STOP #5: West Hawk Lake Impact Crater

West Hawk Lake has long held the reputation as one of the deepest lakes in Manitoba, (112 m). This fact, coupled with the roughly circular shape, suggested to geologists the possibility that the depression in which the lake rests may be an old meteorite crater. Detailed studies of rocks around the margin of the lake, and core from three holes drilled from the ice to a maximum depth of 727 m, indicate a meteorite impact genesis. The evidence is as follows:

1. Below the water in the lake basin there is a 94 m thick accumulation of unconsolidated sediments resting on top of bedrock. This means that the actual depression is over 205 m deep. furthermore the depression is closed and would not be produced by normal erosional processes.

2. The bedrock below the lake and the unconsolidated sediments consists of a badly broken up and disturbed zone of "fall back breccia", 330 m thick, and beneath that a zone 200 m thick of bedrock containing fractures. These zones are characteristic of known meteorite impact craters.

3. The broken bedrock is characterized by a number of mineralogic, crystallographic and textural changes that are produced by instantaneous application of high stress. These are called shock metamorphic features and are undoubtedly produced by impact.

The original crater was circular, about 3700 m in diameter and about 1500 m deep. The diameter of the meteorite required to produce such a crater has been estimated at 85 m; the velocity has been estimated at 20 km/sec. The energy released on impact would have been equivalent to a 25 megaton explosion. Such energy would generate sufficient heat to cause the meteorite to vaporize.

The age of the impact crater is problematic. The unconsolidated sediments that have accumulated in the crater do not contain fossil remains. They resemble sediments elsewhere in Manitoba that are 60 million years old, suggesting that the crater is older. On the other hand, the crater is devoid of rocks we would expect to find there if it existed 180 million years ago. Our best estimate at present is about 100 million years.