Ice Margins in Low Relief Landscapes

5.3.1 Glacier Margins

In East Antarctica the majority of the ice margin terminates in the sea. Relatively small parts of the ice margin terminate on land in small coastal oases. The largest oases in East Antarctica are the Vestfold Hills and Bunger Hills (Fig. 5.1). Recent investigations of the Quaternary history of these areas has suggested that the ice margin during the last glacial maximum was thinner and less extensive than previously thought (Colhoun et al., 1992) and that deglaciation was almost complete by 10,000 years BP (Fitzsimons and Domack, 1993). These conclusions are clearly controversial as they contradict data from the Ross Embayment (Denton et al., 1989) and marine seismic and core data in East Antarctica (Domack et al., 1991). As the mode and pattern of ice advance and retreat have implications for the interpretation of palaeo-climate and ice dynamics, it is vital to have appropriate depositional models for landforms and sediments.

In Vestfold Hills the edge of the continental ice sheet runs from north to south, and the southern limit of the ice-free area is formed by the Sorsdal Glacier, which is the major outlet glacier of the area and forms a small ice shelf. The hills consist of a complex low-relief topography composed of valleys at and below sea level and ridges up to 158 m in altitude. Glacial sediments and landforms are absent from most of the ice-free area and are concentrated close to the glacier margin.

The mean annual temperature of the Vestfold Hills is -10.2°C (Schwerdtfeger, 1970) which is, on average, warmer than Antarctic stations of similar latitude (Burton and Campbell, 1980). Although no precipitation data are available, snowfall is light (probably <250 mm per year) and rainfall is very rare. Melting of snow and ice is restricted to the short summer (December to February). There is a strong diurnal component to the melt activity, which usually ceases between

Figure 5.1 Map showing the location of the Vestfold, Larsemann and Bunger hills and the McMurdo dry valleys.

9 pm and 10 am when air temperatures are below or close to 0 °C and the sun has a low angle of incidence.

In many East Antarctic coastal oases the ice-sheet margin has a complex form, and distinct features such as an ice cliff are difficult to recognize. Consequently it is necessary to define some terms used in this chapter: 'ice edge' is used to describe the terminus of a glacier where it is sharp and easily recognizable (Fig. 5.2a) and 'ice margin' is used to describe an ice-terminus that it is not clearly recognizable (Fig. 5.2b). Within an ice margin an apparent ice edge is often recognizable as an ice cliff beyond which an area of ice-cored moraine occurs (Fig. 5.2b). The width of the ice-cored moraine or debris-covered glacier is highly variable and can range from tens of metres to several kilometres. The term outer ice edge is used to define the actual glacier terminus where ice movement ceases (Fig. 5.2b).

The three pre-eminent characteristics of the ice margin at Vestfold Hills are its variable shape, the presence of a large sinuous ice-cored moraine (Figs. 5.2 and 5.3), and the abundance of large snow drifts (Fig. 5.3). The ice-sheet margin has a convex form and descends rapidly from 300 m above sea level within 2 km of the margin to approximately 100 m above sea level at the margin. Where the ice flows into the sea, the ice margin forms 20—40 m high cliffs. On land the margin is considerably more complex, often with multiple cliffs and snow drifts (Fig. 5.4a).

The sinuous ice-cored moraine that dominates the ice margin at Vestfold Hills is a broad, discontinuous ridge of coarse debris, 100—300 m wide and about 20 km long that occurs inside the ice margin (Fig. 5.3). The debris is, on average, less than 0.5 m thick but accumulations up to 1.5 m thick occur on the sharp-crested ridges. The sinuous inner moraine contrasts with other moraines that occur in front of ice cliffs which have sharp-crested ridges. Moraine ridges beyond the ice margin are much higher (up to 20 m) and much shorter than the inner moraine ridges (less than 1 km long). Most are ice-cored and unstable, as indicated by the occurrence of numerous sediment flows, slumps and other mass movements (Fitzsimons, 1990).

Inner moraine

Inner moraine

I-Ice margin

Apparent ice edge Outar Ice edge

I-Ice margin

Apparent ice edge Outar Ice edge

Figure 5.2 Ice-margin nomenclature. A) Simple ice margin with an ice cliff and inner moraine formed by basal ice cropping out on the glacier surface. B) Wide ice margin with an apparent ice margin separated from an outer ice edge by numerous outcrops of basal debris.

Figure 5.3 Oblique aerial photographs of the ice margin at Vestfold Hills. A) Looking toward the ice margin with S0rsdal Glacier at right. Note the large sinuous inner moraine (arrowed) and snow drift partially concealing the ice margin. The light, turbid lakes are connected to the proglacial drainage system and the dark ones are not. The ice margin is about 10 km long. B) Ice margin looking toward the coast. Note the deep snow drifts downstream of the inner moraine and partly frozen lakes and fiords. The numerous dolerite dykes belie the lack of unconsolidated sediment over the landscape.

Figure 5.3 Oblique aerial photographs of the ice margin at Vestfold Hills. A) Looking toward the ice margin with S0rsdal Glacier at right. Note the large sinuous inner moraine (arrowed) and snow drift partially concealing the ice margin. The light, turbid lakes are connected to the proglacial drainage system and the dark ones are not. The ice margin is about 10 km long. B) Ice margin looking toward the coast. Note the deep snow drifts downstream of the inner moraine and partly frozen lakes and fiords. The numerous dolerite dykes belie the lack of unconsolidated sediment over the landscape.

In other parts of the Vestfold Hills the ice margin is buried by snow drifts and forms in a low-angle ramp. Where the margin is not buried, cliffs up to 30 m high occur near the ice-cored moraines and at the heads of fjords (Fig. 5.4a). In these steeper sections the debris is concentrated below the ice cliffs (Figs. 5.4a and b) forming narrow, sharp-crested ridges up to 10 m high.

(C)
500 m

Figure 5.4 Topographic profiles of the Vestfold Hills ice margin. A) Cliffed margin with inner moraines beyond the apparent ice edge. B) Ramp margin with two sets of inner moraines. C) Ramp margin with a large snow wedge and two inner moraines. D) Multiple ice cliffs and snow wedge remnants with a folded inner moraine beyond the apparent ice edge. E) Ramp margin with numerous outcrops of basal debris.

Figure 5.4 Topographic profiles of the Vestfold Hills ice margin. A) Cliffed margin with inner moraines beyond the apparent ice edge. B) Ramp margin with two sets of inner moraines. C) Ramp margin with a large snow wedge and two inner moraines. D) Multiple ice cliffs and snow wedge remnants with a folded inner moraine beyond the apparent ice edge. E) Ramp margin with numerous outcrops of basal debris.

In the southeastern corner of the hills, where the Sersdal Glacier forms a distinct outlet glacier, the ice margin has a convex profile and has multiple ice cliffs (Fig. 5.4d). The slightly deformed, basal debris zone is unconformably overlain by clean white ice. This unconformity appears to record a former ablation surface that has been buried by ice which accumulated in situ. A zone of ice-cored moraine with numerous sharp-crested ridges parallel to the ice edge occurs beyond the main ice cliffs.

The structure of the ice margin at the Vestfold Hills is revealed by exposures of the basal debris zone in ice cliffs and gullies that cross the ice margin. Deformation structures range from relatively undeformed debris bands to intense deformation characterized by recumbent folds and shear structures (see Fig. 4 in Fitzsimons, 1990). Deformation structures in the basal zone of the ice cap can be divided into large-scale features, which involve the entire basal debris zone, and small-scale features which occur within the basal zone. The most prominent large-scale deformation structure is the upwarping of the basal debris zone to crop out on the surface of the glacier and form a large, sinuous ice-cored moraine (Figs 5.3 and 5.4). Exposures of basal debris reveal structures that vary from slightly deformed stratified ice (Fig. 5.5a) to complex multi-phase folding and shearing (Fig. 5.5b). A section through an ice-cored moraine in the southeastern corner of the hills shows that moraine ridges can form along the axes of a series of large recumbent folds that have amplitudes over 15 m (see Fig. 4c in Fitzsimons, 1990).

Measurements of debris concentrations in the basal zone ice are consistently below 10 per cent by volume. Debris concentration in individual bands is highly variable with most of the debris concentrated close to the bed. Unusually high concentrations occur in rare debris lenses of sorted fluvial sediments that have been entrained by the glacier. Most of the debris consists of silt and sand-sized particles with larger clasts either dispersed or occurring in small lenses. Gravel clasts are dominantly subrounded, rarely angular.

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