And Deposits In The Continuous Permafrost Zone

The thick, continuous permafrost of northern Canada either survived under cold-based Wisconsinan ice, formed under the marginal fringe of the receding ice sheet, formed immediately after release from a warm-based ice margin, or formed after emergence from the sea or after drainage of glacial lakes. In all these cases, because the basal ice and water temperatures were necessarily higher than current mean annual air temperatures, the permafrost would have thickened postglacially towards equilibrium with air temperature. Permafrost aggradation was most rapid at the start and slowed exponentially thereafter (see below). In other words, it is safe to assume that permafrost has never disappeared during postglacial time within the zone of continuous permafrost, except in taliks due to local water bodies.

If we accept these conclusions, certain interpretations commonly applied to landforms and deposits south of the permafrost zone, and sometimes within the permafrost zone, are not tenable within the zone of continuous permafrost. For example, kettles in permafrost cannot result from the melting out of deeply buried ice blocks. In the continuous permafrost zone, a kettle can only form upon the melting of exposed ice that is clean or nearly clean. Because the equilibrium base of permafrost is hundreds of metres below the kettle and because permafrost aggraded rapidly in early postglacial time, the ice block could only have melted from the surface downward, and melting was arrested as soon as a surface debris cover equal to the active layer thickness had accumulated. In the continuous permafrost zone, kettle-like basins are prominent features of ridged and hummocky moraine belts (e.g. Fyles, 1963; Dyke and Savelle, 2000; Dyke and Hooper, 2001), typically occupying 10—30% of the surface area, in places up to 50%. Indeed, they are essentially restricted to such settings. Exceptionally, these basins are floored by bedrock, the kettle penetrating entirely through the moraine. Many others penetrate much of the apparent moraine thickness, and others are shallow. Kettles also form basins at the heads of outwash and in ice-contact deltas, but rarely in distal outwash. They have a wide variety of shapes and sizes, with elongations imparted only by bounding or adjacent ice-contact features. Depth seems roughly proportional to area. Similar basins in similar geological contexts south of permafrost are also interpreted as kettles.

These observations demonstrate that kettle-like basins are characteristic of ice-marginal accumulations in the zone of deep, continuous permafrost. The common interpretation of similar kettled terrain south of permafrost is that the kettling and associated processes occurred well after, often millennia after, deglaciation with the slow removal of ground ice. Some of these southern kettles probably did form in this way, but we suggest that many, particularly the large and deep ones, are essentially contemporaneous with deglaciation. Kettled terrain in both the south and the north is also typically interpreted as 'dead ice' or ice stagnation terrain. However, in the continuous permafrost zone, kettle-like basins are essentially restricted to the prominent morainal belts. If kettles signify stagnation, an inference with which we disagree, then most of the major end moraines of the northern Laurentide Ice Sheet were formed by stagnation. Below we develop the alternative case that these moraine belts were formed by active ice margins.

Permafrost imposes the further restriction that basal meltout till, characterized by the preservation of delicate englacial debris-rich ice structures due to slow basal melting (Shaw, 1982), cannot form. Such till can only form in the continuous permafrost zone if basal melting of permafrost progressed to the surface, passing up through the entire thickness of the putative buried or stagnant ice. There is no evidence that such an event ever occurred and every reason to believe that instead permafrost thickened rapidly in early postglacial time. Therefore, interpretations of glacial landform genesis within the zone of continuous permafrost that start with the inference that the features are comprised wholly or partly of basal meltout till (e.g. Sharpe and Nixon, 1989) are untenable.

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