

CENTRAL STABLE REGION OF THE UNITED STATES 



55 



the Chautauqua arch, and the Ozark dome made up one continuous 

 broad arch which left the Transcontinental Arch at right angles and 

 veered eastward in southern Missouri. 



The dome was uplifted again slightly in the late Mississippian (Plate 

 6). This time the movement was not in company with the Ellis and 

 Chautauqua arches, but apparently with the Hunton arch to the south- 

 west in Oklahoma (Dott, 1934). The great Pennsylvanian transgression 

 nearly, if not entirely, covered the dome (Plate 6), and no recurrences 

 of uplift during the Pennsylvanian or Permian have been described. The 

 Devonian and Mississippian uplifts left the dome wrinkled with very 

 gentle narrow folds that trend in a northwest direction. 



The Arkansas Valley lies south of the Ozark dome and north of the 

 complexly folded and thrust-faulted Ouachita Mountains. It is a structural 

 basin as well as valley, and will be described in Chapter 14 under the 

 heading, "Ouachita System." 



Cambridge Arch 



A number of wells which have penetrated "granite" have been drilled 

 through the Pennsylvanian formations in a line running northwesterly 

 across Nebraska (Ballard, 1942). Isopach maps along this row of wells 

 suggest that the central Kansas arch, well known from many wells, 

 continues northwestward to the Black Hills and beyond to the south- 

 eastern corner of Montana. The arch across Nebraska is known as the 

 Cambridge arch (Plate 7). Geologic contacts determined from both sur- 

 face and subsurface data, however, do not reveal the arch, because it lies 

 mostly within the Precambrian rocks of the larger Transcontinental Arch 

 (Plates 4 and 5). No wells have yet been drilled to the Precambrian 

 northeast of the Cambridge-central Kansas arch, and therefore the bound- 

 aries of the pre-Pennsylvanian formations along the Transcontinental Arch 

 may have to be shifted considerably at a later date. 



NORTHWESTERN INTERIOR BASINS AND ARCHES 

 Williston and Alberta Basins 



The Williston basin was first thought of as a gentle Tertiary downwarp 

 in western North Dakota and eastern Montana, and was named after 



the town of Williston, N.D., on the Missouri River. Cretaceous strata 

 were known to underlie the Tertiary and these to cover Paleozoic rocks 

 of the extensive region of South and North Dakota, Montana, south- 

 western Manitoba, and southern Saskatchewan. With the discovery of 

 commercial oil in 1951 in North Dakota, the term Williston basin became 

 applied to the Paleozoic strata more particularly than to the Tertiary 

 or Mesozoic, and with the drilling of many holes the distribution of 

 formations and systems has become well known. Isopach maps of the 

 several systems important in the Williston basin are shown in Figs. 5.17, 

 5.18, and 5.19. 



A vast region in Alberta, western Saskatchewan, northeastern British 

 Columbia, and the Mackenzie area of the Northwest Territories is a 

 continuation of the Paleozoic sequence of the Williston basin, and the 

 accompanying maps show the close relationship of the geology of the 

 two regions, although they are generally treated separately in oil field 

 parlance. The term "Alberta shelf" has been applied to the Paleozoic 

 sedimentary province under the Great Plains of western Canada, because 

 it is a shallowing shelf region to the Cordilleran geosyncline or Alberta 

 trough on the west for most of the systems (Webb, 1954). It is also 

 commonly referred to as the Alberta basin as a region for oil exploration 

 and structurally as the Alberta syncline. During the Devonian period a 

 broad basin did develop (see Fig. 5.17C), but otherwise the region can 

 more properly be called a shelf. The syncline developed as the result of 

 Cretaceous and Tertiary subsidence, mountain building on the west, and 

 sedimentation, but the synclinal axis is not reflected under the Great 

 Plains in the thicknesses of any of the pre-Cretaceous systems. 



The Cambrian strata are dominantly clastic with a sandstone generally 

 at the base and a sequence of green and maroon shales and light gray 

 calcareous siltstones and fine-grained sandstones above. These beds were 

 deposited unconformably on a Precambrian terrane as the seas invaded 

 the shield region from the west and southwest (Fig. 5.17A). 



The Ordovician beds are extensive under the Williston basin but 

 generally absent on the Alberta plains. The outcrops in Manitoba contain 

 a 50- to 100-foot basal, white quartz sandstone with interbedded shales 





