16 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Silurian (Plate 4) 



The Transcontinental Arch became very wide and well-defined. The 

 Michigan Basin, in which extensive deposits of salines accumulated, and 

 one in Pennsylvania and West Virginia took lasting form. The Ozark 

 dome and Texas arch became prominent. 



Devonian (Plate 5) 



During Devonian time, the Transcontinental Arch rose, but was only 

 gently emergent; and strata previously deposited across its site were 

 removed by the close of the period, except for the Colorado sag where 

 100 to 200 feet of beds remained. In Canada, the great arch bifurcated 

 into a broad arch west of Hudson Bay and another one east of Hudson 

 Bay, but this condition probably did not arise until the close of the 

 Devonian. During the Devonian the arches were at least partly sub- 

 mergent, because Manitoba fossil faunas are very similar to those of 

 Michigan and the Hudson Bay region. 



Transverse arches also developed. The Ellis-Ozark uplift extended 

 from Kansas around the south end of the Illinois-Indiana-Kentucky basin 

 to the Nashville dome, and thence northward to the Cincinnati dome. 

 The Illinois-Indiana-Kentucky basin, with the intervening Kankakee arch 

 or area of much less subsidence came into prominence for the first time. 



A basin of subsidence centered in Pennsylvania during the Devonian, 

 and sediments were supplied from the eastern Taconic orogenic belt 

 which was being elevated adjacent to the basin and undergoing unrest 

 premonitory to the Acadian orogeny. The basin sank mostly in late 

 Devonian time, and its dominantly clastic and subaerial sediments coarsen 

 toward the east. The western and northern marine facies constitute the 

 classic Devonian section of the continent. 



A Devonian trough extended northward through New England, the 

 Maritime Provinces, and Newfoundland, in which much volcanic material 

 was deposited along with various clastic sediments. In New England sedi- 

 mentation was mostly east of the main Taconic belt, but in Quebec it 

 occurred directly on the eroded Taconic structures. The entire region, 

 beginning perhaps in mid-Devonian time in places, gradually became 



involved in the great Acadian orogeny. The strata were folded, intruded, 

 metamorphosed, and thrust-faulted to form a complex of dominantly 

 crystalline rock. The Acadian belt extended southward through the 

 Crystalline Piedmont of the eastern United States, where numerous 

 large batholiths were emplaced and considerable metamorphism occurred. 



At the close of Devonian time a belt of orogeny, the Antler, formed j' 

 in the central part of the western geosyncline. The belt continued active 

 by way of folding and thrusting through Pennsylvanian, Permian, and 

 Mesozoic time with a number of phases of orogeny fairly accurately 

 documented. It effectively separated the miogeosyncline on the east from 

 the eugeosyncline on the west. 



Devonian and Silurian strata have been identified on the Pacific margin 

 of the continent in the Klamath Mountains, and therefore it is concluded 

 that the continental margin was then about where it is now. 



Mississippian (Plate 6) 



Mississippian seas were widespread and in the Rocky Mountain region 

 a small basin subsided 10,000 feet along the Idaho-Montana boundary. A 

 long eastward-extending basin sank through central Montana, and is 

 known as the Big Snowy. A broad eugeosyncline of poorly known limits 

 extended through northern California, southern Oregon, and north- 

 western Nevada west of the Antler orogenic belt. The amount of subsid- 

 ence is unknown. The Antler oros;enic belt in central Nevada was 

 marked by major thrusting and complex folding. 



The Transcontinental Arch sagged gently through its central area and 

 was covered, but by the close of Early Pennsylvanian time it had risen 

 enough to have suffered erosion, and the Precambrian was again exposed. 

 The Texas arch was covered in central Texas and the Ozark-Nashville 

 arch was severed from the Transcontinental Arch. 



In latest Mississippian or earliest Pennsylvanian time, a deep and prob- 

 ably large basin sank rapidly in eastern Texas, southern Oklahoma, and 

 western Louisiana, and received about 17,000 feet of clastic sediments. 



The La Salle anticlinal belt first began to rise at the close of the 

 Mississippian and continued to grow during the Pennsylvanian. It split 

 the Illinois-Indiana-Kentucky basin in two parts. 



