ABSTRACT

The Western Canada Sedimentary Basin contains two major features: 1) a thick western section stretching from Montana to the southern Northwest Territories, and 2) the Williston Basin, centered in North Dakota. The Phanerozoic wedge, clearly more than 6 km thick at the mountain front, thins northeastward above the crystalline Shield. Deformation of the Foreland Thrust Belt (FTB) affected strata varying in age from Helikian to Tertiary. The sedimentary wedge produced loading, subsidence, and a series of geographically different depocentres.

The Peace River Arch and Western Alberta Ridge were topographically high features in the Devonian, but the former became a basin in the Early Carboniferous. Two other ridges are discussed: the Tathlina Arch and the ill-defined axis between the Alberta and Williston basins (often called the Sweetgrass Arch, which is best seen south of Alberta where it was a positive element in the Paleozoic and the Cenozoic).

One dramatic feature in northeast British Columbia and the Northwest Territories is a fault complex, which shows over 1000 m of vertical displacement at the Devonian level. Other features include the Northern Rocky Mountain Trench, with over 400 km of Cretaceous-Tertiary dextral displacement; and the northeast trending Great Slave Lake Shear Zone between the Peace River Arch and the Tathlina Arch, with up to 700 km of displacement, again dextral.

Other features present include salt solution and reef compaction phenomena. Muskeg/Prairie Evaporite salt has been dissolved around reefs; and solution is also evident at the sub-Mesozoic unconformity.

Compaction associated with reefs is present at several horizons and can be mapped residually.

Sections show the estimated shortening across the FTB (150 km in southern Alberta). However tectonic style, grain, thrust sheet thickness and displacement within, and adjacent to the FTB, change to the north. Details of the geology in this western deep frontier will become better understood as deeper exploration continues with the expanding demand for natural gas. One question to be resolved is the relationship between ancient axes and currently visible structures.

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