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STRUCTURAL GEOLOGY OF NORTH AMERICA 



. IDA HO 



UTAH ' | 



Fig. 26.1. Major geologic features of Colorado Plateau. Black areas are lava fields; stippled 

 areas are Early Tertiary sediments; horizontally dashed areas are Cretaceous sediments; and 

 cross-ruled areas are laccolithic mountains. S.R.S., San Rafael Swell; C.C.U., Circle Cliffs uplift; 

 S.L.C., Salt Lake City; P., Provo; F., Filmore; B., Beaver; Fl., Flagstaff; PC, Prescott. Lava fields 

 in the Great Basin not shown, especially in the St. George area. 



Another significant type of structure in the Colorado Plateau is the 

 laccolithic mountain. There are several clusters of laccolithic intrusions, 

 and these have produced the mountains known as the Henry, La Sal, 

 Abajo (or Blue), La Plata, Ute, and Carrizo. See index map, Fig. 26.1 

 and Fig. 33.7. Also, another high mountain, Navajo, is probably a lac- 

 colithic structure. There are several major volcanic fields, one in the High 

 Plateaus of Utah; one just south of the Grand Canyon in Arizona, the 

 San Francisco Mountains; and one in eastern Arizona and western New 

 Mexico, the Datil field. The flexures, laccoliths, volcanic fields, and other 

 features are described in the following pages. 



ASYMMETRICAL ARCHES AND BASINS 



In a very broad way the Monument uplift, with Permian beds exten- 

 sively exposed in the core, is the center of the Plateau, and is nearly sur- 

 rounded by Cretaceous and Tertiary basins. Auxiliary uplifts break the 

 continuity of the surrounding basins or render the general pattern ir- 

 regular (Figs. 26.1 and 19.2). The San Rafael Swell, the Circle Cliffs 

 uplift, and the Uncompahgre uplift lie just inside the Cretaceous and 

 Tertiary basins on the west, north, and northeast, and the Kaibab (Fig. 

 26.8) and Defiance (Fig. 26.9) uplifts break the continuity of the Cre- 

 taceous basins on the south and southwest. The uplifts in Utah and the 

 Kaibab in Arizona are characterized by a sharp monoclinal flexure on the 

 east side, broad tops, and gently dipping west flanks (Fig. 26.7). In the 

 high, desert climate of the Plateau these monoclines are grand features 

 of the scenery. The arches are all believed to be of Laramide age, and 

 are crossed indiscriminately by the master streams that drain the Plateau. 

 The streams are, therefore, either superposed or antecedent. The greatest 

 scenic spectacle in the Plateau, in the minds of many people, is the gorge 

 of the Colorado River (Grand Canyon) across the Kaibab uplift. The 

 river here has cut through the entire Paleozoic section and also well into 

 the Precambrian. 



The Cretaceous and Tertiary basins, aside from the Black Mesa of 

 Arizona, are bounded on the outside by major Laramide uplifts, and the 

 deepest parts or troughs of the basins he close to these uplifted and 

 mountainous areas. The Tertiary section in the Uinta Basin is 10,000 



