CENTRAL STABLE REGION OF THE UNITED STATES 



39 



The "backbone" was also not very strong in resisting deformation. In its 

 southwestern part, as previously mentioned, it was the site of Pennsvlva- 

 nian and Cretaceous-Tertiary mountain building, and its other parts have 

 been almost completely covered by Pennsylvanian, Permian, Mesozoic, 

 jand Cenozoic strata, in places of considerable thickness. 



Wisconsin Dome 



The area of central Wisconsin was probably uplifted several times in 

 the Paleozoic, but evidence both for time and spatial relations is scarce 

 and, therefore, all the geologic boundaries cannot be definitely fixed. 

 The isopach maps of the Ninth Annual Field Conference of the Kansas 

 Geological Society have been used as the chief source of information in 



i making the interpretations shown on the maps of this book. The isopach 

 maps generally show the existing thickness of the various formations or 

 groups, and their compilers say that the original thickness and extent 

 over the Wisconsin dome area is not certain. However, some of the 

 formations thicken basinward under cover of protecting formations, and 



! such contacts can be projected and the limits before burial located ap- 

 proximately. 



Two pre-Devonian times of significant uplift are recognized; the first 

 preceded the deposition of the St. Peter formation in Early Ordovician 

 time, and the second followed the deposition of the Silurian beds. During 



; the second uplift, an arch was formed that extended southeastward from 

 Wisconsin into Illinois, almost to the city of Kankakee (Fig. 220, Ninth 

 Annual Field Conference, Kansas Geological Society). 



By the close of Mississippian time, a pronounced dome had appeared 

 (Plate 6). A strip of Cambrian sediments extending southwest from the 

 Keweenaw peninsula of Michigan indicates that the dome was separated 

 from the Transcontinental Arch by a fairly broad, gentle syncline. A 

 broad, noselike uplift extended southeastward from the Wisconsin dome 

 in approximately the position of the post-Silurian arch and connected with 

 the Kankakee arch of Illinois and Indiana (Plate 6). How far the 

 Mississippian sediments spread over the dome area is not ascertainable, 

 but following the late Mississippian uplift they were eroded back appre- 

 ciably. 



Colorado and Arizona 



The rise of the Ancestral Rockies in late Mississippian and Pennsylva- 

 nia!) time destroyed the Transcontinental Arch in Colorado. The pre- 

 Pennsylvanian sediments present are very thin, and cover the arch 

 throng!) central Colorado in a /one 100 miles wide. The zone was i 

 dently the site of a gentle sag in the arch norma] to its length, and as 

 Burbank's (1933) map shows, it lines up almost precisely with the 

 Wichita trough that others have shown in Oklahoma and Kans.is. It 

 seems, therefore, that the Wichita trough extended northwestward toward 

 the Colorado sag, and not in the direction of the Amarillo Mountains in 

 the Panhandle of Texas as has been suggested by some writers. 



Arizona was mostly above water during the early Paleozoic (Stoyanow, 

 1942). The Mazatzal orogeny of Precambrian time (sec previous dis- 

 cussion in this chapter) produced a chain of mountains that extended 

 from southwestern Arizona to southwestern Colorado with subparalli 1 

 folds and thrust faults trending northeastward (Huddle and Dobrovolny, 

 1950). 



The orogeny and associated intrusions took place after the Mazatzal quartzite 

 was deposited. The mountains subsequently were well worn down by erosion, 

 but the very resistant Mazatzal quartzite formed ridges along the core of the 

 old mountain chain. The ridges served to separate the basins in which the 

 rocks of the Apache and Unkar groups were deposited. . . . Both were con- 

 siderably eroded before the Troy quartzite and Tapeats sandstone of Cambrian 

 age were deposited. . . . After the deposition of the Cambrian sandstones. 

 Mazatzal land probably was up-arched slightly and eroded, because the 

 Martin formation in central Arizona rests on a surface of some relief. Then- 

 are neither Ordovician nor Silurian rocks in central Arizona, and probably 

 there never have been any. Cambrian rocks may have extended through the 

 Mogollon sag, and a considerable thickness of them may have been removed 

 from Mazatzal land during the long erosional interval between the retreat oi 

 the Late Cambrian seas and the spread of the Late Devonian sets. The 

 gradual burial of the mountains and Mazatzal land before Pennsylvanian tunc 

 is summarized diagrammatically in Fig. 4.4. Because the Martin formation 

 was not deeplv eroded prior to the deposition of the RedwaD limestone, prob- 

 ably no diastrophic disturbances of Mazatzal land occurred at the close of 

 the Devonian. After the Mississippian limestone was laid down, however, 

 Mazatzal land again was uparched. as shown by the great erosional reduction 



