The Cenomanian-Santonian Colorado Formations of Eastern Southern Saskatchewan and Southwestern Manitoba

The Cenomanian–Santonian Colorado formations of eastern southern Saskatchewan and southwestern Manitoba comprise (from oldest to youngest), (1) the Belle Fourche Formation (predominantly greyish black shale with basal “Fish Scales” zone, medial quartzose Okla Sandstone and upper Ostrea calcarenite, bentonite and black shale), (2) the Second White Specks or Favel Formation (Keld and Assiniboine members) of coccolithic and shell calcarenite with subordinate marlstone and shale, (3) the Carlile Formation, inclusive of the Morden black shale and Boyne repetitive sequences of coccolithic and shell calcarenite with subordinate marlstone and shale, and (4) in eastern Saskatchewan, the erosional remnant of the Santonian argillaceous Niobrara Formation of western Saskatchewan under the Campanian Milk River (Gammon) Formation. All units below the Niobrara crop out in the bluffs of the Manitoba escarpment in the east of the study area and are traceable westward through cored and geophysical well log sections. Current usage in Saskatchewan treats the ‘Fish Scales’ as a formation separate from the Belle Fourche. In Manitoba it is a basal marker bed of the Belle Fourche, and, in view of its less than 10 m thickness, is treated as such in this study. The Belle Fourche upper contact is drawn on the ‘X’ Bentonite, which in this study is taken as the uppermost of a series of thin bentonites above the Ostrea beloiti beds. The Okla Sandstone is prominent above the Fish Scales zone in the northern part of the study area and is developed as graded laminae of fine grained, quartzose sand and black shale. The Belle Fourche shale is thickest in southwestern Manitoba, and is transitional through the ‘X’ Bentonite bed into the Keld Member of the Favel formation by means of upward-shallowing sequences of black, noncalcareous shale, calcareous shale and coccolithic calcarenite. The Keld Member can be divided into three downlapping sequence sets which appear to be imbricate clinoforms that attenuate to the west and northwest in Saskatchewan. The basal set, Keld A, is capped by a subaerial unconformity in the east and a patterned hardground in the west. Keld B and C are likewise differentiated on the basis of sharp, irregular contacts as well as by changes in depositional pattern. The Assiniboine Member is recognized by its multiple, sharp-based, thin sequences of bituminous black shale, inoceramid-dominant bioclastic limestone with intercalated bentonites. It is partially truncated to the north and west in Saskatchewan under the black shale of the Morden Member of the Carlile Formation. The Morden is thickest in the eastern half of the study area, but attenuates westward into the lower third of the dark grey, argillaceous Carlile Formation of western Saskatchewan, and everywhere is in erosional unconformity on the Second White Specks Formation. Conversely, the Boyne parasequence sets of coccolithic calcarenite and marlstone thicken westward, but likewise grade into the dark grey shale of the western Carlile Formation. In the study area, the Morden – Boyne pair is a stratigraphic and sedimentological re-iteration of the underlying Belle Fourche–Second White Specks (Favel) parasequence sets. The Turonian Carlile Formation on a west to east transect, underlies the Santonian Niobrara Formation in the extreme west of the study area, and, toward the east, the Campanian Milk River (Gammon) and Lea Park (Pierre) formations by virtue of their off-lapping basal unconformities.

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