26. 



COLORADO PLATEAU 



GENERAL GEOLOGY 



i 



The Colorado Plateau is one of the world's show places, not only for 

 the tourist but for the geologist. Badlands, high escarpments, and deep 

 gorges leave few geologic secrets covered if they are searched for. Early 

 geologists such as Gilbert, Powell, and Dutton, who first explored the 

 Plateau geologically, made it classical territory. Their line drawings of the 

 physiographic features and their diagrams of the structures still stand as 

 masterpieces. The contributions in the years since these early investiga- 

 tions have been on the details. 



A bird's-eye view of part of the province may be obtained from a 

 stereogram of Gilbert's (1877), reproduced here in Fig. 26.2. Pictures of 



the Grand Canyon, Zion National Park, the natural bridges of San Juan 

 County, Cedar Breaks, and Bryce National Monument are commonplace 

 and serve to identify small spectacular parts of the great region. The 

 major structures of the plateau are shown in the index map of Fig. 26.1. 



The paleotectonic and paleogeologic maps of this book show clearly 

 that the Colorado Plateau was a shelf area adjacent to the westward lying 

 Cordilleran geosyncline. Parts of Arizona were Precambrian terrane until 

 the Pennsylvanian. From Mississippian times to the close of the Creta- 

 ceous, shallow seas covered the entire province, with the exception of the 

 Uncompahgre and Zuni ranges of the Ancestral Bockies, which were 

 finally buried in Triassic time. 



The Paleozoic section of the Grand Canyon is given in Fig. 26.4, and a 

 number of Mesozoic and late Paleozoic sections of various parts of the 

 Colorado Plateau are reproduced in Figs. 26.5 and 26.6. Beferences to de- 

 tailed stratigraphic studies in the plateau may be obtained from the two 

 figures. 



The paleogeographic and tectonic development of the region in 

 Paleozoic time has been treated in Chapters 6 and 15. 



In brief, the pre-Laramide history of the Plateau is as follows. When 

 the Cambrian seas invaded the area, a relief in places, at least, of about 

 800 feet existed (Sharp, 1940), and the surface was entirely buried by the 

 Cambrian sediments. In general, the absence of Ordovician and Silurian 

 strata throughout the Plateau, with only a disconformity marking their 

 place, indicates either gently emergent conditions during these periods 

 or that, toward the close of the Silurian, the region became emergent, 

 and any beds that were deposited during the interval were removed. The 

 Mississippian was one of limestone deposition, but the Pennsylvanian was 

 one of considerable crustal unrest with the building of the Ancestral 

 Bockies and the subsidence of the Paradox basin. The western margin 

 of the Plateau was the transition from shelf to miogeosyncline in Paleozoic 

 time and later in Cretaceous and early Tertiary time the site of mountain 

 building and accumulation of tiiick orogenic deposits. See Chapter 22 on 

 the Central Bockies. 



Although horizontal strata dominate the landscape, several monoclines 

 were formed which are the steep flanks of large asymmetrical anticlines, 

 some 30 miles across and 100 miles or more long. 



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