250 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



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J LIMESTONE »N0 DOLOMITE 



UNCOMPAHGRE 

 UPLIFT 



Fig. 15.13. Pennsylvanian deposits of the Paradox basin. After Herman and Barkell, 1957. 



Pre-Pennsylvanian Setting 



The total thickness of the Paleozoic formations present in central 

 Colorado by the end of Lower Mississippian (Leadville) time was only 

 1000 feet. In southwestern Colorado, only 400-500 feet existed, and in the 

 northern part of the Front Range, they were still thinner. Since the fairly 

 pure Mississippian limestones occur in areas close to the Pennsylvanian 

 ranges and no lithologic changes are evident in the limestones as the 

 ranges are approached, the Mississippian seas probably spread over the 

 sites of the highlands (Lovering, 1933). 



Evidence of thinning, probably by erosion, is evident when isopachs 

 are worked out, and it appears that some of the ranges first began to be 

 expressed in latest Mississippian time, as illustrated in Fig. 6.6. The New 

 Mexico arch of Mississippian age exposed Precambrian rock over much 

 of central New Mexico. 



Uncompahgre and Colorado Ranges 



The Uncompahgre and Colorado ranges were flanked by basins as 

 indicated on the map of Fig. 6.7; the Paradox, the Central Colorado, and 

 the Denver. The extreme and abrupt facies changes of sediments de- 

 posited against their flanks is the evidence of the sharp uplifts. One of 

 the flanking basins, the Paradox, is illustrated in Fig. 15.13. The south- 



west margin of the Uncompahgre Range was a fault scarp, and the thin 

 pre-Pennsylvanian sedimentary veneer was soon stripped from the rising 

 block, with the Precambrian crystallines furnishing flood deposits of 

 arkose to the adjacent subsiding basin. During part of Pennsylvanian 

 time evaporite conditions prevailed and four evaporite sequences — 

 cyclothems — resulted (Herman and Rarkell, 1957). This part of the 

 Hermosa formation is the Paradox facies or member. 



The Molas is Atoka in age and the Hermosa spans the Des Moines, 

 Missouri, and Virgil. The Cutler extends on into the Permian. The time 

 of the most vigorous uplift, then, is clearly Atokan through to the begin- 

 ning of the Permian. 



The Pennsylvanian and Permian sediments overlap the gently beveled 

 edges of the older Paleozoic rocks and rest on Precambrian crystallines 

 in the cores of the old ranges ( Lovering, 1933; Rurbank, 1933; Glockzin 

 and Roy, 1945). See Fig. 15.14. The crystalline rock was the source of 

 many of the Pennsylvanian and Permian strata which are commonly 

 coarse and arkosic near the old landmasses. For instance, Rrill (1944) 

 describes the sediments of the central Colorado basin as consisting mostly 

 of red and gray arkoses, arkosic conglomerates, sandstones, siltstones, and 

 gypsum which thicken to 13,000 feet in the deepest part of the basin. 

 Lateral variations are abrupt and extreme. During the most active time 

 of uplift of the adjacent ranges, the coarse elastics were deposited as 

 deltas along the margins of the trough, and the fine-grained sediments 

 were carried into the center. Identical mineral assemblages in the elastics 

 on both sides of the basin indicate that the exposed bedrock of both the 

 Uncompahgre and the Colorado Range was much the same. 



Pedemal Uplift 



The Pedernal landmass, named by Thompson (1942) from the Pedemal 

 Hills, is a large north-south-trending range in east central New Mexico, 

 about midway between the Rio Grande and the Pecos rivers. Red shales, 

 sandstones, variegated shales, and limestones of Permian age rest directly 

 on igneous and metamorphic rocks of Precambrian age in an area ex- 

 tending from the eastern side of the Sacramento Mountains, Otero 



