18 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



of maximum subsidence and fill later became the locale of the great 

 Nevadan batholiths. 



Orogeny continued in central Nevada, and a small deep trough in 

 western Utah filled with sandstone, shale, and limestone. Extensive 

 shelf seas stretched eastward and southward. 



The Colorado and Uncompahgre ranges of the Ancestral Rockies 

 remained as islands in the surrounding seas. 



The previously compressed Marathons were elevated epeirogenically, 

 and in front of them several basins sank to considerable depth. The 

 platforms of little subsidence between were the sites of the previous 

 Pecos and Diablo ranges. The Anadarko basin also subsided appreciably. 



The Carboniferous basins and adjacent areas of New England were 

 intensely deformed either in Late Pennsylvanian or Permian time, and 

 in places intruded by granitic batholiths. The deformation is not defi- 

 nitely dated, but presumably it occurred after the beds of the Permian 

 basin of Pennsylvania and West Virginia had been deposited. It seems 

 probable, also, that folding in Pennsylvania and West Virginia occurred 

 at this time. 



The crustal movements and spread of seas in Late Pennsylvanian 

 and Permian time profoundly altered the geologic outcrop pattern of the 

 continent. The greatest change comes from the extensive overlap of the 

 pre-middle Pennsylvanian structures by the Upper Pennsylvanian and 

 Permian sediments. All the Transcontinental Arch southwest of Wis- 

 consin was buried, the structures of Kansas and parts of the Ozarks dome, 

 the Wichita and Arbuckle mountain systems, and the, ranges of west 

 Texas vanished beneath the deposits. Only the Colorado and Uncom- 

 pahgre ranges of the Ancestral Rockies remained visible, not because of 

 renewed uplift, but because of considerable relief inherent from their 

 original development. 



Triassic (Plate 9) 



Eugeosynclinal conditions continued in the west with extensive vol- 

 canic accumulations. Crustal unrest continued in central and western 

 Nevada. In northern Utah a basin subsided and collected 8000 feet of 



carbonates and elastics. Eastward the Triassic deposits are largely con- 

 tinental and red. An emergent corridor connected the Canadian Shield 

 with northern Mexico and southern Arizona. 



The Colorado and Uncompahgre ranges still stood as islands in the 

 surrounding deposits. 



The Marathon-Ouachita orogenic belt of earlier development was still j| 

 mountainous and had a broad piedmont generally free of deposits. The 

 mountainous belt may have risen gently as its rocks were eroded and 

 carried away, but orogeny there had ended. 



Within the metamorphic and igneous core of the Paleozoic orogenic 

 belts of the Atlantic margin, a zone of high-angle faults dropped basins 

 and raised blocks of mountainous proportions. Volcanism was a prom- 

 inent accompaniment of the faulting. The basins were the site of ac- 

 cumulation of thick, red, clastic sediments which were mostly derived 

 from the uplifted, adjacent blocks. The basins are narrow and long, and 

 because of their fault origin, their size was probably not much larger 

 originally than now. The faulting and igneous activity ran its course in 

 Late Triassic time, and the orogeny is known as the Palisades. 



Jurassic (Plate 10) 



The Cordilleran geanticline developed in Early Jurassic time and 

 separated a western trough effectively from an eastern. The western 

 again was one of extreme subsidence, and about 30,000 feet of volcanics, 

 black shale, and other sediments accumulated in it. Central Nevada con- 

 tinued to experience orogeny, and thrusting of large proportions oc- 

 curred. Late Jurassic was also a time of considerable batholithic intrusions 

 in central and northern California and possibly western Nevada. 



The eastern trough was generally the site of marine transgression and 

 deposition, but the Jurassic deposits are less extensive than the earlier 

 Permian, Triassic, and the later Upper Jurassic and Cretaceous. The 

 Jurassic overlap on the Paleozoic strata of Montana, Alberta, and Saskatch- 

 ewan, particularly on the Mississippian, is striking. The Mexican geosyn- 

 cline began to form. It was separated on the north by a peninsula, the 

 Coahuila, from the seas of the Gulf of Mexico. The wide basin of the 



