fans. Only a few small-scale valleys can be seen on seismic profiles (Lee et al., 1991).
3. Mass-failure
features
A number of slide/slump deposits occur on the upper to lower slope areas along the entire margin at water depths of 300 to 1500 m (Fig. I>. This facies is well defined by both rugged surface morphology and disorganized or remoulded internal structures. On
H.J. Lee et al./Sedimenrary
Geology 104 (1996) 39-51
43
Table 1 Summary of slide scar characteristics in the western margin of the Ulleung Basin (modified after Chough et al., 1992) Slope gradient (degr.)
- I .o- 10.0
Water depth Cm)
300-700
Scar depth Cm)
20-70
Scar area (km*)
45-275
Removed sediment (slide/slump) volume
bulkdensity
mass
(km31
(Mg/m3)
(kg X lo’*)
2-8
1.4-1.6 a
0.8-4.8
a Estimated with the ODP data for the upper 100 m of silty clay from the Japan and Yamato basins (Nobes et al., 1992). as well as measured values of surficial sediment in this study.
airgun profiles, these features are characterized by hummocky or blocky, hyperbolic surface reflectors, often accompanied by partially transparent, hyperbolic or mingled subsurface reflectors (Figs. 2 and
3). With a relief on the order of 5-10 m, individual slide sheets reach up to 50 m in thickness and 10 km in length. At their base discontinuous glide planes are often traceable. On some seismic profiles, the
Fig. 3. Airgun profile (A) and line drawing (B) from the western margin of the Ulleung Basin (for location, see Fig. 1). The mid-slope sediments were dislocated sliding.
downslope
leaving irregular
slide scar (XT). Step-like appearance
of the slide/slump
(HS) suggests retrogressive
HJ. Lee et al./ Sedimentary Geology 104 (1996) 39-51
44
failed mass has a step-like appearance, strongly suggesting retrogressive sliding (Fig. 2). In plan view, the scars upslope from the slide masses are crescent-shaped, measuring up to 275 km* in area and 70 m in depth (Table 1). In general, the eastern and southern slopes of the Ulleung Basin are more heavily moulded by mass failure than the western slope (Figs. 2 through 4). At the base of slope, the slide masses rest on the stratified basin-floor turbidite sequence (Figs. 2 and 3) and are occasionally fringed with transparent debris-flow deposits (Fig. 4). Debris-flow deposits at water depths of about 1100-2100 m form a transitional facies between the slide/slump facies on slopes and turbidites of the basin plain (Figs. 1 and 4). They are acoustically
transparent, lens-shaped masses and commonly overlie previously deposited slumps and older debris-flow deposits at the base of slope. On the basin floor, turbidites form an extensive layered sequence (Figs. 1 through 3).
4. Mud facies associated with slope stability Variations in slope stability directly affect depositional processes on the basin floor and hence its type of dominant mud deposit, turbidite or hemipelagic sediment. A turbidite sequence is generally found at water depths greater than 2000 m, although episodic turbidite intervals also occur at water depth of about 1000 m (Lee et al., 1991). According to the dating of
B
Fig. 3 (continued).
H.J. Lee et nl./Sedirnenrary
several distinct ash layers (Arai et al., 1981), the turbidite deposition started at least 75,000 years B.P. (Fig. 5). These turbidites are always overlain by uniform, l-2 m thick, hemipelagic sediments (Fig. 5). The sharp boundary between the turbidite and hemipelagic mud approximately coincides with the beginning of the Holocene (9300 years B.P.). 4.1. Late Pleistocene turbidites-unstable
slope
Numerous thin turbidite units form a finely laminated section (Fig. 5). Parallel-laminated mud, designated E, following Piper’s scheme (19781, is the dominant unit in the Ulleung Basin. It consists of olive grey and grey olive (5Y5/2-5Y3/2) mud. The unit repeats itself, or it is commonly followed by
Geology IO4 (1996) 39-51
45
homogeneous mud (Es) or laminated silt CT,) of the subsequent turbidite. The individual units usually have sharp lower boundaries and rarely exceed 1 cm in thickness. Some laminated muds contain foraminiferal tests or tuffaceous fragments; they are concentrated near the base as discrete lenses and aggregates, and elongate materials are oriented parallel to the bedding plane. The homogeneous mud largely contains materials finer than 30 pm, consisting of terrigenous as well as biogenic ooze components. Some homogeneous muds, however, are slightly bioturbated with abundant pyrite filaments indicating pelagic settling under suboxic conditions. Grading is evident in many parallel laminated layers and in some homogeneous muds. A core from the lower slope (core 25) shows a mixture of mud with semi119
-800
Fig. 4. High-resolution (3.5 kHz) profile from the southern margin of the Ulleung Basin (for location, see Fig. I). The hyperbolic (KY) and blocky slump (BS) occur mainly on the middle slope and debris-flow deposits (D) at the foot of the blocky slump.
slump
46
H.J. Lee et al./Sedimenrary
consolidated mud-clasts, floating in the mud matrix. Mud clasts are up to 6 cm in diameter and poorly sorted; some clasts retain original laminae.
0,
100
PI03
PI05
Geology 104 (1996) 39-51
Sedimentation rates and recurrence interval of turbidity-current events can be precisely determined by using well correlatable ash layers including the
PI06
-25
-24
1
FI
-, 200
1.
93ooYBP
- Fl El .--_
200 El,2
FI .__.
E3 2 300
I
E2
L
300 FI
400
_. 500
500
3 600
Td -Td
E3 El
600 Td E3
\_
7oc
700
ecti
800
Fig. 5. Description of sediment cores from the Ulleung Basin floor. For location of cores, see Fig. 1. The uppermost ash layer is called the Ulleung ash (Oki ash) erupted from the Ulleung Island at about 9300 years B.P. The second and third layers are dated at about 21,OOO-22,ooO years B.P. and 25,00%35,COO years B.P. or 75,000 years B.P., respectively (Arai et al., 1981). Note a sharp lithological boundary, correlatable among cores, between upper hemipelagic clay and lower turbidite sequences, located roughly at the Ulleung ash layer horizon. Modified after Chough et al. (1984) and Chough and B&k (1984-85).
HJ. Lee et al./ Sedimentary Geology 104 (I996) 39-51
Ulleung (Oki) ash (9300 years B.P.), Aira-Tn ash (22,000 years B.P.), and Aso- ash (75,000 years B.P.) (Arai et al., 198 1) (Fig. 5). This yields an average sedimentation rate of 40 cm/1000 years and a minimum recurrence interval of 50 years for the mud turbidites (Bahk and Chough, 1983; Chough and Bahk, 1984- 1985). The acoustically identifiable zonal distribution of mass-failure deposits in the basin (Fig. 1) suggests that slide/slumps on the slope are the precursors of turbidity currents. The thickness of individual layers of turbidite muds tends to decrease toward the basin centre (Fig. 51, due to the thinning of individual flows. Assuming that one turbidite bed originated from one slumping event, we can roughly evaluate their mass flux. The area of turbidite deposition can be approximated by the 2000-m isobath, amounting to 5000 km2. With an average layer thickness of 2 cm and wet bulk density of 1.5 g/cm3 (Table I), one turbidity current event is estimated to deposit a muddy sediment mass on the order of 0.15 X 10 I2 kg on the basin floor. The estimation of failed slope masses is much more complicated because of the uncertainties in dimension and geometry of the slumps across the entire slope. In addition, smallscale slumps and scars remain undetected, because they are below the limit of seismic resolution. Using the estimated values for the slumps on the western margin of the Ulleung Basin (Chough et al., 1992) may only yield part of the full range of individual slope-failure masses (0.8-4.8 X lOI kg, Table 1).
47
Nonetheless, this initial estimation clearly suggests that most of the failed slope masses were merely displaced as slides or slumps to the lower slope, that is, in a nondisintegrative mode. Less than about 10% of the mass only was transported through turbidity currents to the basin floor. 4.2. Holocene hemipelagic sediments-stable
slope
Hemipelagic sediments are rich in diatoms and fine-grained terrigenous material and contain minor amounts of silicoflagellates, spicules, and volcanic glass. Foraminiferal sand grains are rare, partly due to the dissolution of carbonates by highly oxygenated bottom water activated since the postglacial sea-level rise (Chough et al., 1984). The sediments are poorly sorted, olive to olive grey (5Y4/3 or 5Y5/2) muds, and intensely bioturbated largely by benthic deposit feeders. Chondrites and rind burrows are identified occasionally, but the muds are usually completely mottled and yield a strong smell of biogenie gases. The hemipelagic sediments on the western and southern margins have been extensively analysed geotechnically as well as sedimentologically (Chough and Lee, 1987; Lee et al., 1991, 1993; Chough et al., 1992) (Table 2). They are highly plastic and watery, and on the southern slope the muds are slightly overconsolidated and highly compressible. According to the infinite slope stability analysis (Lee et al., 1991, 19931, the entire slope would seldom experi-
Table 2 Geotechnical properties of topmost sediments on the western and southern margin of the Ulleung Basin (modified after Lee et al., 1991, 1993) Province
Texture sand (%I
Shelf (<3OOm) upper slope (300-700 m) Middle slope (700- 1400 m) Lower slope (> 1400 m)
CaCO, silt (%)
clay (o/o)
OM (o/o)
G
Atterberg limits (o/o) 1,
W,
20-100
2-50
<5
I .5-6.5
25-75
5-15
5-15
2-5
2.65-2.70
O-40
O-20
25-70
20-75
6.0-9.5
50-200
2-10
5-10
5-10
2.55-2.60
40-95
60- 130
15-50
45-90
8.0-10.0
140-220
2-5
2-10
5-10
2.60-2.65
60-100
E-140
20-40
55-80
8.0-9.5
140-210
l-5
o-5
5-15
2.55-2.60
55-85
85- 135
- 0 o-5
O-65
W = water content; SS = shear strength, OM = organic matter; G = grain specific gravity; I, = plasticity index; W, = liquid limit. a Averaged for the uppermost 1 m interval.
48
H.J. Lee et al./Sedimentary
Table 3 Results of infinite slope stability analysis for sediments s
Y/Y 0.22-0.23
from the southern margin of the Ulleung Basin (after Lee et al., 1993) 9’ (degr.)
OCR at 1.5 m
F
TfO- ’ cm*/s)
k, (fraction of gravity)
18-23
2-4
X=-1
0.08-0.28
C,
Pdegr.) <4
Geology 104 (1996) 39-51
0.64- 1.28
1.11-1.86
a = slope angle; ‘y/y = ratio of submerged to total bulk densities; S = ratio of undrained shear strength to consolidation stress; C, = compression index; c v = coefficient of consolidation; $J’= effective angle of internal friction; OCR = overburden consolidation ratio; F = factor of safety; k, = critical pseudostatic horizontal earthquake acceleration at failure.
ence any slope instabilities caused by overburden of sediment itself because of low slope gradients ( < 4”) and high shear strength (S > 0.6) (Table 3). The triaxial test results of the sediment from the southern margin also indicate that the effective frictional angle (4’) of sediment, equivalent to the maximum stable slope, ranges from 18 and 23” (Lee et al., 1993). The calculated values of the critical pseudostatic horizontal earthquake acceleration at failure (k,) amount to 0.08-0.28 (Table 3). Assuming that the Yangsan Fault on land (Fig. 1) is the nearest probable seismic centre for the surrounding margin of Ulleung Basin, Chough and Lee (1987) and Lee et al. (1993) have converted the k, values to the actual magnitudes of earthquake intensity greater than 7, which have never occurred during the 1905-1982 seismic measurements in the area. Consequently, the infinite slope stability analyses suggest that the surficial sediment of the Ulleung Basin slope is presently stable. Such a stability must have maintained
I 0
I
20
I
40
1
60 Thousand
throughout the Holocene so that the entire seafloor is covered with only the thickness of hemipelagic sediments (Fig. 5).
5. Sea-level fall as a possible cause of intensifying slope instabilities Since the end of the last interglacial (at 123,000 years B.P.) when sea-level was approximately 6 m higher than at present, the eustatic sea-level tended to lower to about - 120 m at the peak glacial maximum (at 18,000 years B.P.), superimposed by a number of minor fluctuations (Chappell and Veeh, 1978; Dawson, 1992). From the curve of Shackleton (1987), the individual sea-level fluctuations, or relative glacio-marine sea-level falls, can be estimated to average 30 m in magnitude and 10 ka in time duration, except for the marked drops of approximately 50 m between 123,000 and 115,000 years
I
80
I
100
I
120
I 140
years 6.P
Fig. 6. Global sea-level curve of Late Quatemary glacial period, based on planktonic and benthonic “0 data (modified from Shackleton, 1987). Note that since 123,000 years B.P. when the sea-level stood about 6 m higher than at present, the sea-level continuously lowered to about - 120 m until the last glacial maximum (18,000 years B.P.) with a number of minor fluctuations.
H-I. Lee et al./Sedimentary
B.P. and 90 m between 30,000 and 18,000 years B.P. (Fig. 6). Therefore, the representative rate of sea-level fall appears to range from 3 to 10 m per 1000 years in the Late Pleistocene. Such a glaciomarine sea-level lowering directly reduced the hydrostatic pressure on the near-surface sediments. As a result, there was generation within the sediment of some excess pore pressure, which may have either dissipated effectively or been sustained for some time, depending largely on the permeability of the sediments. Another probable cause for generating remarkable excess pore pressure is associated with the formation of gas hydrates, which occur well in submarine, near-surface, fine-grained sediments below water depths greater than about 300400 m (Shipley et al., 1979; Field and Kvenvolden, 1985; Kvenvolden and McDonald, 1985). Gas hydrates are in the solid hydrate form, clathrates (MacLeod, 1982; McIver, 1982). As confining pressure, i.e. hydrostatic pressure, decreases sufficiently to disturb their equilibrium conditions, some of the solid gas hydrates at the base of the zone begin to decompose into free gases and water, further yielding excess pore pressure. Once generated at the gas hydrate base, increased excess pore pressure may well reduce the stability of the overlying sediment mass, and sometimes trigger slope failure under dynamic loading by earthquakes and waves &unmerhayes et al., 1979; Jansen et al., 1987). It is interesting to note that the massive slope failures in the Ulleung Basin occur consistently below a water depth of 300 m (Fig. 11, the shallow limit for gas-hydrate formation. The heavy discharge of biogenic gases in a number of deep-core samples also suggests the likelihood of its presence. The dominance of slope instabilities in the Ulleung Basin, in turn, warrants studies concerning gas hydrates in this region. In concert with the maximum sea-level fall, glacial sea weather would have also produced storm waves that were sufficiently intense to influence the upperslope sediments, although present-day offshore stability investigations normally eliminate wave-control problems. In the subarctic Gulf of Alaska, Lee and Edwards (19861, using a wave-loading equation of Seed and Rahman (19781, have estimated a maximum water depth of 81 m, to which the maximum probable storm waves in this area (37 m in height;
Geology 104 (1996) 39-51
49
300 m in wavelength) would cause nearly flat-lying, clayey-silt surficial sediment to fail. If we assume that modem subarctic sea conditions are more or less comparable to those of the glacial period in the East Sea, adding the maximum sea-level fall (120 m> to the Lee and Edwards’ estimated value of 81 m yields a ‘maximum probable wave-influenced water depth’ over 200 m, corresponding to the region of shelfbreak or uppermost slope. This rough evaluation suggests that the storm waves could have played a significant role in agitating sediments on the upper slope during the glacial period. In the Ulleung Basin, voluminous slope failures (Fig. 1) occur at water depth greater than 300 m. This depth exceeds by more than 100 m the ‘maximum probable wave-influenced water depth’. This means that most of the seismically detectable slope failures on the Ulleung Basin margin may have been triggered exclusively by seismic shaking. Nevertheless, it is still plausible that small-scale slope instabilities might have been initiated near the uppermost slope by cyclic loading exerted by infrequent giant storm waves during sea-level low stand.
6. Conclusions The lithologic boundary between homogeneous hemipelagic clay and the laminated turbidite sequence that occurs l-2 m below the seafloor of Ulleung Basin approximately coincides with the Holocene-Pleistocene boundary at about 10,000 years B.P. The underlying pre-Holocene turbidites consist mostly of parallel-laminated, foraminiferal/ tuffaceous mud. Their sedimentation rate averages 40 cm/1000 years with a minimum recurrence time of 50 years during the Late Pleistocene. A rough estimation of the coupled slide-turbidite mass flux indicates that most of the mass failures in the basin were nondisintegrative, contributing only meager portions of failed sediment to the basin floor. In contrast, the Holocene hemipelagic sediments are intensely bioturbated, carbonate-deficient, diatomaceous muds. These watery silts and clays are highly plastic and compressible. The variation in mud facies on the Ulleung basin floor most likely was caused by glacio-marine eustatic sea-level lowering. Excess pore pressures, as a
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result of differing hydrostatic pressures, may have degraded the regional slope stability, facilitating earthquake-triggering of seabottom sediments. The culmination of sea-level fall might have allowed storm waves to affect the uppermost slope area, as a potential mechanism for inducing small-scale slope instabilities. On such an increasingly destabilized slope, mass failures occurred frequently, yielding a bulk of late Pleistocene turbidites in the basin centre. However, subsequent Holocene sea-level rise has excluded the glacial adverse effects to slope stability. The uniform deposition of hemipelagic mud over the entire basin area reflects a stable slope phase, consistent with the results of infinite slope stability analyses.
Acknowledgements The research was supported through grants to SKC by the Basic Science Research Institute Program (19961, Ministry of Education. Seismic profiles were provided by Dr. C.M. Kim (Korea Institute of Geology, Materials and Mining). We thank the Geological Survey of Japan and the Ocean Research Institute for allowing us access to the cored samples. The manuscript was reviewed by Profs. G. Einsele and T. Shiki, and an anonymous reviewer. All this help is gratefully acknowledged.
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