Debris entrainment and transfer in polythermal valley glaciers

Abstract

Modes of debris entrainment and subsequent transfer in seven “normal” and five surge-type glaciers in Svalbard (76–79° N) are outlined in the context of the structural evolution of a glacier as the ice deforms during flow. Three main modes of entrainment and transfer are inferred from structural and sedimentological observations: (i) The incorporation of angular rockfall material within the stratified sequence of snow/firn/superimposed ice. This debris takes an englacial path through the glacier, becoming folded. At the margins and at the boundaries of flow units the stratified ice including debris is strongly folded, so that near the snout the debris emerges at the surface on the hinges and limbs of the folds, producing medial moraines which merge towards the snout. The resulting lines of debris are transmitted to the proglacial area in the form of regular trains of angular debris. (ii) Incorporation of debris of both supraglacial and basal character within longitudinal foliation. This is particularly evident at the surface of the glacier at the margins or at flow unit boundaries. It can be sometimes demonstrated that foliation is a product of strong folding, since it usually has an axial planar relationship with folded stratification. Foliation-parallel debris thus represents a more advanced stage of deformation than in (i). Although the presence of basal debris is problematic, it is proposed that this material is tightly folded ice derived from the bed in the manner of disharmonie folding. The readily deformed subglacial sediment or bedrock surface represents the plane of décollement. (iii) Thrusting, whereby debris-rich basal ice (including regelation ice) and subglacial sediments are uplifted into an englacial position, sometimes emerging at the ice surface. This material is much more variable in character than that derived from rockfalls, and reflects the substrate lithologies; diamicton with striated clasts and sandy gravels are the most common facies represented. Thrusting is a dynamic process, and in polythermal glaciers is probably linked mainly to the transition from sliding to frozen bed conditions. It is not therefore a solely ice-marginal or proglacial process.

Quantification of the ice-cored moraines' short-term dynamics in the high-Arctic glaciers Ebbabreen and Ragnarbreen, Petuniabukta, Svalbard

“…Boulton, 1967Boulton, , 1968Boulton, , 1970aBoulton, , b, 1972Bennett et al, 1996Bennett et al, , 2000Hambrey et al, 1997;1999;Hambrey, 2001, 2003;Lyså and Lønne, 2001;Sletten et al, 2001;Lønne and Lyså, 2005;Midgley et al, 2007Midgley et al, , 2013Ewertowski et al, 2012). Quantitative studies of Svalbard glaciers, however, have concentrated mainly on transformations in glacier geometry, dynamics and mass balance (e.g.…”

Satujökull glacial landsystem, Iceland

“…This assemblage reveals that, when the glacier occupied its Little Ice Age maximum limit, large scale sub-marginal debris entrainment created a wide belt of controlled moraine (Evans, 2009) at the snout, while subglacial deformation produced a fluted till further up ice (Benn, 1994). Such an assemblage is indicative of a polythermal snout (Hambrey et al, 1999;Evans, 2009), as would be expected for a high altitude plateau icefield in Iceland (Evans, 2010).…”

Structure and sedimentology of George VI Ice Shelf, Antarctic Peninsula: implications for ice-sheet dynamics and landform development

“…Furthermore, both the George VI and McMurdo ice-shelf moraines are different from those produced by terrestrial glaciers, which are the product of a variety of processes, such as thrusting (e.g. Lønne & Lauritsen 1996;Hambrey et al 1997Hambrey et al , 1999Bennett 2001), delivery of debris to an active margin (e.g. Lukas 2005;Bennett & Glasser 2009;Benn & Evans 2010) or ice stagnation (Evans 2003).…”

“…Recumbent folding at the base of a glacier (Hudleston 1976) and upright folding with flowparallel axes (Hambrey et al 1999) are common processes for elevating basal debris in polythermal ice masses. Given the relationships between the two foliations, it seems that the second process is the most plausible for delivering Palmer Land debris to Alexander Island: basal debris was folded within stratified ice in a grounded zone of flow convergence, becoming aligned parallel to S 1 and then transposed into the plane of foliation S 2 as the ice became compressed against Alexander Island.…”