The Continental Slope and Adjoining Deep Sea System

The main large-scale morphological elements on continental slopes are trough mouth fans, gullies and channels, and large submarine slide scars and corresponding accumulations (Fig. 12.8). Some gullies and channels may be formed directly in association with continental ice sheets (e.g. Vorren et al., 1998), but many of these and the slides do not. Thus, they will not be treated here. Here we will discuss the main glacigenic depo-centres on the continental slope, the trough mouth fans, and deep-sea drifts and fans directly related to glacigenic sediment input.

12.2.1 Trough Mouth Fans

12.2.1.1 Distribution of Trough Mouth Fans

On glaciated continental margins, fan or delta-like protrusions occur in front of many glacial troughs or channels crossing the continental shelf and ending on the shelf break (Figs 12.5, 12.6, 12.7 and 12.8). Many of these protrusions surrounding the Norwegian-Greenland Sea

Continental Shelf Sea Fan Delta

Figure 12.12 A) Huntec DTS high-resolution seismic reflection profile from the Halibut Channel, Grand Banks of Newfoundland. B) Geoseismic interpretation of A. Till tongues (wedge-shaped units of incoherent reflections) are interbedded with glacimarine sediments. The upper till tongue has been cut by a series of interpreted subglacial meltwater channels which have fragmented the till tongue into a series of erosional remnants. (Adapted from Moran and Fader (1997).)

Figure 12.12 A) Huntec DTS high-resolution seismic reflection profile from the Halibut Channel, Grand Banks of Newfoundland. B) Geoseismic interpretation of A. Till tongues (wedge-shaped units of incoherent reflections) are interbedded with glacimarine sediments. The upper till tongue has been cut by a series of interpreted subglacial meltwater channels which have fragmented the till tongue into a series of erosional remnants. (Adapted from Moran and Fader (1997).)

were noted by Nansen (1904). Vogt and Perry (1978) pointed out that these protrusions are probably prograded deltas and attached fans. Vorren et al. (1988, 1989) proposed naming these features, which also occur on other glaciated margins, 'trough mouth fans' (TMF). Not all troughs ending on the shelf break have fans at their mouths, but on the eastern margin of the Norwegian-Greenland Sea they are particularly numerous (Fig. 12.14). On the Greenland continental margin the Scoresby Sund TMF is well developed (Dowdeswell et al., 1997) and other fans around Greenland are identified by Funder (1989; Fig. 12.3). Although not originally denoted as TMFs, similar accumulations are described from the eastern continental margin of Canada (Aksu and Hiscott, 1992; Hiscott and Aksu, 1994, 1996) and from the continental margin off northwest Britain (Stoker, 1995). Bathymetric features indicate two TMFs in the eastern Arctic Ocean, namely in front of the St Anna Trough and Franz Victoria

SUBAQUATIC LANDSYSTEMS: CONTINENTAL MARGINS 305

SUBAQUATIC LANDSYSTEMS: CONTINENTAL MARGINS 305

Figure 12.13 Detailed bathymetric map of an area of the southwestern Barents Sea showing a hill-hole pair. The hill (Steinbitryggen) is interpreted to consist of sediments tectonically displaced from the depression (Sopphola). The volume of Sopphola is equal to that of the ridge, which is partly shaped into a westerly trending streamlined form. Possible bathymetric expressions of boundaries between individual thrust masses is shown. Isolated depressions and a possible remnant of a glacitectonic hill southwest of Sopphola are also indicated. (Redrawn from S^ttem, 1990).

Figure 12.13 Detailed bathymetric map of an area of the southwestern Barents Sea showing a hill-hole pair. The hill (Steinbitryggen) is interpreted to consist of sediments tectonically displaced from the depression (Sopphola). The volume of Sopphola is equal to that of the ridge, which is partly shaped into a westerly trending streamlined form. Possible bathymetric expressions of boundaries between individual thrust masses is shown. Isolated depressions and a possible remnant of a glacitectonic hill southwest of Sopphola are also indicated. (Redrawn from S^ttem, 1990).

Trough. TMFs in Antarctica include the Crary Fan in the Wedell Sea (Kuvaas and Kristoffersen, 1991; Batist et al, 1997), the Prydz Bay fan (Hambrey, 1991) in the western Ross Sea, in the Bransfield Basin, and on the Wilkes Land continental margin (Anderson, 1999).

12.2.1.2 Morphology and Architecture of Trough Mouth Fans

The TMFs vary in size and shape. In the North Atlantic-Nordic Seas the smallest are found in the north off the archipelago of Svalbard and in the south off the British Isles. These are several orders of magnitude smaller than the largest, the Bear Island TMF (Table 12.1). The smallest TMFs have in general the steepest slopes (Table 12.1).

At least during the Late Quaternary, TMFs have been the sites of intense debris flow activity. Damuth (1978) was the first to indicate their presence. Vorren et al. (1988, 1989) found that the debris flows, seen in cross section, are bundled in sets of lenses separated by high-amplitude reflections. The middle part of the fans is dominated by a mounded seismic signature in cross section (Fig. 12.15), representing sections through debris flows. The debris flows are deposited in bathymetric lows between older deposits. The flows end on the lower fan. The highamplitude reflections between each lens-set probably represent periods of low sediment

Greenland Interstadials
Figure 12.14 Bathymetric map showing the location and extent of trough mouth fans in the Norwegian Sea. (Adapted from Vorren et al., 1978).

input/erosion during interstadials or interglacials. Later mapping by seismic and by side-scan sonars has confirmed that the debris flows are the main building blocks of the younger part of the TMFs in the Norwegian-Greenland Sea (Vogt et al., 1993; Laberg and Vorren, 1995, 1996a, b; King et al., 1996; Sejrup et al., 1996; Dowdeswell et al., 1996, 1997). Similar flows have been reported from the continental margin off northwest Britain (Stoker, 1995) and the eastern Canadian continental margin (Aksu and Hiscott, 1992; Hiscott and Aksu, 1994).

Vanneste (1995) suggested three basic types of TMF:

1. mostly stable TMFs characterized by the absence of large-scale mass-wasting deposits (e.g. Scoresby Sund TMF)

2. unstable TMFs characterized by the presence of large-scale mass-wasting deposits (e.g. the Bear Island TMF)

3. TMFs associated with deep sea-fan systems in their distal parts (e.g. Crary TMF).

TMF

Kongsfjorden

Isfjorden

Bellsund

Storfjorden

Bear Island

North Sea

Sula Sgeir

Radius (km)

55

50

70

190

590

560

50

Width upper (km)

40

45

55

130

250

165

70

Width lower (km)

60

75

85

210

550

300

85

Depth upper (km)

0.2

0.25

0.15

0.4

0.5

0.4

0.2

Depth lower prox. (km)

2.4

3.0

2.7

1.3

Depth lower distal (km)

2.0

3.0

2.3

2.7

3.2

3.5

1.3

Area (km2)

2,700

3,700

6,000

35,000

21 5,000

142,000

3,100

Gradient (upper)

1.8°

0.8°

0.6°

Gradient (middle)

1.9°

3.2°

1.8

1.0°

0.4°

0.8°

1.3°

Gradient (lower)

0.2°

0.2°

0.3°

Radius = radius along the longest axis; width upper = width at shelfbreak; width lower = maximum width of the lower fan; depth upper = depth at the shelf break; depth lower prox. = depth at the base of the proximal part of the fan; depth lower distal = depth of the base of the distal part of the fan; area = total fan area.

Gradients of the upper, middle and lower slope along the longest fan axis of the Stor^orden and Bear Island TMF according to Laberg and Vorren (1996b) and according to King et al. (1996) for the North Sea TMF. Average fan gradient is given for the Kongs^ord TMF, the Is^ord TMF, the Bellsund TMF and the Sula Sgeir TMF. (After Vorren and Laberg, 1997).

Table 12.1 Size and shape of most of the northwest European trough mouth fans (TMFs).

Table 12.1 Size and shape of most of the northwest European trough mouth fans (TMFs).

Ul o

12.2.1.3 Debris Flows

King et al. (1998) suggested that the debris flows constituting the TMFs should be termed 'glacigenic debris flows' and Vogt et al. (1999) use the term 'glacigenic mudflows'. Here we will use glacimarine debris flows to make a distinction between glacigenic debris flows and mudflows that occur in the subaerial environment. The dimensions of the glacimarine debris flows vary: between 0.5 km and 40 km wide, between 5 and 60 m thick, from less than 10 km up to 200 km long, covering areas up to 1880 km2, and with volumes from 0.5 to 50 km3. There is a clear tendency towards larger fans having the larger and more voluminous debris flows.

The debris flow sediment is a homogeneous diamicton (Laberg and Vorren, 1995; King et al., 1996, 1998; Elverhei et al., 1997; Vorren et al., 1998). It contains 30-55 per cent clay, 30-50 per cent silt, 10-30 per cent sand and usually less than 10 per cent gravel. The grain-size distribution and water content are compatible with the youngest till units on the outer shelf (e.g. King et al., 1998; Vorren et al., 1998).

Transport to and accumulation at the shelf break: A generally accepted model (Fig. 12.16) is that glacigenic sediments were brought to the grounding line as a deforming till layer (Boulton, 1979; Alley et al., 1989). This probably resulted in a build up either of 'till-deltas', according to the model of Alley et al. (1989), or 'diamict aprons' (Hambrey et al., 1992), or 'grounding line or zone wedges' (e.g. Powell and Alley, 1997; Anderson, 1997) along the glacier terminus. The glacigenic sediments could also have continued directly downslope. Sediments deposited in the till deltas are probably inherently unstable, and not well preserved on a sloping subsurface (Dimakis et al., 2000). However, there are examples on the seismic records across the shelf break, which could be interpreted as till deltas (Vorren and Laberg, 1997).

Release factors: The morphology, showing slide scars on the present uppermost slope surface and a chaotic seismic facies on the upper fan, indicates that several sediment slides were released near the shelf break (Laberg and Vorren, 1995). The slides may have been triggered by:

1. build up of excess pore pressure due to high sediment input (Dimakis et al., 2000)

2. earthquakes

3. oversteepening

Figure 12.15 Segments of 3.5 kHz profiles across debris flow deposits on the Bear Island TMF. A) Profile across a debris flow deposit on the upper fan. Here the debris flows are identified from their surface relief. The reflection defining the base disappears immediately beneath the flow deposit (arrows). B) Profile across a debris flow deposit on the middle fan. In this area the debris flows are identified from their surface relief. The reflection defining the base disappears immediately beneath the flow deposit (arrows). C) Profile from about 2000 m water depth. Here the base reflection is clearly seen and the debris flow surface sometimes mirrors highs at the base. D) Profile from the middle fan. The deposit is characterized by a positive relief and the underlying deposits seem to be unaffected by the younger flow. E) Profile from the lower fan illustrating a slightly irregular surface relief. The underlying acoustic parallel unit seems to have been left unaffected by the deposition of the flow. F) Profile from the lower fan illustrating a relatively smooth surface relief. The underlying acoustic parallel unit seems to have been left unaffected by the deposition of the flow. (After Laberg and Vorren, 2000).

Break Slope
Figure 12.16 Conceptual model showing the main sedimentary processes on the shelf break and upper slope during the presence of an ice stream at the shelf break.

4. ice loading (e.g. Mulder and Moran, 1995), and/or

5. seepage of shallow gas.

The repeated release of slides on a regional scale suggests a trigger mechanism common to these particular settings. Thus, most likely the slides were released by build up of excess pore pressure and oversteepening. Dimakis et al. (2000) has calculated that after 95-170 years of high sedimentation rates, failures will take place that will remove the top 10-30 m of the deposited sediments.

Flow behaviour: The debris flows move downslope following bathymetric lows between older deposits (Aksu and Hiscott, 1992; Laberg and Vorren, 1995). Flows that move further downslope than their forerunners spread out laterally, resulting in a width and thickness increase on the lower fan. Generally, the debris flows containing the largest sediment volume have the longest run out distance (Laberg and Vorren, 1995).

Many of the observed debris flows have a large run out distance on low-gradient slopes, particularly on the Bear Island and North Sea TMFs. This indicates low viscosity behaviour. The mobility of debris flows involves an important contribution from excess pore fluid in allowing long run out distances.

Laberg and Vorren (2000) have shown that from at least 1600 m water depth debris flows erode and probably incorporate substrate debris, but further downslope they move passively over substrate sediments. The theory of hydroplaning of the debris flow front may explain why the debris flows moved across the lower fan without affecting the underlying sediments. When hydroplaning is established, the moving debris flow head is substantially decoupled from its bed and, as shown in experiments, runout distance and head velocity become independent of debris flow rheology (Mohrig et al., 1998; Elverhoi et al., 2000).

12.2.2 Deep-Sea Drifts and Fans

The Pacific continental shelf of the Antarctic Peninsula has typical glacial morphology, the main morphological elements being represented by banks separated by troughs. The continental slope is steep. On the continental rise are elongated sedimentary mounds bounded by erosional channels. The sedimentary mounds are sediment drifts. In contrast to trough mouths, elsewhere there is no morphological expression of fan growth on the Antarctic Peninsula slope. Rebesco et al. (1998) explain this by sediments supplied to the trough mouth having low shear strength. They therefore slide down the relatively steep continental slope and feed the turbidity current channels and drift systems (Fig. 12.17). The primary source of drift sediments is, therefore, thought to be from turbidity currents travelling in the channels that traverse the continental rise in deep areas between drifts. The fine-grained components of turbidity currents are entrained in a nepheloid layer within the ambient bottom currents and are then redeposited by those currents in response to sea floor topographic control. Thus, much of the glacial sediment transported to the shelf break accumulates in drifts and deep-sea fans rather than in TMFs in this setting. Also in other steep continental slope areas, glacigenic sediments transported through turbidity channels have been shown to accumulate in deep-sea fans (e.g. Vorren et al., 1998; Anderson, 1999).

Bathymetry Moraine Continental Shelf

Figure 12.17 Synthetic schematic model showing the spatial relationship of the main physiographic elements on the continental margin west of the Antarctic Peninsula. Large glacial troughs traverse the continental shelf from the areas between the main islands, structurally guided towards the topographic lows of the mid-shelf high. Large prograding wedges develop on the outer shelf beyond the topographic highs of the mid-shelf high, next to the outward continuation of the major glacial troughs. Giant sediment drifts separated by large channel systems are present on the upper continental rise in between the prograding wedges. A large deep-sea fan, ponded against an oceanic basement high, is present on the lower continental rise beyond the lowermost reaches of the drifts. (After Rebesco et al., 1998).

Figure 12.17 Synthetic schematic model showing the spatial relationship of the main physiographic elements on the continental margin west of the Antarctic Peninsula. Large glacial troughs traverse the continental shelf from the areas between the main islands, structurally guided towards the topographic lows of the mid-shelf high. Large prograding wedges develop on the outer shelf beyond the topographic highs of the mid-shelf high, next to the outward continuation of the major glacial troughs. Giant sediment drifts separated by large channel systems are present on the upper continental rise in between the prograding wedges. A large deep-sea fan, ponded against an oceanic basement high, is present on the lower continental rise beyond the lowermost reaches of the drifts. (After Rebesco et al., 1998).

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