292 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



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stone, sandy oolite, and shale. The medial Rierdon is largely limy shale 

 and nodular limestone. The upper Swift is dark noncalcareous shale and 

 flaggy glauconitic sandstone. The Morrison consists of fine-grained green- 

 ish gray clay shale and fresh-water limestone. From the structural point of 

 view, it is significant that the Swift overlies the other formations uncon- 

 formably (see cross section, Fig. 18.1), and indicates that the Sweetgrass 

 arch rose in early Late Jurassic time in about the same position and 

 with the same detail as that produced by later Laramide movement. The 

 arch consisted of two domes, a northern and a southern; the southern one 

 was the site of greatest uplift and erosion. It is also believed that the 

 southern dome rose gently and remained above sea level during the 

 deposition of the Sawtooth and Rierdon formations, just preceding early 

 Upper Jurassic uplift. 



The Morrison was deposited conformably on the Swift; following 

 Morrison time, the arch was again elevated slightly, and several low 

 anticlines and synclines were formed. Subsequent erosion removed part of 

 the Jurassic beds from the crests of the anticlines. Erosion was most pro- 

 nounced on an anticline extending approximately north-south through 

 the north dome (Kevin-Sunburst). The Morrison, the Swift, and part of 

 the Rierdon were removed along the crest of the anticline and westward 

 for an unknown distance. Over this area the earliest Kootenai sands and 

 gravels (Cut Bank sandstone) were deposited, while the area east and 

 south continued to undergo erosion. It was not until Sunburst time (a 

 sandstone member of the Kootenai) that the entire area received sedi- 

 mentation. 



By the close of Mid-Jurassic time (Callovian) the Sweetgrass arch 

 had spread as a land area of very low relief to include central and western 

 Montana and northwestern Wyoming. See Plate 12. 



The Ancestral Rockies had been overlapped still more in Early and Mid- 

 Jurassic time, and during the Late Jurassic were entirely buried save for a 

 few peaks in the Front Range of Colorado. 



Fig. 18.1. The Sweetgrass arch in Jurassic time, after Cobban, 1945. Left, isopach map of 

 the Sawtooth formation. Right, isopach map of the Rierdon formation. Ruled areas were 

 exposed Paleozoic strata just before deposition of the Swift formation. The crest lines of the 

 Kevin-Sunburst dome and the south "arch" are those of the present, and together they make 

 up the Sweetgrass arch. 



