23 



CENTRAL MONTANA ROCKIES 



GENERAL FEATURES 



The structures included under the name, Central Montana Rockies, are 

 those in Montana east of the Canadian and Montana Rockies and north 

 of the Wyoming Rockies. The boundary southeasterly of the Foothill belt 

 (see Fig. 23.1) is not clearlly defined and is drawn chiefly for the pur- 

 pose of discussion. The transition from the Cordilleran geosyncline to 

 the shelf is approximately along the west side of the Foothill belt and 

 along the east side of the central Rockies, so that the central Montana 

 Rockies are developed from the shelf. In Chapter 5 it will be recalled 

 that the Big Snowy basin formed in an east-west direction through 

 central Montana in Mississippian and early Pennsylvanian time and 



merged into the Williston basin in eastern Montana, the Dakotas and 

 southern Saskatchewan and Manitoba. Also an arm of the Beltian basis 

 extended eastward through the Little Belt Mountains. 



In the Laramide orogeny a major zone of domes and monoclinal flex- 

 ures formed in an east-west direction approximately in the site of the 

 older Big Snowy basin, and in addition, six smaller, subcircular moun- 

 tain groups evolved about igneous centers. Some striking en echelon fault 

 zones also developed. The mountain groups rise imposingly 2(K)0 to 

 5000 feet from the plains. The primary cause of the entire assemblage 

 of mountain groups is probably magmatic. Major intrusions into the 

 Precambrian rocks domed up the Paleozoic and Mesozoic veneer, and in 

 places central vents and associated dike swarms broke through to the 

 surface or fed sill and laccolithic intrusions into the Cretaceous strata 

 a short distance below the surface. A certain amount of horizontally 

 acting crustal stress was relieved at about the same time as the intru- 

 sions. This is attested by the en echelon fault zones. The magmatic 

 theory is elaborated upon in Chapters 19 and 36 which discuss the 

 igneous provinces of the western United States. 



CENTRAL ZONE OF UPLIFTS 



The central zone of uplifts extends from the Little Belt Mountains on 

 the west to the Porcupine dome on the east. The Big Snowy Mountains 

 comprise a prominent and perhaps the best-known dome. See Figs. 23.2 

 and 23.3. The Madison limestone has proved very resistent to erosion 

 and forms the surface rock of a large central area. The Big Snowy 

 dome has a length of about 25 miles and a width of 12 miles; the 

 structural relief is 12,000 feet on the steep southern flank. 



The dome of the Little Belt Mountains is much larger in horizontal 

 dimensions than the Big Snowy, but not so complete. Its southwestern 

 half is composed of Beltian strata intruded by granodioritic batholiths 

 and the domal structure obliterated by sharp folds and thrust faults. Its 

 northeasterly flank is gentle and stretches beyond the Highwood vol- 

 canic group to the Blood Creek syncline with a structural relief of 

 11,000 feet. 



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