Figure WCSB-1. Western Canada Sedimentary Basin showing thickness of sedimentary strata above Precambrian basement.
A sedimentary basin such as the Western Canada Sedimentary Basin is a large geological feature comprised of thick sedimentary deposits. It is a repository for sedimentary rocks, many of which were deposited above sea level or in shallow seas but are now found at considerable depth as a result of subsidence of the crust that accompanied sedimentation. The Western Canada Sedimentary Basin forms the northern portion of a much larger basinal feature that extends from the Gulf of Mexico to the mouth of the Mackenzie River. Geographically, it underlies the interior plains physiographic province and extends from the eastern edge of the Canadian Rocky Mountain system to the southwestern margin of the Canadian Shield. In simplest terms, the Western Canada Basin may be regarded as a wedge of sub-horizontal sedimentary strata above Precambrian crystalline basement. This wedge has a maximum thickness of about 5,500 m along the edge of the Rocky Mountain Foothills, and generally thins northeastward to a zero-edge along the margin with the Canadian Shield (see Figure WCSB-1). This simple wedge is modified by a thickening to about 3,300 m in the Williston Basin in southern Saskatchewan. There is marked thinning over a major structure in southern Alberta (Sweet Grass Arch).
Consolidated rocks that comprise the sedimentary wedge range in geological age from Cambrian to late Tertiary. The Cambrian strata were formed about 550 million years ago and rest on older deformed crystalline rocks of Precambrian age. The youngest consolidated rocks were deposited about five million years ago. Much of the upper surface of the sedimentary wedge is masked by unconsolidated glacial drift laid down by continental ice sheets during the Pleistocene Epoch (about two million to 11,000 years before present). Western Canada Sedimentary Basin strata can be referred to two broad divisions that reflect contrasting geological conditions: the lower succession of Cambrian to Jurassic age (to about 350 million years before present) is composed largely of carbonate rocks, with an important component of evaporite minerals (anhydrite, halite and potash), and was formed before the major uplift of the Canadian Cordillera; the upper succession of mid-Jurassic to Tertiary age, which consists mainly of shale and sandstone, was deposited following major mountain building and uplift in the Cordillera.
Laurence Vigrass
