418 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



MILES 



Fig. 26.14. Ground plan and cross sections of intrusive bodies of Mt. Ellen, Henry Mountains, 

 Utah. 1, Corrall Ridge laccolith; 2, Durfey Butte laccolith; 3, South Creek Ridge laccolith; 4, 

 Slate Creek laccolith; 5, Cooper Ridge laccolith; 6, Granite Ridges laccolith; 7, Butler Wash 

 laccolith; 8, Mt. Ellen stock; 9, shatter zone; 10, Ragged Mountain bysmalith. Cross sections il- 

 lustrate relation of a laccolith, such as No. 2, to the sedimentary rocks, and also of a bysmalith, 

 such as No. 10. The top of the stock has been cut off to show the shatter zone and hard rock. 

 The lower cross sections are somewhat enlarged views of the igneous bodies 2 and 10, and 

 show the relations to the sedimentary beds. After Hunt, 1954. 



6. In the early Tertiary, the entire section above the salt was arched up, 

 and the salt intruded slightly but not more than through the late 

 Jurassic formations. 



7. The entire Colorado Plateau was elevated in mid-Tertiary time, and 

 coincident with the uplift the laccolithic intrusions occurred which are 

 described under the next heading. As a result of the uplift, ground- 

 water commenced to circulate more freely, the salt masses were sub- 



jected to solution, and collapse of adjacent and overlying strata 

 occurred. 

 8. With the uplift of the plateau, several partial cycles of erosion have 

 followed, and the salt anticlines have been excavated and in places a 

 gypsum residue over the salt exposed. See last diagram of Fig. 26.13. 



LACCOLITHIC MOUNTAINS 



The Henry Mountains were first described by Gilbert ( 1877 ) , and with 

 his publication they became classical for the laccolithic type of mountain. 

 He pictured the laccolith as a tack- or mushroom-shaped, concordant plu- 

 ton — as a centrally thickened sill which has arched the beds above it and 

 has been fed through a conduit from below. 



Hunt (1954) has restudied the Henry Mountains in detail and shows 

 them to be concordant tongues extending outward from a central, trunk- 

 like stock. They are like semicircular, conical fungus growths on tree 

 trunks. The largest cluster is illustrated in Fig. 26.14. Ninety-five percent 

 of the intrusive rock is diorite porphyry, and the rest is monzonite 

 porphyry. According to Hunt, 



The Henry Mountains are located in the structural basin that is one of the 

 major folds of the Colorado Plateau [Fig. 26.15]. The basin is the antithesis of 

 the adjoining Circle Cliffs uplift and San Rafael Swell, being of the same size 

 and form, only inverted. The basin is sharply asymmetric and its trough is 

 crowded against the steep west flank; the deepest part is 8,500 feet structurally 

 lower than the neighboring uplifts. 



Each of the divisions of the Henry Mountains is a structural dome several 

 miles in diameter and a few thousand feet high. In general, the domes have 

 smooth flanks, but on most of them are superimposed many small anticlinal 

 noses that were produced by the laccoliths. At the center of each of the domes 

 is a stock, around which the laccoliths are clustered. The stocks are cross-cutting 

 intrusions, mostly surrounded by a shatter zone, which consists of highly 

 indurated sedimentary rocks irregularly intruded by innumerable dikes, sills, 

 and irregular masses of porphyry. 



The dome of Mount Ellen, the largest dome in the Henry Mountains, has a 

 broad, plateau-like upper surface that is marked with many small anticlinal 

 folds. On this mountain the laccoliths were injected in all directions from the 

 stock. The Hillers dome is the highest and steepest of the domes and the anti- 

 clinal folds over the laccoliths around it are restricted to the north and northeast 



