394 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



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Fig. 25.6. Cameron Pass cross sections. Upper section north of tear fault; lower section south 

 of tear fault. Refer to Fig. 25.5. After Gorton, 1953. Tc, Chugwater fm.; Jm, Morrison and 

 Entrada; Kcl, Dakota ss.; Ks, Benton sh.; Kn, Niobrara Is.; Kp, Pierre sh. 



ginal gravity flow movements auxiliary to the vertical in the Colorado 

 and New Mexico Rockies. See discussion under New Mexico Rockies. 



A transverse fault, called the Independence Mountain, is prominent at 

 the north end of North Park. It trends N. 65° W., dips at a low angle 

 northward, and has been mapped for 20 miles (Rlackstone and de la 

 Montagne, I960). Precambrian gneisses have been thrust southward over 

 all formations from Triassic Chugwater to Paleocene Coalmont. The 

 overlapping nature of the Coalmont indicates that it was derived from 

 previous uplift, and some subthrust folds suggest previous folding also. 

 The thrust is of Eocene age and is offset by post-late Miocene normal 

 faults. Isolated Precambrian rocks resting on the Coalmont formation 

 south of and at lower elevations than the trace of the thrust are con- 

 sidered by Blackstone and de la Montagne to have reached their position 

 by gravity sliding, but they do not propose gravity sliding for the main 

 thrust sheet. 



Transverse Porphyry Belt 



Many small instrusive bodies in Colorado will be noted on the map of 

 Fig. 25.2. They are largely concentrated in a narrow diamond-shaped 

 belt that extends from the southwest corner of Colorado, even from the 

 adjacent states of New Mexico, Arizona, and Utah, northeastward through 

 the San Juan Mountains and the Sawatch Range to the Front Range, and 



across it to Boulder City. They compose the so-called porphyry belt, and 

 most of the state's mineral deposits are localized in it. The belt trends 

 nearly normal to the belt of thrusting. See Fig. 25.1. All the small stocks 

 are Laramide in age and their intrusion generally accompanied the moun- 

 tain building. 

 According to Lovering and Goddard ( 1938b ) : 



The late Cretaceous and early Eocene (Laramide) igneous rocks or "porphy- 

 ries" of the mineral belt are readily distinguished from all but a very few of 

 the pre-Cambrian rocks, and many different varieties are so distinctive in 

 appearance as to justify correlation between districts separated by several 

 miles. See Fig. 25.7. These igneous rocks are commonly medium- to fine- 

 grained and nearly all are prophyritic. Some of the early rocks of mafic or 

 intermediate character are holocrystalline, and thin dikes of widely differing 

 composition have felsitic to glassy textures. These intrusives show a wide range 

 in chemical and mineralogic composition, and include dikes as mafic as limburg- 

 ite, as silicic as alaskite, and as alkalic as aegirite syenite. Most of the intrusive 

 rocks are intermediate or siliceous porphyries whose compositions range from 

 hornblende diorite or biotite-quartz monzonite. 



The mineralization followed the intrusion of the Eocene porphyry. The 

 extensive lead-silver deposits of Leadville, the iron-zinc deposits of Gillman, 

 the molybdenite deposits of Climax, the lead-silver deposits of Montezuma. 

 Silver Plume, and Georgetown, the gold deposits of Gilpin County and Central 

 City, and the tungsten deposits of Nederland were formed at this time. Although 

 most of the mineralization in the San Juan Mountains occurred in Miocene 

 time, some deposits of Eocene age were formed near the centers of early 

 Tertiary intrusion in that region. 



Sangre De Cristo Range 



The Wet Mountains are almost separated from the Front Range by the 

 Canyon City embayment, but in the early history of the Ancestral Rockies 

 and the later history of the Laramide orogeny, the two were closely as 

 sociated. The eastern border of the Wet Mountains is characterized by 

 Laramide overturning or overthrusting of the Carboniferous and Meso 

 zoic formations toward the east. The western border is one of sharply up 

 turned beds and steep faults. 



West of the Front Range in the area of overlap on the ancestral Colo- 

 rado Range and near the center of the Carboniferous basin, sharp folds 

 and several large thrust faults have resulted in a chain of ranges, prin- 

 cipal of which are the Sawatch and Sangre de Cristo. They extend to the 



