Glacial debris release and moraine deposition

Exposure of debris-rich basal ice in many Canadian High Arctic subpolar glaciers during snout downwasting results in the formation of controlled moraine (Fig. 19.2). Preservation potential of these moraines is low, however, owing to sediment redistribution during melt-out. Hummocky till veneers interspersed with glacifluvial outwash occur on valley floors where piedmont glaciers have receded onto surrounding uplands, leaving buried glacier ice at lower elevations. Where debris accumulates by rockfall below bedrock cliffs, this process may produce supraglacial lateral moraines, medial moraines and occasional ice-cored rubble cones.

At the margins of plateaux icefields and upland outlet glaciers, debris turnover is low and moraines are rare. This is reflected by the occurrence of trimlines formed of lines of boulders or rubble veneers. However, the major outlet glaciers that occupied the fjords and valleys of the region during the last glaciation deposited till sheets and extensive lateral moraines. This is predominantly a function of the thermal characteristics of larger glaciers, the soles of which reached pressure melting point.

Thrust-block moraines are formed by proglacial glacitectonic thrusting of glacifluvial, glacilacustrine, and emergent glacima-rine sediments on valley floors where compressive stresses in the glacier snout are transmitted to unconsolidated sediments (Fig. 19.2). Thrust-block moraines of the region typically comprise relatively intact blocks of sediment displaced in en echelon arcs. Individual blocks are commonly tens of metres high, hundreds of metres wide, and, en echelon, may be hundreds of metres long. Bedding in the displaced blocks generally dips back towards the glacier snout, suggesting that they are imbricately stacked blocks partially rotated during thrusting. However, some moraines comprise blocks with bedding dipping away from the snout, indicating that the glacier was responsible for deep-seated wedging of proglacial material (Evans & England, 1991). Thrust blocks can

Figure 19.1 Location map of the Queen Elizabeth Islands, Canadian High Arctic and contemporary ice cover.

Figure 19.2 Landsystems model for subpolar glaciers (plateau icefields and piedmont lobes) of the Canadian High Arctic: (A) blockfield/residuum; (B) thrust-block moraine; (C) ice-cored lateral moraine; (D) trimline moraine; (E) glacier-ice-cored protalus rock glacier; (F) periglacial protalus rock glacier; (G) raised, former ice-contact deltas; (H) lateral meltwater channels.

Figure 19.2 Landsystems model for subpolar glaciers (plateau icefields and piedmont lobes) of the Canadian High Arctic: (A) blockfield/residuum; (B) thrust-block moraine; (C) ice-cored lateral moraine; (D) trimline moraine; (E) glacier-ice-cored protalus rock glacier; (F) periglacial protalus rock glacier; (G) raised, former ice-contact deltas; (H) lateral meltwater channels.

act as a source for later sediment entrainment during glacier overriding. This often gives rise to controlled moraine ridges being superimposed on the inner thrust blocks during glacier downwasting.

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