58 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



t- PRESENT PRECAMBRIAN SURFACE 



Fig. 5.19. Thickness maps of Williston and Alberta basins: Jurassic and Lower Cretaceous. Also 

 contour map on Precambrian surface. Jurassic, after Webb (1954), Francis (1957), and Peterson 



the Rocky Mountain formation. Farther north in adjacent parts of Yukon 

 and Northwest Territories equivalent sandstones with a chert member at 

 the top attain a maximum thickness of 1200 feet. The erosion surface 

 on the Mississippian in the Peace River region has local sharp relief, and 

 beds believed to be Pennsylvanian and Permian cover the surface and 

 range up to 500 feet thick. 



In the Montana and South Dakota area (see map, Fig. 5.18F) elastics 

 predominate over non-elastics, and clean quartzose sandstones are the 

 rule, making up the Quadrant sandstone in central and western Montana 

 and the Tensleep sandstone over the Wyoming shelf. In the southern 

 part of the Williston basin a wedge of Pennsylvanian is preserved, and 

 consists of dolomite interbedded with sandstone, red shale, and evapo- 

 rites. 



(1957); Lower Cretaceous, after Webb (1954) and Reeside (1944); Precambrian surface, from 

 Tectonic Map of Canada (1954) and Moss (1936). 



Triassic time was marked by widespread emergence, but in the Peace 

 River Country a thick sequence of marine elastics, impure limestones and 

 anhydrite accumulated. Thicknesses up to 3000 have been measured in 

 the adjacent Rockies. 



A group of red beds has been charted across part of the Williston 

 basin by Ziegler ( 1956 ) . The beds lie between the Permian Minnekahta 

 limestone and the Piper beds of the Jurassic. See map, Fig. 5.22. A lower 

 shale and siltstone unit is thought to correlate with the Spearfish red beds 

 of the Black Hills which are Triassic, and three overlying units, a salt, a 

 siltstone and sandstone, and an upper salt are thought to be lower 

 Jurassic but may also be Triassic. 



The Jurassic beds in Alberta have about the same distribution as the 

 Triassic except for a wider transgression in the southern Foothill belt 



