Low-level debris transport is the transport of debris at or close to the base of the glacier. Debris may be derived directly from the bed by subglacial entrainment or indirectly from debris that falls onto the glacier surface and finds its way to the bed via crevasses and the downward movement associated with extending flow or basal melting (Figure 7.1). This debris remains at the base of the glacier until it is either deposited subglacially or released at the glacier snout or margin. Alternatively, it may be elevated onto the glacier surface along thrusts or by upward flow within the ice formed by compressive flow, a phenomenon common near the glacier snout.
Once entrained, basal debris becomes concentrated both laterally and vertically during glacier flow. The vertical extent of the basal debris may be increased by folding or thrusting of debris-rich basal ice. Debris can also become concentrated laterally around bedrock obstacles. As a uniform debris-rich basal ice
Figure 7.8 (A) The concentration of basal debris into distinct streams around subglacial bedrock obstacles. (B) A typical flute in the lee of a boulder on a glacier forefield in Svalbard. Former ice flow was towards the camera. (A) Modified from: Boulton (1974) in Glacial Geomorphology (ed. D.R. Coates), George Allen and Unwin, figure 12, p. 62. (B) Photograph: N.F. Glasser
Figure 7.8 (A) The concentration of basal debris into distinct streams around subglacial bedrock obstacles. (B) A typical flute in the lee of a boulder on a glacier forefield in Svalbard. Former ice flow was towards the camera. (A) Modified from: Boulton (1974) in Glacial Geomorphology (ed. D.R. Coates), George Allen and Unwin, figure 12, p. 62. (B) Photograph: N.F. Glasser layer approaches a bedrock obstacle its flow accelerates due to enhanced creep around the sides of the obstacle, such that less debris-rich ice is carried over the top of the obstacle (Figure 7.8). This mechanism explains how it is possible to develop a complex and laterally dispersed basal sediment layer.
The basal transport zone can be divided into two subzones: (i) a zone of traction in which debris is moved along the bed of the glacier; and (ii) a zone of suspension in which debris is transported immediately above the glacier base. The transfer of debris between these two zones is controlled primarily by pressure melting and regelation. Pressure melting on the up-ice side of an obstacle causes particles to move downwards, while the freezing-on of ice and debris on the down-ice side causes debris to move upwards. Folding, particularly against bedrock steps, may also cause particles to move upward from the zone of traction.
Debris transported at a low level within a glacier is often referred to as actively transported because it is altered during glacial transport. Particles in transport within the zone of traction experience considerable modification through the processes of crushing and abrasion (communition). They are typically spherical and rounded, and usually have a bimodal or multimodal grain-size distribution. The grain-size distribution is typically composed of three separate populations: (Figure 7.9) (i) large rock particles, lithic fragments; (ii) mineral grains produced by crushing of the rock fragments; and (iii) submineral-sized particles produced by the abrasion of mineral grains. In an ideal environment in which debris is neither removed nor added, the relative importance of these two populations should change as the transport distance increases: the fine mineral population should grow at the expense of the coarse population (Figure 7.9; Box 7.2). In practice this is rarely observed because of the constant addition of new material.
Roundness should also increase with transport distance as the corners of a particle are blunted and smoothed off by particle abrasion. However, observations suggest that roundness does not increase indefinitely, but reaches a terminal roundness. This reflects the fact that rock particles are also crushed subglacially as they are transported, which increases their angularity, at the same time as they are abraded. The degree of roundness a particle can achieve will be controlled by the length of time between crushing events, which will depend on its strength and the force applied. The stronger or more resistant a particle is to crushing, the more rounded it may become. Particles transported at the base of glacier are also characterised by faceted and striated surfaces, which often give clasts a bullet-shaped appearance (Figure 7.10).
Particles in transport within basal ice also develop a strong particle fabric, as elongated particles become aligned with the direction of ice flow. A preferred orientation develops parallel to the direction of flow although subsequently particles may also become orientated in a direction transverse to flow. This is because the orientation that provides the least resistance to flow is the one in which the long axis is parallel to the direction of flow. This property is of particular importance because glacial sediments may inherit this particle fabric.
Material can also be transported under a glacier by subglacial meltwater streams and through sediment deformation (e.g., as part of a deforming bed or during glaciotectonism). Figure 7.11 represents an attempt to summarise these low-level debris entrainment and transport mechanisms.
TO alr er
Shape of this distribution reflects the original rock
Fine peak caused by abrasion of mineral grains
Fine peak caused by abrasion of mineral grains
Figure 7.9 The grain-size distribution of subglacially transported debris is composed of three distinct populations: (i) lithic or rock fragments; (ii) mineral grains produced by crushing of the rock fragments; (iii) fines produced by the abrasion of individual mineral grains.
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