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ORIGINAL
Large bedrock slope failures in a British Columbia, Canada fjord: first
documented submarine sackungen
Kim W. Conway
1
&J. Vaughn Barrie
1
Received: 1 December 2017 /Accepted: 17 January 2018
#Crown 2018
Abstract
Ve r y l a r g e ( > 6 0 × 1 0
6
m
3
) sackungen or deep-seated gravitational slope deformations occur below sea level along a steep fjord
wall in central Douglas Channel, British Columbia. The massive bedrock blocks were mobile between 13 and 11.5 thousand
radiocarbon years BP (15,800 and 13,400 BP) immediately following deglaciation. Deformation of fjord sediments is apparent in
sedimentary units overlying and adjacent to the blocks. Faults bound the edges of each block, cutting the glacial section but not
the Holocene sediments. Retrogressive slides, small inset landslides as well as incipient and older slides are found on and around
the large failure blocks. Lineations, fractures and faults parallel the coastline of Douglas Channel along the shoreline of the study
area. Topographic data onshore indicate that faults and joints demarcate discrete rhomboid-shaped blocks which controlled the
form, size and location of the sackungen. The described submarine sackungen share characteristic geomorphic features with
many montane occurrences, such as uphill-facing scarps, foliated bedrock composition, largely vertical dislocation and a
deglacial timing of development.
Introduction
The German term “Sackung”(plural “Sackungen”)signifiesa
type of terrain sagging which commonly (but not exclusively)
occurs in mountainous terrains (Helm 1932;Hutchinson
1988) where it often forms a diagnostic landform including
uphill-facing scarps, elongate double-crested ridges and ten-
sion cracks (Pánek et al. 2015). These features are found in
alpine areas globally, including California (McCalpin and
Hart 2002), Italy (Ambrosi and Crosta 2005), Alaska (Li
et al. 2012) and Canada (Schwab and Kirk 2002). Montane
sackungen are normally linear deformations located along
ridge crests and commonly occur in tectonically active areas
(Agliardi et al. 2001; Sanchez et al. 2010) but are not a form of
slide or creep as intimated by Zischinsky (1966; cf. also Poisel
and Preh 2004,2008). An important distinction between
sackungen and landslides is the lack of significant lateral
spreading in the former (Poisel and Preh 2004), though a
bulge at the base of sackungen is frequently present.
Sackung is not synonymous with deep seated gravitational
slope deformation (DSGSD) but is considered to be a partic-
ular type of DSGSD (Soldati 2013). Sackungen are frequently
associated with deglaciation of steep alpine landscapes
(Hutchinson 1988). Some authors, however, reject the require-
ment for glaciation as a precursor to sackung formation (e.g.
Pánek et al. 2015) and point to a lack of evidence for close
timing of sackung formation and deglaciation. Others attribute
the genesis of these features to a combination of climatic and
tectonic processes (e.g. Gutiérrez et al. 2005). Foliated meta-
morphic rocks are key lithologies in the development of
sackungen in the European Alps (Crosta et al. 2013)andthey
may also occur in a variety of other lithologies including soils
and sedimentary rocks. Sackungen have to date not been iden-
tified in submarine environments as these features are typical-
ly associated with mountain range and ridge top settings, not
valley floors. Accurate dating of the timing of initiation and
activity of sackungen has been problematic at many sites.
While sackungen are normally slow, predominately verti-
cal deformations, they have in some cases been shown to be a
geological hazard stemming from sudden block sagging with
devastating effects (Forcella 1984; Brückl 2001; Hewitt et al.
2008; Bianchi Fasani et al. 2014). In British Columbia (BC),
Canada, sackungen have been reported at several locations on
land (e.g. Thompson et al. 1997) and would have been prone
to tsunami generation if occurring below the sea surface.
*Kim W. Conway
kconway@nrcan.gc.ca
1
Geological Survey of Canada, Natural Resources Canada, 9860 West
Saanich Road, Sidney, BC V8L 4B2, Canada
Geo-Marine Letters
https://doi.org/10.1007/s00367-018-0533-y
Thus, some of the largest tsunamis ever recorded have oc-
curred in fjords as a result of slope failures (Bornhold and
Thomson 2012). For example, a 524 m tsunami wave devel-
oped from a landslide in Lituya Bay in SE Alaska in 1958
(Weiss et al. 2009). The potential for large rock slides and
sackungen to trigger tsunamis in coastal BC is therefore of
concern as large events causing significant casualties have
marked the past in this region (Bornhold et al. 2007). The
British Columbia coast is earmarked by steep slopes, seasonal
extremes of soil moisture, macrotidal conditions, and the
highest seismicity in Canada, all of which increase the poten-
tial for slope failures in the form of slides and sackungen. In
coastal environments, both submarine and subaerial slope fail-
ures may be initiated. Because such events generally take
place in relatively shallow and confined waterways, they pres-
ent a serious hazard for tsunami wave generation (Mosher
2009; Bornhold and Thomson 2012).
The Kitimat Arm and Douglas Channel fjord system is
under consideration for the development of extensive shore-
line installations and infrastructure for a variety of industrial
projects mainly related to hydrocarbon port facilities. The
proximity of large submarine landslides to the proposed
infrastructure development has spurred research into the
natural hazards associated with large slide masses and
other submarine landslides in the region (Conway et al.
2012;ConwayandBarrie2015). The largest bedrock
failures in Douglas Channel were described as possible
submarine sackungen by Conway and Barrie (2015)and
Shaw et al. (2017), and a late glacial timing was proposed for
emplacement.
The purpose of this paper is to document the processes
governing the development of these first identified submarine
sackungen. It presents a detailed chronology for emplacement
of these very large deformations, and provides information
pertaining to the degree and nature of any potential geohazard
which they may represent.
Regional setting
The coastline of British Columbia is dominated by steep
coastal mountains and deep fjords which indent the coastline
by up to 170 km. The Cordilleran hinterland of the mainland
fjords is predominantly composed of plutonic bedrock
(Wheeler et al. 1991). Douglas Channel, in particular, is a
steep-sided fjord with depths up to 460 m (Macdonald et al.
1983). The northern portion of the BC coastal region was
subject to extensive glaciation in the last glacial maximum
(LGM), with ice thicknesses up to 2 km originating from
inland ice depocentres (Clague 1984). Ice streams filled fjords
locally to at least 1,000 m during the LGM and occupied the
fjords from 25–16 ka BP. A discrete ice lobe extended beyond
the mouth of Douglas Channel until about 13,000 years ago
(Clague 1984). Sea levels were high due to isostatic depres-
sion of the region, leaving glaciomarine sediment at or above
200 m elevation between 10 and 13 ka BP.
Bedrock geology of the Douglas Channel/Kitimat Arm re-
gion of the Canadian Cordillera was studied by Roddick
(1970). The geomorphology and orientation of Douglas
Channel are thought to have been fault controlled (Duffell
and Souther 1964; Roddick 1970; Holland 1976). The late
Quaternary of the Kitimat region has been earmarked by gla-
ciation and rapid sea-level change (Bornhold 1983;Clague
1985). During the LGM the fjord system was filled with
ice. As ice margins retreated, thick sequences of mud,
sand and deltaic gravels infilled basins in coastal set-
tings as relative sea level fell. The present-day seafloor
of Douglas Channel is underlain by up to 800 m of
unconsolidated sediment (Bornhold 1983). Holocene
sediments are up to 90 m thick in the deeper basin areas of
Douglas Channel (Shaw et al. 2017) and overlie several hun-
dred metres of glacial sediments including till (Bornhold
1983). The Hawkesbury Island marine study area (Fig. 1)
was fully covered by ice until 12 ka BP.
The oceanography of the Douglas Channel fjord system is
the subject of ongoing study (Johannessen et al. 2015)and
was last reviewed by Macdonald et al. (1983). The fjord has
a well-developed estuarine circulation and bottom water ex-
change occurs on an annual timescale through channels to the
continental shelf. Particles accumulate in the fjord from rivers
and streams entering both at the head and along sidewalls of
the fjord system. Sediment accumulation has been uneven due
to variable tidal currents at the seafloor (Shaw et al. 2017)and
Holocene deposition varies from 0 to 90 m in thickness, and in
some areas the glaciomarine section is exposed and eroding.
Regionally, landslides were mapped in Douglas Channel and
Kitimat Arm and other coastal areas by Conway et al. (2013),
and large submarine failures and tsunami potential of failures
within Douglas Channel were documented (Conway et al.
2012; Thomson et al. 2012).
Within the study area, extensive north–south oriented line-
aments show evidence for possible faulting (Conway et al.
2012). However, there has been only very limited seismicity,
with a total of eleven magnitude <3.0 earthquakes in the last
25 years in the proximity of the study area (Geological Survey
of Canada (GSC), unpublished data). Understanding the pro-
cess and timing of submarine slope failure is critical to gaug-
ing the hazard represented by slope instability. The
tsunamigenic potential of creep or slowly moving failures is
much reduced compared to any rapidly occurring event. In
addition, it is understood that slope instability and failure
was much more common during the immediate post-glacial
time period than in more recent (Holocene) times (St. Onge
et al. 2004). However, the presence of submarine failure de-
posits in the present-day landscape is not necessarily an indi-
cation of an ongoing hazard (Conway et al. 2013).
Geo-Mar Lett
Materials and methods
Marine geoscience investigations have been undertaken to
study previously defined slope failures in Douglas Channel
(Conway et al. 2012; Conway and Barrie 2015)inorderto
assess the nature, age and origin of the largest failure features
and shallow faulting of the seafloor. Multibeam data were
collected in Kitimat Arm and Douglas Channel by the
Canadian Coast Guard Ship CCGS Vecto r between 2007
and 2010 using a hull-mounted Kongsberg-Simrad EM2002
multibeam echo-sounder system before 2009, and a
Kongsberg-Simrad EM 710 system operating at 70–100 kHz
system mounted in a gondola after 2009 in water depths great-
er than 50 m. Inshore areas of >5 m depth were surveyed by
the CCG Launch Otter Bay using a EM 3002 multibeam sys-
tem. Data were gathered using the Kongsberg SIS system and
processed using the CUBE extension of CARIS-HIPS soft-
ware. Data were then exported as 5-m resolution grids with
confidence intervals of 0.1% of water depth vertically and 2 m
laterally. Lateral errors arise from navigational limitations.
The grids were then imported into ESRI ArcInfo© GIS soft-
ware to allow visualization. Perspective, three-dimensional
views of samples of the multibeam datasets were created using
Fledermaus© software.
Marine geophysical surveys were completed on CCGS
Vector cruise 2013007PGC in November 2013 and from
CCGS John P. Tully in October 2014. Roughly 220 line km
of high-resolution seismic sparker data were collected using a
400 joule Huntec deep-tow seismic (DTS) system during the
2013 survey (Fig. 2). In addition, 500 line km of chirp profile
data were collected duringboth cruises, operating at a nominal
frequency of 3.5 kHz. Kingdom Suite software allowed pro-
cessing and analysis of sparker and chirp sub-bottom data.
A Benthos piston coring system with a 1,000 kg head
weight allowed cores to 8 m length to be collected (Fig. 2).
A digital camera (GSC-A 4000) was employed in 2014 and a
Fig. 1 Location of the Douglas
Channel/Kitimat Arm study area
with regional topography and
bathymetry. Extracted from
Conway and Barrie (2015)
Geo-Mar Lett
large volume (0.5 m
3
) grab sampler was used during the 2013
cruise. The remote operated vehicle ROPOS (Dive R1877)
was deployed in September 2015 to examine the head scarp
of a large slope failure (cf. block A below). Light detection
and ranging (LiDAR) data were collected onshore, adjacent to
the marine study area, between April and June 2015 using a
Riegl VQ-580 LiDAR system.
GeoTek© multi-sensor core logger (mscl) systems served
to analyse the cores for physical properties including gamma-
ray density, magnetic susceptibility and high-resolution imag-
ing. Note that these data are not reported in the present
study. Cores were visually logged after splitting. Sediment
textures were described conforming to the Wentworth scale
(Wentworth 1923) from grain size datasets (GSC unpublished
data) which were calculated by the method of moments with
analysis of mud fractions by Micromeritics Sedigraph©, sand
by settling column, and gravel by sieve. Cores were rendered
as graphic logs after the method of Boyles et al. (1986).
Accelerator mass spectroscopy radiocarbon dating of se-
lected core samples was performed at Beta Analytic Inc.
Ages are presented and discussed in conventional radiocarbon
years before present (BP). A reservoir correction of 800 years
was applied to shell dates (410 global reservoir and a 390 year
local marine reservoir Delta-R correction). Conversion of ra-
diocarbon years to calendar years was accomplished using the
algorithm of Fairbanks et al. (2005).
Results
Multibeam bathymetric data illuminate two areas of massive
slope failure along the south-eastern margin of Douglas
Channel (Fig. 1). Seabed sample locations, geophysical survey
transects and figure locations are shown in Fig. 2. The fjord
wall appears to have failed where two scallop-shaped hollows
located along the edge of the fjord wall are closely associated
with detached blocks apparent on the fjord floor. The northern
failure (block A) appears to originate from sea level to 100 m
depth along the shoreline (Fig. 3) whereas, to the south, block
B has a source from 75 to 100 m depth (Fig. 4). The bedrock
composition of the blocks is inferred to be derived directly
from the Hawkesbury Island coastal lithology which, accord-
ingtothemappingbyRoddick(1970), consists of a diorite
(igneous) rock type at block A and a gneissic-diorite migmatite
at block B. ROVobservations at block Awere able confirm this
lithology, and also discriminate well-developed joints and frac-
tures on the vertical wall of the head scarp.
The failure masses rest on the fjord floor where a drape of
mud covers the surface morphology of the base of the block at
approximately 350 m water depth at block A and 400 m at
block B. The margins of the failures are mantled with thick
recent sediments (Figs. 3and 4) which also infill the fjord. The
seafloor morphology suggests that a portion of each failed
block extends for some distance onto the fjord floor, where
Fig. 2 Douglas Channel study
area. Insets location of Cruise
2013007 cores and figures
relative to blocks A and B.
Extracted from Conway and
Barrie (2015)
Geo-Mar Lett
they have been subsequently buried by sedimentation
(Figs. 3and 4). Bathymetric profiles across the blocks
(Figs. 3and 4) indicate that they have moved into place after
detachment and that this motion has moved block A down-
slope by up to 350 m and block B by up to 400 m. The profiles
also show that the evacuated head scarps of both blocks are
nearly vertical. Relative block movements are not the
same along the north–south oriented centre axes of the
blocks relative to the source of the failure, and both blocks
indicate more downslope movement on the southern side
of the mass than on the northern side. The volumes of
the failed blocks are calculated to be 62×10
6
m
3
for block A
and 70×10
6
m
3
for block B (Thomson et al. 2012).
Perspective views of block A looking northwards up
the channel (Fig. 5) show that the block is set within a
zone of general slope instability where relatively small
slides are superimposed on the large bedrock masses.
Small slides occur both on the back, landward-facing
slope of block A and also at the crest of the same
block, facing seawards. An adjacent, somewhat muted
scarp appears to define a potential incipient failure im-
mediately south of block A (Fig. 5). This area is brack-
eted by block A to the north and retrogressive, small
slides to the south at the base of the fjord slope. In
addition, an older failure scarp is apparent further to
the south (Fig. 5).
Fig. 3 Detailed multibeam
imagery of block A. Bathymetric
profiles extracted from multibeam
bathymetry data are shown with
locations at right. See Fig. 2for
location. Profiles show general
shape of block detachment from
Hawkesbury Island. Profile 5
shows 100 m scarp where block
was removed. Extracted from
Conway et al. (2012)
Geo-Mar Lett
Sedimentary units and geochronology
Coring results from Douglas Channel describe a sequence of
three widespread surficial units (Fig. 6). A lower unit (here
designated unit 1) is a laminated to massive grey clay with
sparse sand and gravel clasts. The unit, up to 6 m thick
(2013007PGC cores), contains sand grains and gravel clasts
of various lithologies which are matrix supported in the clay.
Unit 1, dated to 10,110±30 to 10,950±40 radiocarbon years
before present (C14yBP; Fig. 6, Table 1), also yielded one
young age of 4,630±30. An interbedded silty clay to variably
coloured well-sorted clay (unit 2) overlying the gravel clay
unit yielded ages from 10,470 to 10,200 C14yBP. The clay
beds vary from 10 to 40 cm in thickness and are burrowed in
some intervals. The clays, coloured blue-grey to blue-green,
are a markedly different colour than the bracketing grey to
dark grey mud unit.
The topmost unit (unit 3) is an olive, soft and massive
bioturbated silt which is up to 5.5 m thick in the cores
(Fig. 6). The unit is completely bioturbated and is entirely
massive in character. Cores sampled variable unit 3 sediment
thicknesses but nowhere did the unit contain bedding or lam-
inations. Ages in unit 3 ranged from 4,250±30 C14yBP at
144 cm depth in core 2014007018 to 10,230±30 C14yBP at
429 cm depth in core 2013007003 (Table 1). The uppermost
unit has been accumulating at a variable rate in Douglas
Fig. 4 Detailed multibeam
imagery of block B showing
detachment area. Bathymetric
profiles extracted from multibeam
bathymetry data are shown with
locations at right. See Fig. 2for
location. Note pronounced north-
trending submarine fractures at
the south end of Hawkesbury
Island. Extracted from Conway
et al. (2012)
Geo-Mar Lett
Channel and the range of radiocarbon ages indicates sedimen-
tation throughout most of the Holocene.
At two core sites (2013007006 and 2013007007) adjacent
to blocks A and B (Fig. 2), no unit 3 sediments have accumu-
lated and unit 1 (glaciomarine gravelly and laminated grey
clay) formed the entire core. Two core sites (2014007017
and 2014007018), distal from the blocks, were sampled to
allow study of regional sedimentation (Fig. 2). These recov-
ered a similar lithologic section and provided similar radiocar-
bon ages. Estimated sedimentation rates were 0.7 and 0.9 mm
per year at these sites respectively during the deposition of
unit 3. No stratified sections indicative of events were ob-
served in the cores. Multi-sensor core logger data indicate that
unit 3 is completely unstratified, resulting from uninterrupted,
slow and well-bioturbated sedimentation.
Sedimentary sequence
The lowermost unit (unit 1) is related to the latest stages of
deglaciation of the study area. Isolated gravel clasts and coarse
sand grains were deposited as ice-rafted debris (IRD). Silt and
very fine sand laminae likely represent glaciomarine pulses
and underflow deposition of sediment from turbid glacial melt
water. The radiocarbon ages obtained in this unit correspond
with a late deglacial time of deposition from 11–10.5 ka BP.
The clay unit (unit 2) which overlies unit 1 is a stratified,
anomalous deposit evidenced by the variable colour hues in
the well-sorted beds. Radiocarbon ages of 10.5 to about
10.2 ka BP, blue-grey sediment colour, and slightly bioturbat-
ed nature of the unit indicate that it records the final phase of
deglaciation, before the onset of a truly Holocene climatic and
oceanographic environment.
The topmost unit in the sequence (unit 3) is of Holocene
age and is completely bioturbated, massive and olive in col-
our. The recent sedimentation rate is variable throughout
Douglas Channel due to variability in the velocity of seabed
tidal currents and to geostrophic effects which modify currents
and influence deposition in the fjord. At some locations
Holocene sediments are up to 90 m thick, while in other areas
which are subject to seafloor scour and even erosion, no sed-
iment has accumulated. Sedimentation rates in unit 3 were
found to be less than 1 mm per year at all core sites. Recent
sedimentation was absent on blocks A and B and these areas
are non-depositional. The massive Holocene sedimentary se-
quence is nowhere interrupted by graded sand beds or silty
laminae.
Glaciomarine sequence deformed by blocks A and B
In the periphery of block A deformation, as imaged in sub-
bottom profile data, suggests impact on a deforming front of
the well-stratified glacial marine sediments of unit 1.
Deformed and gently folded or buckled glaciomarine sedi-
ments are observed overlying and adjacent to the seaward
edge of the blocks. The folds appear as muted anticlines of
100 to 300 m width with indications of brittle deformation at
the boundaries of the weakly developed folds which may be
up to 50 m thick (Fig. 7). The folded, deformed reflectors
do not appear as one deformed section but as several
intervals forming a deformed complex overlying the
bedrock failure. Localized folds are, in some cases,
capped by debris flows. In sub-bottom profile data at
the boundary of block A, brittle deformation of the sec-
tion may be clearly seen (Fig. 8).
At block B the deformed interface is draped, and the up-
permost sequences have been folded and deformed succes-
sively (Fig. 9). This implies that the blocks were not emplaced
in an instantaneous event during a single dislocation. Brittle
deformation at block B is seen at bounding faults and this is
capped by sediments which show subsequent draping and
further folding followed by debris flow deposition (Fig. 9).
The glacial sequence is draped by undeformed horizons which
include the uppermost (latest) glacial sediments.
Fig. 5 Perspective views of block A and surrounding area using
Fledermaus software. aOblique view looking north shows several
instability features adjacent to block A, including an older slide scarp
and a possible incipient slide bracketed by block A and smaller
retrogressive slides. bOblique view looking east towards block A
shows several small slides (at n) on the top and the back slope of the
main slide, and a close-up of what may be a developing slide or sackung
scarp. Extracted from Conway et al. (2012)
Geo-Mar Lett
Identification of late quaternary faults
Evidence of active faulting is seen at three locations in the sub-
bottom profile data. Reflector offsets are apparent in the gla-
cial section at the southern edge of the flank of block A
(Fig. 8). Further to the south, block B is bounded on both sides
by fault offsets which penetrate from the bedrock through the
overlying glacial sediments (Fig. 9)withreflectorsoffsetupto
60 m. The sediments have been dislocated by the block-
bounding faults in a brittle fashion. The data do not resolve
the initiation of the faults, presumably located deep in the
bedrock.
Near the southern end of Gribbell Island (see Figs. 1and 2
for location), seismic profiles indicate that the glaciomarine
section is faulted beneath the seafloor (Fig. 10). The deforma-
tion coincides with the strike of a prominent lineament which
sections Gribbell Island. The deformation of the geologic unit
is post-depositional and not an on-lapping or draping
relationship of reflectors to an adjacent or buried sur-
face. The fault does not entirely cut the glacial section
nor is the Holocene sequence penetrated (Fig. 10). The
top of the faulted section has been eroded and this
sequence has been buried by subsequent glacial and
Holocene deposition which remains undeformed. The
faulted section displays little vertical offset.
LiDAR data
The Hawkesbury Island shoreline is the source of
blocks A and B found along the eastern edge of
Douglas Channel. Regionally extensive north–south lin-
eaments can be seen in bathymetric and topographic
data in this area. LiDAR data (Fig. 11) show the gross
nature of the shoreline structural elements in the vicinity
of block A where joints and faults appear to define
lozenge-shaped bedrock slabs which demarcate potential
detachments from the shoreline above the block. During
a ROV transect along the back scarp of block A, well-
developed foliation and fracture planes were noted in
the bedrock wall, as well as density and salinity anom-
alies indicating freshwater flow through horizons in the
bedrock.
Discussion
The bedrock walls of Douglas Channel would have been
greatly smoothed, and the fjord floor excavated during mas-
sive ice movement through the entire fjord system between 25
and 14 ka BP (Clague 1985). If the blocks had been emplaced
before the last glaciation, then the head scarps would have
Fig. 6 Douglas Channel piston core lithology, ages and correlation. See Fig. 2for locations. Extracted from Conway and Barrie (2015)
Geo-Mar Lett
been greatly modified, and the rock blocks eroded and round-
ed and traces of the block motion largely removed during
long-term glacial ice movement through the fjord. The
dislocation of blocks A and B must have occurred dur-
ing regional deglaciation and after ice had locally reced-
ed from the Douglas Channel failure sites. Examination
of multibeam data from all other BC fjords surveyed to
date does not reveal any similar submarine failures of
this scale and form (Conway et al. 2013.). The process-
es and timing of ice advance and deglaciation would
have been quite similar in adjacent BC fjords, suggest-
ingthatlocalvariationinrockpropertiesandstructural
relationships play a role in the development of the failure
blocks in Douglas Channel.
Fig. 7 Huntec DTS profile
showing stratified and deformed
glaciomarine sequence at block
A. See Fig. 2for location.
Extracted from Conway and
Barrie (2015)
Table 1 Radiocarbon ages of
samples from Douglas Channel
piston cores. A local marine
reservoir correction (Delta-R =
–390 years) and global reservoir
correction (–410 years) were
applied to shell dates for a total
radiocarbon marine reservoir
correction of –800 years.
Extracted from Conway and
Barrie (2015)
Core number Depth in
core (cm)
Beta analytic
lab. number
Material Conventional radiocarbon
age (years BP)
Calendar age
a
2013007001 113 384642 Shell 8,920±30 10,096±87
2013007001 156 384643 Shell 6,900±30 7,714±33
2013007001 226 384644 Wood 4,630±30 5,362±55
2013007002 266 384645 Shell 8,300±30 9,315±58
2013007002 282 384646 Shell 8,790±30 9,798±81
2013007002 340 384647 Shell 9,700±30 11,160±34
2013007003 394 384648 Shell 9,830±30 11,229±15
2013007003 429 384649 Shell 10,230±30 11,977±62
2013007003 502 384650 Shell 10,470±30 12,415±60
2013007003 581 384651 Shell 10,950±40 12,834±46
2013007003 667 384652 Wood 10,110±30 11,729±77
2013007004 160 384653 Shell 10,460±40 12,393±81
2013007005 311 384654 Shell 7,560±30 8,377±16
2013007005 363 384655 Shell 10,200±30 11,922±73
2014007017 208 405125 Shell 6,710±30 7,576±19
2014007017 301 405126 Wood 6,170±30 7,065±60
2014007017 432 407367 Shell 9,030±30 10,209±14
2014007017 453 405127 Shell 9,500±30 10,755±80
2014007018 144 405128 Wood 4,250±30 4,832±19
2014007018 450 405129 Wood 8,770±30 9,753±72
2014007018 536 405130 Shell 9,920±30 11,292±37
2014007018 592 407368 Shell 10,330±40 12,124±78
a
Fairbanks et al. (2005) calibration algorithm
Geo-Mar Lett
Slope instability—blocks A and B
Late glacial sediments are seen to be deformed and folded in
high-resolution seismic data in proximity to blocks A and B
while the Holocene section is not affected. The bounding faults
that demark the edges of blocks A and B do not penetrate the
upper part of the glaciomarine sequence. The undeformed
upper portion of this glacial sequence would date from the
latest phase of deglaciation when ice had receded from the
immediate area and up Douglas Channel towards Kitimat
Arm, where an ice front was present by roughly 11.5 ka BP
(Bornhold 1983). The timing of the active period of the motion
of the blocks is thus estimated at between 13 and 11.5 thousand
radiocarbon years BP (15,800 and 13,400 cal years BP).
Fig. 8 High-resolution sub-
bottom profile adjacent to block
A. aHuntec DTS geophysical
profile. bInterpreted section
showing faulted sediments. See
Fig. 2for location. Extracted from
Conway and Barrie (2015)
Fig. 9 High-resolution sub-
bottom profile adjacent to block B
showing faulted strata below the
Holocene and late glacial section.
See Fig. 2for location. Extracted
from Conway and Barrie (2015)
Geo-Mar Lett
Seismic data indicate that the deformation of the glacial
sequence adjacent to and overlying blocks A and B was in-
cremental and punctuated by intermittent loading or impact on
the well-stratified section. Observations in the sub-bottom da-
ta of a series of low dip angle and relatively broad folds and
buckled sequences in juxtaposition to brittle fault deformation
at block edges are consistent with incremental as opposed to
instantaneous disruption of the sequence. Were an instanta-
neous impact of the very large blocks A and B to collide with
a well-bedded stratigraphic sequence, it would imply the
Fig. 10 High-resolution sub-
bottom profile south of Gribbell
Island. aHuntec DTS
geophysical profile. bInterpreted
section showing faulted
sediments. See Fig. 2for location.
Extracted from Conway and
Barrie (2015)
Fig. 11 Multibeam and LiDAR
data collected near block A (left),
and location of main faults,
fractures and joints along the
Hawkesbury Island shoreline
(right)
Geo-Mar Lett
creation of thrust features within the sedimentary section.
Such thrust features are not observed in the sub-bottom data,
and a gentle and discontinuous deformation style of low angle
folds is seen with only minor small massive intervals noted.
Such massive chaotic seismic units, indicative of debris flows,
are largely restricted to the inshore areas of the blocks and are
found to intercalate and cap both the folded as well as the
undeformed glacial sections at some locations.
Sea levels during the time period of active movement
(15,800 to 13,400 cal years BP) would have been changing
rapidly with inundation of deglaciated landscapes as ice re-
ceded. Isostatic loading of the coastal mountains would have
ensured that the crust remained depressed as ice receded and
allowed inundation to occur, though the existence of a pro-
nounced glacial forebulge to the west complicates this sea-
level scenario somewhat (Hetherington et al. 2004). Sea levels
were 200 m above present at Kitimat at 10 ka BP (Clague
1985). Blocks A and B formed in a setting which had been
newly deglaciated, where sea levels were not stable and where
fractures and faults would have been conduits for melt water
and provided lubrication during block movement.
Small slides originating near the top and from the sides of
block A were composed of gravelly grey glacial marine clay
and these are post-glacial in age. These small translational
slides may have been initiated from the motion of the block
in late glacial time or alternately are due to subsequent, more
recent Holocene slope instability. Some small slope failures at
the edges of the blocks are also composed of glaciomarine
sediments, and the chronology of these slides is uncertain as
much of the area adjacent to the blocks is non-depositional.
Evidence of regional landscape instability is reflected in cored
sequences including unit 2, an anomalous clay deposit. The
unit is similar in texture, stratigraphic position and colour to
clays which record glacial lake outburst flood events in the BC
shelf and fjords to the south (Conway et al. 2001; Blais-
Stevens et al. 2003). Further study of cored intervals of unit
2 could confirm this interpretation. Thick deposits of region-
ally distributed clays have been attributed to such outburst
floods in the Douglas Channel region (Shaw et al. 2017).
Recent faulting
Examples of fault offsets in glacial sediments are observed in
the high-resolution profile data. Seismic data indicate about
10 m of vertical offset at one site adjacent to and offshore of
the southern limit of block A (Fig. 8) and also at block B at
both ends of the block (Fig. 9). Faulting has also affected
glacial sediments at the southern end of Gribbell Island where
strong surface lineations have also developed. The faulting at
the margins of blocks A and B is related toblock emplacement
and accompanied the downward movement of the blocks.
Deformation of the glacial section related to emplacement of
the bedrock masses is apparent at both sites. The seismic data
through to the base of the resolved interval indicate brittle
deformation and offset reflectors on discrete planes. The trend
of the faults, which are roughly at a 30 to 45° angle, suggests
that they may be synthetic or normal faults related to a main
system of north–south faults. The shallow submarine faults
combined with the onshore fractures of the margins of
Douglas Channel suggest that a distributed style of deforma-
tion may exist. Minimal vertical offset is seen on the Gribbell
Island fault trace, which is unsurprising considering the fault
is likely part of a strike slip pattern of local faulting. The
identified fault trace aligns with a well-defined lineament
which bisects Gribbell Island and extends offshore to the
south, seen in multibeam datasets (Conway et al. 2012).
This north–south trending fault was probably reactivated by
the isostatic effects of ice loading. It is also possible that the
modern stress regime is implicated in this faulting. Precision
GPS monitoring in the region has defined broad north–south
dextral shear (Mazzotti et al. 2003,2011), which is opposite to
the left-lateral strike slip motion inferred for the Douglas
Channel area. While these motions are not mutually exclusive,
the left-lateral motion along Douglas Channel must be accom-
modated by block rotation within the larger right-lateral shear,
tectonic situation.
The structural control on major physiographic elements of
the Douglas Channel/Kitimat Arm area has been inferred to be
north–south trending (Duffell and Souther 1964; Bornhold
1983). The main structural alignment in the adjacent region
to the west and north is associated with the well-described
Grenville Channel fault system which trends NW–SE
(Nelson et al. 2011). While no faults are indicated on geologic
maps of the area (Roddick 1970), a series of foliation direc-
tions are indicated as being coast parallel in the area of the
slides on Hawkesbury Island. Duffell and Souther (1964)and
Roddick (1970) both suggest that faults offset the two sides of
Douglas Channel and Kitimat Arm, both of which represent
structurally controlled north–south physiographic landforms
(Holland 1976). The sense of motion of the inferred faults
was up and to the north on the eastern side relative to the
western side of Douglas Channel (Roddick 1970).
Sackungen
Geological and geophysical data indicate that blocks A and B
were emplaced in a punctuated fashion and not catastrophical-
ly. The deformation observed in the glaciomarine sediments
peripheral to the blocks is consistent with a slow sagging
impact as opposed to catastrophic dislocation from instanta-
neous failure. The cored sequences and radiocarbon chronol-
ogy indicate that the blocks date from the deglaciation of
Douglas Channel. The attributes of the blocks including mas-
sive size, thickness and shape, limited downslope and periph-
eral deformation and the uphill-facing scarps are all consistent
Geo-Mar Lett
with the features having origins as sagging blocks, or
sackungen sensu Helm (1932) and Hutchinson (1988).
Bedrock structural control of the shape and dimensions of
blocks A and B is apparent in sub-bottom and topographic
data. LiDAR data show the main trend of joints and faults
above block A on Hawkesbury Island. It is inferred that this
clear structural pattern has contributed to the generation of the
sackungen in Douglas Channel. This is conceptually illustrat-
ed in Fig. 12. Observations of well-developed foliation planes
in the bedrock wall of the scarp at block A, and water density
anomalies at these layers indicate freshwater flow focussed
along these horizons. Flow of this kind along planar disconti-
nuities in the bedrock would have provided lubrication during
movement of the bedrock masses.
Dating of sackungen movement rates on land is normally
accomplished by boreholes collected in the back scarp of the
deformed area which is typically infilled during its develop-
ment. Typical rates of sackung movement measured on land
are a few mm to a few cm per year (Forcella 1984; Hippolyte
et al. 2012). In the case of the Douglas Channel, the back
scarps are not infilled, so that this method of dating is not
possible. The area is non-depositional due to high ambient
seabed tidal currents keeping the displaced masses swept clear
of recent sediments. Blocks A and B have downslope dislo-
cations of 300–400 m. If these blocks are associated with
sackung style deformation, then 300–400 m of movement
would have been accommodated between about 13,000 and
11,500 radiocarbon years BP (15,800 and 13,400 BP) when
ice receded from the outer Douglas Channel landwards to
Kitimat Arm (Bornhold 1983), and before the latest
glaciomarine sediments were deposited. This would give an
estimated sackung development rate of about 13–17 cm per
year. This more rapid rate of movement may be related to the
submarine setting of these features, where presumably lubri-
cation along bounding surface planes would be more
complete.
Compared to blocks A and B, the typical appearance of
sackungen on land is somewhat different because normally
an uphill-facing scarp is the principal indication of the struc-
ture. In addition, the developing upslope gap or head scarp is
commonly infilled to some extent by slope wash and other
processes. In the marine examples of the present study, a deep
and vertical head scarp remains. The sackung style of slope
and bedrock deformation has not been observed in other
British Columbia fjords examined to date (Conway et al.
2013), and these examples are the first to be identified from
a submarine setting. In addition to vertically down relative
motion, some lateral downslope movement of the sackungen
blocks A and B is apparent. The kinematics and mechanisms
of sackungen development represent a continuum with some
elements of early stage landsliding involved in somedescribed
sackungen (Hutchinson 1988).
Conclusions
The Douglas Channel sackungen were emplaced very shortly
after deglaciation over a period of several hundred to possibly
as long as 2,400 years between approximately 15,800 and
13,400 calendar years ago. They have been inactive or only
slowly moving during the last 11,000 years and are among the
most precisely dated sackungen globally. Faults detected in
the sub-bottom seismic data and onshore in topographic
datasets are evidence of bedrock structural control of the form
Fig. 12 Illustration of submarine
sackung development in Douglas
Channel. aBefore sackung
formation during glaciation. b
Formation of sackung controlled
by faults and joints following
deglaciation. cBedrock and
sediment on fjord wall before
sackung formation. dBedrock
and sediment deformation after
sackung formation
Geo-Mar Lett
and scale of the bedrock failures. Numerous small landslides
associated with the submarine sackungen suggest ongoing
slope instability may still exist. Detailed analysis of LiDAR
and structural geology data is underway to examine the re-
gional context in which the sackungen occur.
Acknowledgements Gwyn Lintern provided vital support as project lead-
er, and at sea support by Greg Middleton and Peter Neelands is gratefully
acknowledged. John Shaw provided insightful discussions. Thanks to Burg
Fleming for a very helpful review that improved the paper.
Compliance with ethical standards
Conflict of interest The authors declare that there is no conflict of inter-
est with third parties.
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