497 submarine mass movements in the upper saguenay fjord

497 submarine mass movements in the upper saguenay fjord

SUBMARINE MASS MOVEMENTS IN THE UPPER SAGUENAY FJORD,
(QUÉBEC, CANADA), TRIGGERED BY THE 1663 EARTHQUAKE
J. LOCAT, F. MARTIN, P. LOCAT, S. LEROUEIL, C. LEVESQUE, J.-M. KONRAD
Laval University, Québec, Canada, G1K 7P4
R. URGELES, M. CANALS
GRC Geociències Marines, Departament d’Estratigrafia, Paleontologia i Geociències
Marines, Universitat de Barcelona, Campus de Pedralbes, E-08028 Barcelona, Spain
M. J. DUCHESNE
INRS-ETE, Québec, Canada
In: J.Locat and J. Mienert,Eds., Submarine Mass Movements and their Consequences, Kluwer, 2003
Abstract
Extensive submarine mass movements have been mapped in the upper reaches of the
Saguenay Fjord, Québec, Canada. Our analysis indicates that they were most likely
triggered by the 1663 earthquake which shook a large area in Eastern Canada. Signs of
instabilities have been observed at various places and although the triggering mechanism is
the same, the types of mass movements differ largely including slides, spreads and flow
failures in the soft Holocene sediments. Seismic and morphological investigations have
revealed the presence of a fault -like deformation possibly linked to terrestrial fractures
suggesting that the epicentre of the 1663 earthquake may be located in the area.
Keywords: Fjords, earthquakes, failures, morphology, dating
1. Introduction
The Saguenay region has been the locus of many natural disasters over the last 400 years.
Well known are the 1663 St. Jean Vianney landslide (Lasalle and Chagnon 1968, Schafer
and Smith 1987) which was triggered by a major earthquake, the St. Jean-Vianney landslide
of 1971 (Tavenas et al. 1971), the 1988 earthquake and its related liquefaction phenomena
and mass movements (Tuttle et al. 1990; Lefebvre et al. 1992, , Pelletier and Locat 1993),
and the major flood event of 1996 (Pelletier et al. 1999). Signatures left by the various
catastrophic events are either recorded in bedrock (fault re-activation (?), rock slides), soils,
or in sediments in lakes or in the Saguenay Fjord.
The later event, the 1996 flood, called for a detailed investigation of the fjord bottom. One
of the main objectives of this investigation was to assess the stability of the newly deposited
layer since the recurrence of new landslide events could lead to the exposure of the old
contaminated materials. The first step for assessing slope stability is a good knowledge of
the relief, so that bathymetric data from the fjord bottom was essential (Urgeles et al. 2002,
Locat and Sanfaçon, 2002). Such data were already available from a survey carried out in
1993, but for monitoring purposes additional cruises were undertaken in order to collect
new swath bathymetry data, as well as a complete set of geophysical data and various types
of sediment samples. The integration of all the data collected over the last few years lead us
to the conclusion that numerous slides were triggered by the 1663 earthquake. We will
show hereafter that our extensive coverage of this part of the Fjord supports the views of
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Syvitski and Schafer (1996) who concluded on a major basin collapse as a result of the
1663 earthquake and we propose that unique amplitude of the records is related to the fact
that the earthquake epicentre was very close. In addition we will illustrate how a single
triggering mechanism can generate very different signatures.
Figure 1. Shaded relief image of the study area (square) illuminated from the West from GTOPO-30 topography
(Earth Resources Information Systems Data Center, 1996) showing the main physiographic units along the St.
Lawrence River. Earthquake epicentres from 1600 onwards are plotted from the Canadian National Earthquake
database (Natural Resources Canada, 1999) and clearly show the Charlevoix seismic zone (CSZ). White dots
generally correspond to old earthquakes for which the depth is unknown. The focal mechanism of the 1988
earthquake is also plotted (Modified from Urgeles et al. 2002).
2. Geological Setting
The particular geological environment and Quaternary history of the Saguenay area is at the
origin of many of the above mentioned disasters. Of interest to us is the translation of these
catastrophic events into gravitational phenomena which eventually left a distinct signature
such as a landslide (sub-aerial or submarine), debris flow or turbidite deposits.
The Saguenay area is mostly composed of Precambrian rocks of metamorphic origin. A few
hundred million years ago, the area was tectonically active and the Saguenay graben
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structure was formed (Hébert and Lacoste 1998). The morphological evidence of this
graben is shown in Figure 1 by a series of sub-parallel linear depressions in the bedrock.
The main area of seismic activity is in the Charlevoix region (Figure 1). The most recent
major event took place near Baie des Ha! Ha! in 1988, reaching 6.3 on the Richter scale,
and reminding us that the area is seismically active.
During the Pleistocene, the area was covered by ice sheets several times with the last being
the Wisconsinian glaciation (Lasalle and Tremblay 1978). Since the Saguenay graben is
more or less oriented in the same direction than the glacial flow, it became a preferred path
for ice flow and resulted in a deep excavation of the bedrock. In places, it is now filled with
as much as 1000 m of sediments (Syvitski and Praeg 1989, Locat and Syvitski 1991).
Figure 2. Multibeam survey (1997) of the Upper Saguenay Fjord and the location of some slides (sun is from the
East). PFS: Ponte-du-Fort slide, SV: sand volcanoes, GB: gliding block; white dashed line: schematic outline of
large liquefaction failure.
The final retreat of the Wisconsinian ice sheet, which was completed about 10 000 years
ago in the area (Lasalle and Tremblay 1978), was followed by a land emergence varying
from about 140m on the North side of the graben to 120m to the South (Bouchard et al.
1983). The sedimentary environment was such that fine grained sediments were deposited
in brackish waters, rapidly uplifted and then leached of their salts (in the pore water). This
resulted in the formation of quick clays. From existing data on terrestrial emergence in this
region (Locat 1977), it is clear that most of the recent uplift took place in the first few
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thousand years immediately following glacial retreat. It is no surprise that such a geological
combination yields a situation prone to major disasters: seismic activity (a way of
dissipating stored strain energy due to glacial loading and unloading) and quick clays.
3. Methodology: multibeam and seismic surveys, coring
The data presented in this paper is largely based on multibeam bathymetry acquired with
the Simrad EM 1000 mounted on board the catamaran RV F.G. Creed. Surveys were carried
out with a 200% coverage and the data was corrected for various aspects either related to
the ship movement, positioning or tide (Hughes-Clarke et al. 1996). The seismic surveys
were conducted using an IKB SEISTEK system equiped with a positioning antenna directly
mounted on the floating array. Coring was done using various techniques including a box
core, a Lehigh and piston cores. In 1999, we had the opportunity to use the Marion
Dufresne Calipso core and we were able to recover a 28m long core in the centre of the
Baie des Ha! Ha! (see Figure 5d).
4. Morphology of the Upper Saguenay Fjord area
The Upper Saguenay Fjord has a Y-shape with one arm of the Y being the Bras Nord and
the other one being the Baie des Ha! Ha! (Figure 2). The Fjord in that area has water depths
ranging from 0 to 225 m and a width between 3 to 5 km. The Saguenay River flows into the
Bras Nord while the Mars, Ha! Ha! and du Moulin rivers flow into the Baie des Ha! Ha!.
The regular tide in the area is about 4 m.
The most active segment of the upper Saguenay Fjord is the Bras Nord where the
accumulation rate is measured in centimetres per year at the delta head (Locat and
Leroueuil 1988, Perret et al. 1995) so that the Bras Nord is being filled more rapidly than
the Baie des Ha! Ha!. The water depth in the Bras Nord ranges from about 10 m at the
mouth of the Saguenay River to 200 m at the confluence with the Baie des Ha! Ha!. The
Baie des Ha! Ha! itself has a water depth which rapidly descends to about 100 m near the
head (at La Baie city) to 200 m downstream. In addition to the landslides who will be
described later, the sea floor of the Baie des Ha! Ha! has a few striking features such as a
scarp (in Fig. 2) and a gliding block (GB in Fig. 2), and sand volcanoes (SV in Fig. 2) all
located near the centre of the Baie des Ha! Ha!, in the vicinity of a sharp and more or less
straight escarpment (see also Fig. 5). Above the escarpment, the slope is 0.6° and below it,
it is 0.2°.
The slopes on either side of the fjord are quite different, in particular in the Baie des Ha!
Ha!, the northern escarpment is steep with slopes exceeding 40° while the south shore slope
is gentler and gullied (particularly in the area just below the escarpment and the Point-duFort slide) with slope locally at an angle of about 15°. Gullying appears to be more deeply
developed near the centre of the Baie des Ha! Ha! where that marine deposit may be
thicker. The slope on the north side is controlled by the bedrock while the southern side is
dominated by Quaternary sediments (Laflamme sea clays). The Bras Nord is bisected by a
shallow channel which follows more or less the centre of the fjord, potentially acting as a
conduit for mudflows or debris flows originating from the upper reaches of the fjord. This
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501
channel terminates by a large depositional fan which is cut by two channels. The surface of
the fan is draped with mega-ripples. This fan could have been formed by the accumulation
of the sediments mobilized at the time of the 1663 St. Jean Vianey sub-aerial flow slide.
5. Mass Movements in the Upper Saguenay Fjord
The multibeam data has shown the Saguenay Fjord to be a location where landslides take
almost all of the possible forms and therefore provides a unique opportunity to study their
morphology. Along the fjord margins of both the Baie des Ha! Ha! and the Bras Nord,
several features indicative of mass wasting are present. As already indicated in previous
studies (Locat and Bergeron 1988, Pelletier and Locat 1993) the only triggering which
could have caused these mass movements is an earthquake. Because of the
geomorphological settings, triggering factors (see Hampton et al. 1996, and Locat and Lee
2002) such as erosion, overloading by sedimentation and wave action are ruled out. Another
aggravating factor could be the presence of high pore pressure in the ground related to local
aquifer but this remains to be investigated.
Figure 3. Three-dimensional images of different landslide types encountered in the Saguenay Fjord. (a): Spread
with its escarpment shown by the black arrow (view is from N200°, 20° elevation, north-west illumination). (b):
Flow (view is from N250°, 20° elevation, north illumination). (c): slide (view is from N200°, 35° elevation, northwest illumination). (d): Multiple complex headwall scarps in flow (view is from N290°, 30° elevation, north-west
illumination); (e) Position of the seismic line shown in Figure 4. White arrows indicate flow direction. The
location of these mass movements is indicated in Figure 2.
The mass movements observed in Figures 2, 3 and 5 can be divided into two main groups:
(1) mass movements where only scars are left, i.e. the failed material has left the slide area
and likely moved towards the centre of the basin as pounded material (Perret et al. 1995);
and (2) those where most of the sediment involved in the slide remains within the slide area.
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The largest flow slide was mapped in the centre of Baie des Ha! Ha! extending down into
the deeper part of the fjord (Figure 2 and 5). The upstream part of this flow slide is also
shown in Figure 5a and is described with more details below.
Figure 4. (a): Seismic line across portion of the sidewall slide at the entrance of the Bras Nord (see slide (a) in
Figure 3); (b) same line but at uniform scale.
5.1 FLOWS
Along the fjord walls, several scars are observed without direct evidence of the deposits. In
all cases, the head scarp is located along the fjord wall and appears located in the Laflamme
sea clays. In Figure 3b, the failed area is also bounded by one prominent scar, about 10m
and facing West (see also Hampton et al. 1996 for a description of this feature). The flow
direction has been along the gradient and in the eastern channel cutting into the fan. A
seismic line (Figure 4) through the lower part of the failure zone indicates that some debris
are left in the form of small mound of sediments, about 5m high, on a slope of about 2°.
The other flow slides site in Figure 3d has similar characteristics. On this image, we can
distinguish two clear failures both bounded by clear escarpments. In this case also, there is
little sediments left inside the failed area, and the flow direction has been along the gradient
and towards the centre of the basin. The failed area has involved not only some clay in the
fjord wall, but also some recent sediments on the sea-floor itself, to a depth of about 10 m.
The slide most likely involved both parts of the wall and more recent clayey sediments.
The major flow slide of Figure 5a, covers about one half of the Baie des Ha! Ha! area. This
slide is the most spectacular feature in this part of the fjord. It is limited in the upstream
direction by a 7 to 8 metre escarpment, which can be traced on the adjacent northern flank
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of the fjord (Figure 5b). In the downstream area of the slide there is a rafted block, almost 1
km2 in area, which shows signs of displacement or stretching (this block is visible in the
lower part of Figure 5b as a split mass; see also Fig. 2). Just around this block is a
morphology which suggests that this part of the sea floor was only partially liquefied at the
time of this event.
The swath data show a flat relief in the most distal parts of the fjord, indicating that the
flowing sediments were impounded there (Figure 2). Such relief is especially manifest west
of the scarp in the centre of Baie des Ha! Ha! indicating deposition there of the sediments
resulting from liquefaction event (Figures 2 and 3). The sediments involved in this event
travelled across the fjord bottom for a distance of about 10 km.
All along the fjord wall shown in Figure 3c are a series of mass wasting features. Near the
top of this Figure a black arrow locates the headwall of the flow slide and its extension onto
the sea-floor. The blocky debris seen next to it could come either from the flow slides or
from the nearby escarpment.
Figure 5. Major flow failure event in the Baie des Ha! Ha! (b) triggered by an historical earthquake dated in 1663
(see fault-like feature on-land in ‘a’ (white arrow), and its offset in the sediment in ‘e’). Core in ‘c’ give the age of
the related earthquake accumulation of 1663 and ’d' shows the geotechnical profile obtained at the site with the
Marion Dufresne in 1999 (Core MD992223). Location is given in Figure 2.
5.2 SLIDES AND SPREADS
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Along the escarpment of the Bras Nord (Fig. 2) there are two sectors showing spread
failures (Fig. 3a and c). In Figure 3a we can see a failure with the main headwall indicated
by the black arrow. At that location we can also see some linear features which could
represent a retrogression process. The escarpment appears to have mostly cut into recent
sediments and not into the older and stiffer Laflamme sea clay.
In the lower portion of Figure 3c we can see another spread failure parallel ridges. As the
failed mass spreads over the sea floor, it can also push the sediment near the front and form
pressure ridges. At this particular site, we can see, near the escarpment, that the upper part
of the failed mass lies almost like a slab. The black arrow points at the limit of the head wall
of this slide which has apparently involved some Laflamme sea clays. Downstream from the
slide headwall are a series of sub-parallel ridges which represent the spreading of the failed
mass along the topographic gradient.
Another interesting failure has been studied in detail and reported in this volume by Locat
et al. (2003) and is related to the Pointe-du-Fort slide (PFS in Figure 2). This slide extends
over a length of about 1 km and the thickness is about 10m. When it spreads on the sea
floor, it turned downstream following the gentle slope. The displaced mass appears to have
detached from the wall of the fjord and may have been disturbed by, or inter-acting with,
the passing of the major liquefaction flow failure sediments coming from the central part of
the Baie des Ha! Ha!.
In general, slides and spreads, usually originate at the base of the fjord walls where slopes
are gentler, about 3°, but higher than those of the fjord bottom which do not exceed 0.2°
(Figure 2), and they travel for distances not exceeding 1 km. Also of interest if the height of
the various escarpments observed for all the slides which varies between 5 and 10m. At
such a water depth and on low angle slopes, a mechanical back analysis by Locat and
Bergeron (1988) has shown that they could be mobilized only under stress modification
caused by a large earthquake.
6. The 1663 earthquake
The largest natural disaster that has taken place in the Saguenay Fjord area is probably
associated with the February 5th 1663 earthquake. The earthquake epicentre was believed to
be located in the nearby Charlevoix seismic zone (CSZ, Figure 1) based on observations by
Jesuits priests (Smith, 1962). However, recent discoveries, both morphological and
structural data presented hereafter, is proposing that the epicentre was rather located in the
Saguenay area.
Amongst the most striking on-land features associated with this event is the Saint Jean
Vianney I flow slide (Lasalle and Chagnon, 1968), which contributed a huge amount of
terrigenous sediments into the Saguenay fjord (Schafer and Smith 1987). Other evidence
includes the occurrence of a major rock avalanche located on the east margin of the
Saguenay graben (Mont Éboulé slide, Locat et al. 1997) which, according to a 14C date,
occurred less than 1000 years ago. In addition to these elements, very fresh mass movement
morphologies and seismic traces (see Figures 3 and 4) in the Saguenay Fjord have been
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considered by Syvitski and Schafer (1996) as part of a major “basin failure” that they also
attribute to the 1663 earthquake. The final element to support the location of this earthquake
in the Saguenay area is the recent discovery of a recent fault-like feature on the edge of the
fjord (Fig. 5a) which appears to be directly related to a submarine escarpment and a major
liquefaction flow slide (Figure 5b and e).
The multibeam and seismic surveys along with coring data have been assembled in Figure 5
to present the various evidences obtained from the centre of the Baie des Ha! Ha! which is
believed was very close to the epicentre of the slide. The multibeam survey shown in Figure
5b was at the origin of our investigation. This peculiar escarpment was surveyed in 1997
with a SEISTEK seismic equipment which revealed that the layers on both sides of the
escarpment did not match. This already indicated that some relative displacement could
have taken place here. It must be clear that the escarpment itself correspond to the headwall
of a major flow failure which happens to cross the estimated direction of the fault-like
feature. Later we observed that the fjord wall above sea level were showing a clear faultlike escarpment (Figure 5a) which is adjacent to the submarine escarpment which could also
be followed on land over a distance of 600m with a vertical offset of about 0.6m. Note here
that the actual ground displacement very likely took place along pre-existing fracture in the
bedrock. In 1999 we were able to get a Calypso core just below the escarpment which
revealed that the sediments consisted mostly on stacked debris flow deposit with thin
hemipelagic layer in between (see the geotechnical profile in Figure 5d). During the
summer of 2002 we also collected a box core near the base of the escarpment and its
CATSCAN image is shown in Figure 5c with a sharp transition at the contact between the
1663 failure surface of the large liquefaction failure as shown by hemipelagic sediments
sitting just on top of debris flow-like deposit. The same core (Fig. 5c) shows the 1996 flood
layer on top. If we consider a sedimentation rate between 0.5 and 1.0 mm per year, before
1996, the interface would correspond to the 1663 earthquake event. A similar age was also
estimated for the Ponte-du-Fort slide (Locat et al. 2003). Therefore, major flow failure
headwall escarpment seen in Figure 5e, which appears to cross the fault line, was caused by
the 1663 earthquake. In addition, Martin (2002) presented cyclic test results and carried out
analysis using SHAKE and found that for an earthquake of magnitude 6.7 with an epicentre
located 25 km away from the site, the first 15 metres of the sediments near the escarpment
could be liquefied.
All this information leads us to conclude that most of the recent submarine mass movements
were triggered by the 1663 earthquake. In addition, because of the extent of the
synchronous various instability features observed in the region (both below and above sea
level) we can suggest that the epicentre of the 1663 earthquake was near the sector of the
Baie des Ha! Ha!.
7. Discussion and Conclusion
The Upper Saguenay Fjord has recorded the signature of many natural disasters, the largest
one being related to the 1663 earthquake. Many types of mass movements have been
recorded with all the same trigger. The extent of the mass wasting process along the side
wall of the fjord and the sea-floor of the Baie des Ha! Ha! reveal an event which was
extremely violent. As part of the COSTA project, a large effort is put into linking geology,
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geomorphology and geomechanics into improving our understanding of the major event.
Over the last few years, most of our effort had been on morphology and geotechnical
properties. We recently developed a significant geophysical program which will go in the
detailed analysis and integration of both geophysical data and core descriptions into the
development of sedimentary models and facies (Duchesne et al. 2003, this volume) that will
improve our understanding of this major natural disaster and may bring new or modified
interpretation of the mass wasting phenomena which took place in Upper Saguenay area.
Our accumulated knowledge, so far, clearly indicate that the major epicentre of the 1663
earthquake was located in the vicinity of the Baie des Ha! Ha! and most likely in a sector
where may fault-like features have been mapped both in the marine sediments an on-land.
This work fully supports the conclusions of Syvitski and Schafer (1996) to the effect a large
basin failure took place on February 5th 1663 as a result of this large earthquake. There is
still a lot of uncertainties about evidences of recent tectonic activity in the area, but many
features and event point towards local intense seismic activity in recent times, which is
something to take into account in geo-hazard risk assessment for the region.
From a geomechanical view point, the Saguenay Fjord offers a unique environment to study
the triggering and development of mass movements. The size of the various mass
movements is at a scale that can be more easily mastered than large events recorded in
many locations (Locat and Lee 2002). It is our hope that the Upper Saguenay Fjord be
developed as a natural laboratory for the study of marine geohazards. As our scientific
knowledge of the fjord increases the usefulness of this natural laboratory expands.
8. Acknowledgements
The authors would like to thank the Natural Sciences and Engineering Research Council of
Canada for their support into Saguenay post-déluge and COSTA-Canada projects. We also
thank Alcan of Canada Ltd for their contribution towards Saguenay post-déluge project. We
also acknowledge the financial support of the Québec Ministry of Education, especially for
the postdoctoral fellowship provided to R. Urgeles as part of Cataluna-Québec cooperation
which has been very helpful in improving our understating of various mass wasting
processes in the Saguenay Fjord. We also thank the captains and crew members of the
various vessels where data for this work was acquired.
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