The Jurassic Subsurface in Southern Alberta

Abstract Sediments of Middle Jurassic and Upper Jurassic age are present in the extreme southern plains and in the foothills and mountain areas of Alberta. On the plains the northern boundary of the Jurassic is roughly crescentic in shape, approximately coinciding with the South Saskatchewan River and its tributary, the Oldman River, and from there trending northwestward, passing a few miles east of Calgary. On the Alberta plains, Jurassic strata are readily divisible into three formations, which were originally described and named in Montana, and which are, in ascending order, Sawtooth, Rierdon, and Swift. In the Alberta foothills, Jurassic strata are called the Fernie group, which is roughly the equivalent of the three formations of the plains. Jurassic strata in southern Alberta rest unconformably on the Rundle formation of Mississippian age. On the plains they are overlain unconformably by the basal sandstones of the Blairmore formation which have been correlated with the Cutbank and Sunburst sands of Montana. Post-Jurassic erosion truncated the formations so that they wedge out northward; consequently, the Swift being the uppermost has a very limited distribution in Alberta, having been eroded completely from the crest of the Sweetgrass arch and remaining only in the extreme southeastern corner of Alberta and in a narrow belt paralleling the foothills. The Rierdon and Sawtooth formations extend northward approximately to the limits of the Jurassic as defined above. In the western foothills the Fernie is overlain conformably in most places by Kootenay sandstone and shale of Lower Cretaceous age. The Sawtooth formation consists of two sandstone members, separated by green, pyritic, non-calcareous shale. The upper sand or its equivalent is a reservoir for oil and gas in several small fields in Alberta. The formation has a maximum thickness of about 235 feet. The Reirdon formation is made up of alternating gray calcareous shale and greenish-gray limestone with some pyrite, glauconite, and ironstone, and has a maximum thickness of 250 feet. The Swift formation lies unconformably on the Rierdon and has a maximum thickness of 150 feet. It consists of glauconitic sandstone, siltstones, concretions, dark gray shale, and generally has chert pebbles at the base. Facies change from west to east in the Fernie group of the foothills to the Ellis group of the southern Alberta plains; but the changes are not so pronounced as to preclude the making of satisfactory correlations. On the east flank of the Sweetgrass arch the Ellis group can be readily correlated with the Gravelbourg, Shaunavon, and Vanguard formations of Saskatchewan. However, they are not the exact equivalents of these formations as some overlapping occurs.