COLORADO PLATEAU 



421 



mapped (Witkind, 1958). They are mostly intrusive into the Morrison, 

 Burro Canyon, Dakota, and Mancos formations. 



UPHEAVAL DOME 



Upheaval dome is a small circular structure of most peculiar and spec- 

 tacular nature. It is described by McKnight (1940) as follows: 



The Upheaval dome . . . lies about 4 miles east of the Green River at the 

 head of a short canyon (Upheaval Canyon) that cuts through the Wingate 

 cliff (Fig. 26.7). It is circular in ground plan and consists of a conical dome 

 surrounded by a ringlike syncline. The diameter of the dome, measured through 



• center from the axis of the syncline on one side to the axis on the other, is 2 

 miles. The outer flank of the syncline is uniformly half a mile wide, making 

 the complete diameter of the affected area 3 miles. Outside of the very sharply 

 defined line along which the strata dip in abrupdy toward the syncline, the 

 regional low dip to the north has been undisturbed. The inward dip on the 

 outer flank of the syncline is generally between 15° and 30°; the outward 



i dip on the central dome ranges between 30° and 90°, though generally between 

 40° and 60°. 



, Upheaval dome is considered by McKnight to be a salt dome rather 



/than a cryptovolcano, because of the occurrence of thick salt beds at 



moderate depth, and because deformation apparently took place slowly. 



Gravity and aeromagnetic surveys related to the geology have led 



Joesting and Plouff (1958) to the following conclusions: 



1. Uplift totalling some 2,000 feet of comparatively dense, magnetic base- 

 iment rock at Upheaval Dome and Grays Pasture. The uplift took place before 

 !the deposition of the White Rim sandstone member of the Cutler formation of 

 Permian age, and may have coincided with tectonic activity during Pennsyl- 

 vanian and Permian times in other parts of the Paradox basin. 

 , 2. Formation of a salt dome centered at the present Upheaval Dome, 

 possibly controlled by the basement uplift. Plastic flow of salt continued, 

 probably intermittently, until late Triassic (Wingate time). 

 < 3. Further doming due to salt flow, possibly in Tertiary time, coincided 

 with renewed flow of salt in the nearby salt anticlines. The rim syncline formed 

 during this period as a result of thinning of salt around the dome and subsidence 

 of everlying beds. 



4. Intrusion of igneous rock into the salt dome, probably coincident with the 

 late Tertiary igneous activity in other parts of the Colorado Plateau (Hunt, 

 1956, pp. 42-53). The igneous intrusion was comparatively small. It did not 

 displace all of the salt in the core of Upheaval Dome, but it may have been 

 responsible for some of the additional upward movement. 



VOLCANIC FIELDS 



Peripheral Fields 



Several volcanic fields lie around the periphery of the Colorado Plateau: 

 the High Plateaus field in southwestern Utah, the Unikaret or Mt. Trum- 

 bull field of northeastern Arizona, the San Francisco field of north- 

 central Arizona, the Datil field of southeastern Arizona and southwestern 

 New Mexico, the Mt. Taylor and Jemez fields of northwestern New- 

 Mexico, the San Juan field of southwestern Colorado, and several small 

 fields in western Colorado. See particularly Fig. 33.7. These are all de- 

 scribed in Chapter 33. 



Hopi Buttes and Navajo Volcanic Fields 



Scores of volcanic necks, dikes, and lava-capped mesas rise from the 

 high plateau of northeast Arizona and the adjacent parts of Utah and New- 

 Mexico. These are the remnants of a volcanic field that formerlv covered 

 many thousands of square miles. Erosion has so far dissected this field 

 that the original cones have disappeared, the sheets of lava have been dis- 

 membered, and the old volcanic conduits now rise as giant towers, re- 

 vealing their inner structure with singular clarity (Williams, 1936). The 

 largest volcanic cluster is the Hopi Buttes, an isolated field 35 to 40 miles 

 on a side (Fig. 33.7). 



The surface on which the flows of the Hopi Buttes area were erupted 

 was one of low relief and is now nowhere far from an elevation of 6000 

 feet. According to Williams (1936): 



Such valleys as existed on this old surface must have been choked by showers 

 of ash during the opening stages of volcanic activity. The streams were re- 

 peatedly dammed, forming playas and ponds, which seem to have been united 

 ultimately into a lake of great extent. This may be spoken of as Hopi Lake. Its 

 deposits stretch as far north as Jedito Wash and south to the vicinity of the Five 

 Buttes, a distance of almost 35 miles; in an east-west direction they are trace- 

 able for 50 miles, from near Ganado to Dilkon. 



Although the original cones have long since been removed, there is no 

 difficulty in recognizing where they once existed. Erosion, acting rapidly on the 

 surrounding sediments, has left the crater- and conduit-fillings as conspicuous 

 towers, the summits of which cannot be more than a few hundred feet below 

 the tops of the former cones. It is not surprising, therefore, that the intrusive 

 rocks are indistinguishable from the surface flows. 





