William George Adam

School of Physical and Geographical Sciences, Keele University, Keele, Staffordshire ST5 5BG, UK

Many interpretations of glacial sediments to reconstruct the basal ice layers of former glaciers have focused upon melt-out tills and glacigenic sediment flows, relying on the preservation of diagnostic glacially derived structures (e.g. Lawson, 1979b; Ham & Mickelson, 1994). This approach can be strengthened if the interpretation did not rely solely upon the preservation of diagnostic glacial structures but was coupled with another technique. For example, Hambrey et al. (1999) have provided evidence that a sedimentological difference may exist between basal ice facies at glaciers in Svalbard, using the technique of displaying the RA (aggregate roundness) and C40 (shape) indices on a covariant plot (Benn & Ballantyne, 1994). Knight et al. (2000) suggested that

Figure 88.1 Covariant plot of C40 and RA indices with samples from stratified facies, debris bands and the ice-marginal moraine, Russell Glacier, western Greenland.

Co-variant plot of C40 and RA indices, Russell Glacier.

A Debris Band ■ Stratified Facies A Moraine Debris Bands □ Moraine Stratified Facies

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particle-size differences between debris samples from different basal ice facies at Russell Glacier, west Greenland, were transferred to and preserved in the ice-marginal moraine, so that the distribution of particle-size characteristics in moraine sediments could be used to reconstruct patterns of subglacial thermal regime and ice flow of former glaciers.

Adam & Knight (2003) applied Benn & Ballantyne's (1994) method to identify another sedimentological distinction between basal ice facies at the Russell Glacier using covariant plots of aggregate roundness (RA index) and aggregate shape (C40 index) of samples from different ice facies and adjacent moraines. Multiple samples of 50 pebbles each were taken from the stratified facies, from debris bands, from the moraine where the stratified facies was the dominant source of sediment supply and from the moraine where the debris bands were the dominant source of supply. Figure 88.1 displays the covariant plot of the C40 and RA indices with samples from the stratified facies, debris bands and samples from the moraine at the site referred to by Adam & Knight (2003). The results from the Russell Glacier indicate that there is both a particle-size difference and a morphological difference in clasts between basal facies, which are transferred to and preserved in the ice-marginal moraine.

Observations at Svinafellsjokull, an outlet glacier of the Vatnajokull ice cap, also provide insight into sedimentologically distinctive characteristics within the basal layer. This research was conducted to test Adam & Knight's (2003) conclusions. A covariant plot was built from sample points taken from the strat ified facies, from the debris bands and from the moraine. However,the resultant covariant plot shows that there is no significant difference recorded in the pebble-sized debris between the stratified facies and debris bands at Svinafellsjokull. The C40 index values of the stratified facies and debris bands range from 8 to 24 and the RA index values range from 4 to 22. Figure 88.2 indicates a complete morphological overlap between the two facies. Figure 88.3 displays the relative proportions of different particle sizes in the stratified facies, debris bands and the dispersed facies basal layers at Svinafellsjokull. The stratified facies is sedimentologically distinctive in the amount of fine-grained material it contains, with almost 60% of its total volume comprising silt-sized debris, but almost 30% of the dispersed facies debris is also of silt-size. Therefore when examining debris from the moraine it is not possible to ascertain simply by the presence/absence of fine-grained material what basal ice layer the sediment originates from.

It is because there is no distinctive particle-size marker in any one facies that the particle-size distribution cannot be used as an interpretive tool at Svinafellsjokull as it was at Russell Glacier. The morphology of the pebble-sized debris at Svinafellsjokull also fails to display the contrast found in the stratified facies and debris bands at the Russell Glacier. Only if distinctive sedi-mentological characteristics in the basal ice layer are transferred to and preserved in ice-marginal moraines can a sedimentologi-cal interpretation of basal ice and hence subglacial environments be built.

Co-variant plot of C40 and RA indices, Svinafellsjokull.

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A Debris Band ■ Stratified Facies A Moraine Debris Bands ° Moraine Stratified Facies

Figure 88.2 Covariant plot of C40 and RA indices with samples from stratified facies, debris bands and the ice-marginal moraine, Svinafellsjokull, Vatnajokull ice cap, southern Iceland.

Figure 88.3 Relative proportions of different particle sizes in the stratified, debris band and dispersed basal ice facies.

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