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



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some thrusts were formed. The Pennsylvanian and Permian Marathon 

 and Coalhuila systems are, therefore, thought to have extended consider- 

 able control over the later Laramide structures. 



Sonoran Rockies 



i 



, The Sonoran Rockies include three geomorphic provinces, namely 

 from east to west, the Sierra Madre Occidental, the parallel ranges and 

 valleys, and the Sonoran desert. From coarse conglomerates along the 



i western margin of the Mexican geosyncline, it is clear that the orogenic 

 belt in western Sonora continued active, and at least twice in late Cre- 

 taceous time rose sharply and crowded the sediments eastward. The early 

 Laramide structures thus created are obscure, first for lack of field work, 

 and second because other younger orogenies have been superposed, and 



i much Tertiary lava covers them. 



: The Paleozoic and Mesozoic sediments are of a mainland assemblage, 

 as far as known, and the volcanic assemblage of the Pacific border systems 

 is absent. On the other hand, the Permian beds of the Coahuila peninsula, 



,a considerable distance to the east, have much volcanic material. 



Colorado Plateau 



The Colorado Plateau is a rudely circular and lesser deformed part of 

 the crust within the broad zone of Laramide orogeny. A sedimentary 

 Veneer of about 6000 to 10,000 feet overlies a Precambrian basement, 

 jmd over much of the Plateau the strata are nearly flat. Several large 

 jnonoclinal flexures of Laramide age break the monotony of the flat- 

 lying beds. 



The monoclines are the steep flanks of asymmetrical anticlines in size 

 mich like those of Wyoming, Colorado, and New Mexico, but with about 

 lalf as much vertical uplift. Consequently, it is believed, no thrusts have 



developed through gravity gliding as in the ranges where the Precambrian 

 cores are so broadly exposed, and topographic relief is so much greater. 

 In post-Laramide time, the Colorado Plateau became the site of con- 

 siderable intrusive and extrusive igneous activity, but it must not be 

 inferred that the igneous activity was confined to the Plateau. It was 

 equally pronounced in the more severely deformed belts to the east and 

 west. The Plateau includes part of the Ancestral Rockies in its eastern 

 part and contains along its western edge some Reltian (?) strata, but 

 most of the Precambrian is a pre-Beltian crystalline complex. 



RELATION OF BELTS OF DEFORMATION TO CRUSTAL CONSTITUTION 



The outer ranges of the Rockies were all developed in the shelf zone 

 of the westward-lying Paleozoic Cordilleran geosyncline, and shelf con- 

 ditions of deposition continued through the Triassic and Jurassic. But with 

 the coming of Cretaceous time, rather thick masses of sediments accumu- 

 lated locally over the former shelf, particularly in Upper Cretaceous 

 basins incident to the early uplift of the Wyoming, Colorado, and New 

 Mexico Laramide ranges. 



The belt of deformation in the shelf extends into the region of the 

 Devonian Transcontinental Arch without effect. The thick Proterozoic 

 metasediments, perhaps all of Beltian age, are shown as well as possible 

 in relation to the Laramide belts of deformation in Fig. 19.4, and do 

 striking coincidence is noted, except locally, perchance in the Uinta 

 Range and the mountains of central Montana. If the relations as depicted 

 are correct, then only one conclusion seems warranted, namely, that the 

 belts of deformation are due to deep-seated causes, not influenced par- 

 ticularly by deeply filled troughs or basins, nor by the crystalline base- 

 ment with a thin veneer of sediments. 



