COLORADO AND NEW MEXICO ROCKIES 



393 



PICEANCE BASIN 



Fig. 25.4. Section along the Colorado River from the Gore Range to Rifle and the Piceance 

 basin. Tgr, Green River fm.; Tw, Wasatch fm.; Kwf, Williams Fork fm.; Ki, lies fm.; Kmc, Mancos 

 sh.; Kb, Benton gr.; Kd, Dakota ss.; Jm, Morrison fm.; Je, Entrada ss.; \ Chinle sh.; Cpp, 

 Permian and Pennsylvanian; CD, Miss, and Dev.; OC, Ordovician and Cambrian. After Bench 

 et a/., 1948. 



veloped as independent units, not much controlled by the ancestral uplift. 

 The flanks of the central Colorado ranges are replete with thrusts, and 

 apparently reflect the superior uplift of the Front Range. 



Harms (1961) has studied the sandstone dikes of the eastern margin 

 of the Front Range south of Denver and presents a convincing case for 

 granite tectonics there. Large Laramide faults place Precambrian rocks in 

 contact with sediments as young as Tertiary in age. The stratigraphic 

 displacement in places is 15,000 feet and the structural relief from 15,000 

 to 25,000 feet. He concludes that the stress distribution causing the in- 

 fection of the sandstone dikes was governed by dip-slip movement along 

 ,.steeply westward dipping, convex upward fault surfaces, and that, there- 

 [fore, the major structures outlining the flank of the range are high-angle 

 reverse faults which steepen with depth. 



Uplift of the Front Range began in middle Pierre time while the Denver 

 jbasin was sill being downwarped, and from that time till well into the Paleocene 

 ,the central part of the range moved upward at an ever increasing rate. Parts 

 of it rose above the ocean during Fox Hills time, and at the beginning of 

 'Denver time large areas were shedding pre-Cambrian debris to the east and 

 west. Intense folding and faulting occurred at the edges of the basins of 

 .deposition where the troughs merged with the old positive element about the 

 [end of Denver time and outlined the Front Range as it now is (Lovering and 

 Goddard, 1950). 



>i 

 North Park Thrusts 



J An unusual example of thrusting in the general Front Range region, 

 and as indicated previously one that represents considerable horizontal 



movement, is at Cameron Pass. Here the Never Summer Range borders 

 on the southern part of the Medicine Row Mountains adjacent to North 

 Park. A tear fault extending along the Middle Fork of Michigan Creel 

 separates two patterns of thrusting (Gorton, 1953). See map of Fig. 25.5. 

 Although they developed simultaneously, each produced its own struc- 

 tures. The block on the north exhibits two thrusts, as shown in the upper 

 section of Fig. 25.6, whereas the block on the south is interpreted to have 

 one thrust. All thrusts have been folded, and the sequence of events 

 appears to Gorton as follows: 



1. Folding, probably in Late Cretaceous. 



2. Thrusting, post-Middle Park and pre-North Park. 



3. Open folding. The quartz monzonite stock was emplaced during this 

 stage or immediately afterward. 



The Renton Gulch thrust klippe and the downfolded Never Summer 

 slice would be interpreted by some geologists as detached gravity slide 

 blocks, and the amount of compressional orogeny minimized. The writer 

 is inclined to view vertical uplift as of paramount importance with mar- 



Fig. 25 5. Thrusts of the Cameron 

 Pass area, Never Summer Range, 

 Colorado. After Gorton, 1953. 



