Structural Glaciological Controls on Debris Entrainment and Transport

An understanding of structural glaciology and its role in controlling debris entrainment and transfer is essential to the interpretation of the landsystem developed at Svalbard glacier margins (Hambrey and Lawson, 2000). Debris entrainment in Svalbard glaciers takes place by a variety of mechanisms that can be linked to the evolution of ice structures (Hambrey et al., 1999). The most significant structures include stratification, folded stratification, foliation, the deformed basal ice zone, thrusting and thrust-related recumbent folding. These modes of debris entrainment are outlined below.

4.2.1 Incorporation of Rockfall Material

Primary stratification is inherited from snow accumulation and superimposed ice in the accumulation area, and is sometimes supplemented by rockfall material from the glacier headwalls (Hambrey et al., 1999). This material is typically angular, and becomes incorporated englacially as a result of burial and folding of the stratified sequence. Converging flow, where multiple accumulation basins supply a narrow tongue, promotes folding (Fig. 4.5a). Component flow units may be reduced in width by more than 50 per cent. Fold styles typically range from open 'similar' to less common chevron and isoclinal types, each commonly associated with an axial-planar foliation. Fold axes tend to be parallel to flow and plunge gently up-glacier. As debris-laden folded stratification intersects the glacier surface in the ablation area, 'medial moraines' emerge from a single point-source or multiple point-sources, producing down-glacier widening spreads of surface debris (Fig. 4.5b, c). As in temperate glaciers (e.g. Meier and Post, 1969; Lawson, 1996), this relatively simple structure can be complicated by surges, giving rise to 'looped' moraine structures in which the debris layers originate from the stratification (Hambrey and Dowdeswell, 1997).

4.2.2 Entrainment of Debris at the Bed

Svalbard glaciers carry a large basal debris load, commonly reaching a thickness of several metres, with debris concentrations in some zones approaching 100 per cent. Although poorly understood at present, the principal processes of entrainment are considered to be regelation, water flow through the crystal vein system, bulk freezing-on, folding and shearing (Knight, 1997). Where observed in sections, layers of ice facies are commonly subjected to repeated shearing and isoclinal folding. The typical coarse-clear ice crystal character of basal ice, combined with varying proportions of debris allow it to be distinguished from coarse-bubbly ice derived from snowfall, when glacitectonically transferred to higher level positions within the glacier. Research into the composition and glacitectonic transport of the basal debris layers of Svalbard glaciers is an ongoing area of research.

Figure 4.5 Structural glaciology and debris transport in Svalbard valley glaciers. A) Oblique aerial photograph of a typical Svalbard cirque glacier with a single glacier tongue fed by multiple accumulation basins. Note the prominent debris stripes on the glacier surface and their continuation onto the forefield. B) Angular debris emerging from stratification on the surface of Sagabreen. The debris thickens down-glacier to form supraglacial debris stripes. C) Supraglacial debris stripes and their continuation onto the forefield at Midtre Lovenbreen. Individual stripes tend to consist of a single lithology, which in some cases can be traced to source areas in the accumulation basins. D) Debris-poor up-glacier dipping structures interpreted as thrusts in a lateral ice cliff on Kongsvegen. Note how the features rise asymptotically from the bed. The leftmost feature is 'blind', and does not reach the glacier surface. E) Debris-rich structure

Figure 4.5 Structural glaciology and debris transport in Svalbard valley glaciers. A) Oblique aerial photograph of a typical Svalbard cirque glacier with a single glacier tongue fed by multiple accumulation basins. Note the prominent debris stripes on the glacier surface and their continuation onto the forefield. B) Angular debris emerging from stratification on the surface of Sagabreen. The debris thickens down-glacier to form supraglacial debris stripes. C) Supraglacial debris stripes and their continuation onto the forefield at Midtre Lovenbreen. Individual stripes tend to consist of a single lithology, which in some cases can be traced to source areas in the accumulation basins. D) Debris-poor up-glacier dipping structures interpreted as thrusts in a lateral ice cliff on Kongsvegen. Note how the features rise asymptotically from the bed. The leftmost feature is 'blind', and does not reach the glacier surface. E) Debris-rich structure

interpreted as a thrust on the surface of Midtre Lovenbreen. The feature contains well-sorted sand and gravel, retaining original depositional characteristics indicative of a subglacial derivation. Glacier flow is from right to left and the plane of the thrust can be seen immediately above the ice axe. F) Small debris-rich structure interpreted as a thrust on the surface of Kongsvegen. The feature is composed of basal material, with high proportions of subrounded and striated clasts. Glacier flow is from left to right.

4.2.3 Association of Debris with Longitudinal Foliation

Longitudinal foliation is a structure common to all glaciers, being the product of tight folding, or simple or pure shear. In glaciers dominated by converging flow, longitudinal foliation pervades the width of a glacier (Hambrey and Müller, 1978). Svalbard glaciers commonly have pervasive near-vertical foliation throughout their widths, although this foliation varies in intensity, being strongest at the zones of confluence of adjacent flow units and at the margins. Debris is associated with foliation in two ways.

• Moderately well-sorted angular debris of supraglacial origin, which has been folded so tightly that the original stratification has been transposed into foliation (Fig. 4.6). Alternatively, the debris is dispersed around the hinge of a more open fold, which is intersected by an axial planar foliation (Hambrey et al., 1999).

• Debris showing basal characteristics (i.e. a wide variety of clast shapes, clast surface features such as striations and facets on appropriate fine-grained lithologies, and poorly sorted texture), disseminated through, or layered within, coarse-clear ice. This debris is isoclinally folded or sheared within foliation, particularly in ice-marginal areas and occasionally within the lower reaches of medial moraines. The folding mechanism that allows basal debris to reach the surface is unclear, but it is possible that in the thicker parts of the glacier a deformable bed of debris is folded within the overlying ice in the zone of converging flow (Hambrey et al., 1999). Again, the folding is in axial-planar relationship with the foliation.

plan VIEW

CROSS-SECTION

plan VIEW

Medial moraines

Snout

Medial moraines

Stratification with/without rockfall debris

Stratification with/without rockfall debris

Supraglacial debris B'

Supraglacial debris B'

ebris layer from rockfall

Basal debris ebris layer from rockfall

Basal debris

Supraglacial debris (medial moraines)

Supraglacial debris (medial moraines)

Snout

B. Cross-section of snout area during recession from Neoglacial maximum

Sandy Diamicton gravel

Basally derived supraglacial debris

Sandy Diamicton gravel

Basally derived supraglacial debris

Basal décollement

Proglacial landform/ sediment assemblage r

Basal debris

Englacial landform/ sediment assemblage Thrust

Figure 4.6 Schematic diagram illustrating the origin of structures in a typical polythermal Svalbard valley glacier.

Basal décollement

Proglacial landform/ sediment assemblage r

Basal debris

Englacial landform/ sediment assemblage Thrust

Figure 4.6 Schematic diagram illustrating the origin of structures in a typical polythermal Svalbard valley glacier.

4.2.4 Debris Entrainment by Thrusting

Thrusting is the most controversial mechanism of debris entrainment. Swinzow (1962) suggested that this was a valid mechanism for entrainment in the margin of the West Greenland ice sheet, but an alternative view is that incorporation of debris is a passive process as ice flowlines turn upwards towards the ice-frontal margin (Weertman, 1961; Hooke, 1973). Examination of structural relationships and mapping of numerous Svalbard glaciers suggests that thrusting is indeed a valid process for debris entrainment in polythermal glaciers (e.g. Hambrey et al., 1996, 1999; Hambrey and Dowdeswell, 1997; Murray et al., 1997; Glasser et al., 1998a).

Thrusts in Svalbard glaciers tend to be new structures, unrelated to pre-existing structural inhomogeneities. They are particularly common on the glacier surface within several hundred metres of the snout (Fig. 4.5d, e, f,). Few thrusts are actively-forming today, except in glaciers that are actively surging (Glasser et al., 1998b; Hambrey et al., 1999). Thrusts incorporate whatever material lies on the bed or within the basal zone, including basal till, glacifluvial and glacimarine sediment. The sediment thickness associated with thrusting may reach several metres, and original sedimentary structures can be preserved. At the opposite end of the spectrum, thrusts may be marked simply by thin layers of debris-rich ice of basal origin or films of fine debris along a well-defined plane. Displacements along thrusts are variable, and these are often difficult to evaluate due to the lack of clear marker horizons in the glaciers. Where these marker horizons exist, measured displacements range from only a few centimetres to several metres. However, larger displacements of tens of metres are required to raise basal debris to the ice surface.

Geometrically, thrusts show an asymptotic relationship with the bed and emerge at the glacier surface at angles ranging from 15° to 70°. Most layers associated with prominent debris-ridges dip at ~30° or less. Thrusts tend to be arcuate in plan, mirroring the geometry of the ice margin. In places, many thrusts can be seen to intersect at low angles. Thrusts may be laterally continuous for several tens of metres, although significant quantities of debris usually only occur on a fraction of this length. Intersecting thrusts tend to promote accumulations of debris on the glacier surface (Bennett et al., 1999). Debris-bearing thrusts are particularly well-developed in polythermal glaciers, where the thrusting process is facilitated by the transition from sliding bed conditions to frozen bed conditions at the margin and snout (Clarke and Blake, 1991; Hambrey et al., 1999). At such locations, high water pressures can facilitate the upward displacement process of englacial and subglacial debris (Glasser et al., 1999). Thrusting is not confined to polythermal glaciers, although large-scale debris-incorporation in temperate glaciers generally requires particular topographical conditions, such as ice-flow against a reverse slope (Glasser and Hambrey, 2002).

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