Abstract
This was a study of subglacial deformation till genesis from a modern temperate glacier, at Skálafellsjökull, Iceland. Detailed microscale properties of till samples (from Scanning Electron Microscope [SEM] and thin section analysis) were examined from a glacial site with in situ subglacial process monitoring and an exposed subglacial surface in the foreland. Two lithofacies were examined, a grey sandy till derived from the ash and basalt, and a silty reddish brown till derived from oxidized paleosols and/or tephra layers. These also represented a clay-content continuum from low (0.3%) to high (22.3%). The evolution from debris to subglacial till was investigated. This included a reduction in grain-size (21% for grey lithology, 13% reddish brown lithology), and reduction in rounding (RA) (32% for the grey lithology, 26% for the reddish brown lithology), and the quantification and analysis of the different grain erosion/comminution processes in the resultant till. It was shown that the microstructures within a till were dependent on shear strain and glaciological conditions (deformation history). The low clay content tills were dominated by linear structures (lineations and boudins, and anisotropic microfabric) whilst the higher clay content tills were dominated by rotational structures (turbates and plaster, and isotropic microfabric). These results are important in our understanding of the formation of both modern and Quaternary tills and informs our reconstruction of past glacial dynamics.
Highlights
Temperate glacier motion is due to creep, sliding and subglacial deformation; and so subglacial hydrology and sediment deformation play a key role in modulating glacier behaviours (Boulton and Jones, 1979; Murray and Dowdeswell, 1992)
[SK18, SK19] and B3 [SK20, SK21]; those that are predominately silt); and those that have a range of grain sizes
Allowing for the complexities discussed above, our results show the reduction in grain size from englacial debris to till for the grey lithology was at least 21% and for the reddish-brown lithology was at least 13%
Summary
Temperate glacier motion is due to creep, sliding and subglacial deformation; and so subglacial hydrology and sediment deformation play a key role in modulating glacier behaviours (Boulton and Jones, 1979; Murray and Dowdeswell, 1992). Understanding the subglacial environment must include a combination of in situ subglacial experiments (Fischer and Clarke, 2001; Murray and Porter, 2001; Hart et al, 2011a), geophysical and geodetic survey (Woodward et al, 2003; Smith et al, 2007; Hart et al, 2011b), geotechnical experiments (Kamb, 1991; Iverson et al, 1998; Sane et al, 2008; Altuhafi et al, 2009; Iverson, 2010) and sedimentology (Hart and Rose, 2001; Evans et al, 2006; O Cofaigh et al, 2011; Trommelen et al, 2014) Such studies have illustrated that the processes within the subglacial environment can vary rapidly on both a spatial and temporal scale, with 20e85% of glacier motion occurring in a subglacial deforming layer (typically to 0.3 m depth) (Truffer et al, 2000; Boulton et al, 2001; Cuffey and Paterson, 2010). The resultant till is a mixture of all these processes
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