Sinkhole near Norman Wells, NWT

This sinkhole is approximately twenty miles southeast of Norman Wells, NWT. I took the following photos Sep 2, 2000. These photos are on my website as I thought it was quite interesting. I'd also like an explanation of what might have caused this. It's quite hard to get a good estimate of the size from the air but I'd guess that the sinkhole would be about 150 yards long and 75 yards wide. Note that this is very close to the tree line so the trees are quite small.

From NWT govt Oil and Gas Geology this terrain could be "Middle Devonian carbonates, Ordovician to Silurian carbonates, Cretaceous sandstones."

Note that this region has lots of permafrost. Where the ground is permanently frozen. All the buildings in town are 3 or more feet off the ground. Otherwise the heat from the building would thaw the ground and the building would gradually sink. Worse parts of the building would sink more than other parts.