LATE CRETACEOUS AND EARLY TERTIARY ROCKY MOUNTAIN SYSTEMS— THE LARAMIDE OROGENY 



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Permian and Mesozoic trough sediments and the extent to which it over- 

 lapped the Paleozoic sediments of the geanticlinal area. 



Precambrian rocks are not exposed at the surface in many ranges, but 

 from central Utah northward to western Montana, those of Proterozoic 

 (Reltian) age become increasingly widespread. In western Montana, 

 most of the Laramide Rockies are in the Beltian strata, and this zone 

 extends to the northwest in eastern British Columbia. The crystalline 

 complex, supposedly everywhere older than the Beltian, is exposed in 

 the trough zone only in the Wasatch and Raft River Mountains. In the 

 shelf ranges the opposite is true; the crystalline complex is exposed in 

 many of the cores of the ranges. The extent of the Beltian trough, as 

 well as can be determined, is shown in comparison with the Laramide 

 belts of deformation in Fig. 19.3 and will be referred to later. 



Thrust faults dominate the structure in the trough zone. The overriding 

 sheets of greatest displacement and shallowest dip moved mostly east- 

 ward, but several thrusts, especially in Montana and Canada, have moved 

 westward. 



Southern Arizona Rockies 



The southern Arizona Rockies consist largely of Precambrian rocks 

 of several ages, both igneous and sedimentary. The ancient rocks were 

 veneered with a thin Paleozoic cover, and in places with thin Triassic 

 and Jurassic strata. The Cretaceous Mexican geosyncline extended north- 

 westward into the southernmost part of Arizona, and its strata are there 

 thrown into folds and thrust sheets. Part of the Cretaceous accumulations 

 were lavas. See the Paleotectonic maps for the details of the setting for 

 the Laramide orogeny. Intrusive rocks of Laramide age are abundant, and 

 they are associated with valuable ore deposits. A succession of volcanic 

 episodes spread through the Tertiary, and probably some are early enough 

 to be considered Laramide. 



\[ Wyoming Rockies 



The Wyoming Rockies consist in part of the shelf facies of Paleozoic, 

 1 Triassic, Jurassic, and Lower Cretaceous rocks, in part of thick clastic 



deposits of Late Cretaceous age and in part of Beltian (?) and pre- 

 Beltian crystalline rocks. The most conspicuous ranges are sculptured out 

 of great asymmetrical anticlines in which the Precambrian rocks .in- 

 exposed in the cores. The Black Hills, Big Horn, Laramie, and Wind 

 River ranges are eroded in such folds. The anticlines are asymmetrical 

 to the extent of overturning and thrusting in places, and began to rise 

 in Late Cretaceous time, while the broad basins between sank and 

 received thousands of feet of sediments. Examine the paleotectonic map 

 of the Late Cretaceous. The thick Upper Cretaceous sediments are 

 generally involved in late phases of the compressional orogeny. Paleocene 

 and Eocene sediments have accumulated in the basins to considerable 

 thicknesses in places, and certain phases of deformation marginal to the 

 basins have deformed them also. 



The northwest corner of the state of Wyoming became the site of 

 considerable volcanic activity in middle and late Eocene time, and the 

 pyroclastics and lavas of the Absaroka Range and Yellowstone Park were 

 mostly exuded at that time. 



Central Montana Rockies 



The central Montana Rockies consist of a general east-west assemblage 

 of monoclinal flexures, domes, and belts of en echelon faults. Numerous 

 bodies of igneous rocks consisting of stocks, laccoliths, radiate dike sys- 

 tems, and various extrusions lie in a belt approximately transverse to the 

 sedimentary rock structures. 



During the Beltian epoch of the Proterozoic, a trough extended east- 

 ward into central Montana and may have predetermined the location of 

 the central Montana Laramide structures. See map, Fig. 19.4. Also, in 

 Mississippian times a broad east-west basin through central Montana sub- 

 sided and received over 2000 feet of beds, appreciably more than on 

 either side. See paleotectonic map, Plate 5. This basin, like the Beltian. 

 may have helped to determine the position that the later Laramide struc- 

 tures took, or else the coincidence in space of all three means that some 

 deep-seated influence has been at work repeatedly from Beltian times to 

 Laramide. 



