Figure 651 Ice composition structure and deformation at the base of Suess Glacier in the Taylor Valley Antarctica

Fitzsimons, 1996; Fitzsimons et al., 1999; Lorrain et al., 1999; Sleewaegen et al., 2003). Holdsworth & Bull (1970) demonstrated that the effective bed of Meserve Glacier (basal temperature —18°C) occurred along the tops of boulders that protruded from the glacier bed into the basal ice. Holdsworth & Bull recorded a debris-rich amber basal ice layer up to 0.6 m thick and demonstrated that salts from the glacier substrate and/or the amber ice extended up to 6 m above the glacier base. These observations, later verified by Cuffey et al. (2000c), tend to reinforce the view in the glaciological literature that cold-based glaciers are inefficient agents of erosion.

However, the thickness, composition and structure of basal ice of glaciers in the McMurdo dry valleys that rest on unconsoli-dated sediments are distinctly different from those confined to the valley sides, such as Meserve Glacier. Glaciers that reach the valley floor are characterized by a thicker basal ice layer, higher debris concentrations, frozen blocks of sediment within the basal ice, and in some cases well-developed ice-marginal moraines (Fitzsimons, 1996; Humphreys & Fitzsimons, 1996). Analysis of the composition of the ice exposed at the margins of some of these glaciers suggests that at least part of the basal ice has been derived from accretion of marginal water and sediment. Tunnels excavated into several of these glaciers have provided an opportunity to closely examine the composition, structure and behaviour of basal ice and glacier beds.

At Suess Glacier the basal ice sequence is approximately 3.8 m thick and can be divided into five main units on the basis of physical appearance, debris concentration and disposition of debris (Fig. 65.1). The layers include clean englacial ice (4—3.7m), amber ice (sensu Holdsworth, 1974) (3.7-2.9 m), a solid layer of frozen sediment (2.9-2.4 m), a basal laminated facies (2.4-0.0 m) and the substrate (Fig. 65.1). The clean englacial ice is underlain by amber ice that contains dispersed particles and fine aggregates, which give the ice a greenish to amber colour. The amber ice is underlain by a broken layer of frozen sand and fine gravel that contains well preserved sedimentary structures. The laminated ice that lies below the main debris band consists of multiple layers of clean bubbly ice to complexly deformed layers of ice with very high

Figure 65.2 Photograph of the stratified ice facies in the basal ice zone of Suess Glacier 26 m into the tunnel. The solid facies occurs at the roof of the tunnel at this location.

debris concentrations (Fig. 65.2). Occasional boulders up to 1.2 m in diameter were observed in the laminated ice up to 2 m above the bed. Ice with relatively low debris concentrations contained abundant ductile structures (Figs 65.3-65.5), whereas structures in the solid layers indicate predominantly brittle defor

Boundinage Structure Photo

Figure 65.3 Boundinage structures 1.7m above the base of Suess Glacier. The boudins have formed from a 10-mm-thick layer of sand. The glacier flow direction is from right to left.

Boundinage Structure Photo

Figure 65.5 Sheared recumbent folds 0.8 m above the base of Suess Glacier. The glacier flow direction is from right to left.

Figure 65.3 Boundinage structures 1.7m above the base of Suess Glacier. The boudins have formed from a 10-mm-thick layer of sand. The glacier flow direction is from right to left.

Figure 65.5 Sheared recumbent folds 0.8 m above the base of Suess Glacier. The glacier flow direction is from right to left.

Figure 65.6 Photograph of a 4 m shaft excavated through part of the basal zone of Wright Lower Glacier showing layers of sand interbedded with ice. The glacier flow direction is from top to bottom.

Figure 65.4 A folded layer of gravelly sand 1.0 m above the bed of Suess Glacier. The heads of the bolts are 10 mm in diameter and the glacier flow direction is from right to left.

Figure 65.6 Photograph of a 4 m shaft excavated through part of the basal zone of Wright Lower Glacier showing layers of sand interbedded with ice. The glacier flow direction is from top to bottom.

mation. The presence of large blocks of sediment within which sedimentary structures are well preserved suggests that at least part of the basal ice in the glacier has been formed by erosion and entrainment of the frozen bed of the glacier.

Similar observations made at Wright Lower Glacier demonstrated that large layers of the frozen sand occur within the basal ice (Fig. 65.6). The presence of these layers demonstrates that there is no clear distinction between basal ice and the glacier substrate.

The configuration and composition of basal ice in glaciers in the McMurdo dry valleys that rest on unconsolidated sediment show that there is a continuum of ice-sediment mixtures that range between basal ice and frozen debris. The case for this con tinuum is supported by studies of glaciated permafrost environments, which have been interpreted as the product of coupling between glaciers and frozen substrates (Astakhov et al., 1996).

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