Continentalslope and oceanbasin morphology and sediments

The angle of the continental slope offshore of high-latitude shelves varies from <1° to >10°. The gradient of about 13,000km or 70% of Arctic continental margins was analysed by O'Grady & Syvitski (2002). They found that lower slope angles were associated with convergent modern and full-glacial ice flow and, by implication, with more rapid rates of sediment delivery (Dowdeswell & Siegert, 1999). Side-scan sonar and seismic-reflection surveys of Arctic and Antarctic continental slopes have shown that large submarine fans formed from glacier-derived debris are present beyond many, but not all, cross-shelf troughs (e.g. Bart & Anderson, 1996; Dowdeswell et al., 1996; Vorren et al., 1998; O Cofaigh et al., 2003). Seismic stratigraphical investigations show that such margins are usually prograding; that is, the continental shelf edge has built out seaward through hundreds of thousands to millions of years, during successive periods of high full-glacial sediment delivery (e.g. Larter & Vanneste, 1995; Vorren et al., 1998; Anderson, 1999).

These fans, forming cone-shaped depocentres beyond the shelf break (Fig. 30.3), have particularly low slope angles (0.5 - 2°). They have areas of 104-105km2 and are made up largely of stacked series of debris flows, derived from diamictic sediment delivered by fast-flowing ice streams to the shelf break and upper slope

Figure 30.3 Oblique view of the Bear Island Fan, Norwegian-Svalbard margin, with debris flows observed on 6.5 kHz longrange side-scan sonar imagery shown. (Modified from Taylor et al., 2002.)

during full-glacial conditions (e.g.Vorren et al., 1989; Dowdeswell et al., 1996, 1997; King et al., 1996). Packages of debris flows are separated by thin layers of fine-grained hemipelagic debris, representing successive interglacials or interstadials (King et al., 1996). Acoustically laminated silts and clays with a substantial ice-rafted component also contribute to full-glacial sedimentation on some fans, suggesting that ice streams may shift their position between successive full-glacials (Taylor et al., 2002). Sometimes, large slope failures or slides are also present, either on the fans themselves or on adjacent parts of polar continental slopes (e.g. Laberg & Vorren, 1993; Dowdeswell et al., 1996; Vorren et al., 1998). Characteristic features include failure scars on the upper slope, and zones of deposition of the failed material, often associated with large blocks, on the lower slope and beyond (e.g. Dowdeswell et al., 1996, 2002b).

Between these high-latitude fans, the continental slope is sometimes relatively steeper. Canyons are occasionally present (Taylor et al., 2000), especially where the rate of glacial sediment delivery is low. On the Lofoten margin, for example, a series of canyons is located seaward of what was probably a relatively restricted full-glacial drainage basin of low ice flux, located between two much larger, fast-flowing systems (e.g. Ottesen et al., 2005). In general, canyons are much less common on glacier-influenced margins than at lower latitudes, probably because glacial sediments are delivered from line sources (i.e. the seaward ice-sheet margin), rather than from the quasi-point sources provided by lower-latitude riverine activity (e.g. Dowdeswell et al., 1996).

However, rates of full-glacial sediment delivery directly to the upper slope vary considerably with the dynamics of the ice margin (Dowdeswell et al., 1996, 2002b). Fast-flowing ice-stream margins, often tens of kilometres in width, with an ice flux several orders of magnitude greater than slower moving margins, release considerably greater quantities of icebergs, meltwater and sedi ments to the upper slope (e.g. Dowdeswell & Siegert, 1999; Dowdeswell & Elverhoi, 2002; Siegert & Dowdeswell, 2002). Thus, submarine fans represent major sedimentary depocentres on high-latitude ice-influenced margins.

By contrast, relatively steeper slopes of 5-10° are also found offshore of some Antarctic cross-shelf troughs (O Cofaigh et al., 2003; Dowdeswell et al., 2004b), which are known to have contained major fast-flowing palaeo-ice streams (O Cofaigh et al., 2002; Dowdeswell et al., 2004a). In these locations, such as the Marguerite Bay trough and slope system west of the Antarctic Peninsula, sediments are inferred to be delivered rapidly from a full-glacial ice stream. Debris largely bypasses the relatively steep slope to be deposited as sediment drifts, and more distally as tur-bidites, in the deep-ocean basin beyond (e.g. Rebesco et al., 1996; Pudsey, 2000; Dowdeswell et al., 2004b).

Marine geophysical studies have identified several further submarine landforms associated with ice-influenced continental slopes and the deep-sea basins beyond. Systems of submarine channels are well-developed in some high-latitude settings (e.g. Mienert et al., 1993; Hesse et al., 1997; Dowdeswell et al., 2002b, 2004b). In the 250,000km2 Greenland Basin, for example, a branching network of channels is present on the upper slope, coalescing into a few major meandering channels on the abyssal plain beyond (Fig. 30.2d). The channels reach several kilometres in width, up to 100 m in depth, and are several hundred kilometres in length (Mienert et al., 1993; Dowdeswell et al., 2002b; O Cofaigh et al., 2004). Beyond the channel margins, acoustically laminated levees, related to successive downslope-flow events, are built up by turbidity-current transport and deposition of debris during overbank flow events (O Cofaigh et al., 2004). In the most distal and lowest gradient parts of the Greenland Basin, the channels lose their definition and deposit extensive lobes of relatively sandy sediment (Dowdeswell et al., 2002b).

Extensive areas of sediment waves and well-sorted contourite drifts are also clearly defined submarine landforms in some high-latitude slope and basin settings, particularly in interfan areas, where along-slope transport of fine-grained sediments is the dominant transport process (e.g. Kenyon, 1986; Kuvaas & Leitchenkov, 1992; Rebesco et al., 1996; Pudsey, 2000). In the series of sediment drifts at the base of the continental slope on the west side of the Antarctic Peninsula, individual sediment drift mounds cover thousands of square kilometres and rise several hundred metres above the general elevation of the sea floor. The drifts are usually separated by channel systems draining from the slope to the abyssal basin beyond (Plate 30.2).

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