52 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Nemaha Range 



A very sharp uplift, the Nemaha Range, trends south-southwest from 

 Omaha through southeastern Nebraska across Kansas into northern Okla- 

 homa, but is now buried. See Figs. 5.13 and 14.1 and Plate 6. It came 

 into mountainous relief during early Pennsylvanian time, because the 

 Mississippian strata are tilted up and truncated along its sides. Uplift 

 and dissection were sufficient to expose Precambrian crystalline rocks in 

 the core before burial. See cross section of Fig. 5.14. Structural relief is 

 3600 feet in the central part of the range, and the east flank is so steep 

 and straight that a block-fault movement has been visualized ( Lee et al., 

 1946). The range was eroded rapidly so that the Pennsylvanian strata, 

 partly derived from the range itself, encroached on its flanks and, to- 

 gether with much exotic material perhaps in part from the early Ouachi- 

 tas, finally buried the range. The present depth of the "granite" at the 

 Kansas and Nebraska line is only about 400 feet (500 feet above sea 

 level), but at the Kansas and Oklahoma line, it is over 3000 feet below 

 the surface (2500 feet below sea level). 



The Nemaha Range contrasts strongly with the central Kansas arch in 

 relief and symmetry. The Nemaha Range has 3600 feet of relief, whereas 

 the arch has 1500. The range has a very steep eastern front and gentle 

 back slope, whereas the arch is symmetrical and gentle. The nearly north- 

 south trend of the Nemaha Range is unlike the northwest trend of the 

 broad, gentle arches, and this sets it apart from the arches as a different 

 structural type. It resembles the Colorado Range of the Ancestral Rockies, 

 and therefore the characterization of it as a range is more appro- 

 priate than as an arch, anticline, or ridge, as it has variously been 

 called. 



Bourbon Arch 



Slightly north of the site of the previous Chautauqua arch, a later but 

 narrower one rose in early Pennsylvania time. It was probably a shallow 

 platform between the Forest City basin on the north and the Cherokee 

 basin on the south (Moore and Jewett, 1942). See Fig. 5.13. 



Ozark Dome 



At present, the Ozark dome is a broad, nearly circular area of Cambrian 

 and Ordovician limestones, surrounded by escarpments of Mississippian 

 limestone. In the east central part, knobs of pre-Cambrian crystalline 

 rocks project through the Cambrian and Ordovician strata to the surface. 

 The crystalline outcrops occur in southeastern Missouri, the area of the 

 St. Francis Mountains, and the strata dip everywhere away from them 

 (Croneis, 1930). The dome itself spreads over two-thirds of the state, 

 and also into northern Arkansas where the Boston Mountains make up 

 the southern flank. The Precambrian surface had considerable relief, and 

 the younger strata were deposited on it with initial dips in places up to 

 30 degrees (Bridge, 1930). 



The first major unconformity in the Paleozoic succession around the 

 Ozark dome, especially on the west side in the Forest City basin, is at the 

 base of the St. Peter sandstone. Lee et al. (1946) summarize the subsur- 

 face geology in maps and cross sections and show that subsidence took 

 place in the Ozark region in pre-St. Peter time, while upwarping took 

 place in southeastern Nebraska and northeastern Kansas (the Nebraska 

 arch). The structural relief between basin and uplift was about 2000 

 feet. 



With the coming of St. Peter time, the crustal movements were re- 

 versed, and the Ozark basin now started to rise as the Nebraska arch 

 started to subside. At the end of Silurian time, widespread erosion oc- 

 curred, with the greatest amount around the rising Ozark dome. The 

 Devonian strata not only rest on the truncated Silurian and older rocks 

 around the dome, but themselves in turn are truncated and covered by 

 the Mississippian strata. 



The Mississippian overlap is most extensive and very well known from 

 many well records on the west side of the dome. Consult the geologic 

 map of the close of the Devonian, Plate 5, and cross sections of Figs. 5.15 

 and 5.16. The unconformity indicates that the dome was again uplifted in 

 late Devonian time and considerably eroded. The pre-Mississippian geo- 

 logic maps of the region (Moore and Jewett, 1942; Wrather, 1933; Lee 

 et al., 1946), together with surface outcrops, indicate that the Ellis arch, 



