Box 81 Were Deforming Glacier Beds Widespread

During the early 1990s there was a paradigm shift within glacial geology which involved the increasing acceptance of the idea of subglacial deformation as a key glacial process responsible not only for many subglacial tills but also for associated landforms such as drumlins. Not all researchers have accepted the idea that glacier bed deformation was extensive beneath former mid-latitude ice sheets. At the heart of this question lies a debate over the interpretation of the glacial evidence and about how sediment actually deforms. Subaerial sediments, such as those found on hill slopes, tend to fail in a plastic fashion as explained by classic soil mechanics. These sediments have a finite strength known as the yield strength, above which they fail. The rate of subsequent deformation - strain rate - is independent of the stress applied. Sediment that behaves in this way tends to fail along a discrete horizon or surface. There is one school of thought which argues that this type of model best explains what goes on beneath a glacier. This view has been supported by observation beneath the Whillans Ice Stream in Antarctica and by laboratory based studies. If this model is correct then deformation will occur in a very thin zone and the development of a thick deforming layer is unlikely. Promoters of this model suggest that thick deforming layers seen in the geological record reflect the ploughing of ice bed keels and boulders through sediment along with the cumulative effects of multiple failures. Piotrowski et al. (2001) argued eloquently for this model and identified a range of sedimentological observations that favour this interpretation. They suggested that bed deformation was consequently restricted beneath former mid-latitude ice sheets.

The alternative is that subglacial sediments fail via some form of non-linear viscous rheology in which strain rate is dependent on the applied stress. Consequently deformation will occur through a greater thickness of sediment rather than along a discrete horizon, thereby giving a thicker deforming layer.

This type of model is consistent with the original observations of subglacial deformation reported in Box 3.4 and with the development of a thick layer of deforming sediment.

Hindmarsh (1997) reconciles these two schools of thought rather elegantly. He acknowledged, as most researchers now do, that at a small-scale till fails in a plastic fashion. During such events the rate of deformation is independent of the stress regime. He suggests, however, that the net integration of multiple small-scale plastic failures is best approximated by a viscous flow law. He draws the analogy with ice, which on an atomic scale behaves in a manner similar to plastic deformation, but the net effect of individual crystal dislocations is non-linear viscous type behaviour. He argues that a non-viscous approach explains the geological products of subglacial deformation such as patterns of erosion, deposition and the formation of glacial bedforms such as drumlins. Boulton et al. (2001) responded to the points raised by Piotrwoski et al. (2001), arguing for widespread bed deformation and demonstrating how similar glacial sequences can be interpreted in very different ways.

Source: Boulton, G.S., Dobbie, K.E. and Zatsepin, S. (2001) Sediment deformation beneath glaciers and its coupling to the subglacial hydraulic system. Quaternary International, 86,3-28. Hindmarsh, R.C.A. (1997) Deforming beds: viscous and plastic scales of deformation. Quaternary Science Reviews, 16, 1039-56. Piotrowski, J.A., Mickelson, D.M., Tulaczyk, S., et al. (2001). Sediment deformation beneath glaciers and its coupling to the subglacial hydraulic system. Quaternary International, 86,3-28. Piotrowski, J.A., Mickelson, D.M., Tulaczyk, S., et al. (2001) Were deforming subglacial beds beneath past ice sheets really widespread? Quaternary International, 86, 139-50.

One potential exception to this is in cold arid environments, such as Antarctica, where ablation may occur by sublimation. Sublimation is the direct vaporisation of ice without it passing through a liquid phase and consequently ice removal may be achieved without disturbing the debris to the same degree as more conventional meltout. Sublimation can occur in both subglacial and supraglacial locations, but the focus has been on subglacial till formation via this mechanism (sublimation till; Table 8.1). For post-depositional deformation to be avoided, sublimation must occur below stagnant ice. Sublimation till may preserve some of the debris structure inherited from the ice, such as stratification and englacial fold structures.

8.1.2 Supraglacial Till

Melting on the surface of a glacier, driven by solar radiation, releases debris that can produce supraglacial meltout till (Figure 8.7). This debris may be confined to debris

Figure 8.7 Supraglacial debris on a Himalayan glacier. [Photograph: N.F. Glasser]

transported at a high-level within the glacier or alternatively may incorporate debris from basal ice if it is elevated to the surface at the glacier snout (see Section 7.4).

As the debris accumulates on the glacier surface it first accelerates melting, because dark surfaces absorb more heat than reflective ones, but then insulates the surface from further melting as the debris thickness increases. Variation in the thickness of the debris causes variation in insulation and therefore variation in the surface-ice topography. The debris becomes concentrated into ridges and mounds of buried ice. This debris is unstable and prone to slumping and surface redistribution. Due to the almost constant movement of debris on the glacier surface it rarely retains any of the characteristics of the debris-rich ice from which it is derived. Scree-like characteristics, crude bedding and downslope clast fabrics may develop due to the flow or fall of material down ice-cored slopes. Several different facies of supraglacial moraine till may be identified, depending on the thickness of the original supraglacial debris cover and upon the level of fluvial reworking (Figure 8.8). The debris on a glacier surface is commonly concentrated into ice-cored ridges, which may trace the outcrop of debris-rich structures on the glacier surface. The characteristics of a supraglacial meltout till are summarised in Table 8.1. The clast content is usually dominated by sediment typical of high-level transport (see Section 7.1), although basal debris may also be present where it has been elevated by flow compression at a glacier snout. Consequently, clasts show a broad range of characteristics, but are dominated by angular particles, with a non-spherical form and few surface striations. The size distribution is also typically coarse and unimodal with a clast fabric that is

Tension cracks in till surface

Tension cracks in till surface

Gravity sorting of partices

Englacial and subglacial melt

Englacial and subglacial melt

Tension cracks become boulder filled

Tension cracks become boulder filled

Figure 8.8 Progressive development of supraglacial meltout till with down-wasting of buried ice. Note the complex range of processes involved and the constant reworking of the sediment. [Modified from: Eyles (1979) Canadian Journal of Earth Science, 16, figure 5, p. 1348]

Figure 8.8 Progressive development of supraglacial meltout till with down-wasting of buried ice. Note the complex range of processes involved and the constant reworking of the sediment. [Modified from: Eyles (1979) Canadian Journal of Earth Science, 16, figure 5, p. 1348]

unrelated to ice flow, and generally poorly developed, although locally strong fabrics may occur where sediment has fallen or slumped, scree-like, down ice-cored slopes. Supraglacial meltout tills are also typically poorly consolidated with a low bulk density.

Supraglacial debris is often saturated and located on a constantly changing ice surface due to surface ablation. As a result, supraglacial meltout till is highly unstable and consequently subject to downslope flow. Sediment that has flowed in this way is know as 'flow till' and its properties will depend on the water content, the debris character, the surface gradient and whether the surface over which the debris is moving is composed of ice or debris (Figure 8.9). In general, the greater the water content the more fluid the debris flow (Figure 8.10). There are many types of mass movement that can re-mobilise supraglacial debris although it is possible to recognise three main types.

Figure 8.9 Large debris flow lobe (flow till) on the surface of Kongsvegen, Svalbard.

[Photograph: M.R. Bennett]

Figure 8.9 Large debris flow lobe (flow till) on the surface of Kongsvegen, Svalbard.

[Photograph: M.R. Bennett]

1. Mobile flows. These are thin, highly fluid, rapid flows, which are erosive and show crude size-sorting with coarse particles tending to settle to the base of the flow. The particles are usually strongly orientated in the direction of flow.

2. Semi-plastic flows. These are thick and slow moving tongues of debris, which are erosive. They may show size-sorting, with coarse particles settling to the base of the flow, and their upper surfaces may be sorted by the flow of meltwater. Fold structures and a weak particle orientation may develop. These types of flows usually result from the failure and flow of the downslope edge of sediment at the base of a laterally retreating ice-cored slope.

3. Creep. Slow, downslope movement of debris, which is not visible to the naked eye. This may occur either as a general non-channelised lobe of debris or as a more-or-less continuous sheet of creeping mass. Particles are rarely orientated in the direction of flow.

In practice, although it is possible to recognise different types of flow on modern glaciers, ancient flow-till deposits simply consist of numerous flow packages, of varying type stacked one on top of the other. Consequently, flow tills are characteristically very diverse in nature. The character of a unit of flow till consisting of several flow packages is summarised in Table 8.1. The clasts within flow tills show a broad range of characteristics, but are dominated by particles that are angular and have a non-spherical form. Clasts are commonly not striated or faceted. The deposit is usually dominated by sediment typical of high-level transport, but subglacially transported particles may be present. In general the size distribution is coarse and unimodal, although individual flow packages may be locally well sorted. Flow tills have a variable fabric, although individual flow packages can have strong fabrics, reflecting the former slope down which flow occurred. This type of sediment is poorly

Type IV

Type IV

Type II

Meltwater flow

Type II

Meltwater flow aa

Type I

Type I

Thin basal shear

Pushed material

Thin basal shear

Pushed material

Sediment or basal ice

Sediment or basal ice

-Supraglacial debris

-Supraglacial debris

Basal ice

Ablation scar: ■■■^-■■■■■■■■-'^mmm i Failure and mixing of ""i ice, water and sediment Flow of sediment and water '

Basal ice

Scar

Ablation scar: ■■■^-■■■■■■■■-'^mmm i Failure and mixing of ""i ice, water and sediment Flow of sediment and water '

Type II

Type III-fan

Channel Ji Levées iff possible

Depositional lobes

Type II

Scar

Type IV

Channel Ji Levées iff possible

Depositional lobes

Stacked,

Individual lobes coalesced lobes

Source area

Mid-section

Depositional area

Source area

Mid-section

Depositional area

Single or Pool of water coalesced fans and sediment

c ac

Figure 8.10 The characteristics of flow tills at the Matanuska Glacier (Alaska). (A) Variation of flow type with water content. (B) Morphology of the source and depositional area of different types of flow. [Modified from: Lawson (1982) Journal of Geology 90, figures 3, 5 & 13, p. 282,

consolidated with a low bulk density, although occasionally flow packages may be closely packed. Individual flow packages may sometimes be visible within a flow-till unit, and crude sorting within some flow packages may be present. The base of some flows may contain a concentration of larger clasts. Sorted sand and silt layers may be common, associated with reworking by meltwater on top of individual flows. Some flow packages may have erosional bases and small folds may also be present in certain flow types. In general they are highly variable and diverse sediments.

8.1.3 Distinguishing Different Types of Till

Distinguishing tills in the field can be challenging, particularly when dealing with ancient lithified tills, known as tillites, which record periods of ancient glaciation within the geological record (see Box 1.1). Deducing the process history of tillites is an important goal when studying such deposits (Table 8.1). It is worth considering a few of the key sedimentary properties and the information that they may contain, before discussing the broader issue of external facies relationships that are key to placing glacial sediments in their wider context.

The collection of clast-fabric data is one of the most traditional approaches to the description of tills. It has been used widely to infer information on ice flow, patterns of strain and as a basis for inferring till type, although its utility has been disputed by some (Box 8.2). The clast fabric of a till is defined by two properties: (i) the compass orientation of elongated particles; and (ii) the dip or angle at which those particles are inclined to the horizontal within the sediment. This information

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