Icefacies formation and types

As indicated above, the characteristics of glacier ice are known to vary significantly through individual glaciers in ways that affect the value of flow parameter A in Equation (2), and hence the response of the ice to applied stress. The most systematic occurrence of significantly different ice within glaciers occurs in the basal zone, because the interaction of the glacier with the bed can lead to the development of layers near the bed of distinctive regelation ice, and to metamorphism of overlying meteoric ice (Weertman, 1957). Regelation ice is formed by melting and refreezing of the ice in contact with the bed, owing to local changes in the melting point resulting from stress concentrations (Fig. 63.1). The melting process allows liquid water to interact with material at the bed, and therefore to acquire a distinctive ice flow

Erosional unconformity

Regelation

Figure 63.1 Schematic diagram illustrating the process of formation of distinctive basal regelation ice by interaction of the glacier at its basal boundary with bed roughness elements. This process, which contributes to basal motion, is known as Weertman (1957) regelation, and is probably most effective for bed roughness elements with dimensions 10—1-100m. It produces layers of basal ice with distinctive physical and chemical characteristics.

Figure 63.1 Schematic diagram illustrating the process of formation of distinctive basal regelation ice by interaction of the glacier at its basal boundary with bed roughness elements. This process, which contributes to basal motion, is known as Weertman (1957) regelation, and is probably most effective for bed roughness elements with dimensions 10—1-100m. It produces layers of basal ice with distinctive physical and chemical characteristics.

chemical signature, and the refreezing process allows debris to be entrained into this chemically enriched ice.

As a result of these processes, thick layers of distinctive ice are often found in the basal zones of glaciers and ice sheets in both temperate and cold conditions (Hubbard & Sharp, 1989; Knight, 1997). Temperate glaciers, in particular, can have basal ice layers that make up a significant proportion of the total depth of the ice. Hubbard et al. (2000a), for example, found basal ice layers up to 14.5 m thick in ice with a total depth of 45 m at Tsanfleuron Glacier in the Swiss Alps. Fisher & Koerner (1986) identified a basal layer 8 m thick of ice with enhanced solid impurity content in an ice core 340 m deep in the Devon Ice Cap in Arctic Canada. Simoes et al. (2002) reported a layer 89 m thick near the base of the Vostok core containing entrained bedrock particles.

In terms of dynamic response, the fact that ice with significantly different characteristics occurs in the basal zone—rather than anywhere else in the glacier system—is particularly important because it is the basal zone where the driving shear stress is greatest, and also where local stress concentrations occur due to bed roughness.

In order to simplify analysis of ice types, and especially in order to characterize complex sequences of apparently chaotic basal ice, various typologies of ice facies have been developed. Lawson's description and classification of ice types at the terminus of the Matanuska Glacier (Lawson, 1979b) was the first application of a systematic stratigraphical facies approach to ice types. He denoted the upper horizons of clean meteoric glacier ice formed by snow metamorphism as englacial ice facies, and distinguished between two basal ice facies. The basal dispersed facies, usually found stratigraphically between upper englacial facies and lower stratified facies, had fewer bubbles and higher sediment concentration than englacial ice. The stratified facies, of which there were three subfacies, was typically strongly layered, and had significantly higher debris concentrations than dispersed-facies ice.

Lawson's non-genetic facies approach to ice classification, and its three basic ice facies, has been widely adopted, with slight variations. Hubbard & Sharp (1995), for example, distinguished seven facies in the western European Alps, although several of their facies might be considered subfacies (Knight, 1997). In some environments, local names have been used: for example, Holdsworth (1974) denoted a basal ice facies with debris and bubble characteristics like those of Lawson's dispersed facies as 'amber ice' because of its distinctive colour, and this term continues to be used (e.g. Fitzsimons et al., 2000).

Recently, there has been a move towards genetic classifications of basal ice facies, although there is often a strong overlap between genetic interpretations and purely stratigraphical typographies. Knight (1997), for example, distinguished two genetic basal ice facies, the first comprising ice-debris mixes entrained at the bed in various ways. In a non-genetic classification, this would include stratified ice and its subtypes. The second comprises ice that has been affected by metamorphic processes near to the bed, and is basically dispersed facies ice. In terms of mechanisms of emplacement, the significance of tectonic effects in determining facies assemblages is also becoming increasingly clear. Waller et al. (2000), for example, outlined the way in which post-accretion glaciotectonism had affected basal ice facies at the Russell Glacier in Greenland, such that inferences about subglacial conditions based on a twofold genetic approach to facies classification would be problematic.

Essentially, then, there are three main ice-facies groups. The majority of a glacier is comprised of meteoric englacial ice (Fig. 63.2). This ice may have varying crystallographic and ice-fabric characteristics, but physically and chemically is relatively homogeneous. In the basal zone, there may be ice adjacent to the bed that has been significantly affected by its interactions with the bed in various ways, and this is called 'stratified ice' (Fig. 63.2). Between the englacial ice and underlying stratified ice is ice with some debris but less bubbles than meteoric ice, referred to as dis-persed-facies ice.

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