Jurassic Stratigraphy of the Sweet-Grass Arch—Manitoba Section

Abstract In southern Saskatchewan and Manitoba the interval between the late Paleozoic and late Jurassic epeirogenies is represented in stratigraphic order by continental red beds, evaporites, carbonates, and clastics, all of which are assigned to the Jurassic System. The sequence is in excess of 1,400 feet thick in south-central Saskatchewan and thins to the north, west, and east. Depositional and erosional thinning are complementary and the northern limits are determined by late Jurassic to post-Jurassic truncation. The system was divided (Milner and Thomas, 1954) into the Watrous, Gravelbourg, Shaunavon, and Vanguard formations, each representing widely recognizable lithologic units. Proof of Jurassic age is available for the Gravelbourg, the Shaunavon, and Vanguard formations, which appear to represent the Bajocian to Kimmeridgian time interval. The evaporites of the Watrous formation possibly represent the lower Bajocian or earlier Jurassic stages. A minor unconformity of Middle Jurassic age is indicated by the occurrence of chert on top of the marine carbonates of the Gravelbourg. The evidence for this unconformity is strongest in central and southeast Saskatchewan. There the anhydrite of the Watrous formation thickens markedly and the evaporitic conditions appear to have lasted longer than in western Saskatchewan. In the same area the marine sediments above the unconformity thin and gradually change facies. The carbonates to the west are substituted by clastics, and coarse clastics appear in increasing amounts in an easterly direction. It would appear that a separate basinal area existed toward the end of Middle Jurassic time in Manitoba, receiving clastic sediments from the north and east. The Shaunavon formation cannot be traced lithologically in this facies district, but approximately equivalent picks can be made from electric and radiation logs. The lithologic character of the uppermost Gravelbourg, Shaunavon, and Vanguard formations in Manitoba resembles closely that of the Sundance formation in North and South Dakota. Callovian time is represented by the lower part of the Vanguard formation. A slight unconformity is recognizable between the lower and the middle Vanguard at the basin rim in western Saskatchewan. The middle Vanguard is largely marine in the basin center, but around the basin rims, brackish, or locally continental, sediments represent this member. Most of these sediments appear to be of Oxfordian age. The lithologic character of the upper Vanguard together with the fact that it contains reworked fragments of older Jurassic faunas indicates redeposition of material from the truncated basin flanks into the center. The hiatus between Upper Jurassic and Lower Cretaceous is recognizable only along the basin rims. In the deeper portions of the Jurassic basin a more gradual change from marine to brackish into continental deposition appears to have taken place between Upper Jurassic and Lower Cretaceous time. Truncation in these portions appears to have occurred later, possibly in post-Jurassic time, and, to a lesser degree than on the rims. In the basin center there is no evidence of truncation but merely of a transition from marine into brackish sedimentation.