Debris entrainment and transfer in polythermal valley glaciers

Modes of debris entrainment and subsequent transfer in seven "normal" and f ive surge-type glaciers in Svalbard (76-79 degrees N) are outlined in the c ontext of the structural evolution of a glacier as the ice deforms during f low Three main modes of entrainment and transfer are inferred from structur al and sedimentological observations: (i) The incorporation of angular rock fall 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 inclu ding debris is strongly folded, so that near the snout the debris emerges a t the surface on the hinges and limbs of the folds, producing medial morain es which merge towards the snout. The resulting lines of debris are transmi tted to the proglacial area in the form of regular trains of angular debris . (ii) Incorporation of debris of both supraglacial and basal character wit hin 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 usuall y has an axial planar relationship with folded stratification. Foliation-pa rallel debris thus represents a more advanced stage of deformation than in (i). Although the presence of basal debris is problematic, it is proposed t hat this material is tightly folded ice derived from the bed in the manner of disharmonic folding. The readily deformed subglacial sediment or bedrock surface represents the plane of decollement. (iii) Thrusting, whereby debr is-rich basal ice (including regelation ice) and subglacial sediments are u plifted into an englacial position, sometimes emerging at the ice surface. This material is much more variable in character than that derived from roc kfalls, and reflects the substrate lithologies; diamicton with striated cla sts 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.