Permafrost Features under Arctic Lakes, District of Keewatin, Northwest Territories

In the District of Keewatin, west of Hudson Bay, numerous shallow lakes occupy depressions on the perennially frozen, glaciated terrain north of the treeline. Many lakes in the vicinity of Kaminak Lake have extensive shallow areas that are characterized by features of probable periglacial origin. Some features, such as polygonal patterns, frost-heaved boulders, and mudboils, are similar to those of the subaerial landscape.Digitate, cobble-covered ribs and boulder-filled troughs that commonly form a crenulate pattern on the shallow shelves adjacent to till-covered shores are thought to be the subaqueous equivalents of mudboils that are common on the adjacent till plains. They are composed of till and are underlain by an undulating frost surface that is raised beneath troughs and depressed under ribs.Holes with or without raised rims often occur singly or in clusters on loose, sandy silt bottoms in water depths less than 2.5 m. A frost table underlies the bottoms of these holes at depths of 30 to 50 cm in early August. Holes may be either sites of strudel scour or sites of final points of attachment of winter ice to the frost table, just before the buoyancy of the ice caused it to break free from the bottom in the spring, extracting frozen sediment from the surrounding unfrozen sediment, leaving a hole.The features described are restricted to water depths that are probably equivalent to the average maximum thickness of winter ice. Thus, they represent areas where the lake is seasonally frozen to the bottom, and may be restricted to portions of lake basins underlain by perennially frozen ground.