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STRUCTURAL GEOLOGY OF NORTH AMERICA 



At least 30 volcanic necks have been recognized. A few are isolated, but most 

 occur in clusters or are aligned in a northwest-southeast direction, parallel to 

 the adjacent dikes. The typical neck is circular in plan, though some are oval 

 and merge gradually with dikes. 



The typical Hopi vent was opened by the explosive drilling of a cylindrical 

 pipe, and doubdess a pyroclastic cone or maar-like depression was formed 

 about the orifice. Subsequendy, upwelling lava filled the crater and finally 

 spilled over the rim in broad floods. The evidence of this history is abundant. 

 There is hardly a neck without a jacket of inwardly dipping pyroclastic debris, 

 made up of lava and sedimentary fragments than range in size from the finest 

 dust to blocks many yards in diameter. Normally, the dip of these ejecta in- 

 creases both upward and inward. Inbedded friction breccias bordering the 

 necks are extremely rare, and in general the enclosing sandstones and shales 

 remain undisturbed. 



Hack ( 1942 ) has found that the explosive pipes or diatremes are nu- 

 merous, and that most of them occur in a dense cluster within an area of 

 800 square miles. He writes as follows: 



In general the diameter of the vents decreases as erosion increases. In areas 

 where dissection has been slight, at levels above the Hopi Buttes surface, ex- 

 plosion pits are generally 3000 to 4000 feet in diameter. In many places, the 

 initial explosion pit is overlain by domes of lava which have pushed outward, 

 spilling over the sides, crumpling and pushing out the underlying and border- 

 ing tuffs and sediments. In a few places these lava eruptions were of sufficient 

 duration to form rather continuous flows. The more deeply dissected diatremes 

 range in diameter from 500 feet to 2000 or 3000 feet. In general the material in 

 them is less well bedded, and, if pyroclastic, is coarser. 



The volcanism occurred in Pliocene time, because vertebrate bones of 

 that age have been found in the Hopi lake beds ( Bidahochi formation ) . 



The igneous rocks of the Navajo region differ from those of the Hopi 

 Buttes because they contain a paucity of lava flows. Also, according to 

 Williams (1936): 



. . . most of the Navajo volcanic necks are made up predominandy, not of 

 columnar lava, but of coarse tuff-breccia and are crowded with fragments of 

 plutonic rock, chiefly of granitic type. Petrographically, also, the two provinces 

 differ markedly; in the Hopi Buttes, monchiquitic rocks are typical, while in 

 the Navajo region these are far subordinate to minettes. Probably the Navajo 

 vents were more explosive. Indeed, many of them can never have erupted lavas. 

 How closely they resemble the well-known diatremes of the Schwabian Alb, 

 the Bhongebirge, and Central Scotland will be apparent from what follows. 

 Like those explosive vents, many of the Navajo volcanoes seem to be scattered 



at random, without regard to pre-existing structures. None is located on a fault. 



Monument Valley, with its fantastic, castellated crags, is carved from the De 

 Chelly sandstones and the Moenkopi shales that occupy the broad and gently 

 rippled top of a domical uplift, bordered on the south and east by the sharp 

 monocline of Comb Ridge and on the west by less-prominent folds that traverse 

 the Rainbow Plateau. On the summit of the upwarp, dips of more than 30 

 degrees are rare, but in the flanking folds they may reach 60 degrees. Many 

 intrusions are to be found along the Comb Ridge monocline, extending from 

 the village of Kayenta in an arc to the San Juan River. These tend to follow a 

 strong system of joints, approximately normal to the trend of the monocline. In 

 Monument Valley itself there is much less regularity in the trend and distribu- 

 tion of the intrusions. 



All the necks rise boldly from the surrounding sediments, despite the fact 

 that they consist almost entirely of tuff-breccias. The few thin dikes which cut 

 the breccias are not responsible for this resistance to erosion; it results, rather, 

 from the compactness of the neck fillings, for the fine tuffaceous matrix has 

 been indurated by hot solutions, and much of it is cemented by calcite. The 

 absence of strong joints, such as cut the adjacent sandstones, is, doubdess, 

 another contributing factor. 



Shiprock, Agathla, and Alhambra Rock are some of the well-known 

 necks referred to by Williams. 



Two examples of cauldron subsidence in the Navajo region have been 

 described (Williams, 1936), one at Buell Park, north of Fort Defiance, 

 near the Arizona-New Mexico line, and one at Indian Wells in he Hopi 

 Buttes. The first is about 2/2 miles in diameter and collapsed at least 1000 

 feet. The second is J£ by K mile in diameter, and it collapsed perhaps 

 100 or 200 feet. 



HIGH PLATEAUS OF UTAH 



The High Plateaus of Utah are generally considered a subprovince of 

 the Colorado Plateaus physiographic province. They lie along the western 

 margin of the Colorado Plateau as shown in Fig. 26.1. The individual 

 relief features are indicated in Fig. 26.16, and bold escarpments 2000 to 

 6000 feet high between valley floor and Plateau top are common. 



The High Plateaus span the transition from Paleozoic miogeosyncline 

 to shelf, and also contain the thick clastic deposits of the Cretaceous and 

 early Tertiary described in Chapter 22 on the Central Rockies. The thick 

 Jurassic evaporite trough also lies within them. In other words, the 



