Mudboils and rib-and-trough structures in a shallow tundra lake

Location: Southwestern Nunavut, Canada

Virtually no lacustrine sediment covers the diamicton (probably till) that forms the bottom of this small, shallow tundra lake in southwestern Nunavut, Canada. This lake has several features that are not commonly seen together. The polygonal features are apparently frost (ice wedge) polygons of a significant size. They break up a surficial cover of diamicton that is ornamented with mudboils and rib-and-trough structures (see discussion for image 0120). From this image, I conclude that the lake is less than 2 to 3 meters deep throughout, at least during freeze-up in the fall, that at some point in the winter and its ice cover is in contact with the bottom throughout so that the frost cracks can form. I also conclude that permafrost underlies the lake; when the ice melts in the spring, bottom waters can enter the frost crack fissures, freezing and keeping them open. During the thaw season, because of the lack of normal fine-grained lacustrine sediment cover, the diamicton is deformed by rib-and-trough and mudboil processes. Note that fine-grained sediment has accumulated in the depressions over the ice wedge fissures. See image 0129 for a more-distant view of this lake. See detailed discussion for image 0240.

Updated 05/06/2010 AW