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STRUCTURAL GEOLOGY OF NORTH AMERICA 



sides. Mount Pennell is similar to Mount Hillers, only smaller. The Holmes 

 dome is broken by faults, and its top is wrinkled with some minor anticlinal 

 noses. The Ellsworth dome has no anticlinal folds to mar its symmetry, although 

 it is broken by a few faults. 



Several lines of evidence indicate that the laccoliths were injected radially 

 from the stocks: (1) the laccoliths are tongue-shaped in plan and make a 

 radial pattern around the stocks; (2) dike-like ridges on the roofs of the lac- 

 coliths trend away from the stocks; (3) the laccoliths are bulged linearly and 

 the axes of the bulges radiate from the stocks. 



Coherence and competency of the invaded rocks appears to have been an 

 important factor controlling the stratigraphic distribution of the laccoliths. Pre- 

 Jurassic formations, estimated to be about 5,000 feet thick, consist of well- 

 bedded, relatively coherent, alternating thin competent and incompetent units 

 in which very few laccoliths were injected. Overlying this is 1,200 feet of 

 competent and highly coherent sandstone of the Glen Canyon group (Wingate, 

 Kayenta and Navajo formations), containing even fewer laccoliths than the 

 underlying zone. Overlying this sandstone is the San Rafael group and lower 

 half of the Morrison formation of Jurassic age, aggregating a thickness of about 

 1,000 feet, and consisting of incoherent, incompetent, poorly-bedded rocks and 

 interbedded competent layers. About 15 per cent of the total volume of the 

 laccoliths is in these formations. The sequence including the upper half of the 

 Morrison and the Cretaceous formations, aggregating a thickness of about 2500- 

 3000 feet, consists largely of incoherent, incompetent shale in very thick units 

 separated by thin competent layers. By far the greatest number of laccoliths, 

 and at least 70 per cent by volume, are in this zone. Through this zone the 

 laccoliths are concentrated along the thin competent layers. 



Because the stratigraphy and structure of the Colorado Plateau is fairly 

 uniform, the similarity in form of intrusion, geologic structure, and igneous 

 rock types at the several laccolith mountains in the Plateau reflect close simi- 

 larity of the igneous processes involved. The mountains are believed to repre- 

 sent a series of examples of one igneous process that was arrested at various 

 stages of completion. 



Navajo Mountain, a structural dome containing no exposed igneous core, 

 represents the least advanced stage of the process. Mount Holmes, whose dome 

 is slightly higher than Navajo Mountain and whose center contains a small 

 stock from which a moderate number of dikes, sills, and very small laccoliths 

 radiate, represents the next more advanced stage. The process is still farther 

 advanced at Mount Ellsworth, where the doming is steeper and higher than at 

 Mount Holmes and where a moderate size stock surrounded by a shatter zone 

 occupies the center of the dome and abundant dikes and sills intrude the flanks. 

 On Mounts Pennell and Hillers which illustrate the next most advanced stage, 

 the doming is much steeper than on Mount Ellsworth, the stocks at the centers 

 of the domes are much larger, the flanks of the domes contain abundant dikes 

 and sills and in addition, to the north and northeast, huge linear, tongue-like 

 laccoliths were injected. The dome of Mount Ellen covers a much greater area 



than the domes of the other mountains, and the laccoliths radiate in all direc- 

 tions from the stock (Hunt, 1954). 



The La Sal Mountains are made up of three separate masses both 

 topographically and geologically, and each mass consists of a stock and 

 a cluster of radiating concordant intrusions into the sediments (lacco- 

 liths). According to Hunt (1958): 



The intrusions are in the midst of a series of salt anticlines and synclines 

 whose axes trend northwest. Although the folding and attendant faulting in 

 the area around the La Sal Mountains are chiefly the result of late Late Cre- 

 taceous or early Tertiary deformation, the structural history is complicated 

 because there has been repeated plastic deformation of the salt beds and the 

 strata arched over them. These structures antedate the intrusions and are 

 not believed to be causally related to them. 



North La Sal Mountain is located on an anticline, South La Sal Mountain 

 is in a faulted syncline, and Middle La Sal Mountain is in an area of gende 

 homoclinal dips between these two structures. The North La Sal Mountain 

 forms a dome 10 miles long and 5 miles wide, and the uplift on it exceeds 

 6,000 feet. This dome is greatly elongated northwesterly, parallel to the axis 

 of the anticline in which it is located. The Middle Mountain dome is nearly 

 circular in plan, about 5 miles in diameter, and about 3,500 feet high. At 

 Mountain dome is 6 miles long, 4 miles wide, and about 6,000 feet high. At 

 the center of each of these domes is a stock, and radiating from each stock 

 are laccoliths. The domes are attributed to the physical injection of the stocks. 

 In the North and South La Sal Mountains the laccoliths spread in the salt 

 beds of late Paleozoic age; in the Middle La Sal Mountain the laccoliths spread 

 in shale of late Cretaceous age. 



The petrology of the laccolithic groups is discussed in Chapter 33, but 

 of interest here is the conclusion reached by Hunt that in the closing 

 stages of fusion, crystallization, and intrusion of the North Mountain 

 stock four pipelike masses of explosion breccias formed as diatremes were 

 blasted through the arched roof. 



The Abajo Mountains consist of two unequal parts, the smaller and 

 northern division being an isolated dome known as Shay Mountain. It is 

 believed to be underlain by a stock. The southern and main division con- 

 sists of two parts, East Mountain and West Mountain, where central 

 stocks are exposed with surrounding shatter zones and clusters of radi- 

 ating laccoliths. A fourth stock is postulated although not exposed, and 

 altogether around the four stocks 31 laccolithic intrusions have been 



