CHARACTERISATION OF A SUBMARINE FLOW-SLIDE AT POINTE-DUFORT, SAGUENAY FJORD, QUEBEC, CANADA.
P. LOCAT, S. LEROUEIL
Dept. of Civil Engineering, Université Laval, Québec, Canada, G1K 7P4
J. LOCAT
Dept. of Geology and Geological Engineering, Université Laval, Québec,
Canada, G1K 7P4
M. J. DUCHESNE
National Scientific Research Institute on Water, Earth and Environment, Québec,
Canada, G1S 2L2
Abstract
The submarine flow-slide of Pointe-du-Fort is situated on the south shore of the
Saguenay Fjord, near the mouth of the Baie des Ha!Ha!, Quebec, Canada. About 1.5
million m3 of material, constituted of clayey silt rhythmites and thin sand layers, were
involves in the slide. Surface of rupture would have reach a till contact. Seismic surveys
over the displaced mass revealed a multiphase accumulation of the debris. Stratigraphic
position of the debris link the event to the 1663 (Ms~7) earthquake. The slide was
approximately dated to be over 260 years old (from present day) using sedimentation
rates.
Keywords: Submarine slide, mass movement, Saguenay Fjord, COSTA-Canada,
geotechnical properties.
1. Introduction
Starting in 1993, a set of 4 multibeam surveys (SIMRAD EM1000) of the Saguenay
Fjord in the Province of Quebec, Canada, were collected on the Research Vessel (RV)
of the Canadian Coast Guard Ship (CCGS) Frederick G. Creed. These yielded a detailed
bathymetric map of the fjord (Fig. 1). This map provided a unique tool to identify
submarine morphological features such as spreads, flows, rafted block, etc. (Locat et al.
this volume; Urgeles et al. 2002; Locat et al. 2000) in the vicinity of the Baie des
Ha!Ha! and of the Bras Nord (Fig. 1b). From these investigations, a landslide was
identified in a sector called Pointe-du-Fort on the south shore of the Saguenay Fjord,
near the mouth of the Baie des Ha!Ha! (Fig. 1). This slide was chosen to study both the
failure and the post failure mechanisms affecting the transformation of the material from
the intact to remoulded state, and the associated mobility.
Gravity corers such as box core, Lehigh, Stacore and Benthos piston cores were used to
collect samples during scientific cruises on RVs Alcide C. Horth (2000), CCGS Martha
L. Black (2001) and Coriolis II (2002). Seismic reflection surveys were also carried out
during these scientific cruises and onboard of two other RVs namely the CCGS Denis
Riverin (summers 2000 and 2001) and the CCGS Willy Desraspe (summer 2002).
521
522
Locat et al.
Morphological characteristics and geotechnical properties of the intact material as well
as the slide debris will be presented hereafter.
Figure 1. Localisation map (a). Map (b) shows the position of cores (black dots), vane tests (VT), boreholes
(BH), seismic line and sections mentioned in the text.
Flow-slide at Pointe-du-Fort, Saguenay Fjord
523
2. Slide morphology
The source area shows hummocky surfaces that correspond to debris in the slope and
pinnacles of till or rock (Fig. 2). Three main gullies are observed. Transverse profiles AA’ and B-B’ (Fig. 2) are bathymetric profiles extracted from the multibeam data. These
profiles show that these gullies are more developed near the crown (A-A’) where,
currently, a stream drains, at low tides, some tide waters from the tidal flat. The
thickness in the centre of the slope (lateral cross section B-B’) and width of the slide are
about 14.5 m and 300 m respectively. The source area affected by the slide is about 105
000 m2. The volume is approximately equal to 1.5 millions m3.
Figure 2. Transverse bathymetric profiles in the main scarp. Location of cross sections is indicated in
Figure 1b.
Figure 3. Longitudinal profiles of the flow-slide at scale 1v:1h. Location of profiles is indicated in Figure 1b.
The slide debris is tongue shaped and is slightly curved following the topographic
gradient of the fjord bottom (Fig. 1). The displaced mass rests in an average water depth
of 160 m. The horizontal and vertical distances from the crown to the tip of the
displaced material are respectively 1070 m and 160 m (see Cruden & Varnes 1996 for
definition of landslide features). The fahrböschung angle is about 7,5°. The angles with
the horizontal for the surface of rupture is 24° and for the fjord bottom in the zone of
accumulation is 1,4° (Fig. 3). The displaced mass is 730 m long and 300 m wide, and
the area covered by debris is about 150 000 m2. Seismic surveys (see Duchesne et al.,
this volume), show that the debris are thinner and narrower near the head wall and
present an average thickness of 13 m. The toe of the displaced mass shows a frontal
angle at rest of 18°. Volume of debris is approximately 1.95 millions m3. The estimated
volume increase is about 30% of the volume before displacement.
524
Locat et al.
3. Geotechnical characterisation of the intact material
3.1 LOCAL GEOLOGY
Near the Pointe-du-Fort, bedrock on both sides of the Baie des Ha!Ha! consists of a
granulitic gneiss from the Cap à l’Est Complex. A regional shear zone linked to the
deformation zone of St-Fulgence and a local fault (undetermined movement) are
situated at less then 1000 m on the west side of the starting zone of the slide (Hébert &
Lacoste, 1998). Rondot (1979) noticed an intersection of two lineaments at less then 1
km on the east side of the slide and traced a E-W fault line that approximately goes
through the starting zone. However, no evidence of faulting was noticed in the surficial
deposits during field works on the tidal flat. In borehole BH1, located about 500 m back
of the crown (Labo SL, 2001), bedrock was reached at a depth of about 20 m (BH1, Fig.
4). The first 2 m of the bedrock were cored. The RQD (Rock Quality Designation)
value is 80% for the first meter and 94 % for the second meter.
The bedrock is mantled by a very dense till which is about 20 m thick in the sector of
interest (see borehole BH1, Fig. 4). The first escarpment above the tidal flat, near the
shore line, is cut into the till deposit where it is about 18 m high with evidence of the
water table at about 4 m from the top.
3.2 INTACT MATERIAL IN THE STARTING ZONE
The stratigraphy in the starting zone was established using seismic refraction surveys,
vane tests and boreholes on the tidal flat, and seismic reflection surveys on water near
the main scarp (Fig. 4). This information was completed by observation of outcrops on
the tidal flat. Toward the north, from the shoreline, the tidal flat shows a hard till surface
for the first 180 m. The last 120 m of the tidal flat surface shows sparse rounded
boulders of 3 m or less in diameter over soft clayey silt. Vane tests VT1 and VT2 reach
respectively depths of 13 m and 18 m before reaching a stiff layer, which is thought to
be the till.
As observed in the borehole BH2 located about 100 m back from the crown beside VT2
(Figs. 1 and 4), the clayey silt shows thin sand layers varying from 2 to 5 mm in
thickness. Sand layers show an irregular rhytmicity ranging from 1 to 15 cm. The sand
layers are inclined with the slope. The geotechnical profile (Fig. 4) of the clayey silt
shows water content of about 40%, the average plasticity index is 23% and the liquidity
index varies from 1.13 at 1 m to 0.87 at 10m deep. The vane shear strength of the intact
soil regularly increase from 38 kPa at 1m to 100 kPa at 17 m deep. Sensitivity evaluated
from vane tests varies between 6 and 14. It would, however, be in order of 50 on the
basis of intact vane shear strength and liquidity index. Pore-water salinity measured at 5
m and 10 m is 10 g/l.
Seismic line WD0206_01_19 on Figure 4 shows a thickness of about 20 m for the
clayey silt and a contact with the till that follows the slope angle. Down slope on the
same seismic line (Fig. 4), at the foot of the clayey silt, debris was observed, and
represented by hyperbolic reflectors in contact with the till.
Flow-slide at Pointe-du-Fort, Saguenay Fjord
525
Figure 4. Stratigraphy of the back scarp and geotechnical profile of the layered silty clay (BH2). A coordinate
0,0 represents the head of the back scarp. High tides in this sector vary by ± 2,5 m. (a) is the geotechnical
profile of BH2-VT2 and (b) is the seismic profile from the line WD0206_01_19 indicated in Figure 1b.
4. Geotechnical characterization of the debris
Lehigh core AH0005_LE3 and Benthos piston core MB01_04 were taken at the
locations indicated in Figure 1. The corresponding geotechnical profiles are shown on
Figures 5a and 5b. CAT-scan images on the side give an idea of the structure within the
first 6 m of the displaced mass. These images come from other cores taken in the same
sector as the two geotechnical profiles (see Fig. 1). The first 1,5 m are characterized by
4 facies: A1: a light grey-brown clayey silt which is the rapidly deposited layer that
followed the 1996 flood; A2: a dark grey silty clay with strong bioturbation and organic
matter, traces of sand and gravels (hemipelagic layer); B: a light grey silty clay, spotted
with black organic matter, poorly bioturbated; C: a light grey silty clay with fine
laminations, without any trace of bioturbation or organic matter.
Below a depth of about 1,5 m, facies D (Debris) corresponds to a mix of mud clasts
(about 5% of the volume) in a matrix of fine material. Observed mud clasts on CATscan images have diameters ranging from 5 mm to 40 mm. The matrix is constituted of
526
Locat et al.
Figure 5. Geotechnical profiles of the displaced mass: (a) the first 1,6 m, Lehigh core AH0005_LE03; and (b)
Benthos piston core MB01_04 from 0 to 6 m deep. The CAT-scan images are in (a) the Lehigh core
COR02_03 and in (b) representative sections of Benthos piston core COR02_12.
Flow-slide at Pointe-du-Fort, Saguenay Fjord
527
medium grey silty clay with traces of sand, gravels and organic matter. Figure 5b shows
that facies D can be divided in 2 sub-facies from the CAT-scan images: D1 is
characterized by chaotic laminations, shear zones, convolute structures and large mud
clasts with sub-rounded edges; D2 presents rounded mud clasts in a matrix of fine
material without laminations or other visible structures.
Geotechnical profiles (Fig. 5a and 5b) show 5 zones. In the upper zone, from 0 to 0.6 m
(facies A), the shear strength, measured with the fall cone, regularly increases with the
overburden stress up to 4.5 kPa. From 0.6 to 1 m, (facies B) shear strength decreases
and remain close to 1.5 kPa. Whereas the water content increases to a value of 140%.
This latter phenomenon is characteristic of a massive arrival of sediments (Perret,
1995). From 1 to 1.35 m, shear strength increases to reach values of about 17 kPa
whereas water content decreases to a value of 60% (facies C). Below this depth, facies
D1 shows water content of about 85% and an average liquidity index of 1.06. The shear
strength, measured with the fall cone, is about 11 kPa for the intact material and about
1.4 kPa for the remoulded material. Geotechnical properties of facies D2 present
differences from facies D1. Facies D2 shows water content of about 65%, an average
liquidity index of 0.85 and a shear strength about 16 kPa for the intact material and
about 2.7 kPa for the remoulded material. The density profile (Fig. 5b) shows 4
signatures which reflect the separation between the main facies (A,B,C and D).
5. Discussion
5.1 SEQUENCE OF EVENTS
Seismic profiles over the displaced mass show 4 observations (see Duchesne et al., this
volume): (1-) the underlying layer of the displaced mass is about 5 m thick and shows
chaotic reflectors; (2-) a scour surface at the base of the debris from the Pointe-du-Fort
and the top of the underlying layer; (3-) reflectors within the mass which looks like
channels or multiple deposits in the long axis of the displaced mass, reflecting multiple
surges of debris. These reflectors are seen only from the toe to approximately the half of
the displaced mass; and (4-) a low tone reflector over the first 1.5 m of the fjord bottom
that seems to correspond to the facies A1, A2, B and C.
Information from seismic profiles shows that the Pointe-du-Fort flow-slide was
deposited over a layer of debris. This underlying layer could corresponds to debris
coming from upstream due to liquefaction (Urgeles et al. 2002) or basin collapse
(Syvitski & Schafer, 1996) triggered by the 1663 (Ms~7) earthquake (see Locat et al.
this volume). An escarpment averaging 7 m height was left by this event in the centre of
the Baie des Ha!Ha! (Fig. 1a).
Based on the analysis of the core MB97_17, St-Onge and Hilaire-Marcel (2001) stated
that a light grey silty clay layer with laminations, found at depths of 74 to 109 cm, is the
turbidite from the 1663 earthquake. The geochemical properties and isotopic analysis of
the layer (St-Onge and Hilaire-Marcel, 2001) confirm that the material is a slightly
carbonated clay with a low organic matter content, characteristic of the post-glacial
Laflamme sea clay encountered on land in this region. Also, they noticed high density
528
Locat et al.
and susceptibility values at the base of the turbidite. Facies B and C in Figure 5a
correspond to this description. Moreover, the depth of facies B and C are approximately
the same as in the core MB97_17. Also, Figure 5a shows that the Layer B was rapidly
deposited because water content is near by 140 % (Perret et al., 1995). This layer of
light grey silty clay corresponds with the base of the low tone reflector on the seismic
lines over the debris.
5.2 DATING
The sedimentation rate from St-Onge and Hilaire-Marcel (2001) can be used to date the
slide event. These authors proposed a sedimentation rate of 0,2 cm·yr-1 at the location of
the core MB97_17, taken at a depth of 165m, near the Pointe-du-Fort (see Fig. 1b). If
we applied this sedimentation rate to facies A2 (about 52 cm), which is a hemipelagic
deposit over the facies B and C, we obtain an approximate age of 260 years. However, a
date approximation based on modern sedimentation rates has to be used with precaution
because of several episodes of catastrophic sedimentation, which were recorded in the
surveyed area (Praeg and Syvitski, 1991). These episodes might induce irregular
sedimentation, which, in turn, can caused problems when computing an age based on
sedimentation rates. If these episodic events were frequent since 1663, it would lead to a
younger age evaluation. On the other hand, there are no historical records that could
justify these since 1663, except the 1996 flood event. Even though the method used to
date the deposits is not accurate, the calculation tends to link the flow-slide with the
1663 earthquake.
5.3 GEOTECHNICAL PROPERTIES
Comparison of geotechnical profiles at Figure 4 and 5 show differences between the
properties of intact material and those of the displaced mass (facies D1 and D2). For the
same liquidity index (IL about 1) water content of intact material and debris are
respectively 40% and 85%. This mean a liquid limit increase of about 45% from the
source to the depositional areas. The presence of organic matter in the debris and the
volume increase by 30% of the failed mass suggest an incorporation of organic matter
within the mass movement. Future descriptions and analysis of other cores will help to
understand this changes of properties.
6. Conclusion
The stratigraphic situation of the displaced mass and the datation by sedimentation rate
of the hemipelagic layer (facies A2) over the debris appears to link the Pointe-du-Fort
flow-slide to the 1663 earthquake. The volume of material involved in the slide is about
1.5 million of m3 of clayey silt rhythmites and sand layers. Surface of rupture seems to
have reach the till contact. CAT-scan images and core analysis from the debris show
mud clasts supported in a matrix of fine material. Seismic reflexion surveys show a
multiphase accumulation of the debris mass.
Flow-slide at Pointe-du-Fort, Saguenay Fjord
529
7. Acknowledgment
This research project is financed by NSERC Canada (Natural Sciences and Engineering
Research Council) via the COSTA-Canada project. Thanks to all the crewmembers on
board of the vessels mentioned in the paper. Special thanks to colleagues at Laval
University and INRS who have been involved in the project, especially to Ms.
Christiane Levesque for the correction of the text. Réjean Couture and Denis Demers
are acknowledged for constructive reviews of the manuscript.
8. References
Cruden, D.M. & Varnes D.J. 1996. Landslide types and processes. In: Landslides: investigation and
mitigation, Special Report 247, Transportation Research Board, pp. 36-75.
Duchesne, M.J., Locat, P., Long, B.F., Massé, M. and Locat, J. 2003. The Pointe-du-Fort mass movement
deposits, upper Saguenay Fjord, Canada : a multiphase build-up. This volume.
Hébert, C. and Lacoste, P. 1998. Géologie de la région de Bagotville. Service géologique du Québec, rapport
géologique # RG 97-06, 22 p with map.
Les Laboratoire S.L. inc. 2001. Expertise de stabilité, chemin de la Batture, Ville de La Baie. 11 p + annexes.
Locat, J., Martin. F., Locat, P., Leroueil, Levesque, C., Konrad, J.-M., S.Urgeles, R., Canals, M. and Duchesne
M. 2003. Submarine mass movements in the upper Saguenay Fjord, (Québec Canada) triggered by
the 1663 earthquake. This volume.
Locat, J., Urgeles, R., Schmitt, T., Houareau, L., Martin, F., Hill, P., Long, B., Simpkin, P., Kammerer, E. and
Sanfaçon, R. 2000. The morphological signature of natural disasters in the Upper Saguenay Fjord
area, Quebec, Canada. Proceedings of the 53rd Canadian Geotechnical Conference, Montreal,
109-116.
Perret, D., Locat, J. and Leroueil, S. 1995. Strength development with burial in fine-grained sediments from
the Saguenay Fjord, Quebec. Canadian Geotechnical Journal, 32 : 247-262.
Preag, D.B. and Syvitski, J.P.M. 1991. Marine geology of Saguenay Fjord. Geological Survey of Canada,
Open File Report, 2395, 14 sheets.
Rondot, J. 1979. Reconnaissance géologiques dans Charlevoix Saguenay. Ministrère des Richesses Naturelles
report # DPV-682, 44p with map.
St-Onge, G. & Hillaire-Marcel, C. 2001. Isotopic constraints of sedimentary inputs and organic carbon burial
rates in the Saguenay Fjord, Québec. Marine Geology, 176: 1-22.
Syvitski, J.P.M. and Schafer, C.T. 1996. Evidence for an earthquake-triggered basin collapse in Saguenay
Fjord, Canada. Sedimentary Geology. 104: 127-153.
Urgeles, R., Locat, J., Lee, H. et Martin, F. 2002. The Saguenay Fjord, Quebec, Canada : integrating marine
geotechnical and geophysical data for spatial seismic slope stability and hazard assessment.
Marine Geotechnics, 185: 319-340.