The Pliocene Shelburne Mass-Movement and
Consequent Tsunami, Western Scotian Slope
D.C. Mosher, Z. Xu, and J. Shimeld
Abstract Submarine mass-movement is a significant process along continental
margins, even along passive margin slopes. Interpretation of seismic reflection
profiles along the Scotian margin, for example, indicates the Cenozoic section is
dominated by mass transport deposits (MTD) at a spectrum of scales. Occasional
exceptionally large MTDs are observed which seem particularly foreign in a passive
continental margin setting. The Shelburne MTD was recognized from exploration
industry seismic reflection data along the western Scotian Slope. It is a buried Plio/
Pleistocene feature that extends in excess of 100 km from the upper slope to the
abyssal plain and maps to an area in excess of 5,990 km2 and a volume >862 km3. Its
features demonstrate that it is a frontally-emergent MTD with a slump portion and a
debris flow/run-out portion. Tsunami simulations were generated for this event, one
assuming the slump portion generated the tsunami, the other, both the slump and
debris flow contributed. For a mass movement comparable in scale to the Shelburne
MTD, these simulations demonstrate that the city of Halifax, Nova Scotia, would
be impacted within 70 to 80 minutes by a 13–25 m high wave, depending on the
MTD source volume (slump or slump and debris field).
Keywords Submarine landslide • mass-failure • mass-transport deposit • tsunami • geohazard • seismic reflection
D.C. Mosher () and J. Shimeld
Geological Survey of Canada, Natural Resources Canada Bedford, Institute of Oceanography,
1 Challenger Dr., Dartmouth, NS, Canada B2Y 4A2
e-mail: dmosher@nrcan.gc.ca; jshimeld@nrcan.gc.ca
Z. Xu
Canadian Hydrographic Service, Fisheries and Oceans Canada, Maurice-LaMontagne Laboratory,
PO Box 1000, Mont-Joli, Quebec, G5H 3Z4 Canada
e-mail: Zhigang.Xu@dfo-mpo.gc.ca
D.C. Mosher et al. (eds.), Submarine Mass Movements and Their Consequences,
Advances in Natural and Technological Hazards Research, Vol 28,
© Springer Science + Business Media B.V. 2010
765
766
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D.C. Mosher et al.
Introduction
On November 18, 1929, a submarine landslide off the Grand Banks of Newfoundland
caused a devastating tsunami (Mosher and Piper 2007), demonstrating that even
passive continental margins are prone to submarine geologic hazards. The landslide
and ensuing turbidity current severed 12 undersea trans-Atlantic communication
cables (Doxsee 1948) and the tsunami claimed 28 lives on the south coast of
Newfoundland. Mosher and Piper (2007) showed from seafloor multibeam bathymetric data that there is little telltale geomorphologic evidence that a catastrophic
event as recent as 80 years ago occurred; the slope morphology looks much the
same as the remainder of the continental margin. In contrast, seismic imaging of the
eastern Atlantic margin has led to the understanding that submarine mass failure is
a significant process along the continental slope; mass transport deposits (MTD)
probably comprising in excess of 50% of the stratigraphic section. These deposits
occur at a spectrum of scales, but occasional exceptionally large (>100’s of km3)
MTDs are observed. The initiating mechanisms of such deposits along passive
margins remain enigmatic. Additionally, given that a seemingly geologically unrecognizable mass-failure generated a tsunami in 1929, there is a question as to the
tsunami-generating capability of these exceptionally large MTDs. The Shelburne
mega-slump is one such mass failure identified on seismic data from the western
Scotian Slope (Fig. 1). Study of its stratigraphic position, morphology and seismic
attributes are used to determine its origin and depositional style, and its metrics are
used to numerically synthesize a resulting tsunami and its potential impact.
1.1
Regional Geology
Rifting of the Nova Scotia margin began in the Middle Triassic to Early Jurassic
(230–175 Ma), followed by Jurassic seafloor spreading (Wade and MacLean 1990).
Thick Jurassic and Cretaceous strata overlie Triassic salt (Wade and MacLean
1990), the latter of which has mobilized into significant salt diapirs and salt canopies that affect even the modern seafloor morphology. Prodeltaic shales accumulated along the Scotian margin during the Tertiary and continued fine-grained
sediment progradation dominated through to the present. Deep canyon incision
during major sea level lowstands occurred through the Tertiary and again in the
Quaternary. In the later Miocene, the Western Boundary Under-current influenced
sediment distribution on the margin (Campbell and Mosher, this volume) creating
unconformities in some locations and sedimentary deposits in others. Rapid sedimentation of largely fine-grained sediments ensued with the onset of terrestrial
glaciation during the Pliocene and ice sheets extended close to the shelf edge in the
Pleistocene, drastically affecting sea level and shelf and slope sedimentation processes (Mosher et al. 2004). Low-grade seismicity along the margin occurs even into
the present, in part related to remnant glacial rebound strain and in part inherited
from old tectonic terrains (Mazzotti 2007).
The Pliocene Shelburne Mass-Movement and Consequent Tsunami
767
Fig. 1 Study area location on the Scotian margin, also showing the location of two adjacent
exploration wells
1.2
Methods
The principal data set used in this research is part of a 2D multichannel seismic grid
(8 × 8 km-spacing) covering the Scotian margin with 35,000 line-km of data. These
data were acquired for with the vessel Geco-Prakla using a 7,918 in3 airgun array
towed at 7.5 m depth and an 8,000 m long hydrophone array consisting of 320
channels with 25 m group interval. The shot point interval was 37.5 m resulting in
106-fold data. Signals were sampled for 14 s at a 2 ms sample interval. Data were
processed with re-sampling to 4 ms, radon multiple attenuation, spherical divergence and exponential gain recovery, deconvolution, brute stack, NMO, DMO
stack, FK filter and Kirchoff migration, time variant filter, FX deconvolution and
relative amplitude scaling. Seismic data were imported into Seismic MicroTechnology Inc. Kingdom Suite version 8.2 interpretation software for imaging and
horizon tracing. Horizons were digitally picked and isochron maps rendered within
this software suite.
The tsunami model used for this study is based on a set of depth-averaged, linearized shallow water equations with the Coriolis force and frictional force
included. The numerical algorithm for the simulation is similar to that proposed by
Heaps (1969), but the dynamics is assembled in a highly sparse matrix. With such
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a dynamics matrix, we can use it in one of two ways: (1) matrix-vector multiplication, and (2) vector-matrix multiplication. The latter is suitable to generating allsource Green’s function (Xu 2007) for tsunami preparedness when the source of a
tsunami cannot be known beforehand. In this study, we know the location and metrics of a tsunami source and we wish to know how the consequent tsunami propagates and affects the near and far fields. This scenario calls for the first way of using
the dynamics matrix; thus the tsunami simulations presented in this paper are
obtained recursively by such matrix-vector multiplications, with the initial vector
related to the area and height of the slump and debris flow. The model is on a
spherical coordinate and grid resolution is 5 minutes in both longitudinal and latitudinal directions.
2
2.1
Results
Seismic Mapping
Investigation of the TGS seismic grid along the Scotian margin led to identification
of many MTDs in the Cenozoic succession. Some of these MTDs are exceptionally
large, involving >100 km3 of sediment. One of the largest and youngest identified
is referred to as the Shelburne megaslump (Shimeld et al. 2003), located adjacent
to the Shelburne G-29 exploration well on the western Scotian Slope (Fig. 1). This
well was drilled in 1985 in 1,154 m water depth to a target depth of 4,005 m. The
Albatross B-13 exploration well, drilled in 1,341 m water depth to a total depth of
4,046 m, is about 10 km to the east of the MTD.
The Shelburne MTD appears to consist of two fundamental parts, based on the
seismic reflection profiles: at the upslope portion, in present water depths of
750–1,350 m, is a <700 ms thick deposit of folded and faulted stratigraphy (Figs. 2
and 3). The base of this deposit rests on top of conformable reflections. The top of
the deposit is irregular because of faults and contorted folds, but it has been infilled
with chaotic reflections, leveling out the terrain. Above this contorted and incoherent unit, there is largely coherent stratigraphy that is ∼600 ms thick at its upslope
extent and 100–200 ms thick above the folds at the outboard end. The contorted unit
truncates abruptly against a ramp of undeformed stratigraphy (Figs. 2 and 3),
marked in Fig. 2 at the downslope position of the labeled slump. Laterally, the unit
defines a sharp contact with undisturbed stratigraphy in a similar ramp-like contact.
This upslope deformed part of the Shelburne MTD maps to an area of 260 km2.
The second part of the Shelburne MTD commences at the aforementioned undeformed ramp. The contorted unit abuts against the ramp in the downslope direction
and then continues on top of undeformed section, creating a deposit of incoherent
and deformed reflections that is initially about a maximum of 400 ms thick. The
base of this deposit defines a high amplitude reflector which appears to occasionally skip to different stratigraphic levels. The top of the unit is highly irregular with
The Pliocene Shelburne Mass-Movement and Consequent Tsunami
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Fig. 2 Seismic profile showing the upslope portion of the Shelburne MTD (highlighted in yellow). The slump transitions to a debris flow in a frontally-confined manner, with a possible slide
portion involved in the transition
Fig. 3 Dip seismic profile showing an enlargement of the slump region of the Shelburne MTD.
It demonstrates a rotational component in the upslope region, frontal confinement downslope, and
over-topping the frontal ramp as a slide/debris flow
relief of 10’s to >100 ms. Internally, the deposit consists of a mélange of coherent
but contorted and incoherent reflections. Beyond about 14 km downslope of the
ramp, internal reflections of the deposit become nearly incoherent and it’s top is
smooth relative to the initial portion. Occasional high amplitude reflection events
appear within the deposits that are coherent over kilometers of distance. The unit
thins distally from the contorted unit, extending 80 km in the downslope direction
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D.C. Mosher et al.
and fanning out laterally (along strike) to the east and west, covering a total area of
about 5,730 km2. Along its lateral and downslope edges, the incoherent unit develops into conformable, high amplitude and coherent reflections.
There are no direct velocity data within the Shelburne MTD, but ties to the
Shelburne G-29 well log data show the equivalent interval has velocities averaging
about 2,000 m/s. This value is in agreement with refraction velocity analyses conducted further to the east in the Torbrook block (LeBlanc et al. 2007). This value of
2,000 m/s was used to convert time–thickness to thickness in meters. As line spacing
is insufficient over the feature to directly derive its spatial dimensions, the total volume was calculated by first determining sediment thickness along line, then generating an isopach map using a grid interval of 800 × 800 m (Fig. 4) with an
inverse-weighted distance technique. The volume of each grid element was calculated
using the thickness value for that element and then all grid elements were summed to
provide total volume for the feature. Using this technique, the average thickness of
the upslope portion of the feature is ∼450 m and its volume is 117 km3, covering an
area of 260 km2 (Table 1).
Fig. 4 Isopach of the Shelburne MTD, upslope perspective view
Table 1 Metrics of the Shelburne MTD
Area (km2)
Slump
Debris
Total
260
5,730
5,990
Thickness (km)
0.450
0.129
Volume (km3)
117
745
862
Dip length (km)
10
80
90
The Pliocene Shelburne Mass-Movement and Consequent Tsunami
771
The second portion of the MTD, with strongly incoherent reflections, covers an
area of 5,730 km2. Its average thickness is 129 m and total volume is 745 km3. The
total volume of mobilized sediment for the Shelburne MTD, therefore, is about
862 km3 affecting a total area of about 5,990 km2 (Fig. 4). The entire unit rests on a
major regional unconformity that spans nearly the entire Scotian margin (Campbell
and Mosher, this volume). With ties to the Shelburne G-29 well, this unconformity
is recognized to be probable mid-Miocene in age. The top of the MTD appears to
be capped by sediment judged to be Late Pliocene to very Early Pleistocene in age.
The mass failure, therefore, likely occurred about the time of the Plio-Pliestocene
boundary.
2.2
Tsunami Simulation
The characteristics of a tsunami generated by a submarine landslide are mainly
determined by the volume, the initial acceleration, the maximum velocity, the water
depth and the possible retrogressive behaviour of the landslide (Harbitz et al. 2006).
Two tsunami models were calculated and simulations were produced (Fig. 5); (1)
the input function used only the metrics of the slump portion of the MTD, and (2) the
input function used the slump and debris flow components. There is little knowledge of volumes of material that may have been displaced as a translational slide
versus slump, yet a slide may be more efficient at generating a tsunami than a slump
(Grilli and Watts 2005). For the purposes of these tsunami solutions, the seafloor is
assumed to have displaced by an instantaneous vertical displacement of the MTD
thickness and area. It is also assumed that the water depth at the time of failure was
equivalent to todays. Wave dissipation and friction are considered in the numerical
solution.
Figure 5a shows the synthesized tsunami wave field, generated by the slump
portion of the MTD, as it strikes Halifax. The initial wave is negative because
of the style of seafloor displacement reflected at the sea surface. This surface
deformation results in initial drawdown impacting most of the central and
southwestern coast of Nova Scotia at about 50 minutes post failure, followed
by a 13 m-high wave striking Halifax at about 70 minutes (Fig. 5b). Figure 5c
shows the tsunami wave field generated by the slump and debris flow combined. In this case, because the centre of deformation is slightly outboard, the
leading negative wave arrival time at Halifax is 60 minutes post-failure and the
first inundation wave reaches Halifax, the capital and principal population
centre at about 75 minutes (Fig. 5d). Due to local topographic wave amplification, it is over 25 m in height. The wavefront of the tsunami impacts the eastern
seaboard of the U.S. coastline after about 180 minutes, again with a leading
negative wave. In the first scenario (slump only), the inundation wave is only
0.13 m high at New York. In the second scenario, with the slump and debris
field, the inundation wave is 3 m at New York. The tsunami wave hits Bermuda
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D.C. Mosher et al.
Fig. 5 Results of tsunami simulations based on initiation by the Shelburne MTD. (a) and (b)
show results from a tsunami generated solely by the slump, whereas (c) and (d) show results from
a tsunami generated by the slump and debris flow. In both cases, the leading wave is negative as
a result of the initial landslide displacement, followed by a positive inundation wave. In scenario
(a), a 13 m high tsunami wave strikes Halifax at about 70 minutes post-failure. In scenario (b), a
28 m high wave strikes Halifax at about 80 minutes post-failure
at about the same time as Halifax; over five times the distance, but the wave
velocity is greater in deep water.
3
Discussion and Conclusion
The two principal units of the Shelburne MTD describe a “frontally-emergent”
submarine landslide as defined by Frey-Martinez et al. (2006), with a blocky initial
portion terminating abruptly against a ramp and then a thinner deposit of chaotic
reflections overtopping the ramp and spreading downslope. This initial portion,
showing folding, contortion and faulting, is interpreted to represent the initial
The Pliocene Shelburne Mass-Movement and Consequent Tsunami
773
slump of the Shelburne landslide, enlarged in the seismic profile on Fig. 3 and
shown as the thickest portion in the isopach map on Fig. 4. This upslope portion
shows clear evidence of seaward dipping faults and landward-verging folds, probably in response to initial extension and rotation during failure (Fig. 3). Outboard,
the faults are landward dipping and folds seaward-verging in response to compression of the mass abutting the ramp (Figs. 2 and 3), thus the slump appears to have
been partially contained by this ramp. Material of incoherent reflections infill low
areas on top of the deposit (i.e. in the synclines of folds and grabens of faults),
likely material that slumped onto the deposit after initial rotation and translation of
the main mass.
The MTD overtops the frontal ramp at its downdip and outboard position and
spills onto what is probably the paleo-seafloor. It spreads downslope and laterally
as a ∼200 to 400 m thick debris flow, although initial coherent stratigraphy within
the deposit probably reflects a slide component. Coherent but contorted reflections
within the flow and its hummocky surface at this upslope portion suggest that the
flow was blocky, perhaps transporting relatively intact blocks of sediment as it
transitioned from a slump to slide to a debris flow. Beyond about 14 km downslope
from the ramp, the surface of the deposit is smoother and internal reflections are
chaotic, suggesting a well-mixed debris flow. Occasional coherent and relatively
continuous reflectors within the deposit may represent internal flow shear or confluence of different branches of the flow. The deposit occasionally skips to lower
or higher stratigraphic levels, likely reflecting the complexities in the paleoseafloor. At its lateral extents, contorted reflections merge into laterally coherent
reflections, likely representing transition of the failure from debris flow to turbidity
current.
The Shelburne MTD maps from available seismic data to a total area of nearly
5,990 km2, and a volume of 862 km3. This volume is five times that of the 1929
Grand Banks landslide (Piper and Aksu 1987; McCall et al. 2005), and equivalent
to the initial failure of the Storegga slide (Storegga is an accumulation of at least
three distinct events) (Haflidison et al. 2005; Mason et al. 2006). Although the trigger mechanism for the Shelburne MTD is beyond the subject of this study, it is
presumably earthquake initiated, as suggested by Mosher et al. (1994) for most
mass-failures on the Scotian margin and as was the case for the 1929 landslide. The
Grand Banks and Storegga landslides are known to have generated tsunamis that
impacted surrounding coastlines. In the case of the 1929 event, wave heights of
3–8 m are estimated with run up elevations of 13 m (Ruffman 1997; Clague et al.
2003). For the Storegga submarine landslide, maximum run up heights of 20 m
above the contemporaneous high tide level are suggested (Bondevik et al. (2005).
Bondevik et al. (2005) and Lovholt et al (2005) modeled tsunamis from this slide,
assuming retrogressive slide behaviour and calculated open water wave heights
between 5.7 and 7.2 m and wave heights in excess of 20 m at a coastal site proximal
to the source (Sula, Norway). Our two tsunami simulations for the Shelburne MTD;
one generated by just the slump portion of the MTD and another by the slump and
debris flow, demonstrate open water wave heights of 2 and 5 m, respectively.
Inundation waves striking Halifax, 200 km from the failure site, arrive 70–80 minutes
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D.C. Mosher et al.
post-failure with wave heights of 13–25 m. If such waves inundated Halifax at the
time of the failure, all evidence onshore would either be now submerged or eliminated by subsequent Pleistocene glaciations. In both scenarios, such events today
would clearly pose significant risk to life and societal infrastructure along the Nova
Scotia coastline.
Acknowledgments The authors would like to express their appreciation to TGS-Nopec for permitting use of the seismic data and to D.C. Campbell and S. Bartel for assistance with interpretation and technical issues. This work was conducted under the NRCan Geoscience for Ocean
Management Program. We also thank the reviewers Drs. K. Moran and A. Zakeri for critiquing
and improving this manuscript.
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