CENTRAL STABLE REGION OF THE UNITED STATES 



61 



By Jurassic time the rise of the Cordilleran geanticline had become ex- 

 tensive (see Plate 10 of Chapter 3), and considerable sediment was 

 shed from it eastward to the subsiding areas of accumulation. Part of 

 the geanticline became engrossed in major mountain building in Early 

 Cretaceous time, and this, The Nevadan orogeny, resulted, in British 

 Columbia, in the uplifting, disruption, and widespread intrusion of the 

 sedimentary rocks of the Cordilleran geosyncline. A new restricted trough 

 or longitudinal basin formed, as shown in Figs. 5.19 and 5.20, in about 

 the position of the present Canadian Rockies. The Nevadan Orogeny 

 engrossed the Selkirk Range on the west as well as a vast region west- 

 ward to the continental margin. The earliest Lower Cretaceous sediments 

 deposited were a thick coal-bearing series, the Kootenay formation, and 

 then after a brief erosion interval elastics of the Blairmore formation 

 spread eastward over the Kootenay and extensively over the Alberta shelf 

 region. See Fig. 5.19H. The coarse elastics along the foothills and front 

 ranges of the Rocky Mountains and maximum thickness there indicate 

 that the rise of the mountain belt on the west was rapid, and that it was 

 suffering active erosion. 



The distribution of Upper Cretaceous sediments is about that of the 

 Lower Cretaceous and follows about the same pattern of thickening 

 westward into the trough. The Upper Cretaceous are much thicker than 

 the lower Cretaceous in the Williston basin and attain thicknesses of 

 4000 feet in eastern Montana and the western part of the Dakotas. The 

 Upper Cretaceous beds reflect the growth of the later Laramide Rockies 

 and become involved themselves in deformation. They, with a central 

 blanket of Tertiary beds, have been deposited and gently folded adjacent 

 to the major belt of mountain building on the west to form the Alberta 

 syncline. 



A contour map of the pre-Paleozoic surface reflects the summation of 

 all subsidences and uplifts in the Alberta-Williston region, and it will be 

 seen (Fig. 5.191) that the center of the Williston basin is about at the 

 international boundary and the North Dakota-Montana line. All told, it 

 now holds over 7000 feet of sediment. Its position and extent are some- 

 what modified by the central Montana and Black Hills uplifts of Late 

 Cretaceous age. The Sweetgrass arch is a strong element of 4000 feet 



Fig. 5.23. Jurassic correlation chart of the Williston basin and adjacent areas. Reproduced from 

 Peterson, 1957. 



relief. The Alberta basin centers between Peace River and Edmonton, 

 and contains there in front of the disturbed belt over 10,000 feet of 

 sediments. Within the disturbed belt the thickness is much greater, and 

 had the Precambrian surface not been broken and deformed in the 

 Nevadan and Laramide orogenies it would lie very deep, indeed. 



Utah-Wyoming Shelf 



The Williston basin and its relation to the Alberta shelf has already 

 been described. Southward through central and eastern Wyoming and 

 the Colorado Plateau of Colorado and Utah relatively thin layers of 

 Cambrian, Ordovician, Devonian, Mississippian, Pennsylvanian, and 

 Permian strata occur. They represent the transition from the geosyncline 

 on the west to the Transcontinental Arch on the east. The influence of 

 the Ancestral Rockies and other land movements in Carboniferous time 



