56 



STRUCTURAL GEOLOGY OF NORTH AMERICA 





B. ORDOVICIAN 



JL*T 





c 



MAN. 



C DEVONIAN 

 "EVAPORITE BASIN 



Fig. 5.17. Thickness map of Williston and Alberta basins: Cambrian, Ordovician and Devonian. 

 Cambrian, after Webb (1954) and Sloss (1950); Ordovician, after Webb (1954) and Sloss (1950); 



and then a sequence of 400 feet of limestones and dolomites. The car- 

 bonates are the chief rocks encountered in wells; the basal elastics appear 

 to wedge out to the northwest (Webb, 1954). 



The Silurian is represented in east-central Alberta Plains by an evapo- 

 rite sequence and is generally included with beds which may be Middle 

 Devonian. The Silurian and Middle (?) Devonian beds are the Elk 

 Point formation of the stratigraphic chart, Figs. 5.20 and 5.21, and con- 

 tain a composite salt thickness of 1200 feet in 1700 feet of beds. The Silu- 

 rian is present in Manitoba, North Dakota, much of Saskatchewan and 

 northern Montana, but with the Ordovician, is absent in the Sweetgrass 

 arch region. It consists of light yellowish gray and yellowish orange, finely 

 crystalline to dense dolomite (the Interlake group). 



The Upper Devonian strata in western Canada are much more wide- 



Devonian with evaporite region, after Webb (1954), Sloss (1950), and Baillie (1955). 



spread than the Middle, and the original extent was still greater. Post- 

 Paleozoic erosion has removed the beds over considerable areas. The 

 Upper Devonian is characterized by thick deposits of limestones, dolo- 

 mites, shales, and evaporites. It marks a time of limestone reef growth 

 on widespread banks with numerous local bioherm and biostrom deposits 

 and abrupt facies, changes, all holding large oil reserves. 



The Devonian succession of the Williston basin is shown on the chart 

 of Fig. 5.21, and its distribution in Fig. 5.17C. It is divided into four 

 major lithologic units, which in ascending order are, Elk Point group, 

 Manitoba group, Saskatchewan group, and Qu'Appele group. The lower 

 three are chiefly carbonates but the upper is composed of red shales 

 and siltstones. An extensive evaporite sequence occurs in the lower Elk 

 Point group and also in the Manitoba group. In north-central Montana 



