CENTRAL STABLE REGION OF THE UNITED STATES 



57 



Fig. 5.18. Thickness map of the Williston and Alberta basins: Lower and Upper Mississippian 

 and Pennsylvanian. Lower Mississippian (Kinderhookian, Osagian, and Meramecian series), after 



a third evaporite sequence occurs at a still higher stratigraphic position, 

 in the top of the Jefferson. 



The Mississippian beds which rest on an erosion surface on the De- 

 vonian are marked at the base by black shale in the Williston basin. The 

 Mississippian is more restricted in the Alberta region, but the beds 

 possibly extended east at the time of deposition as far as the present 

 margin of the Canadian Shield. The beds in Alberta start with a lower 

 dark gray calcareous shale or dark brown-gray argillaceous limestone 

 with fine-grained sandstone beds in the south. The upper beds are buff 

 crystalline to dense limestones. The succession in the Williston basin 

 beginning with the Kinderhookian and Osagian strata is largely lime- 

 stone. These beds make up the Lodgepole and Mission Canyon forma- 

 tions. The Meramecian is dominated by dolomites which compose the 



Webb (1954) and Sloss (1950); Upper Mississippian (Chesterian), after Sloss (1950); Pennsyl- 

 vanian, includes Permian in Canada, after Webb (1954) and Sloss (1950). 



Charles formation. See Fig. 5.22. The Charles contains considerable 

 thicknesses of evaporites. See map, Fig. 5.18D. 



The Upper Mississippian or Chester beds lie in an east-west basin 

 through central Montana, called the Big Snowy. The eastern part of this 

 basin, however, is in the general region of the Williston basin and hence 

 it is considered part of the Williston. The strata are dominantly clastic 

 in contrast to the chemical precipitates of the Lower Mississippian and 

 compose the Kibbey, Otter, and Heath formations. Also part of the 

 overlying Amsden formation is Chester in age. 



The Alberta shelf region was emergent and suffered long-continued 

 erosion during the Pennsylvanian. In the front ranges of the Rockies, 

 however, a thin sequence of sandy dolomites and quartzitic and cherty 

 sandstones are Pennsylvanian and Permian in age, and are known as 



