A SYNTHESIS OF THE DISTRIBUTION OF SUBMARINE MASS
MOVEMENTS ON THE EASTERN CANADIAN MARGIN
D. J.W. PIPER and C. McCALL
Geological Survey of Canada (Atlantic), Bedford Institute of Oceanography, P.O. Box
1006, Dartmouth, Nova Scotia, B2Y 4A2, Canada
Abstract
Published accounts and unpublished multibeam bathymetry and seismic-reflection profiles
have been used to assemble a G.I.S. database documenting the geographic extent of surface
and buried Quaternary submarine mass movements on the eastern Canadian margin. These
range from small failures in fiords to enormous mass-transport deposits on the continental
rise. The patterns of distribution are interpreted in terms of failure processes and large scale
physiographic, geologic and glaciologic variability.
Keywords: Submarine slide, mass-transport deposit, continental margin, fiords
1. Introduction
The Eastern Canadian continental margin rifted asynchronously from south to north in the
Mesozoic and is draped by relatively thin Cenozoic sediments. The entire margin has been
influenced by shelf-crossing glaciation. Much of the upper continental slope, to water
depths of 500 m on the Scotian margin, but to >1000 m off some northern transverse
troughs, is underlain by overconsolidated sediment interpreted as glacial till. Sparse large
earthquakes have occurred on the passive margin, the best known being the 1929 AGrand
Banks@ M = 7.2 earthquake, but the distribution and recurrence interval of seismicity is
poorly known (Keen et al., 1990).
We have prepared a G.I.S. database showing the distribution of known failures within the
first few hundred metres of the seafloor on the eastern Canadian margin, based on published
literature and unpublished seismic data of the Geological Survey of Canada (Atlantic) and
Memorial University of Newfoundland. Figure 1 is a summary of the most prominent
features in the compilation.
2. Types of failure
2.1 FAILURES IN COASTAL SYSTEMS AND ON THE CONTINENTAL SHELF
Post-glacial failures are common in steep, high sedimentation rate areas of coastal systems,
notably the fiords of Baffin Island (Syvitski et al., 1986); the Saguenay Fiord (Urgeles et al.,
2002); and deltas along the North Shore of the Gulf of St Lawrence (M. Duchesne, pers.
comm. 2001). Similar failures are presumably present in less studied fiords of Labrador and
of Devon and Ellesmere islands. Most such failures are slides or retrogressive slumps, either
in prodelta settings or on steep side walls where sediment accumulated from pro-deltaic
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Piper and McCall
plumes.
Figure 1. Map of Eastern Canada showing distribution of major
failures.
Post-glacial failures have
been recognised from
seabed failure scarps and
disturbance of postglacial
sediment
in
a
few
continental shelf areas.
Apparent
retrogressive
failure occurred over tens of
kilometres in the northeastern part of Eastern
Basin of Hudson Strait
(MacLean et al., 2001).
Failure has been recognised
from the steep walls of
Cartwright Saddle transverse trough on the southern
Labrador Shelf (Josenhans,
1983) and less clear
evidence has been seen
elsewhere on the margins of
transverse troughs. On the
east Scotian Slope, in the
area of ground shaking from
the Grand Banks 1929
earthquake, a near-surface
fault offsets the top of the
glaciomarine section and is
associated with severe
disturbance of the overlying
post-glacial strata (Durling
and Fader, 1986).
Sediment
failures
are
common in proglacial settings, both in fiords with modern tidewater glaciers (Syvitski et al.,
1986) and at former ice margins on the continental shelf. The features known as till tongues,
which are generally accepted to be ice-margin diamicts, have been argued by some authors
(e.g., Stravers and Powell, 1997) to be debris-flow deposits, although some appear to be
lodgement till (Piper et al., 2002). Debris-flow or other mass-transport deposits are common
stratigraphically between ice-contact deposits and glaciomarine deposits (e.g. MacLean et
al., 2001, p. 75). Small mass-transport deposits have been detected within some
glaciomarine successions (e.g. Fig. 26, 58 of MacLean et al., 2001), which might be related
to ice readvance (e.g., Fig. 29 of MacLean et al., 2001).
Submarine mass movements on the Canadian eastern margin
293
2.2 FAILURES IN CANYONS AND ON CHANNEL WALLS
Several failure types occur in the heads and walls of the numerous canyons that dissect the
eastern Canadian continental slope. Slopes on canyon walls are commonly 20-40Ε and in
places are vertical. Many canyons head at the downslope limit of glacial till deposition (400
m on the Scotian Slope, 600 m on the Labrador Slope), but a few on the Grand Banks and
Scotian margins cut back into the continental shelf. On the Scotian margin, shelf-indenting
canyons have narrow axial talwegs that channel sandy turbidity currents, in some cases to
the continental rise (Baltzer et al., 1994; Pickrill et al., 2001). Fishermen report rapid
disappearance of sand from the head of Verrill Canyon on the central Scotian Slope and off
Browns Bank on the southwest Scotian Slope. Milne (1897) reported cable breaks from the
Tail of the Banks probably related to rapid disappearance of canyon sands (A. Ruffman,
pers. comm. 1998). The recurrence interval of such sandy flows near the Titanic wreck site
was estimated as hundreds of years (Savoye et al., 1990).
On canyon walls with gradients of more than about 7Ε, the upper few metres of soft surficial
sediment commonly shows failure (e.g., Piper 2001, Fig. 8.5; Josenhans and Barrie, 1989,
Fig. 4.7). Bioturbational excavation or tidal current scouring of loose sands in places leads
to topples of overlying massive mud (Piper and Campbell, 2002).
The floors of many canyons consist of stacked mass-transport deposits, recognised from
seismic-reflection profiling (Mosher et al., submitted), piston cores (Jenner et al., in prep.;
Piper 2001, Fig. 7-17), and submersible observations (Josenhans and Barrie, 1989). The
presence of large, distinct, and in some cases correlatable mass-transport deposits suggests
episodic widespread failure.
Where submarine channels cross the lower continental slope and rise, channel walls
commonly have been steepened by erosional undercutting and have gradients of more than
20Ε. Local topples and rotational slumps have been observed from submersibles in Eastern
Valley of Laurentian Fan (Hughes Clarke, 1988; Piper and Campbell, 2002). Multibeam
bathymetry shows amphitheatre-like retrogressive slump failures on the flanks of channels
on the lower Scotian Slope (Pickrill et al., 2001), although many are tens to hundreds of
thousands of years old. Long-range sidescan sonar shows similar slumps on the walls of
NAMOC (Hesse et al., 1997).
2.3 COMPLEX SHALLOW FAILURES ON THE MID- TO LOWER SLOPE
Complex shallow failures, consisting principally of retrogressive rotational slumps and
debris-flow deposits evolved from them, are widespread on the middle continental slope off
Nova Scotia and Newfoundland. The best documented are around the 1929 Grand Banks
earthquake epicentre on St. Pierre Slope (Piper et al., 1999a), where rotational slumps 5-10
m thick and 15-30 m thick are widespread and pass downslope into blocky debris flows.
The upslope limit of retrogression is at about 500 mbsl, where sediment is more
consolidated as a result of till deposition and iceberg scouring. Similar near surface failures
dating from the late Pleistocene are known on the central Scotian Slope (Piper et al., 1985;
Mosher et al., 1994; Gauley, 2001; Nott et al., 2002). Analogous buried features have been
interpreted on the continental slope off the Grand Banks (Toews, 2003).
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Piper and McCall
2.4 LARGE MASS-TRANSPORT DEPOSITS ON THE CONTINENTAL RISE
Large mass-transport deposits on the continental rise, with deposit thicknesses of tens of
metres and tens of kilometres of lateral extent, probably have a range of sources and
initiation. In general, evidence for rotational slumping is lacking and many of these deposits
are referred to as Adebris-flow deposits@ in the literature.
Three types of failure are tentatively recognised, but many examples of this type of deposit
exist that cannot be assigned with confidence to one of these three types.
(a) Glacially derived trough-mouth fans, where shelf-crossing ice streams discharged
diamict directly to the upper slope. This occurred off Hudson Strait once in the mid to
late Wisconsinan, as well as during earlier glaciations (Rashid et al., in prep). A similar
near-surface trough-mouth fan is visible seaward of Karlsefni trough. Farther south on
the Labrador margin, northern Hopedale Saddle has a large trough-mouth fan that is
draped by about 100 m of younger sediment and a trough-mouth fan is developed in
Orphan Basin (Hiscott and Aksu, 1996). Sparse data suggests that trough-mouth fans are
also developed seaward of major transverse troughs in Baffin Bay, notably seaward of
Lancaster Sound (Aksu, 1984).
(b) Large failures, which appear to have a source on canyon walls on the steep middle
slope. The best known example is the Albatross debris flow (Mulder et al., 1997), which
developed from a middle slope (500-1000 m) failure at about 14 ka and ran out to the
continental rise. The geotechnical properties of mud clasts suggest that a vertical
thickness of up to 40 m of slope sediment failed.
(c) Other large mass-transport deposits on the continental rise, which appear to have
resulted from failure of slabs of middle slope to upper rise stratified sediments up to 100
m in thickness. The best documented examples are on the east Scotian Rise (Piper and
Ingram, 2003) and on the central Scotian Rise just west of Logan Canyon (Piper et al.,
1999b).
In two areas, off Georges Bank (Hughes Clarke et al., 1990) and off the Tail of the Banks
(Savoye et al., 1990), 5-10 km wide mass-transport deposits are recognised on the
continental rise that have distinct imbricate toe thrusts. The example off Georges Bank also
has a clear head scarp. These failures are tentatively interpreted as slides.
3. Causal processes
Numerous processes may trigger or increase the possibility of sediment failure. For each,
the likely occurrence on the eastern Canadian margin is summarized and possible examples
cited. It should be emphasized, however, that the causal processes of most sediment failures
are unknown.
A magnitude 7.2 earthquake was responsible for the best known failure on the Canadian
continental margin, the 1929 AGrand Banks@ event (Piper et al., 1988). Earthquakes have
been implicated in many older failures, on the basis of (a) morphological similarity of
failure to that observed in the 1929 event, i.e. abundant retrogressive slumps affecting the
Submarine mass movements on the Canadian eastern margin
295
upper 5-20 m (e.g., Piper et al., 1985); (b) evaluation of strength properties of seabed
sediments and their response to seismic shaking (Mosher et al., 1994) and (c) the
observation of widespread simultaneous failure in separate submarine drainage systems (e.g.
Piper and Skene, 1998; Toews and Piper, 2002).
Loading on steep slopes is responsible for some prodeltaic failures in fiords (Syvitski et al.,
1986) and may be a process involved in the deposition of trough-mouth fans. It is
presumably a contributing factor to failure in many other settings, but its role is difficult to
separate from other factors.
Oversteepening of slopes may trigger failure on canyon and channel walls, principally by
turbidity current erosion (e.g., Shor et al., 1990; Pickrill et al., 2001). Maintainence of steep
slopes by bottom current erosion, for example around Flemish Cap, may play a role in
triggering failure there. Some steepening may be the result of active salt tectonics,
producing fault offsets in near-surface sediment. Mosher et al. (submitted) have suggested
that movement on faults related to salt tectonics on the Scotian Rise may set up conditions
for common retrogressive failure on the upper rise and lower slope.
Excess pore pressures, including those due to dissociation of gas hydrates, may play a
role in sediment failure (e.g., Mosher et al., 1994). There is no clear spatial relationship
between the sparse areas of observed bottom simulating reflectors (BSRs) and sediment
failures on the eastern Canadian margin. On the Gabriel Lobe in central Flemish Pass, Piper
and Campbell (submitted) found that failures tended to occur in the early stages of
glaciations, when falling sea level might destabilize hydrates. The abundance of failures and
pockmarks on the mid Scotian Slope, near the limit of gas hydrate stability, was interpreted
by Piper et al. (1999b) as evidence for the involvement of gas hydrate in failures.
Pockmarks and gassy sediment are widespread in the area of the 1929 Grand Banks failures
and Christian and Heffler (1993) measured excess pore pressures in this area. Gauley
(2001) suggested that excess pore pressures might be generated on the mid slope by rapid
loading by till tongues on the upper slope. As elsewhere in the world, the role of pore
pressure and gas hydrates in failure is poorly understood and our distribution maps throw no
new light on this issue.
Decollement surfaces and overlying creep folds are observed in several places on the
Scotian Slope at sub-bottom depths of 30 to 100 m (Gauley, 2001; Mosher et al.,
submitted). These decollements are unsupported either at canyon/channel walls or at faultline scarps related to salt tectonics. Mosher et al. (submitted) have suggested that eventual
failure of these decollements may be responsible for some of the large mass-transport
deposits on the Scotian Rise.
Retrogressive failure is readily recognisable on parts of the Scotian and St Pierre slopes
that lack canyons. Retrogression from local zones of steepening was dominant in the 1929
earthquake (Piper et al., 1999a) and Nott et al. (2002) have demonstrated that this is also
the dominant process on the central Scotian Slope. Salt tectonics producing fault scarps on
the upper continental rise (Pickrill et al., 2001; Mosher et al., submitted) localises the
initiation of retrogression here.
Cyclic loading from waves is unlikely to cause the observed failures in water depths >500
m on the continental slope (Moran and Hurlbut, 1986). On the other hand, such loading
296
Piper and McCall
might be responsible for failure of sand in canyon heads at 100-200 mbsl. Ignitive flow
initiated by storm-driven currents (Fukushima et al., 1985) could also erode canyon head
sands.
There is a strong geographic and temporal link between shelf-crossing glacial ice,
particularly ice streams in transverse troughs, and the occurrence of failures on the
continental slope. Both till tongues and debris-flow deposits of trough-mouth fans may
represent input of glacial till from the ice margin. Gauley (2001) and Piper and MacDonald
(2002) observed a correlation between evidence of ice advance and the occurrence of small
Adebris-flow@ deposits that might have resulted from direct destabilization of sediment at
an ice margin. As discussed in 2.1 above, ice-margin conditions appear associated with
failure on the continental shelf and meltwater may cause canyon erosion. Mulder and Moran
(1995) discussed several ice-margin processes, notably bearing capacity failure, that might
be responsible for failure at ice margins. In general, these processes would be effective
close to the ice margin on the upper slope, but do not provide an adequate explanation for
mid slope or deeper water failures. The indirect effects of ice loading on seismicity (Stewart
et al., 2000), however, may be important in triggering failures.
4. Distribution patterns
A few general patterns of distribution of failures are observed. At high latitudes, troughmouth fans are an important style of failure, whereas at lower latitudes, shelf-crossing
glaciation involving warmer and wetter ice caused cutting of canyons by subglacial
meltwater, resulting in different types of failure. Retrogressive slumping appear particularly
abundant on the Scotian Slope, influenced by salt tectonics. Variations in the gradients of
continental margins, principally controlled by basement geology, must have an influence on
sediment stability, but do not appear to be a major control on the regional distribution of
failures. On the Scotian rise, the largest failures are in the pre-glacial section, demonstrating
that glaciation was not a pre-requisite for large failures and suggesting that canyon
excavation by glacial meltwater may have helped stabilise continental slopes by providing
drainage for excess pore pressures (Piper et al., 1999b). As noted above, no general
relationship is noted between the presence of BSRs and the occurrence of failure.
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