Submarine glacial-landform distribution across the West Greenland margin: a fjordshelf-slope transect through the Uummannaq system (70˚ to 71˚N)
J. A. DOWDESWELL1*, K. A. HOGAN2 & C. Ó COFAIGH3
1
Scott Polar Research Institute, University of Cambridge, Cambridge CB2 1ER, UK
2
British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK
3
Department of Geography, Durham University, Durham DH1 3LE, UK
*Corresponding author (e-mail: [email protected])
Today, the Greenland Ice Sheet reaches the sea via a number of fastflowing outlet glaciers that are fed by ice draining from huge interior
basins (Rignot & Kanagaratnam 2006). At the Last Glacial Maximum
(LGM), the ice sheet expanded to reach the continental shelf break
around much of Greenland (Ó Cofaigh et al. 2013a). In the
Uummaannq area, at about 70-71°N (Fig. 1a), there is now a 400 km
distance between the terminus of Rink Glacier, which drains about
30,000 km2 of the ice sheet, and the shelf edge. This provides a
transect from the modern glacier front, through a deep fjord system
and adjacent cross-shelf trough, to the continental slope in Baffin Bay.
The seafloor is now exposed along this transect and the landforms
produced by past glacial activity can be examined using marinegeophysical methods. Deglaciation from the LGM was underway at
the shelf edge in Uummannaq Trough by 14.8 kyr ago and from the
mid-shelf by 10.9 kyr, and ice had probably retreated back into the
fjord system by 9.3 kyr ago (Ó Cofaigh et al. 2013a; Roberts et al.
2013).
The seismic stratigraphy of the continental shelf and upper slope in
central West Greenland (Fig. 1b; Dowdeswell et al. 2014), together
with the convex-outward shape of bathymetric contours at the mouth
of Uummannaq Trough (Fig. 1a), demonstrate that a major
sedimentary depocentre has built up there (Ó Cofaigh et al. 2013b).
The prograding nature and seaward-dip of seismic reflections within
the Pliocene-Pleistocene sediment package confirm the presence of a
major trough-mouth fan that has progressively built-out the
continental shelf into the deeper waters of Baffin Bay (Fig. 1b). In
addition, a seismic section across Uummannaq Trough shows, through
the truncation of sub-horizontal reflections at its margin (Fig. 1c), that
there has been erosion as well as deposition from the likely past
activity of ice streams flowing through the trough. The longer-term
context is therefore of a margin where ice streams have advanced and
retreated through the Uummannaq fjord-shelf system on a number of
occasions over the past few million years. The submarine landforms
that we describe and interpret below are the product of the latest cycle
of ice-sheet growth and decay on Greenland since the LGM about
20,000 years ago.
Description of fjord-shelf-slope landforms
Inner fjords
The seafloor of Rink Fjord and Karrat Isfjord is relatively flat for
the most part, with protruding bedrock pinnacles and ridges (Fig. 2a,
b) (Dowdeswell et al. 2014). The basins in Rink Fjord reach over
1,000 m in depth. Karrat Isfjord is shallower, with water <600 m deep
and appears, from its rougher appearance, to have more bedrock
exposed at the seafloor. Both fjords have steep subaerial and
submarine side-walls, giving the fjords a characteristic ‘u’-shape. This
inner-fjord morphology is similar to that of the inner fjords of the
Scoresby Sund and Kejser Franz Josef Fjord systems in East
Greenland (Ó Cofaigh et al. 2001; Evans et al. 2002).
Within a few kilometres of the modern marine terminus of Rink
Glacier there are two basins over 1,000 m deep, separated by a large
transverse ridge that stretches across the fjord in about 650 m of water
(Fig. 2a-c). The ridge is asymmetrical, with a steeper ice-distal slope
of 22. This ridge geometry is typical of many ice-contact sedimentary
landforms (Benn and Evans, 2010). A well-defined submarine channel
is also present on the otherwise fairly smooth basin floor and extends
for about 4 km seaward from the distal side of the large transverse
ridge (Fig. 2c). The channel is sinuous, up to 350 m wide and 25 m
deep. Inshore of the transverse ridge, there is a deep trough within
which two sedimentary lineations are present which are streamlined
along the fjord axis (Fig. 2c).
Outer Uummannaq Fjord
Outer Uummannaq Fjord has the widest variety of submarine
landforms anywhere within the Uummannaq system (Dowdeswell et
al. 2014). Several sets of streamlined features are orientated
approximately along fjord long-axes, although there is a clear
curvilinear trend in the orientation of the landforms (Fig. 2d-g). In the
inner 20 km or so of the fjord, streamlined lineations and bedrockcored crag-and-tail features are found (Fig. 2e). It appears that the
distally narrowing sedimentary tails of the crag-and-tails originate
from bedrock ridges crossing the fjord. The streamlined features can
be seen in TOPAS sub-bottom profiles in which a roughly 10 m-thick
semi-transparent drape buries the strong reflection in which the
streamlined features have formed (Fig. 2f).
Where Uummannaq Fjord narrows to about 25 km wide, between
Ubekendt Island and Nuussuaq Peninsula (Fig. 2d), the seafloor
morphology is more complex, but remains streamlined in an east-west
direction. Shorter linear features trending first WNW and then WSW
are present (Fig. 2d, g). The less regular areas are probably bedrock at
or near the seafloor, and there is a 20 km2 area where small channels
are imaged with bedrock highs between them (Fig. 2d, g).
Cross-shelf trough
Uummannaq Trough is almost 200 km long and about 20-25 km
wide, with shallower banks generally <400 m deep to either side (Fig.
1) (Dowdeswell et al. 2014). The trough floor is relatively smooth
(Fig. 3a, b, f) compared with the fjord further inshore (Fig. 2d). This,
together with shallow sub-bottom profiles (Figs. 3d, e), indicates that
the seafloor is sedimentary. TOPAS profiles show a strong, prolonged
sub-bottom reflection (Fig. 3d), which is sometimes lineated, overlain
by an acoustically stratified drape up to about 20 m thick. Shallower
banks have an irregular surface, with chaotic furrows (Fig. 3e, f).
Seismic data also show that the shelf is composed of several hundred
metres of prograding sediments (Fig. 1b, c).
Between about 5430’ and 5530’W, the inner-shelf is dominated by
linear to curvilinear streamlined and apparently sedimentary
landforms (Fig. 3a); some are blunt-nosed with streamlined tails, and
have crescentic depressions around their heads (Fig. 3a). West of this,
the seafloor is smoother, but several prominent scarps, up to 40 m
high (Fig. 3c-e), are imaged (Fig. 3a, b). These features have a wedgelike and asymmetrical shape with a steeper seaward face (Fig. 3c, e).
Where their surface is undisturbed, the features are draped by
acoustically semi-transparent or stratified sediment underlain by a
strong, prolonged reflection and, sometimes, by less continuous
weaker reflections (Fig. 3d, e).
From: DOWDESWELL, J. A., CANALS, M., JAKOBSSON, M., TODD, B. J., DOWDESWELL, E. K. & HOGAN, K. A. (eds) Atlas of Submarine Glacial
Landforms: Modern, Quaternary and Ancient. Geological Society, London, Memoirs, xx, 1-4. © The Geological Society of London, 2015. DOI: xx.xxxx/xxx.x.
SUBMARINE GLACIAL LANDFORMS IN UUMMANNAQ FJORD AND TROUGH
Fig. 1. Regional bathymetry and shelf architecture of the Uummannaq fjord-shelf-slope system, West Greenland. The location of subsequent figures is also shown. (a)
Multibeam-bathymetric coverage of the Uummannaq system; (a) is located as a red box on the inset location map of Greenland. UI is Ubekendt Island; RF is Rink
Fjord. Regional bathymetry from IBCAO v. 3.0. (b) 130-km long dip seismic reflection profile showing West Greenland continental shelf architecture comprising
prograding sedimentary units. (c) 110-km long strike profile showing a Late Quaternary glacial trough where erosion has truncated pre-existing reflections (black
arrows). The reflections marked bPP represent the lower boundary of Plio-Pleistocene erosion and glacier-influenced sediments; (b) and (c) are located in (a). Note that
the depth scales in (b) and (c) are approximate and are based on correlating the seafloor reflection with the bathymetric data.
J. A. DOWDESWELL ET AL.
Outermost shelf and slope
The continental slope and outermost Uummannaq shelf are shown in
Figure 3f (Dowdeswell et al. 2014). The trough-mouth <600 m deep
has streamlined sedimentary lineations with a strong sub-bottom
reflection (Fig. 3f, g), buried under a drape of acoustically stratified
sediment. There is also a 10 m-high ridge at about 60°W at about 600
m depth; the basal 12 cm of a 1.42 m-long core here is composed of
stiff diamict overlain by glacimarine mud (Ó Cofaigh et al., 2013a).
As water shallows on either side of the trough (Fig. 1a), irregular
furrows a few metres deep dominate the seafloor in water < 570 m
deep (Fig. 3f). A few irregular furrows occur down to about 850 m
(Fig. 3f).
The continental slope reaches over 2,000 m in Baffin Bay, beyond
the shelf break. In plan, the contours offshore of Uummannaq Trough
bulge outwards, giving a fan-shaped appearance to the slope (Fig. 1a).
On the slope, lobate sedimentary features are present in swath imagery
(Fig. 3f). Sub-bottom profiles show stacking of lobes, particularly
below 1,500 m depth, and individual semi-transparent units can be
traced to the slope base (Fig. 3h). To the south of the trough-mouth,
several slide scars have also been observed on the upper slope
(Dowdeswell et al. 2014).
Interpretation of fjord-shelf-slope landforms
Inner fjord
A relatively smooth seafloor, broken by bedrock ridges and pinnacles
that divide the fjord into several deep basins, is present over much of
Rink Fjord and Karrat Isfjord (Fig. 2a, b). This is interpreted as finegrained basin fill, supplied mainly by rain-out from sediment-rich
meltwater derived from subglacial and glacifluvial sources. Turbid
lateral meltwater streams were observed at the margins of Rink
Glacier during marine-geophysical data collection in August 2009,
and subglacial meltwater is also relatively abundant, especially given
a basal melt rate of about 2.5 m d-1 close to the glacier terminus
(Enderlin & Howat 2013). Suspended sediment plumes were also
observed upwelling from the base of other tidewater glaciers in the
Rink Fjord system (Powell 1990; Mugford & Dowdeswell 2011).
There are probably additional contributions to the inner-fjord basin
fill from several other sources. Rink Glacier has an ice velocity of
about 10 m d-1 close to its margin (Enderlin & Howat 2013), implying
that many of the icebergs that drift through the fjord system are
produced here (Fig. 1a). Icebergs are also sourced from several other
tidewater glaciers entering the fjord. As the icebergs melt, debris rains
out to the seafloor.
The large transverse ridge, asymmetrical in cross-section, extending
across Rink Fjord about 2 km from the modern ice front, is likely to
be sedimentary (Fig. 2a-c). Acoustic penetration is limited, suggesting
that the ridge is composed of diamictic rather than sorted debris.
Lateral moraine ridges are also clearly visible onshore These subaerial
and submarine ridges are interpreted to indicate the ice-front position
of Rink Isbrae during the cool Little Ice Age (LIA), which took place
in West Greenland from about 1500 to 1860 (Dahl-Jensen et al., 1998;
Fischer et al., 1998). Many marine-terminating West Greenland
glaciers show a similar LIA maximum. Similar transverse ridges in,
for example Svalbard and Chilean fjords, are also linked to LIA
glacier growth (e.g. Ottesen & Dowdeswell, 2009; Dowdeswell &
Vasquez, 2013). The remaining large transverse ridges in Karrat
Isfjord are probably bedrock, sometimes with a thin drape of sediment
(Fig. 2b). The floor of the inner fjords has a rougher appearance
adjacent to steep fjord side-walls and where bedrock is also close to
the sea-floor (Fig. 2a, c). In the relatively deep basin inshore of the
moraine ridge, two streamlined landforms are interpreted as megascale glacial lineations (MSGL), formed subglacially before the retreat
of Rink Glacier from this area over the past century (Fig. 2c).
A single 4 km-long sinuous submarine channel was also imaged in
the 1,000 m-deep innermost sedimentary basin (Fig. 2c). It is
interpreted as a turbidity-current channel, formed by underflows of
dense and probably highly turbid water. Flow in this channel likely
contributes intermittently to the build-up of acoustically laminated
basin fill in the inner fjord system. Turbidity currents may be
generated by occasional slope failures on the steep distal face of the
moraine ridge close to the present terminus of Rink Glacier, with
debris flows translating into turbidity currents downslope (Fig. 2c).
Outer fjord and Uummannaq shelf
The well-developed streamlined landforms, orientated parallel to
outer-fjord and trough long axes, are interpreted as a product of
deformational and depositional processes at the bed of a fast-flowing
ice stream in the full-glacial and deglacial Greenland Ice Sheet (GIS)
(Figs. 2e-g, 3a, f). Entirely sedimentary streamlined landforms,
typically with an elongation ratio of >20:1, are interpreted as MSGLs
(Clark 1993). The MSGLs are usually buried by a few metres of finegrained glacimarine sediment, deposited since ice retreat. MSGLs are
observed frequently in cross-shelf troughs in both polar regions (e.g.
Canals et al. 2000; Ó Cofaigh et al. 2002; Ottesen et al. 2005), and
have also been observed forming beneath fast-flowing ice streams in
modern Antarctica (King et al. 2009).
The presence of these subglacial landforms is clear evidence that
the GIS advanced across Uummannaq Shelf to the shelf edge,
probably at the LGM. Radiocarbon dates from the Uummannaq shelf
and upper slope confirm that the MSGLs are linked to the presence of
ice during the last full-glacial period (Jennings et al. 2012; Ó Cofaigh
et al. 2013a). A 10 m-high moraine ridge on the outermost shelf yields
a date of 14.8 cal. kyr from 5 cm above a stiff diamict, which is
interpreted as subglacial till (Ó Cofaigh et al. 2013a); the date
suggests that deglacial retreat from a full-glacial maximum limit on
the outermost shelf was underway by this time.
Where streamlined subglacial landforms have an ice-proximal rock
core and a sedimentary tail, they are interpreted as crag-and-tail
features (Benn & Evans 2010). These distinctive landforms are found
only in the outer fjord at about 52.5°W (Fig. 2d). The bedrock
component of crag-and-tails presumably acted as a high-friction zone
in what was otherwise a mainly sedimentary former glacier bed.
Further offshore, the floor of Uummannaq Trough is entirely
sedimentary, like most high-latitude cross-shelf troughs, due to buildup through progradation of glacier-derived debris (Fig. 3a). Bluntnosed sedimentary drumlins, present on the shelf at about 55°W (Fig.
3a), again represent a sedimentary landform of subglacial origin (Benn
& Evans 2010). Some drumlins have cresentic depressions on their
upstream sides. These depressions may be produced by subglacial
water flow, especially given the presence of palaeo-channels on the
inner shelf (Fig. 2g). Each of the these types of streamlined
sedimentary landform is an indicator of the presence of a former ice
stream, together with its onset zone, in outer Uummannaq Fjord and
Trough.
The sedimentary scarps in Uummannaq Trough, between 55° and
56°W (Fig. 2a-d), and at about 58°W (Fig. 3e), are interpreted as the
relatively steep ice-distal faces of three grounding-zone wedges
(GZWs; Dowdeswell & Fugelli 2012). These depocentres appear
asymmetrical in long-profile, typical of many similar buried features
on the Greenland shelf (Dowdeswell & Fugelli 2012). GZWs are
sedimentary wedges formed by delivery of deforming subglacial
sediment to a marine ice margin that has been in a similar location for
decades to centuries during more general regional deglaciation (e.g.
Mosola & Anderson 2006; Dowdeswell & Fugelli 2012). Faint subbottom reflections suggest that the wedges are probably a few tens of
metres thick (Fig. 3d, e). The presence of GZWs also implies that
retreat in Uummannnaq Trough was episodic, and punctuated by at
least three still-stands, rather than a single catastrophic collapse
(Dowdeswell et al., 2008a), although the duration of the still stands is
still unknown.
Irregular to chaotic linear to curvilinear, v-shaped furrows are the
characteristic landform on the relatively shallow banks to either side
of Uummannaq Trough. There is a fairly sharp depth limit to the
distribution of these features (Fig. 3e). The furrows are interpreted as
ploughmarks produced by iceberg keels eroding the sedimentary
seafloor (Woodworth-Lynas et al. 1991). Ploughmarks have been
observed over very large areas of the Greenland shelf, especially
where water is shallower than about 500 m (e.g. Brett & Zarudzki
SUBMARINE GLACIAL LANDFORMS IN UUMMANNAQ FJORD AND TROUGH
Fig. 2. Marine-geophysical data from Rink Fjord and inner Uummannaq Fjord-Trough system (modified in part from Dowdeswell et al. 2014). (a) Sun-illuminated
multibeam-bathymetric image of Rink Fjord and Karrat Isfjord. (b) Seafloor depth profile along Rink Fjord showing flat-bottomed basins interrupted by bedrock highs.
VE × 37. (c) 3D-view of inner Rink Fjord, looking up-fjord towards Rink Glacier, showing a sinuous channel system (white arrows) and cross-fjord moraine ridge;
behind the ridge two lineations (black arrows) occur in front of Rink Glacier. (d) Multibeam image of submarine landforms in Uummannaq Fjord. Streamlined
landforms indicate a confluence of palaeo-ice flow directions into Uummannaq Fjord and westwards onto the continental shelf (Dowdeswell et al., 2014); crag-and-tail
landforms are arrowed in white. (e) Bathymetric image of the strait east of Ubekendt Island showing streamlined landforms (solid white) and a series of ridges (dashed
and arrowed) at a topographic high that may indicate a former ice-marginal position. (f) 3.5 kHz sub-bottom profile across the crag-and-tail landforms shown in (d). VE
× 25. (g) Multibeam image of streamlined landforms (white) and across-trough channels (arrowed) on the inner shelf; (a), (d) and (g) are located in Fig. 1a. Multibeam
acquisition system: Kongsberg EM120. Frequency 12 kHz. Grid-cell size 30 m. Acoustic profile acquisition system: Kongsberg TOPAS PS 018 parametric sub-bottom
profiler. Secondary beam frequency 0.5-6 kHz. Regional bathymetric contours from IBCAO v. 3.0.
J. A. DOWDESWELL ET AL.
1979; Dowdeswell et al. 1993). An abrupt water-depth limit to
iceberg-keel ploughing is typical of many high-latitude shelves (e.g.
Barnes & Lien 1988; Dowdeswell et al. 1993), and is a result of the
relatively uniform dimensions of the parent ice-sheet terminus from
which the icebergs are derived (Dowdeswell & Bamber 2007).
Icebergs with deeper keels, evidenced by ploughmarks at the mouth of
Uummannaq Trough down to about 850 m (Fig. 3f), are present only
where iceberg fragmentation and overturn have taken place, which
may occasionally lead to iceberg geometries that result in particularly
deep keels. Isolated slope-parallel depressions up to about 40 m deep
in water depths of 850 to 1,085 m of water on the West Greenland
upper slope have also been ascribed to ploughing by huge icebergs
probably produced during break-up of the last full-glacial ice sheet
(Kuijpers et al. 2007). Few icebergs with keels greater than 500 to 600
m are calved from the fast-flowing ice streams and outlet glaciers of
the Greenland Ice Sheet today (Dowdeswell et al. 1992).
Continental slope and Uummannaq Fan
The outward-bulging contours of the continental slope at the mouth
of Uummannaq Trough indicate the presence of a major submarine
fan (Fig. 1a), Uummannaq Fan (Ó Cofaigh et al. 2013b), which is
similar in geometry to trough-mouth fans reported at many locations
on polar continental margins (e.g. Vorren et al. 1998). Prograding
sediments on seismic records from Uummannaq Shelf support this
interpretation (Fig. 1b). Offshore of Greenland, large fans have been
mapped beyond the Disko shelf in West Greenland and Scoresby Sund
and Kangerlussuaq Trough in East Greenland (Dowdeswell et al.
1997, 2010; Ó Cofaigh et al. 2013a; Laberg et al., 2013).
Such high-latitude, glacier-influenced fans are built up
predominantly from the delivery of diamictic debris to the shelf edge
and upper slope when ice sheets advanced to the shelf break during
Quaternary full-glacial conditions (Vorren et al. 1998). The lobate
landforms on Uummannaq slope (Fig. 3 f, h) are interpreted as
glacigenic-debris flows, formed mainly from diamict delivered by the
palaeo-ice stream that advanced through the adjacent trough at the last
full-glacial about 20,000 years ago. The debris flows are stacked on
the slope (Fig. 3h), and are the main building blocks of Uummannaq
Fan (Ó Cofaigh et al. 2013b). Similar stacked glacigenic-debris flows
have been reported from many large trough-mouth fans on polar
continental margins (e.g. Laberg & Vorren 1995; King et al. 1996;
Vorren et al. 1998). Coring on the northern part of Uummannaq Fan
has shown that turbidity currents, iceberg-rafting and hemipelagic
rainout have also contributed to fan sedimentation (Ó Cofaigh et al.
2013b), although turbidity-current channels are absent from the area
of the fan we have imaged using swath bathymetry (Fig. 11f).
Finally, immediately south of the mouth of Uummannaq Trough,
several small slide scars on the upper slope suggest that additional
downslope mass wasting may take place through intermittent slope
failures (Dowdeswell et al. 2014).
Discussion: landform distribution and schematic model
The landform distribution in the Uummannaq fjord-shelf-slope
system is mapped in Figure 4a. The deep inner fjords are blanketed by
fine-grained basin fill between rock pinnacles. The distribution of
streamlined subglacial landforms, and a moraine ridge at the mouth of
Uummannaq Trough, imply that a fast-flowing ice stream reached the
shelf edge on this part of the West Greenland margin. Radiocarbon
dates indicate that this advance took place at the LGM and that outershelf deglaciation had begun by about 14.8 cal. kyr ago (Ó Cofaigh et
al. 2013a). The streamlined subglacial landforms vary in morphology
from crag-and-tail features together with well-defined MSGLs in the
outer fjord, to less well-defined lineations on the shelf. There is also
an area of drumlins and cresentic depressions on the inner shelf, but
the detailed role of meltwater in their formation is not clear. The set of
streamlined landforms described above, and illustrated schematically
in Figure 4b, indicates deformation of water-saturated sediments at a
former ice-stream bed (Ó Cofaigh et al. 2005).
The distribution of subglacial landforms, with the fjords and inner
shelf of the Uummannaq system containing a mix of sediments and
bedrock at the seafloor, and the outer shelf being entirely sedimentary
(Fig 4), is typical of many major ice-stream systems in both the Arctic
and Antarctic (e.g. Wellner et al. 2001; Ó Cofaigh et al. 2002;
Dowdeswell et al. 2010). Drumlins and crag-and-tail bedforms have
been interpreted to indicate the onset-zone of fast ice-stream flow by
some previous workers (e.g. Wellner et al. 2001). The association
between form and flow is less clear in the Uummannaq system, given
that drumlins are found at about 55W at the wide mouth of outer
Uummannaq Fjord (Fig. 4a). The presence of cresentic depressions on
the stoss side of some drumlins, and limited development of channels
nearby (Figs. 2g. 3a), certainly relates to subglacial processes, but
whether to an onset zone or to full ice-stream flow remains unclear.
The fjords of the Uummannaq system show that acoustically
laminated and presumably fine-grained sediments, derived
predominantly from turbid meltwater sources and by rainout from
icebergs, are typical of overdeepened basins within its fjords; similar
to sediments reported from Kangerlussuaq and Nordvestfjord at 68°
and 70°N in East Greenland (Syvitski et al. 1996; Ó Cofaigh et al.
2001). The rates of buildup of this material are, however, often slower
than in the fjords of Alaska, Chile and Svalbard where air and water
temperatures are warmer than in West Greenland (Powell & Molnia
1989). Subglacial and ice-contact landforms produced at the LGM and
during regional deglaciation are often buried by a few to about 20 m
of sediment on the West Greenland shelf, whereas tens of metres have
built up over a similar period in milder fjord settings (e.g. Powell
1990; Dowdeswell & Vásquez 2013). More distal from modern
sources of meltwater, the seafloor on the West Greenland shelf is
dominated largely by streamlined and diamictic subglacial landforms,
indicating the former presence of an ice stream in the Uummannaq
cross-shelf trough, and GZWs that suggest episodic retreat across the
shelf punctuated by still-stands of decades to centuries to build up
substantial transverse-to-flow depocentres.
In environments around Antarctica and North and Northeast
Greenland, which are colder than the West Greenland margin,
meltwater delivery of sediments is much more restricted. Here,
especially in fjord and shelf areas traversed by large numbers of
icebergs, sedimentation is much slower and debris deposited on the
shelf is often diamictic in grain size (e.g. Dowdeswell et al. 1994;
Anderson 1999).
The record of LGM and deglacial landforms is heavily reworked at
water depths shallower than about 450-500 m by the ploughing action
of deep-keeled icebergs. This is typical of many high-latitude shelves
where high fluxes of large icebergs occurred during deglaciation and,
in many cases, continued through the Holocene to the present, fed
from huge (105 to 106 km2) interior ice-sheet drainage basins. The
continuing release of such deep-keeled icebergs is not typical of the
fjords of Alaska, Chile and Svalbard, where Holocene and modern
iceberg production is restricted mainly to smaller and irregular-shaped
icebergs with relatively shallow draughts derived from grounded
tidewater glaciers with catchments of often only tens to a few hundred
square kilometres (e.g. Dowdeswell & Forsberg 1992).
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SUBMARINE GLACIAL LANDFORMS IN UUMMANNAQ FJORD AND TROUGH
Fig. 3. Marine-geophysical data from the continental shelf and slope of Uummannaq Trough (modified in part from Dowdeswell et al. 2014). (a) Sun-illuminated
multibeam-bathymetric image of drumlinsed lineations (solid white) and two backstepping grounding-zone wedges (GZWs) the front of which are marked with dotted
lines. (b) Multibeam-bathymetric image of a large GZW on the middle shelf. (c) Seafloor depth profile over the mid-shelf GZW in (b) showing rapid deepening of the
trough in an offshore (westward) direction. VE × 156. (d) 3.5 kHz sub-bottom profile over the backstepping GZWs in (a). VE × 60. (e) 3.5 kHz sub-bottom profile over
the large mid-shelf GZW in (b). VE × 53. In (d) and (e) faint reflections beneath the GZWs are arrowed. (f) Multibeam-bathymetric image of mega-scale glacial
lineations (MSGL) on the outer shelf, and the Uummannaq trough-mouth fan; (a) and (f) are located in Fig. 1a. (g) Seafloor depth profile across Uummannaq Trough
on the outer shelf showing MSGL (arrowed) and the 35-km wide glacial trough. VE × 42. (h) Down-slope 3.5 kHz sub-bottom profile on the lower slope of the
Uummannaq trough-mouth fan showing stacked wedging units of sediment interpreted as glacigenic-debris flows. VE × 6. Multibeam acquisition system Kongsberg
EM120. Frequency 12 kHz. Grid-cell size 30 m. Acoustic profile acquisition system Kongsberg TOPAS PS 018 parametric sub-bottom profiler. Secondary beam
frequency 0.5-6 kHz. Regional bathymetric contours from IBCAO v .3.0.
J. A. DOWDESWELL ET AL.
Fig. 4. (a) Summary diagram of the distribution pattern of submarine landforms in Rink Fjord and in the Uummannaq Trough-Slope system, West Greenland (modified
from Dowdeswell et al. 2014). (b) Schematic landform-assemblage model for the fjord-shelf-slope sedimentary system in central West Greenland. The model includes
a schematic stratigraphy for the West Greenland continental shelf based on interpreted 2D seismic profiles like that shown in Fig. 1b. The legend for submarine
landforms in (a) is also applicable to the schematic model in (b).
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