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Quaternary International ] (]]]]) ]]]]]]
Subsidence and tectonic controls on glacially influenced continental
margins: examples from the Gulf of Alaska and the western Scotian
Shelf and slope
Michael R. Gipp*
Department of Earth Sciences, University of Toronto, Scarborough Campus, Scarborough, Ont., Canada, M1C 1A4
Abstract
The glacial record on continental shelves and slopes at mid- to high latitudes is dominated by marine strata, but little is knownregarding the large-scale architecture of such deposits. Models showing the gross architecture and facies successions of Late
Cenozoic deposits in the Gulf of Alaska and on the western portion of the Nova Scotian Shelf and Slope of southeastern Canada
have been developed to guide the interpretation of ancient glacially influenced marine sequences and to illustrate the influences of
their tectonic setting.
The processes of deposition (including ice rafting, suspension rain-out, debris flows, and turbidity currents) are the same on both
the Gulf of Alaska and the Scotian Shelf and Slope, yet the large-scale depositional architecture of glacially influenced marine
deposits on both margins differs because sediment preservation is strongly influenced by the impact of tectonics on relative sea-level
changes. Eustatic sea level was lower during major glaciations, enabling ice sheets to advance across the Scotian Shelf, whereas rapid
ongoing subsidence in the forearc basin of the Gulf of Alaska restricted glacial advances to shallow, nearshore areas until the forearc
basin filled with sediments to create a broad shelf. Both shelf and slope deposits are preserved in the Gulf of Alaska, which has
developed by both progradation and aggradation, whereas slope deposits are selectively preserved on the Nova Scotia continental
margin, which has developed by propagation.
r 2002 Published by Elsevier Science Ltd.
1. Introduction
Glacigenic sediments in the rock record consist
predominantly of glacially influenced marine deposits,
which have been deposited numerous time in Earths
history (Eyles, 1993). Typical deposits consists of poorly
sorted diamictites, debris flows, and turbidites (Powell
and Molnia, 1989). The relative importance of different
mechanisms of deposition and the resulting facies
successions are controlled in a complex fashion bytectonic setting, sea-level fluctuations, and climate
change (Boulton, 1990). Attempts to model these
deposits have drawn on extensive geophysical and
geological databases (e.g., Gipp, 1994a; ten Brink et al.,
1995; Steckler et al., 1999), yet they remain poorly
understood in terms of their large-scale architecture and
regional facies distributions. Recent controversy over
the Snowball Earth hypothesis (Hoffman et al., 1998;
Kennedy et al., 2001) has greatly increased the need to
fully understand the marine record of glaciation.
This paper contributes to the discussion by presenting
models of glacially influenced continental margin devel-
opment in two distinct tectonic settingsthe tectoni-
cally passive southeastern continental margin of Canada
(Scotian Shelf and Slope) and the tectonically active
Gulf of Alaska. The model of the Scotian Shelf and
Slope is constructed from offshore seismic profilescombined with core data and a large pre-existing
database. The model of the Gulf of Alaska is
constructed from extensive seismic-scale outcrop.
Representative facies successions together with large-
scale architecture are used to infer the influence of
tectonic setting on subsidence rate and hence margin
development.
On the continental margin of Atlantic Canada,
decades of hydrocarbon exploration, in conjunction
with the Geological Survey of Canada, have resulted in
a database of thousands of kilometres of seismic data,
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pp:1225col:fig::NIL
ED:h:rameshPAGN: n:s: SCAN: radhika
*Current address: Marine Mining Corp., 5829 Fieldon RD,
Mississauga, Ont., Canada, L5M 5K3. Tel.: +1-905-821-9468; fax:
+1-416-421-0949.
E-mail address: [email protected] (M.R. Gipp).
1040-6182/02/$ - see front matter r 2002 Published by Elsevier Science Ltd.
PII: S 1 0 4 0 - 6 1 8 2 ( 0 2 ) 0 0 1 0 9 - X
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thousands of square kilometres of sidescan sonar
coverage and digital multibeam bathymetric data, and
many boreholes and cores. In the Gulf of Alaska, a long
and complete record of Cenozoic climatic deterioration,
glacially influenced marine deposition, and active
tectonism is exposed in outcrops of the 5 km thick
Miocene-to-Recent Yakataga Formation, which can becorrelated with offshore boreholes and seismic reflection
data (Eyles et al., 1991). Both of these margins have
been characterized by warm-based ice with similar
discharge rates during the Pleistocene (Gustavson and
Boothroyd, 1987). The recent (o150 ka) glacial history
of both margins is characterized by advances of
grounded ice across a broad shelf (Eyles, 1988; Piper
et al., 1990b).
2. Nova Scotian Shelf and Slope
The Nova Scotian Shelf and Slope (Fig. 1) is a passive
margin formed by the opening of the North Atlantic
Ocean. Seafloor spreading along the margin has
occurred since the Jurassic, and it has been passive,
but subsiding, since the late Mesozoic (Welsink et al.,1989). On the basis of its current physiography, it can be
divided into an eastern and a western portion.
The western portion is characterized by a broad (100
200 km) shelf with small (o5000 km2) banks and is
among the smoothest continental slopes in the North
Atlantic (Swift, 1985). The slope varies from a gradient
of about 1:7 in waters shallower than 500 m but the
slope in deeper waters is about 1:40. The eastern portion
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111Fig. 1. Scotian Shelf and Slope, on the southeastern margin of Canada, and location of study area. Seismic profiles in Figs. 46 are marked by
dashed lines, cores are marked by circles. Line drawings of sections on upper slope modified from Piper (2001). Canyons seaward of Banquereau and
Sable Island Bank are named. After Atlantic Geoscience Centre (1991) and Gipp (1996). Depths in metres; ebEmerald Basin.
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is characterized by a very broad (>200 km) shelf with
large (B10000km2) banks and a relatively steeper slope
(1:61:30), which is dissected by large submarine canyon
systems.
Fluvial systems incised channels on the Scotian Shelf,
and provided sediments to the continental slope
throughout the Tertiary, particularly during times oflowered sea level, until the first glacial influences in the
Pleistocene (Piper et al., 1990b). Tertiary fluvial
channels have been overdeepened by Pleistocene glacia-
tion, and glacial sediments have draped inner shelf
bedrock highs, and partially filled deeper mid-shelf
troughs (King and Fader, 1986). The outer Scotian Shelf
was a major depocentre for glacially influenced marine
sediments during the Late Wisconsinan, and sediments
of this age unconformably overlie gently dipping
Tertiary strata (Boyd et al., 1988).
The first major (shelf-crossing) glaciation to affect the
Scotian Slope occurred in the middle Pleistocene
(Mosher et al., 1989), probably during isotopic stage
12 (Piper et al., 1994), which appears to have been the
most severe glaciation on the Scotian margin. Conse-
quently, Pleistocene sediments on the Scotian Slope and
Rise are limited in thickness, reaching a maximum of
about 1 km (Swift, 1987).
On the western Scotian Slope, rapid deposition of
fine-grained glacial debris has lead to wide spread small-
scale slope failure, transporting material to the rise in
the form of distal turbidites and debris flows (Swift,
1985). Debris flow deposits on the rise appear to
correspond to major glacial cycles, with deposits
correlated back to isotopic stage 16 (Berry and Piper,1993). Layers of red-brown mud provide direct evidence
of turbidity currents from the Gulf of St. Lawrence at
the end of isotopic stage 12 (Piper et al., 1994).
On the eastern Scotian Slope, hemipelagic sedimenta-
tion occurred at a lower rate and was less prone to
failure (Swift, 1985), and much sediment bypassed the
shelf entirely through submarine canyons, located
seaward of outer shelf banks (Fig. 1), leading to
widespread deposition of turbidites and contourites on
the eastern portion of the rise. Intercanyon areas of the
Scotian Slope are characterized on sidescan sonar
records and digital multibeam bathymetry by recent
activity and slope failure (Piper et al., 1999b; Baltzer
et al., 1994).
3. Gulf of Alaska
The southern continental margin of Alaska (Fig. 2) is
an active, collisional setting, characterized by several
allochthonous terranes that were assembled in their
present-day positions during the Mesozoic and Cen-
ozoic by the northward motion of the Pacific Plate
against the North American Plate (Plafker et al., 1994).
The Yakutat terrane is currently colliding with accreting
onto the North American Plate in the Gulf of Alaska
area, undergoing oblique subduction along the Fair-
weatherQueen Charlotte transform fault (Plafker et al.,
1994). Rotation of the Pacific Plate since 6 Ma has
resulted in the rapid uplift of the FairweatherSt. Elias
orogen, which as the greatest coastal relief on Earth, andis experiencing extremely rapid erosion (Meigs and
Sauber, 2000). This orogen is responsible for the
initiation of Late Cenozoic glaciation and the deposition
of the 5 km thick Late Miocene-to-Recent Yakataga
Formation (Armentrout, 1983; Lagoe et al., 1993).
The Yakataga Formation is deposited within a
forearc basin that has subsided throughout the Tertiary,
with the rate of subsidence increasing since the latest
Miocene. Using an assumed function for porosity of the
Yakataga Formation, Gipp (1996) demonstrated that
this subsidence can be explained by compaction and
isostatic loading in response to rapid deposition of
Yakataga Formation sediments, and may be partially
accommodated by movement along the Dangerous
River Zone. The great thickness of the Yakataga
Formation is attributed to a combination of the
glaciation and rapid uplift (Armentrout, 1983; Eyles
et al., 1991), which has resulted in the greatest sediment
yields in the world (Hallet et al., 1996). The Yakataga
Formation is exposed in coastal mountains because the
central portion of the Yakutat Block is currently
undergoing uplift, which is accommodated along the
Dangerous River Zone (Fig. 2).
The strata of Yakataga Formation provide a detailed
geological and paleontological record of Late Cenozoicclimate change (e.g., Eyles et al., 1992; Lagoe et al.,
1993, 1994; Zellers, 1994). Paleomagnetic, biostrati-
graphic, and K/Ar dating of glauconites from the
lowermost Yakataga Formation established that glacia-
tion in the Gulf of Alaska was initiated between 6.5 and
5 M a (Lagoe et al., 1993) in agreement with sites
elsewhere in the northern hemisphere (e.g., Jansen and
Sjoholm, 1991; Geirsdottir and Eiriksson, 1994). The
latest Miocene glaciation was followed by a warm
interval that lasted through the mid-Pliocene, and by a
major expansion of regional ice cover from 2.52 Ma
(Eyles et al., 1991; Lagoe et al., 1993). The thickness of
the Yakataga Formation can thus be related both to the
longer time of deposition relative to the timing of
Pleistocene glaciation in eastern Canada, and to high
precipitation and rapid (10 m/ky) uplift of coastal
mountains (Meigs and Sauber, 2000).
The Yakataga Formation, which outcrops between
Cape Yakataga and Icy Bay, has undergone about 25%
crustal shortening (Plafker et al., 1994). Exposures on
Middleton Island are virtually undeformed, although
extensive exposures were elevated above sea level by the
1964 Alaska earthquake (Plafker, 1965).
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4. Physiographic characteristics and deposits
Over 200 linekm of airgun and high-resolution
seismic data were collected on the Scotian Slope,
including alongslope and downslope lines. Nineteen
cores were studied in order to characterize facies in shelf
basins, and on the upper, middle, and lower slope
(Fig. 1).
Aerial photographs covering approximately 25 km2 of
outcrops of the Yakataga Formation have been taken in
the Robinson Mountains. Sections representative of
outer shelf/upper slope, middle slope, and lower slope
facies have been studied at Middleton Island, Icy Bay,
and Cape Yakataga, respectively (Fig. 2).
4.1. Scotian Shelf
Cores and high-resolution seismic data collected on
the Scotian Shelf (Fig. 1) can be integrated with previous
work (e.g., King and Fader, 1986; Amos and Knoll,
1987; Piper et al., 1990b; Gipp, 1994b; King, 1996; Stea
et al., 1998) to produce a comprehensive catalogue of
sedimentary facies and structural features. The Scotian
Shelf has been divided into three physiographic zones
the inner shelf, the middle shelf basins, and the outer
shelf banks (King and MacLean, 1976).
The inner shelf slopes gently seawards, but is highly
irregular locally, with its topography controlled by
bedrock. Overdeepended basins and valleys are filled by
acoustically stratified sediments. Five physiographic
zones have been defined, characterized by terminal
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Fig. 2. Summary of tectonics of the Gulf of Alaska, showing the docking of the Yakutat terrane against the Chugach and Prince William terranes by
the motion of the Pacific Plate. Locations of photographs in Figs. 710, including Middleton Island, are marked by circles. Interpreted cross-section
of Yakutat Block modified from Ehm (1983). Location of modern submarine canyons in the northern Gulf of Alaska from Carlson et al. (1990) and
Dobson et al. (1998). Zones of failed sediment on the modern shelf from Schwab and Lee (1992). Location of the shelf break is approximately
denoted by the 200 m contour. Abbreviations as follows: YBYakutat Bay; CYCape Yakataga. Simplified from Plafker et al. (1994).
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moraines, small basins, bedrock outcrop, smaller
moraines, and modern erosion (Stea et al., 1998).
The middle shelf basins are filled with Late Wisconsi-
nan sediments, which unconformably overlie seaward-
dipping Tertiary strata (King and Fader, 1986; Gipp
and Piper, 1989; Piper et al., 1990a, b; Gipp, 1994b).
Core lithofacies (Fig. 3) have been described followingthe scheme of Eyles et al. (1983). Sediments in shelf
basins consist of a sequence of brown and olive-grey
massive (occasionally weakly stratified) muds (Fm or
Fl), which are frequently heavily bioturbated and
contain abundant ice-rafted debris (- -d). Occasional
sandy or silty interbeds are noted. Muds in the lower
sections of the cores also contain abundant rolled
sediment clasts (- -r). Cores from Emerald and LaHave
Basins contain abundant shell material, and dates
obtained have constrained the recovered sediments to
the Late Wisconsinan and Holocene epochs (Gipp and
Piper, 1989; Piper et al., 1990a; Piper and Fehr, 1991).
On the basis of seismic profiles, Gipp (1994b)
identified four acoustic facies in Late Wisconsinan
sediments in Emerald Basin, which were attributed to
the Scotian Shelf Drift, Emerald Silt, and LaHave Clay
facies of King and Fader (1986), and to diffused gas insediment (Fig. 4). Seismic profiles show a basal acous-
tically incoherent layer of uniform thickness (King and
Fader, 1986). The basal layer is overlain by a draped
acoustically stratified layer and topped by a ponded
acoustically transparent layer (Emerald Silt and LaHave
Clay ofKing and Fader, 1986). Other features identified
on seismic profiles are: (1) buried subparallel ridges on
the top of the basal layer (lift-off moraines of King and
Fader, 1986), and interpreted as forming in transverse
basal crevasses (Gipp, 2000); (2) tongue- and lens-
shaped acoustically incoherent units interpreted vary-
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111Fig. 3. Lithological description of cores from the Scotian Shelf and Slope. Lithofacies descriptions follow those of Eyles et al. (1983). Cores are
depicted in order of increasing water depth. Vertical scale in metres. Note compressed vertical scale for cores 87-02 and 87-06. Dates from Gipp and
Piper (1989), Piper and Fehr (1990) and Piper and Skene (1998).
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ingly as till tongues (King et al., 1991) or debris flows
(Gipp, 1994b; Stravers and Powell, 1997); 30 buried
iceberg scours (Gipp, 1993); and (4) surficial or buried
gas-escape features, or pockmarks (Hovland and
Judd, 1988; Fader, 1991).
The outer shelf banks are broad, flat mesas, which cut
across seaward-dipping strata of Tertiary to Quaternary
age (Boyd et al., 1988), and are covered by thick sands
and gravels, which are reworked by shelf currents
(Amos and Knoll, 1987). Few cores have been recovered
from the outer shelf and uppermost slope, because the
piston corer cannot penetrate sands and gravels, and the
core catcher is not strong enough to support sediment
that does not adhere to the liner. Sidescan sonar reveals
the presence of abundant bioherms, predominantly
composed of scallops (Fader, 1991). Seismic profiles
rarely show anything below a highly reflective seafloor,
because there is little penetration by acoustic energy,
and the water is so shallow that multiples are closely
spaced and of greater amplitude than reflections from
depth.
Relict tunnel valleys of Late Wisconsinan age occur at
the surface on the Scotian Shelf between Sable Island
and Cape Breton (Loncarevic et al., 1992) and are
interpreted to have been formed by basal ice melting or
catastrophic release of meltwaters. Similar anastomos-
ing and overdeepened valleys, cut to depths >550 m
below present-day sea level mapped on Sable Island
Bank (Boyd et al., 1988) and on Banquereau (Amos and
Knoll, 1987), are inferred to be older examples of such
features. Boyd et al. (1988) demonstrated that the
volume of meltwaters stored subglacially on the Scotian
Shelf in unnamed basins landward of Banquereau, and
in Brandal, Emerald, and LaHave Basins (Fig. 1) may
have been as large as 490 km3, sufficient to have
maintained a flow of 1 m/s through the largest tunnel
valley for 9 days. Rising sea level, possibly due to local
isostatic downwarping, may have acted as a trigger for
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Fig. 3 (continued).
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the release of these meltwaters by decoupling the ice
sheet from its bed (e.g., Hughes, 1987). Catastrophic
discharge of meltwaters was probably associated with
large discharges of icebergs into the North Atlantic
(Heinrich events, e.g., Heinrich, 1988), recording the
collapse of the ice sheet margin, and acting as a major
influence on submarine canyon development.
4.2. Western Scotian Slope
Sediments on the Scotian Slope typically consist of
greyish brown to reddish brown bioturbated muds,
usually massive, but occasionally laminated (Fm, Fl).
The laminations are frequently deformed. The upper few
metres are of Holocene age (Piper and Skene, 1998), and
are usually free of dropstones, but both dropstones (- -d)
and rolled sediment clasts (- -r) are common in the lower
(Late Wisconsinan) portions of the cores. Chaotic
deposits (C) are noted in some cores (Fig. 3), and are
recognized by large numbers of rolled and distorted
sediment clasts of varying overconsolidation, layered
sediment clasts with widely varying shapes, and blocks
of slightly deformed sediments, with recognizable
internal structures that have been slightly deformed by
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seafloor
debris flow
debris flow 160
180
140 m
160 m
180
200
200
seafloor
lift-off moraines
0 1000 2000 3000 m
seafloorpockmark
hydrocarbon gas
LaHave Clay
0 1000 2000 m
0 1000 2000 3000 m
220 m
240
260
Emerald Silt
Emerald Silt
Scotian Shelf Drift
Emerald Silt
(a)
(b)
(c)
Fig. 4. Seismic profiles from basins on the Scotian Shelf, showing acoustic facies and typical features of proglacial marine sediments in shelf basins,
including (a) Emerald Silt, Scotian Shelf Drift and debris flows; (b) lift-off moraines (Gipp, 2000); and (c) LeHave Clay, disseminated hydrocarbon
gas and surficial pockmarks, or gas-escape structures (Hovland and Judd, 1988). Note vertical exaggeration.
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folding and/or faulting. The chaotic deposits are all
matrix-supported (Cm).
On the basis of single channel airgun acoustic profiles,
four acoustic facies can be defined. Facies S1 is
acoustically stratified, characterized by continuous
coherent internal reflections, which are approximately
equal in both spacing and apparent reflectance. Theinternal reflections are continuous over a few to tens of
kilometres. This facies is predominantly observed near
the surface on the upper part of the slope, to depths of
about 1000 m.
Facies S2 is acoustically stratified, but characterized
by discontinuous internal reflections. The reflections are
coherent, but not continuous, and frequently show
opposing dips at low angles.
Facies S3 is an acoustically incoherent facies, devoid
of internal reflections and often displaying a ponded,
depression-filling geometry and an uneven upper sur-
face, frequently with hyperbolic diffractions. It may
form continuous (although uneven) units or discontin-
uous lenses and wedges. Returns from underlying facies
are strong, suggesting that very little energy is absorbed
by this facies. Facies S2 has been interpreted as
including debris flows, debris avalanches, and rotational
slumps (Piper, 2001). This facies is also observed with a
mounded appearance, particularly on slopes and fre-
quently exhibits diapiric geometry (Fig. 5).
Facies S4 is observed on the outer shelf and upper
slope, and consists of a hard surficial reflection with no
clearly identifiable continuous internal reflections. The
apparent internal reflections are either discontinuous
and high in amplitude, or, more likely, are the result ofscattering at the surface. This facies is only observed on
the upper slope and outer shelf, and represents till or
sands and gravels, and passes downslope into facies S
(Fig. 5).
On the basis of physiographic features interpreted
from seismic profiles, Piper and Sparkes (1987) divided
the Scotian Slope into an upper, middle, and lower
slope. The upper slope, from the shelf break to about the
500 m isobath, is characterized by a steep slope (51), and
a highly reflective seafloor (Fig. 5), which allows little
penetration of seismic energy (facies S4).
Submarine canyons, 24 km wide and up to 500 m
deep, are observed on the upper slope (Fig. 6), seaward
of the outer shelf banks (Fig. 1). As in the shelf basins
and banks, iceberg scouring appears to have been a very
important reworking process (Pickrill et al., 2001).
Pockmarks are not observed, probably because the
sediments are too coarse to entrap gas.
The middle slope, to about 1000 m water depth,
consists of acoustically stratified sediments (facies S1
and S2) dissected by canyons, with a wide range of sizes
(5002000 m wide and 150350 m deep) (Fig. 6). These
canyons are not observed in water depths greater than
1300 m; whereas the canyons on the eastern Scotian
Slope extend to the continental rise (Pickrill et al., 2001).
Canyon geometry compares favourably with modern
submarine canyons investigated by sidescan sonograms
and seismic profiles (e.g., Hugh Clarke et al., 1990;
Pratson et al., 1994). Buried canyons are also observed,
and are o1 km wide and 200 m deep, making them
much smaller than the active canyons (Fig. 6). Theactive canyons commonly show evidence of repeated
incision after partial infill. Canyon C in Fig. 6, for
instance, shows at least two incision events which
happened after the initial canyon-cutting event. Sub-
sequent incisions are probably related to repeated
discharge of sediment-laden meltwater during times of
glacial retreat and repeated sediment failure. The larger
slope canyons on the eastern slope may be incised (and
reincised) by the meltwater discharges from subglacial
tunnel valleys.
Canyon cutting began at about the beginning of the
Pleistocene, likely due to lowered sea levels (Piper et al.,
1999a). Subsequent deposition infilled the canyons on
the western part of the slope, whereas those on the
eastern slope were maintained by erosion (Swift, 1987).
A second episode re-excavated canyons in the latest
Pleistocene (Figs. 5 and 6). On the basis of their
morphology, canyon excavations are thought to have
been caused by turbidites, either related to a lowered sea
level, to the formation of tunnel valleys on the shelf
banks, or to proximal ice margins. The relative lack of
canyons on the western slope has been interpreted to
suggest a more distal ice margin (Swift, 1987), but
according to Grant (1989), major ice streams are likely
to have flowed through Emerald and LaHave Basinsthrough to the Scotian Gulfconsequently, the western
Scotian Slope to Browns Bank Would be expected to
have a proximal ice margin.
The intercanyon seafloor is characterized by numer-
ous features suggestive of slope failure (Piper et al.,
1999b), including large-scale rotational failures which
pass into eroded seabed (Baltzer et al., 1994). The base
of the erosion is commonly planar, corresponding to
particular bedding planes, and therefore represents
block failure (Piper, 2000). Pockmarks are commonly
observed on sidescan sonar profiles on the eastern slope,
particularly between 500 and 900 m depth (Baltzer et al.,
1994), and the presence of free gas within sediments
likely adds to sediment instability (Piper et al., 1999a).
Actual failure may be attributed to fluid overpressures
associated with iceberg loading (Mulder and Moran,
1995), earthquakes (Mosher et al., 1994), or sublimation
of gas hydrates (Piper et al., 1999a).
Sediment cores from the middle slope usually include
a thin (o1 m) surficial layer of sandy silt, which is
commonly bioturbated, and which likely represents a lag
deposit (Piper, 2001). These silts overlie massive to
laminated bioturbated muds and silts (Fig. 3). Drop-
stones and overconsolidated clay rafts are present in
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sediments (o5 m deep), but appear to be absent in the
uppermost (postglacial) sediments.
The lower slope, down to the continental rise
(B2000 m) is characterized by a slope ofB21, and by
the appearance of seismic facies S3, showing lenticular,
ponded, or diapiric geometries (Figs. 5 and 6). Sedi-
ments of facies S3 are interbedded with facies S1.
Further downslope, in water depths >2000 m, there is a
decrease in slope due to the ponding of thicker debris
flows on the continental rise (Fig. 5). Turbidite se-
quences are inferred on the Scotian rise (Berry and
Piper, 1993). Pockmarks are observed on the lower
slope, down to water depths of 2200 m (Piper, 2001),
although they are not as common as they are on the
middle slope.
Cores from the lower slope show a variety of muds,
including sediment rafts and disseminated gravel and
dropstones, but also include chaotic deposits which
incorporate folded and faulted blocks of laminated
muds and concentrically banded clay rafts. Unlike the
muds with sediment rafts, the chaotic facies are
dominated by deformed resedimented material (Fig. 4).
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Fig. 5. Downslope seismic profiles. (a) The change in acoustic character from upper slope sediments characterized by poor penetration to middle
slope sediments with clear stratification and the development of structures, such as gullying is likely due to the downslope thinning of sand sheets that
have been swept off the outer shelf banks, and the predominance of finer grained sediments. (b) Acoustically stratified lower slope sediments show a
variety of deformation structures, including diapirs and slope failures.
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Fig. 6. Seismic profiles from the Scotian Slope, showing submarine canyons and their associated downslope facies. (a) On the upper slope, large
canyons are incised into poorly stratified (presumably sandy) sediments. Feeder channels are not apparent. Channel C may have undergone more
than one episode of canyon cutting. (b) On the middle slope, canyons of all sizes are noted, and buried canyons are apparent. Canyons AC can be
correlated from the upper slope to the middle slope. Note that these canyons have all been incised to different depths. (c) On the lower slope, canyons
are not apparent, but the stratified sediments consists largely of thin, lenticular deposits of variable extent, interpreted as debris flows (Gipp, 1996).
(d) Sediments on the continental rise consists of much thicker (>100 m) debris flows. The seafloor is very uneven, probably due to sediment failure
and diapirism.
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The uppermost (Holocene) sediments of these cores are
commonly bioturbated, and contain no dropstones.
On the western slope, there is a noted downslope
facies change from stratified muds with abundant
sediment clasts, to slump deposits and debris flow.
Debris flows are noted to have started being deposited
dating from the pliocene (Piper et al., 1999a), suggestingthat glaciation is not the only cause of sediment failure.
4.3. Gulf of Alaska
4.3.1. Outer shelf
Outcrops of Yakataga Formation sediments on
Middleton Island form a succession about 1.25 km thick
which is characterized by thick sandy to silty matrix-
supported diamictites, interbedded with graded and
ungraded sands and gravels, and minor laminated and
massive muds (Miller, 1953; Plafker and Addicott, 1976;
Eyles, 1987, 1988; Eyles and Lagoe, 1990).
The diamictites are poorly sorted sandy muds with
abundant dispersed gravels. The gravels range in size
from granules to boulders, and include striated and
faceted clasts. Diamicts may be massive or stratified, but
stratification, where present, is generally weak (Eyles
and Lagoe, 1990). Deformation features are common
and include rafts of muds emplaced within the
diamicities (Fig. 7a) and injection structures (Eyles and
Lagoe, 1990). Other structures noted within the
diamictite include coquinas (Fig. 7b), boulder pave-
ments (Fig. 7c), and deformed laminated sands and silts
(Fig. 7d). Massive diamicts are thought to represent
deposition by a combination of suspension rain-out andice rafting (Miller, 1953), whereas the deformed
diamictites represent remobilized sediment (Eyles and
Lagoe, 1990). They are likely related to the sediment
failure in response to storm-wave loading or tectonic
shock. Similar failures are noted on the present-day Gulf
of Alaska continental shelf (Fig. 2), especially seaward
of major glacial outlets such as Icy Bay and Yakutat
Bay (Schwab and Lee, 1992).
Coquinas are composed of pecten shells on a gravelly
substrate, are usually only a few centimetres thick, and
extend over 100m (Eyles and Lagoe, 1989). Boulder
pavements are lateral concentrations of striated
boulders, which appear to have been loaded into a
softer substrate, and planed off along the former
bedding plane (Eyles, 1988). Laminated sands and silts
include ice-rafted debris, and are frequently strongly
sheared, Similar to proglacial sediments that have
undergone subglacial deformation ( !O Cofaigh and
Dowdeswell, 2001). On Middleton Island, the deforma-
tion is highly localized and occurs near features
suggestive of ploughing (Fig. 7e and f), and may be
related to the impact of floating ice on the seafloor.
Sediments at the base of the exposed section form a
submarine channel system filled with a complex series of
sandy and gravelly facies (Eyles, 1987). Small cut-and-
fill channels, on the order of a few metres across and
deep, are filled with a variety of normally graded,
inversely graded, and ungraded sandstones, diamictites,
and conglomerates (Fig. 8ad). They occasionally un-
dercut their sidewalls (Fig. 8c) and incorporate large
contorted sediment rafts in their fill (Fig. 8d). Largerchannels are also apparent (Fig. 8d), which are up to
500 m across and 70 m deep (Eyles and Lagoe, 1990).
Sediments exposed on Middleton Island glacially
influences on the deposition of outer shelf and upper
slope sediments. The dominant sediment types represent
deposition by suspension rain-out and ice rafting, and
show the influence of bottom traction currents, ice
scour, and sediment failure due to tectonic shock or
storm-wave loading. They have been interpreted to
represent deposition on the outer margin of a glacially
influenced continental shelf (Eyles et al., 1991). Given its
present physiographic setting, it is likely that Middleton
Island is an analogue for the banks on the outer part of
the Scotian Shelf.
4.3.2. Continental slope
Yakataga Formation outcrops between Cape Yaka-
taga and Icy Bay (Fig. 9) consist of massive and
laminated sandstones, massive mudstones, and both
massive and stratified diamictite facies. Sandstones and
mudstones are interpreted as turbidites (Eyles et al.,
1991). Massive diamictites consists of a sandy and silty
matrix supporting scattered clasts and shells and may
display tabular or lenticular geometry. Stratified dia-
mictites are clast-rich and include sediment clastsconsisting of deformed sandstone. These olistoliths
range in size from a few centimetres to some tens of
metres in length.
Features observed in Icy Bay outcrops include
olistoliths loaded onto muds (Fig. 9a); large submarine
canyons, up to 4 km wide and 750 m deep, previously
identified as megachannels (Armentrout, 1983), only
identifiable because of the extent and quality of the
outcrop (Fig. 9a); large (up to 20 m) sediment loads and
diapirism at the tops and bases of muddy units ( Fig. 9b);
and lenticular debris flows interbedded with stratified
sands and silts over a range of scales (Fig. 9c and d).
Many of the features exposed in the Yakataga Forma-
tion near Icy Bay resemble features interpreted from
seismic records on the middle Scotian Slope.
Submarine canyons are filled with basal conglomer-
ates, which fine upwards into turbiditic sandstones and
heavily bioturbated mudstones (Eyles et al., 1992). They
are comparable to submarine canyons described in other
outcropping marine sediments (e.g., Morris and Busby-
Spera, 1988). Microfauna in the canyon-fill sediments
suggest that they formed in outer neritic to upper
bathyal water depths, but debris flows in these canyons
contain displaced shallow-water foraminifera (Lagoe
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et al., 1994). Active submarine canyons in the northern
Gulf of Alaska show a dendritic pattern, which has been
modified in places by compressive ridges around which
some of the canyons meander (Fig. 2). Modern canyons
are commonly 5 km wide and may be greater than 1 km
deep, with individual canyons as much as 12 km wide
(Carlson et al., 1990; Dobson et al., 1998).
Lenticular and tongue-shaped deposits of massive
sands and diamictites (Fig. 9c and d) are exposed near
Icy Bay. Topologically, these appear similar to debris
flows observed in seismic profiles on the Scotian Shelf
and upper slope. In detail, these can be seen to have a
depression-filling geometry, and demonstrate little basal
scour, and are commonly interbedded with turbidites.
The oldest sediments of the Yakataga Formation are
exposed at Yakataga Reef, across a wave-cut platform
raised above sea level by the 1964 Alaska earthquake
(Fig. 10a). The first appearance of dropstones marks the
onset of tidewater glaciation during the latest Miocene
(Lagoe et al., 1993). The strata at Yakataga Reef are
dominated by fine-grained graded sandstones and
mudstones, interpreted as turbidites, with a single
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Fig. 7. Facies and features exposed on Middleton Island. (a) Detail of coquina, which is composed of pectin shells and abundant gravel, and can be
traced laterally over hundreds of metres (Eyles and Lagoe, 1989). (b) Outcrop of boulder pavement exposed on Middleton Island. Note that the
planation of the boulders, which is also striated, dips towards the right at an angle of about 25 1. (c) Finely laminated sands and silts, including
dispersed clasts and sandy and gravelly sediment clasts, representing ice-rafted debris. (d) Mud rafts incorporated into sandy sediments. (e) Possible
ice scour, showing erosive centre and berm. (f) Detail of scour deformation, showing normal faulting of underlying sediments.
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stratified diamictite unit approximately 20 m thick
(Fig. 10d). Individual thin debris flows can be identified
amongst the turbidites as ungraded sands with clay rafts
(Fig. 10b). The turbidites exhibit extensive bioturbation
and carry abundant shells and their imprints.
The diamictite exhibits complex stratification and
numerous rock and sediment clasts and abundant soft-
sediment deformation structures. It exhibits a broadly
channelized geometry, and its basal contact is sharp and
slightly scoured, penetrating into the underlying turbi-
dites (Eyles et al., 1991).
Depositional depths estimated from foraminiferal
assemblages suggest that the sediments have been
deposited glacially derived outer shelf and upper slope
sediments and remobilized downslope, presumably as a
result of seismic shock (Eyles et al., 1991). Such an
interpretation is supported by calculations showing that
several kilometres of sediment could be accommodated
in a shallow forearc basin B500 m deep, assuming
normal compaction and isostatic adjustment in response
to the weight of the sediments (Gipp, 1996).
4.4. Features common to the Scotian margin and in the
Gulf of Alaska
Many glacial marine sedimentary processes are
expected to be common to both study areas, although
their recognition is often difficult because of the
different databases available in the two study areas.
These similarities allow sediment facies types and
structures to be compared, which is a useful exercise
as the types of information revealed in outcrop is
different in scale than that inferred from seismic profiles.
4.4.1. Submarine canyons
Submarine canyons are recognized on both margins,
where they typically have gullied walls and numerous
minor tributary canyons, which feed sediments from the
continental shelf and upper slope to the lower slope and
continental rise (e.g., Dobson et al., 1998; Weaver et al.,
2000).
Submarine canyons on the eastern margins of North
and south America appear to be restricted to areas of
high Neogene sedimentation (Emery and Uchupi, 1984),
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(b)
(c)
(a)
Fig. 8. Details of gravelly cut-and-fill channels on Middleton Island. (a) Detail of side wall of a cut-and-fill channel shows undercutting of incised
diamictite, suggesting erosion by sediment-charged waters under high hydrostatic pressure. Arrow shows bedding plane of incised diamictite. Note
hammer for scale, left of centre. Stratigraphic top to top of photograph. (b) Two cut-and-fill channels, one infilled with normally graded fine
conglomerate and sandstone (black arrows), the other steeply incised into the first. Stratigraphic top to right. (c) Reworked sediments raft
incorporated in channel-fill conglomerate. The deformation of the diamictite suggests that channels have been cut into unlithified sediment.
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including the glaciated margin of North America andthe mouths of large river systems. Similar spatial
distribution has been noted on the eastern Atlantic
margin, with canyons noted on the glacially influenced
portion of the margin (Weaver et al., 2000). Sediment
supply, therefore, plays an important role in their
formation.
Tectonic setting also plays an important role in
governing the formation of submarine canyon systems.
The low tectonic gradient and the presence of a board
continental shelf inland of the Scotian Slope permitted
the storage of large volumes of meltwaters which, when
suddenly released, carved deep canyons across the Sable
Island Bank to the top of the slope (Boyd et al., 1988;
Loncarevic et al., 1992). Such discharges must have
played a key role in the development of the large
submarine canyons on the eastern part of the Scotian
Slope, where broad banks on the outer shelf acted as
grounding points for an ice sheet that could dam up
these waters. In the Gulf of Alaska, by contrast, rapid
subsidence created a steep regional slope and the lack (at
least initially) of a broad shelf meant that there was little
storage space for subglacial meltwaters. Discharges of
meltwater were thus small and of less importance in the
development of submarine canyon systems. Submarine
canyons in the Gulf of Alaska must have formed by thesteady erosion of numerous turbidity currents rather
than a few large discharges, and are thus similar in
origin to the submarine canyons on the western Scotian
Slope. The canyons on the western Scotian Slope are
most deeply incised on the upper slope, and become less
prominent downslope, until they are not visible on the
lower slope (Fig. 6), suggestion that the Icy Bay section
represents the upper to middle slope. The geometry and
dimensions of the Icy Bay canyons are very similar to
those of the Scotian Slope (Pickrill et al., 2001).
Tectonic setting may also influence the orientation of
canyon systems, especially the variability in flow
directions through time. Submarine canyons on the
Aleutian arc are deflected away from regional slope and
into forearc basins by compressive ridges (Dobson et al.,
1991). Present-day upper slope canyon systems in the
Gulf of Alaska are similarly influenced by compressive
ridges (Carlson et al., 1990), which are caused by folding
and thrusting associated with crustal foreshortening
observed on the Yakutat Block (Plafker et al., 1994).
The deep-water fans associated with modern canyons in
the Gulf of Alaska are gradually translated away from
sedimentary point sources by the motion of the Pacific
Plate (Dobson et al., 1998). By contrast, submarine
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Fig. 9. Outcrops and features exposed in the Robinson Mountains near Icy Bay. (a) Large section showing olistoliths (or boundinage?) due to
downslope failure, submarine canyon (at right) and progressive tilting of block due to deformation in the Pamplona Zone. All sediments were
originally horizontal. (b) Large (>30 m) sediment loads observed at two distinct horizons. (c) Lenticular debris flow (maximum thickness about
30 m) and small channels (arrows denote erosive base of channels). (d) Interbedded debris flow and turbidites in Yakataga Formation sediments in
Icy Bay. Section is 2 m thick.
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canyons on the Scotian Slope appear to trend directly
downslope. Little meandering is noted for canyons on
the Atlantic margin of the North America (Emery and
Uchupi, 1984), although it should be noted that a
branching dendritic pattern of tributary channels may
lead to local variations in channel orientation exceeding
901.
4.4.2. Sediment failure
Evidence for diapirism and sediment loading is
present in both study areas, indicating that rapid
downslope sediment reworking is common to both
margins (Figs. 5, 9 and 10). Olistoliths are common in
middle and upper slope outcrops of the Yakataga
Formation (Fig. 9), but there is only circumstantial
evidence for them on the Scotian Slope. Apparent
bedding plane failures in canyons (especially in canyon
A in Fig. 6) may be expected to have produced large
blocks of sediment that have been incorporated into
debris flows, or remain as olistoliths.
Block failures may be inferred on the lower Scotian
Slope (Piper, 2001), and sidescan sonograms and digital
multibeam bathymetry imagery both show relatively
recent large-scale bedding-plane failures on the middle
slope (Baltzer et al., 1994; Piper et al., 1999b). Such
events in older sediments may have resulted in
olistoliths, but features of the scale observed in outcrop
are too small to be resolved in seismic profiles and too
large to be clearly identified in boreholes or piston cores.
Rolled sediment clasts record episodes of sediment
failure and are observed in most piston cores on the
western Scotian Slope. Geotechnical measurements of
the sediment blocks observed within chaotic facies of
piston cores show that many of them are extremely
overconsolidated, suggesting that the depth of burial of
the original sediment (and consequently the failure
depth) frequently exceeded 50 m (Mulder et al., 1997).
Wedge-shaped acoustically transparent bodies on the
Nova Scotia continental margin are lenticular in cross-
section, and have been interpreted as debris flows based
on detailed seismic stratigraphic and geometric work
(Gipp, 1994b) Similar debris flows have been described
from seismic records on the Norwegian Shelf (King
et al., 1991), the Baffin Island Shelf (Stravers and
Powell, 1997), the Newfoundland Slope (Aksu and
Hiscott, 1992), the Hebrides Slope (Baltzer et al., 1998),
and many are recognized on the Scotian Slope (Fig. 6).
Unfortunately, cored intervals through these features
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Fig. 10. (a) Aerial view of Cape Yakataga shows interbedded turbidites and thick diamictite bed. Stratigraphic top is towards the bottom of
photograph. (b) Massive sandstone and muddy sediment rafts in Cape Yakataga turbidites. Arrowed raft is 5 cm in diameter. (c) Muddy facies with
scallop valve impression in Cape Yakataga turbidites. (d) Chaotically stratified diamictite, representing a slump deposit or debris flow. Boulders at
centre is 50 cm in length.
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are very rare, and their sedimentology remains unclear.
One cored debris flow has been described as a chaotic
deposit of overconsolidated sediment clasts in a poorly
consolidated muddy matrix (Piper, 2001).
Debris flows are also present in slope facies within the
Yakataga Formation, particularly near Icy Bay (Fig. 9),
and at Cape Yakataga (Fig. 10). These appear as verythin, apparently tabular beds containing rip-up clasts, as
chaotically bedded diamictites (Fig. 10b), with channe-
lized or wedge-shaped geometries.
Sediment instability on the upper Scotian Slope is
related to the high sensitivity of undisturbed sediments,
meaning that their undrained shear strength is much
greater than their remoulded strength. Failure may be
triggered by cyclical wave loading during storms,
possibly amplified by simultaneous iceberg-seabed im-
pacts (Bolen, 1987), or ice loading (Mulder and Moran,
1995), neither of which is likely to be of significance in
water depths greater than 500 m. Sediment instability on
the middle and lower slope has a number of possible
triggers, including tectonic shock (Mosher et al., 1994),
the migration of hydrocarbon gas (Piper et al., 1999a),
and the inability of pressurized fluids to escape through
a cap of relatively impermeable sediment (Piper, 2001).
The Brand Banks 1929 earthquake resulted in massive
turbidity currents, moving 175 m3 of sediment down-
slope (Piper et al., 1988). Seismic activity has been a
major factor in the Gulf of Alaska (Hampton et al.,
1987), although other possible mechanisms such as gas
seeps and dewatering cannot be ruled out.
4.4.3. CoquinasCoquinas have been noted on Middleton Island
(Fig. 7a), and are composed primarily of pectinids
(Eyles and Lagoe, 1989). The presence of broken valves
and shell debris suggests that local sediment starvation,
wave reworking and scouring of the substrate are a
factor in their formation.
Similarly, scallop beds are commonly detected atop
banks on the outer Scotian Shelf at present, where they
are recognized on sidescan sonograms as small zones of
enhanced seafloor reflectivity and are often less than
2500m2, although larger ones are possible (Fader,
1991). They tend to be very closely spaced, and may
be related to areas of gas venting (Levy and Lee, 1988).
Given the physiographic similarities between Middleton
Island and the banks of the outer Scotian Shelf, scallop
beds and coquinas are probably analogous. Their
abundance on the outer Scotian Shelf also suggests that
they would probably be very common in outer shelf
sediments where these are preserved, and that no great
climatic significance needs to be attributed to them.
4.4.4. Boulder pavements
Planar concentrations of striated boulders are ob-
served in diamictite facies on Middleton Island (Fig. 7b).
On the basis of the parallel striations on all of the
boulder, and their loading into underlying sediments,
these boulder pavements were interpreted to have
formed when boulder and cobble lag horizons were
overridden and striated by a buoyant ice shelf (Eyles,
1988). Lag boulder horizons in the glacial marine setting
may be formed by: (1) wave and current winnowing; (2)iceberg scouring, particularly during storms; or (3)
loading of clasts into fine material by floating ice on a
very shallow or emergent platform.
There is no direct evidence for boulder pavements on
the outer Scotian Shelf; however, the author has
encountered a boulder pavement during a geotechnical
drilling program at the current site of the Hibernia
platform, on the Grand Banks of Newfoundland. This
buried boulder horizon likely resulted from iceberg
impacts on a locally sediment-starved and winnowed
area. As these are conditions we would expect to have
existed on the Scotian Shelf, there may be boulder
pavements in the sediments of the outer banks.
4.4.5. Ice scour
Recognition of ice scour in sediments is a difficult
problem, and few field observations are documented in
the literature (Eden and Eyles, 2001). Possible scour
features are observed on Middleton Island in conjunc-
tion with highly localized distortion, including the
downward piercement of sedimentary horizons and
stretched and faulted sediments below the scours
(Fig. 7). Observations resemble the not only ice scours,
but deformed sediment suggestive of the ice keel
turbates exposed in Pleistocene sediments in theScarborough Bluffs in Toronto, Canada (Eden and
Eyles, 2001). Scouring by floating ice was probably an
important process in the Gulf of Alaska, but due to the
uncertainty of the water depth, it is unclear if this
scouring would be caused by icebergs or by pans of sea
ice.
Ice scour has been an important process on the shelf
and upper slope of the Scotian margin (King and Fader,
1986; Gipp, 1994b; Pickrill et al., 2001). Due to the
extreme depths (often >200 m), the scours are almost
certainly the result of icebergs. Iceberg scour directions
have been used to infer dominant current or wind
directions (Gipp, 1993).
5. Models of the development of the Scotian Shelf and
Slope, and the Gulf of Alaska
On the basis of single- and multichannel seismic data,
piston cores, sidescan sonar and digital bathymetry
images, and study of exposures of sediment along
coastal mountains, generalized sedimentation models
for glacially influenced continental margins have been
developed for the Scotian margin and for the Gulf of
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Alaska. These models are presented as a series of
illustrations showing dominant depositional processes,
tectonic activity, and resultant stratal geometry from a
typical cycle or sequence of glacial cycles.
5.1. The Nova Scotia continental margin
5.1.1. Ice advance
Ice advance (Fig. 11a) occurs during a fall in eustatic
sea level. A thin sheet of glacial ice advances across the
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Fig. 11. Model of Pleistocene development of the Nova Scotia continental margin representing (a) glacial advance, (b) glacial maximum, and (c) late
glacial retreat. Numbers on the diagram label processes referred to in text according to the following: (1) proglacial muds, including sediment raining
from suspension and ice-rafted debris; (2) massive lenticular debris flows at points of sediment instability on slopes; (3) subglacial meltwater stream
outlet, releasing sediment-charged fresh water; (4) icebergs and ice-rafted debris; (5) small debris flows of uncertain geometry; (6) steep-sides incised
channels, filled with gravels and sands; (7) large submarine canyons, which (8) progressively widen and shallow downslope; (9) sediment failure in
intercanyon areas; (10) overdeepened shelf basins carved by subglacial meltwaters under high hydrostatic pressure; (11) subglacial tunnel valleys; (12)
gas-escape structures (pockmarks); (13) bioherms and shell beds on the modern shelf banks; and (14) icebergs (from coastal glaciers) and currents
winnow coarse gravels into lag boulder horizons. Modified after Gipp (1996).
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shelf, crossing Cretaceous and Tertiary seaward-dipping
beds. A thin layer of pre-existing proglacial sediments
(1), deposited during a previous glacial retreat or ahead
of the advancing ice, acts as a deforming bed to aid ice
advance (Hart and Boulton, 1991). The advancing ice
planes off the most recent sediments, locally eroding
into bedrock, especially along pre-existing structuralweaknesses, which become overdeepened (e.g., Ehlers,
1990). Although some of this sediment will remain on
the shelf in the form of a basal or deformation till (King
and Fader, 1986), most of it is removed. If the ice is
sufficiently thick when it reaches the shelf edge, it may
advance a short distance downslope, depositing till
tongues (Mosher et al., 1989), debris flows (2) (Bonifay
and Piper, 1988), or turbidity currents, which may be
funnelled into submarine canyons and gullies (7) (e.g.,
Piper et al., 1988). Other sediment is suspended in
subglacial meltwaters (3) and dropped from icebergs (4)
(Powell and Molnia, 1989).
5.1.2. Glacial maxima
During glacial maxima (Fig. 11b) lenticular bodies of
till (King and Fader, 1986; Mosher et al., 1989) form
along the glacier margin. These sediments may be
reworked downslope into debris flows (5) and turbidites,
draped by hemipelagic sediments. The upper slope
deposits consist of debris flows (composed of eroded
bedrock and reworked shelf sediments), proglacial
sediments, and ice-rafted debris overlain by finer silts
and muds (Eyles, 1993). On the uppermost slope,
ephemeral channels as small as a few metres deep and
10 m wide, are cut by turbidity currents (6) and filledwith sands and gravels.
Intercanyon sediments on the upper and middle slope
may be remobilized (9) by storms, tectonic activity
(Hampton et al., 1987; Piper et al., 1988), gas hydrate
phase changes (Locat, 2001), or by increased bearing
stresses related to the weight of ice on the outer
continental shelf (Mulder and Moran, 1995). The rapid
deposition of sediments on the upper slope leads to
sediment instability and likely slope failure. On the
lower and middle slope, failure occurs because muddy
proglacial sediments are relatively impermeable com-
pared to the underlying sediments, causing sediment
instability where the overburden is thinner. Fluid
overpressures may be cause by migration of ground-
waters down the pressure gradient (from thick over-
burden to thinner overburden) (Dugan and Flemings,
2000), migration of hydrocarbon gas resulting from the
sublimation of gas hydrates (Piper et al., 1999a), or by
dewatering of buried sediments.
Overdeepened troughs form on the shelf at the
boundary between crystalline and sedimentary bedrock
by ice and subglacial meltwaters under high hydrostatic
pressure (10) and act as storage areas and conduits for
meltwaters, which can carve subglacial tunnel valleys
(11) when they are released (Boyd et al., 1988;
Loncarevic et al., 1992). Such sediment-charged melt-
water outbursts are most likely to cascade downslope in
the form of turbidity currents, deepening pre-existing
submarine canyons or gullies (7), or eroding new ones.
Submarine canyons may also form by the progressive
gullying caused by repeated sediment failure. They maybe up to 500 m deep and 4 km wide, but become broader
and shallower on the lower slope (8) for two reasons: (1)
they are filled in by debris flows and turbidites, and (2)
the erosive power of the flows decreases as they disperse
downslope.
The lower slope is characterized by thin lenticular
debris flow deposits downslope of the canyons (2)
(Fig. 6). On the rise, the debris flows are thicker and are
Shingled (e.g., Aksu and Hiscott, 1992; Baltzer et al.,
1998). As these sediments are unstable, subsequent
sediment loading may lead to the development of mud
diapers (Fig. 5).
5.1.3. Ice retreat
As the ice retreats (Fig. 11c), the sequence is
blanketed by a seaward-thinning prism of proglacial
muds (1). Shelf depressions, including overdeepened
basins and tunnel valleys, are infilled by a sequence
consisting of a thin basal till layer (likely deformation
till) covered by heavily bioturbated muds with drop-
stones and sediment rafts (1) (Gipp, 1994b; King, 1996).
The Entire sequence may be over 100 m thick (King and
Fader, 1986), yet despite this thickness, its preservation
potential remains poor, as it is likely to be planed off by
subsequent glacial advances.Where fine sediments are exposed the seafloor is
marked by circular gas-escape structures up to 400 m
wide and 20m deep (12), known as pockmarks
(Hovland and Judd, 1988). The gas is supplied from
older hydrocarbon-bearing strata, phase changes in gas
hydrates, or from the decay of organic material within
the most recent sediments. The eruptive nature of the
gas release from the sediments may become a factor in
sediments instability on the upper slope (Locat, 2001).
Gas seeps in sandy outer shelf sediments do not form
pockmarks, because the gas can seep out without being
trapped by fine sediments, but may become sites for
bioherm accumulation (13) (Levy and Lee, 1988).
Subsequent glacial advances will destroy pockmarks
on the shelf, but pockmarks on the slope may be
preserved through burial.
During interglacials, the sedimentation rate on the
shelf and upper slope declines. Sedimentation within the
shelf basins consists of remobilized inner shelf sediments
reworked by rising sea levels and hemipelagic sediments.
Pockmarks may become greatly enlarged during this
time (Gipp, 1994b). Storm-wave activity reworks sands
and gravels on the outer shelf, occasionally sweeping
sands over the shelf break and down the slope, and also
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rewards upper to middle slope muds, leaving a relatively
coarse lag (Piper, 1991). Sands which are swept into
slope canyons may ignite small turbidity currents,
cutting networks of small channels into the canyon
heads (Baltzer et al., 1994). Canyons which survive infill
during deglaciation are probably enlarged by storm-
driven downvalley surges (Shepard, 1979), headwarderosion (Farre et al., 1983), cold seeps of pressurized
fluids on the lower slope (Dugan and Flemings, 2000),
or lateral expulsion of hydrocarbon gas from sublimated
gas hydrates. Interestingly, the presence of the canyon
may add to the stability of nearby intercanyon
sediments by allowing for pressurized fluids to escape
laterally into the canyon (Piper et al., 1999a). Small
tributary channel feeding submarine canyon systems are
likely to be completely filled during this phase.
Presently, on the Scotian Shelf, a thin (o100 m)
veneer of Quaternary sediments rests unconformably on
Tertiary and Cretaceous bedrock (King and Fader,
1986). Glacial marine sediments which predate the last
glacial cycle are rare (o1 km over 1000+line km
surveyed), and only occur where bedrock highs offer
some protection from glacial erosion.
5.2. Gulf of Alaska
In the Gulf of Alaska, rising mountain ranges
intercepted moisture-laden air masses from the Pacific
Oceanthe resulting orogenic precipitation initiated
alpine glaciers, which supplied large volumes of
sediment to the forearc basin (Hallet et al., 1996). The
absence of a broad continental shelf prevents glacial icefrom advancing far offshore (Fig. 12a). Sediment
accumulates in front of the glacier terminus, and is
resedimented downslope in the form of debris flows (2)
and turbidites. The initial depth of the forearc need not
be significant, as subsidence caused by sediment
compaction and isostatic loading allows it to accom-
modate tremendous thicknesses of sediments (Gipp,
1996).
Submarine canyons (7) are incised on the upper slope
by the entrainment of turbidites, and broaden down-
slope. They do not necessarily follow regional slope, but
will drain towards the centre of the forearc basin, the
location of which will vary through time (Dobson et al.,
1991).
Ice rafting (4) supplies coarse material into the basin,
as dropstones in otherwise fine-grained sediments.
Debris flows and turbidity currents, caused by rapid
deposition on the upper slope, combined with ice push,
tectonic shock, and storm-wave loading, also move
coarse sediments downslope (Hampton et al., 1987).
The rapidly deposited sediments on the upper and
middle slope are unstable and subject to remobilization
by repeated tectonic shock, release of pressurized fluids,
or sublimation of gas hydrates due to changing water
temperatures (cf. Piper et al., 1999a). These sediments
are remobilized downslope as slumps, debris flows,
olistoliths, and turbidity currents (5), where they fill up
the forearc basin, which is subsiding in response to
tectonics and isostasy (Fig. 12b). The balance between
continuing subsidence and sediment accumulation
results in a basin filled with aggrading remobilizedsediments. On the upper slope, gravel- and sand-charged
meltwaters carve small feeder channels (6), which supply
submarine canyons (7) with sediment-laden water from
beneath the glacier. Submarine canyons eventually cross
the forearc basin and carry sediments towards the
trench, and may change regional orientation at different
stratigraphic intervals, in response to tectonic activity
(e.g., Carlson et al., 1990; Dobson et al., 1991). During
deglaciation and interglacial episodes, enough sediment
is introduced to the basin from rising coastal mountain
chains that the ephemeral feeder channels, gullies, and
even the largest submarine canyons may be completely
buried.
As the forearc basin fills with sediment (Fig. 12c), a
broad shelf may be built. The shelf differs in geometry
from the broad shelf on the passive margin, as it is
largely aggradational (Fig. 2). Muddy proglacial sedi-
ments are deposited by suspension rain-out (1) (Eyles
et al., 1991), but during the next glacial advance, the
shelf will have subsided sufficiently to either prevent
grounded ice from advancing across the shelf, or prevent
it from eroding deeply into pre-existing sediments,
allowing their preservation. On the outer shelf, currents
and icebergs may form lag gravels (14), which may
subsequently be overridden by grounded ice to formstriated boulder pavements.
Only when a board shelf is developed is there storage
space for significant volumes of subglacial meltwater to
be stored, and thus for subglacial tunnel valleys (11) to
form. For this reason, the present submarine canyons in
the Gulf of Alaska may be related to catastrophic
discharges of subglacial meltwaters, and in this way they
may differ somewhat from the older canyons exposed in
outcrop around Icy Bay.
Large pockmarks are unlikely, despite the presence of
hydrocarbon seepages from bedrock, because sedimen-
tation rates remain high during interglacials, and pock-
mark formation apparently occurs when sediment rates
are low (Gipp, 1994b). Small unit pickmarks (Hov-
land and Judd, 1988), which are too small to be
observed on seismic profiles, may occur, and may be
observable in outcrop. Seepages of gas may be an
important factor in supporting colonies of macrofauna
on the seafloor, which appear in outcop as coquinas
(13).
Thus, the earliest sediments in the sequence will be
dominated by debris flows, but suspension rain-out
sediments and ice-rafted debris increase in stratigraphic
importance upsection. Debris flows and turbidites are
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important before the forearc basin is completely filled.
Once the shelf extends across the previous forearc basin,
resedimentation becomes only of local importance. The
new continental slope is seaward of the forearc basin
and remobilized sediments drain directly into the trench.
As long as the margin continues to subside, grounded
ice sheets will not entirely remove pre-existing shelf
deposits. Thus, the sequence is capped by a thick
succession of shelf facies.
5.3. Facies successions of glacially influence marine
settings in contrasting tectonic settings
The models described above can be used to construct
facies successions for tectonically active and passive
glacially influenced continental margins. A generalized
facies model depicting the Pleistocene glaciation of the
Scotian margin (Fig. 13) consists of an interbedded
sequence of thick debris flows and turbidites, represent-
ing deposition on the continental rise, grading upwards
into turbidites and thin-bedded debris flows, with
considerable slumped material, olistoliths, and loading
structures, including diapirs, representing deposition on
the lower slope (Figs. 3, 5 and 6). Turbidites become
coarser upsection, and submarine canyons, character-
istic of the middle slope, may be observed in middle
slope strata. Canyons are filled by diamictites, olisto-
liths, debris flows, and turbidites. Intercanyon facies
include debris flows and reworked sediments, and are
capped with dropstone-free sandy silts, representing
winnowed interglacial sediments and sands swept over
the shelf edge. These coarsen upsection where they are
capped by sandstones and conglomerates with small cut-
and-fill channels, near the shelf break (e.g., Fig. 7). Till
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Fig. 12. Model depicting development of the Yakataga Formation in the Gulf of Alaska, representing (a) initial deposition into a relatively narrow,
shallow forearc basin, which (b) subsides, and is infilled by progressively larger amounts of sediment, resulting in (c) a broad shelf, which may become
entirely glaciated. Numbering scheme as in Fig. 9. Modified after Gipp (1996).
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tongues occur on the upper slope when ice advances to
the shelf edge. The sequence is capped locally by tabular
sandstones with coquinas and boulder pavements,
representing outer shelf deposition, or by bioturbated
shelly mudstones with dropstones and clay rafts (Fig. 4),
representing deposition within a basin on the shelf.
The facies model for the Yakataga Formation
(Fig. 13) consists of a basal sequence of turbidites with
debris flows (Fig. 10) containing abundant sediments
clasts, olistoliths and mud diapirs, representing deposi-
tion on the lower slope. These deposits grade upwards
into turbidites and debris flows cut by submarine
canyons (Fig. 9). The orientation of submarine canyons
may change dramatically upsection, due to change in the
location of the forearc basin (Dobson et al., 1991) or the
formation of compressive ridges on the slope (Carlson
et al., 1990).
Atop the middle slope facies are turbidites and sand
sheets, representing upper slope facies, which are over-
lain by a thick succession of grades and ungraded
sandstones and conglomerates characterized by abun-
dant cut-and-fill channels (Fig. 8). The Yakataga For-
mation is capped by a thick shelf sequence (>1 km on
Middleton Island) of diamictites and sandstones, with
coquinas, striated boulder pavements, and other evi-
dence of ice scour (Fig. 7). Successions of heavily
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Fig. 13. Comparison of facies successions on the Scotian margin and Gulf of Alaska. Major changes may be noted in the variability of paleocurrent
directions shown by the large submarine canyons, the geometry of debris flows, and the development of a thick sequence of shelf deposits in the Gulf
of Alaska that is accommodated by rapid subsidence of the forearc basin. Modified from Gipp (1996).
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