38 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



form of an embayment from Texas into southern Oklahoma resulted in 

 the local accumulation of more than 6000 feet of strata, the chief forma- 

 tion of which was the Arbuckle limestone. Another embayment possibly 

 extended to the western Texas region, where later the Pecos Range de- 

 veloped. The pre-Devonian sediments are thin in the Marathon and 

 Ouachita systems as compared with the Appalachian system. 



A rather deep basin formed in Michigan, Indiana, and Illinois in pre- 

 Devonian time, approximately parallel with the Appalachian geosyncline. 

 Its largest and deepest part is the present Michigan basin. 



The great western geosyncline of early Paleozoic time extended from 

 Alaska to southern California. It sank 15,000 to 20,000 feet across Nevada, 

 and at the Nevada-California boundary it contained over 20,000 feet of 

 beds (Nolan, 1943). No information is available farther southwest in 

 California because of the extensive Mesozoic and Cenozoic cover, in- 

 tensive metamorphism, and widespread Jurassic intrusions. The south 

 termination of the geosyncline shown on the map is, therefore, hypo- 

 thetical. The inner trough of the geosyncline becomes progressively deeper 

 to the southwest and undeniably heads into the later Jurassic orogenic 

 belt, which with still younger tectonic elements determines the margin of 

 the continent today. 



TRANSCONTINENTAL ARCH 



General Features 



During Devonian and Mississippian time the great Central Stable 

 Region of North America consisted of three major divisions, a central 

 northeast-southwest-trending Transcontinental Arch, and large basins, 

 shelves, and arches and domes of various sizes on each side (Plates 3 and 

 5). The arch had three peninsular extensions to the southeast, one into 

 Kansas and Missouri, the Ellis and Chautauqua arches and the Ozark 

 dome; one into Wisconsin, the Wisconsin dome; and possibly one into 

 Texas. It is also known to have sagged below sea level in two places 

 where thin lower Paleozoic sediments were deposited, one in Colorado 

 and one in Arizona. Until the rise of the ranges of the Ancestral Rockies 

 and the Wichita systems, the Transcontinental Arch and its flanking 



basins dominated the landscape. The Transcontinental Arch may have 

 bifurcated north of Lake Superior, with one arm extending northward 

 on the west side of the Hudson Bay basin, and the other extending first 

 eastward and then northward along the east side of the basin. This sup- 

 position is based on present Precambrian exposures, but paleontological 

 evidence and newly found erosional outliers suggest that much of the 

 area of the arms may have been submerged in early Paleozoic time. 



Northeast of Colorado 



The arch in Nebraska, South Dakota, and Minnesota was recognized 

 by Schuchert and called Souxia. It was later clearly depicted by Levorsen 

 (1931, PI. 1), and then still later mapped by Ballard (1942). The 

 boundaries of the formations shown on the geologic maps of the close 

 of the Devonian and the close of the Mississippian are those preserved 

 under the extensively overlapping Pennsylvanian strata (Plate 7) which 

 covered most of the arch. Ballard has gathered together the available well 

 records of the area and believes enough data is at hand to establish defi- 

 nitely the existence of the arch and fairly well the formational contacts 

 on either side of it. 



The arch was referred to as the continental backbone by Keith (1928) 

 in his notable paper on "Structural symmetry of North America," and 

 later, also, by Levorsen. The name implies that it was a strong, resistant, 

 centrally located tectonic element with flanking basins and marginal oro- 

 genic belts in bilateral symmetry. With the exception of the peninsulas 

 and sags previously mentioned, the bilateral symmetry of the United 

 States part of the continent in a northeast direction was pronounced until 

 the Pennsylvanian transgression. The building of the Ancestral Rockies 

 altered conspicuously the aspect of the Transcontinental Arch, and then 

 the late Mesozoic and early Cenozoic mountain building disturbances 

 left the southwest half unrecognizable on a geologic map of the present 

 time. 



The Transcontinental Arch appears very dominant on a pre-Pennsyl- 

 vanian geologic map, but this appearance should not be misinterpreted. 

 During the Devonian and Mississippian, the arch was very low-lying and 

 furnished chiefly chemical sediments to its flanking basins (Weller, 1931). 



