424 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



north end of the VVaterpocket fold (Dutton, 1880). At each of these 

 places, Eocene strata lie across the eroded edges of the older folded rocks. 

 It seems probable that the other large, northerly trending folds of the 

 plateau, like the Kaibab uplift, the Defiance anticline (see Fig. 26.1), and 

 the Monument upwarp also were formed at this time. As a first result of 

 this folding, the structural uplifts became topographically high areas, and 

 the structural basins became the sites of deposition of sediments eroded 

 from the highland. San Juan basin in New Mexico and Uinta basin in 

 Utah still preserve their basin sediments; from the other basins such fill 

 as was deposited has been removed. 



Following the monoclinal flexing and the creation of the uplifts and 

 basins in Laramide times, the Plateau was broadly uplifted. The history 

 of epeirogenic uplift, superposition of the major streams across the struc- 

 tures, and the gorge cycle of erosion are not yet well known, although 

 considerable has been written about these fascinating subjects. Longwell 

 (1946) reviews the problem of dating these events, and concludes pro- 

 visionally that the major uplift occurred in the Pliocene. See also Hunt 

 (1956). 



The intrusion of the laccolithic clusters and the main phases of the ex- 

 trusive activity in the volcanic fields probably occurred in the Pliocene. 

 The activity can be dated accurately only in the Hopi Ruttes area, but be- 

 cause of the similar setting of all the volcanic fields in the Colorado Pla- 

 teau, it is safest to assume this age for all until proved otherwise. The 

 volcanic activity continued in several stages down to very recent times, 

 both north and south of the Grand Canyon and in the Datil field. 



Regarding the age of the laccolithic intrusions, Hunt (1956) observes: 



The drainage on the laccolithic mountains is consequent and some of the 

 main tributaries of the Colorado River appear to have been shifted mono- 

 clinally as a result of the doming by the intrusions. For example, the Fremont 

 River swings in a wide arc around the north side of the Henry Mountains and 

 follows the trough between the mountains and the San Rafael Swell. The 

 Dolores River swings in a similar arc around the north side of La Sal Mountains 

 and follows the trough between them and the Uncompaghre Uplift. The head- 

 ward part of Glen Canyon has avoided the domes at the two southern Henry 

 Mountains; the lower part of San Juan River and the adjoining section of the 

 Colorado River have avoided the dome at Navajo Mountain. The adjustment 



of the drainage to the intrusive structures stands in striking contrast to the lack 

 of adjustment of the drainage to the orogenic structures. 



This adjustment would have developed if doming of the laccolithic moun- 

 tains had dammed earlier stream courses, forcing streams like the Fremont 

 and Dolores into new courses. Inasmuch as both streams now follow the struc- 

 turally lowest course possible, they may have been flowing across Tertiary 

 basin sediments when the intrusions occurred, and their courses shifted mono- 

 clinally off the domed areas even though the doming progressed slowly. 



EPEIROGENIC MOVEMENTS AND ISOSTATIC AND 

 SEISMIC CONSIDERATIONS 



The summation of movement in the Colorado Plateau in late Creta- 

 ceous, Paleocene, and Eocene time was downward around its borders, save 

 for the south and southwest parts. The thick deposits in the High Plateaus 

 of Utah, the Uinta and Piceance basins, and the San Juan basin attest to 

 subsidence there, leaving the central and southern parts positive. The 

 general relations may also be viewed as a northward down-tilting. The 

 occurrence of gravels containing Precambrian quartzite on the Mogollon 

 Rim ( Fig. 26.9 ) suggests that this part of the Plateau was low-lying and 

 was receiving debris from mountains to the south (Hunt, 1956). The 

 several asymmetrical uplifts within the Plateau formed at this time also. 



After the development of the marginal basins the entire Plateau was 

 uplifted as a block between 6000 and 8000 feet. The uplift of the southern 

 part was probably greater than the northern, and the impressive Mogollon 

 Rim and Mountain Region of central Arizona was brought into existence. 

 The epeirogenic rise is thought to be contemporaneous with the late 

 Tertiary and Quaternary block faulting to the west with the creation of 

 the Basin and Range province. The magmatic activity represented by the 

 laccolithic clusters, the Hopi and Navajo volcanic fields, and the large 

 marginal fields was also contemporaneous with the broad uplift. 



For the Colorado Plateau mass of the earth's crust to stand 6000 to 8000 

 feet above sea level it has generally been concluded from isostatic con- 

 siderations that roots of "granitic" crust several times as thick must ex- 

 tend downward into the subcrust. However, Tatel and Tuve (1955) 

 believe they recognize the Moho discontinuity under the Colorado Plateau 

 at about 30 kilometers. If so, and thus in the absence of roots, some phe- 



