THE CHRONOLOGY AND RECURRENCE OF SUBMARINE MASS
MOVEMENTS ON THE CONTINENTAL SLOPE OFF SOUTHEASTERN
CANADA
D. J.W. PIPER, D. C. MOSHER, B.-J. GAULEY, K. JENNER, D. C. CAMPBELL
Geological Survey of Canada (Atlantic), Bedford Institute of Oceanography, P.O. Box
1006, Dartmouth, Nova Scotia, B2Y 4A2, Canada
Abstract
High-resolution seismic reflection profiles, multibeam bathymetry, piston cores, and
biostratigraphy from petroleum wells are used to date submarine mass movements on the
continental slope off southeastern Canada. Several different styles of mass movement are
recognised in a variety of geological settings. The chronology allows evaluation of potential
forcing processes, including earthquakes triggered by glacial loading on the continental
shelf and dissociation of gas hydrates related to sea level or bottom-water-temperature
changes.
Keywords: submarine slide, mass-transport deposit, continental margin, recurrence
interval, seismicity
1. Introduction
The Eastern Canadian continental margin is a passive continental margin extending from
Georges Bank in the southwest through the Scotian margin, Grand Banks, Flemish Cap, the
Labrador Sea and Baffin Bay (Fig. 1). The margin rifted in the early Jurassic south of the
Grand Banks and in the early Cretaceous farther north (C.E. Keen et al., 1990). In general,
Cenozoic sediments are relatively thin on the continental shelves and slopes. The entire
margin has been influenced by shelf-crossing glaciation that dates back to the late Pliocene
in Hudson Strait (Piper and deWolfe, 2002), but only to 0.5 Ma on the Scotian margin
(Piper et al., 1994, 2002). 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 (M.J. Keen et al., 1990).
Southeastern Canada has wide continental shelves, notably on the Grand Banks of
Newfoundland, that are cut by deep, glacially excavated transverse troughs. The depth of
the shelf break increases from south to north and is greatest seaward of these transverse
troughs (Piper, 1988). Flemish Cap and Orphan Knoll are remnants of continental basement
separated from the main continental mass during rifting of the continental margin. Modern
circulation along the eastern Canadian margin is southward and westward, in the West
Baffin and Labrador Currents at the surface and the Western Boundary Undercurrent at
depth. Similar patterns existed in glacial times, although the strength and position of the
currents has varied. As a result, sediment introduced to the continental margin by glacial
processes tends to be dispersed southward and westward along the margin. This paper
attempts a synthesis of the chronology of sediment failures on the continental margin. This
299
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Piper et al.
can be done at two different
scales. (1) Failures that are
recognised in the upper 20 m
or so of sediment are
commonly
visible
in
multibeam bathymetry and
sidescan sonar imagery, are
penetrated by high-resolution
seismic systems such as the
Huntec sparker (or even 3.5
kHz profiles), and their ages
can be determined or
reasonably extrapolated from
piston cores. (2) Longer timescale records of failure from
conventional seismic reflection profiles can, in places, be
dated by correlation of
biostratigraphic markers from
ODP or industry exploration
wells. In many areas there is
a lack of sampling for
biostratigraphy
shallower
than the Miocene or Pliocene
and indirect methods must be
used to date seismic markers,
generally
based on assumFigure 1. Map of Eastern Canada showing areas discussed in text.
ptions about the glacial
Arrows show regional gradients in areas discussed in detail.
history of the margin. The 1020 m vertical resolution of
seismic data means that only large failures and thick deposits can be recognised.
Such an assessment has two main benefits. First, understanding the timing of failure may
point towards causal mechanisms. Second, we argue below that in some settings, large
failures are almost certainly earthquake triggered and so provide a proxy for the recurrence
interval of earthquakes, important for risk assessment for offshore structures.
Much of the work reported in this synthesis is detailed in previous preliminary papers,
unpublished reports and student theses, to which reference is made for further detail.
2. Shallow failures
2.1 THE BASIS OF CHRONOLOGY
On much of the eastern Canadian continental margin, 10-m piston cores penetrate only to
Submarine mass movements, S.E. Canada
301
1
the last glacial maximum (18 ka ), although on the outer parts of the margin, some cores are
available that penetrate to marine isotopic stage (MIS) 6 (130 ka). Direct chronology is
available through radiocarbon dating of molluscs or foraminifera. Periodic beds of detrital
carbonate transported by ice-rafting and proglacial plumes from Hudson Strait are known as
Heinrich layers and were deposited on much of the Canadian margin south of 60ΕN. They
have been well dated and provide convenient low-cost chronostratigraphic markers. Similar
beds of brick-red gravelly mud, derived from the Laurentian Channel, can be used as
chronostratigraphic markers on the Scotian Slope. Use of such markers is documented by
Piper and Skene (1998) and Piper and Campbell (2003). Within areas of < 100 km extent,
other lithostratigraphic markers related to ice-rafted or plume transport of proglacial
sediment can be commonly recognised, from colour (e.g. Campbell 1999) or from
abundance or petrography of ice-rafted detritus (e.g. Piper and deWolfe, 2002).
For cores that penetrate beyond the practical limits of radiocarbon dating, at about 50 ka,
generally oxygen isotopes provide a method of recognising MIS 5 (75-127 ka) (e.g.,
Hillaire-Marcel et al., 1994; Hiscott et al., 2001) and thus an extended chronology.
Sedimentation rates on the continental margin vary greatly, so that extrapolation of
sedimentation rates to deeper sections must involve consideration of potential variables. For
example, on the upper Scotian Slope, sedimentation rates were 1 m/kyr during deglaciation
until about 8 ka, but then − 0.05 m/kyr in the mid and late Holocene. the reduced
sedimentation occurred as sea level approached its present position and current winnowing
on the upper slope increased (Piper, 2001), in response to changes in the Labrador Current
(Hillaire Marcel et al., 2001). On Laurentian Fan following the last glacial maximum,
sedimentation rates were > 5 m/kyr during meltwater plume discharge events, but − 0.1
m/kyr during intervening times. In some areas, however, sedimentation rates appear
relatively constant, at least through glacial periods. For example in Flemish Pass (Piper and
Campbell, submitted), where there is good control from both radiocarbon dates and
Heinrich layers. East of Orphan Knoll, Toews and Piper (2002) found almost constant
sedimentation rates of 0.1 m/kyr in glacial MIS 2-4 and 0.18 m/kyr in Holocene MIS 1.
The preferred method for dating mass failures is to locate cores in areas of continuous
sedimentation and to use high-resolution seismic-reflection profiles to correlate failure
features and debris-flow deposits to the cores. This obviates the influence of local
sedimentation effects above a failure deposit, such as winnowing and non-deposition on
high-standing blocks or very high local rates of sedimentation in depressions.
2.2 CASE STUDIES
Detailed studies of the central Scotian Slope have resulted in a chronology of failures over
the past glacial cycle (Gauley, 2001). Radiocarbon dating provides a chronology back to 36
ka (Campbell, 2000) and failures have been recognised from high-resolution Huntec sparker
profiles and from cores, with the positioning of cores based on multibeam bathymetry
1
All ages younger than 60 ka are cited in radiocarbon years
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Piper et al.
(Pickrill et al., 2001). This work shows that failures are rare in the Holocene section, with
only one possible Holocene failure (Jenner et al., in prep) in the eastern part of the study
area, but very common through the glacial section.
On St Pierre Slope, the record of failure is dominated by the 1929 AGrand Banks@ failure
(Piper et al., 1999), but one older failure event is recognised at about 20 m sub-bottom,
close to the last glacial maximum (M. Morrison, pers. comm. 2000). Farther southeast, off
South Whale Basin, there is a widespread failure event dating between Heinrich layers H2
and H3, but younger failures are lacking.
Chronology in both Flemish Pass (Piper and Campbell, submitted) and to the south in Salar
basin is also well controlled by Heinrich layers. In Flemish Pass, there is no evidence of
failure younger than about 22 ka, whereas on the steeper slopes in Salar Basin, three
regional failure events are recognised in the past 40 ka and older failures are interpreted to
date from MIS 5 or earlier.
2.3 IMPLICATIONS OF CHRONOLOGY
In all areas with a good stratigraphic record, the most abundant failures appear to be at or
immediately after the local
maximum Wisconsinan ice
extent. On the Scotian
Slope, there were probably
three advances between 23
ka and 15 ka (Gauley,
2001), whereas on the
Grand Banks the maximum
was probably between H2
and H3, at about 25 ka
(Piper and Campbell,
submitted). The explanation
for this distribution is not
known. Sedimentation rates
were higher during glacials
and there may be a
correlation between frequency of failure and
sedimentation rate (Fig. 2).
Deposition of till on the
upper slope may have
caused abrupt consolidation and expulsion of fluids from underlying sediment. Because of
the influence of the cold Labrador current, it is unlikely that gas hydrates were destabilized
by temperature changes on the Grand Banks margin, although this is possible on the central
Scotian margin. Most likely, the abundant failures are the result of increased seismicity
brought about by glacial loading and unloading. Salt tectonic deformation may have been
influenced by the same loading effects.
Figure 2. Chronology of failures on the Scotian Slope in the past 40 ka
and comparison with St Pierre Slope, South Whale basin, Salar basin
and Flemish Pass. Also shows mean sedimentation rates at the 1000 m
isobath.
Submarine mass movements, S.E. Canada
303
3. Deeply buried failures
3.1 THE BASIS OF CHRONOLOGY
Chronology on the continental slope and rise is based largely on sparse biostratigraphic data
in offshore wells and samples from canyon walls, which have locally provided Pliocene
datums (e.g., Piper and Normark, 1989). Except for ODP wells in the Labrador Sea (as
brought to the Labrador margin by Myers and Piper, 1988) there is no Quaternary control
on seismic stratigraphy below the depth of piston coring. As a result, the chronologic
resolution of the late Cenozoic section on the eastern Canadian margin is poor.
On the continental slope, periodic glacial advances can be recognised from wedge-shaped
units of incoherent reflections on the upper slope that are correlated with shelf crossing
glaciations and probably represent tills. Below this stacked till section is a different seismic
architecture and the change is interpreted as evolution from pro-glacial to pro-deltaic
sedimentation (Mosher et al., 1989; Piper et al., 2002). On the Scotian Shelf, the first shelfcrossing glaciation is dated at MIS 12 (0.45 Ma) (Piper et al., 1994) but farther north in
Hudson Strait it was in the late Pliocene (Piper and deWolfe, 2002). Chronology based on
this Atill-tongue stratigraphy@ must be regarded as speculative because of the lack of
groundtruth.
3.2 CASE STUDIES
Most studies of the longer term record of failure on the continental margin are based on the
recognition of stacked mass-transport deposits on the continental rise. On the eastern
Scotian Rise (Piper et al., 1999; Piper and Ingram, 2003), about 10 discrete mass-transport
deposits, some > 100 m thick, are recognised above a mid-Pliocene seismic marker,
implying a recurrence interval of about 250 kyr. Many of these deposits occur in channel
systems, implying a regional trigger. Between MIS 12 and the base Pleistocene, four
widespread failures are recognised on the central Scotian Slope east of Mohican Channel,
implying a 300 kyr recurrence interval. Farther west, in the Albatross debris-flow corridor,
an estimated recurrence interval of 350 kyr is based on a base-Pleistocene marker (Berry
and Piper, 1993). In Flemish Pass (Piper and Campbell, submitted), a mid-slope basin that
traps sediment from failures in < 1000 m water depth, the recurrence interval of large masstransport deposits is about 400 kyr. The Gabriel lobe, a small depositional lobe seaward of a
slope valley, has a higher recurrence interval, about 100 kyr. In an area remote from direct
glacial input, southeast of Orphan Knoll, Toews and Piper (2002) recognised three masstransport deposits derived from the flanks of Orphan Knoll since MIS 8, or a recurrence
interval of 80 kyr. They saw no correlation between the deposits and periods of falling
eustatic sea level.
Studies of failures from conventional seismic reflection profiles on continental slopes are
complicated by the varied nature of failures in these areas (Mosher et al., submitted). We
summarise studies in two areas: the central Scotian Slope and St Pierre Slope, where
widespread failures have been recognised at discrete horizons from the occurrence of large
buried scarps. A similar pattern is seen at the modern seafloor on St Pierre Slope. Three
widespread buried failures, plus one surface failure, all with failure scarps many tens of
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Piper et al.
metres in height are seen on St Pierre Slope above a MIS 12 marker (MacDonald, 2001),
implying a 150 kyr recurrence interval. On the central Scotian Slope, using similar criteria
to St Pierre Slope, since MIS 12 three large widespread failures are recognised east of
Dawson Canyon (Piper et al., 2002) and two large failures are noted in the ALogan debrisflow corridor@, immediately west of Logan Canyon.
3.3 IMPLICATIONS OF CHRONOLOGY
Large failures, occurring in multiple drainage systems (Toews and Piper, 2002; Piper and
Ingram, 2003) are likely triggered by large passive-margin earthquakes. Care must be taken
to distinguish such failures from debris-flow deposits seaward of transverse troughs,
forming trough mouth fans fed directly by glacial input at the shelf edge (e.g. Hudson Strait:
Rashid et al., in prep.; Orphan Basin: Hiscott and Aksu, 1996). In general, for large masstransport deposits on the continental rise, there is a correlation between recurrence interval
and gradient of the source area. The flank of Orphan Knoll, with a gradient of as much as
15Ε, has a high recurrence interval on the adjacent continental rise. On the Scotian Rise,
recurrence intervals diminish westward, concomitant with a diminution of regional
gradients. Thus the distribution patterns might be interpreted as resulting from relatively
uniform probability of earthquakes, with frequent failure on steeper slopes.
On the other hand, the observed geographic variation might also be explained by proximity
to major tectonic lineaments. The eastern Scotian Rise is close to the Cobequid-Chedabucto
fault and the SW Grand Banks transform margin, the lineament along which the 1929 Grand
Banks earthquake occurred (Bent, 1995). Orphan Knoll lies just south of the Dover fault
and the Charlie-Gibbs fracture zone. In this interpretation, the correlation with gradient may
be largely fortuitous. There is no clear relationship between salt tectonics and failure. Some
of the most active salt tectonics is near the SW Grand Banks transform margin and salt
tectonics is absent north of Salar Basin. More studies are needed to resolve uncertainties as
to the importance of these various factors.
4. Discussion
Globally, dissociation of gas hydrates has been implicated in the occurrence of several
major failures. Supporting evidence has been the spatial correlation of gas hydrates with
failures (e.g. Dillon et al., 2001), or the correlation of failure with times of pronounced
bottom-water warming (e.g. at the Storegga slide, but see Bouriak et al. 2000 for a counterinterpretation) or sea-level fall (Maslin et al., 1998). In all the case studies cited from the
eastern Canadian margin, there has been no compelling chronological evidence for
increased risk of failure at times when dissociation of gas hydrates was likely. Bottom
waters on almost all of the margin are unlikely to have changed temperature significantly
between glacials and interglacials, with the exception of the southwest Scotian and the
southern Grand Banks margins, where Gulf Stream rings impinge during interglacials.
Where a good long chronology is available, as for Orphan Knoll and Flemish Pass, the only
possible correlation of failures with times of falling sea level was for deposits on the small
Gabriel lobe in Flemish Pass and there the correlation is not strong.
Submarine mass movements, S.E. Canada
305
Rather, as argued above, the evidence is that many of the observed failures result from rare
passive margin earthquakes. Spatially, three M>7 earthquakes occurred since 1800 adjacent
to the eastern Canadian margin. Piper et al. (1985) argued for earthquake triggers on the
Scotian Slope on the basis of failure distribution and Mosher et al. (1994) provided
sediment strength estimates that supported this hypothesis. Large, deeply buried failures
recognised on the continental rise are interpreted to largely result from earthquakes. The
apparently greater frequency of shallow continental slope failures during glacial periods is
probably related to seismicity induced by glacial loading and unloading (e.g., Stewart et al.,
2000).
We do not suggest that all large failures are earthquake triggered - some may result from
progressive creep failure, for example (Mosher et al., submitted). However, our
chronological data cannot distinguish between earthquakes and other infrequent effectively
random events. Our information on the distribution of failure deposits suggest that
earthquakes are the likely trigger for many large failures.
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