Subsidence and tectonic controls on glacially influenced continental margins: examples from the Gulf of Alaska and the western Scotian Shelf and Slope

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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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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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Documents

Subsidence and tectonic controls on glacially influenced continental margins: examples from the Gulf of Alaska and the western Scotian Shelf and Slope