50 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



a large fault zone that extends across several states. Local structures are 

 not as well mapped as in the Nashville dome; but as far as known, the 

 perceptible northwest "grain" does not exist. Instead, one or two "highs" 

 have been described on the eastern flank of the dome that trend parallel 

 with the main axis. It may be that with better contouring, a northwest 

 direction of local structures will be noted. 



Kankakee Arch 



The Kankakee arch, as defined by Ekblaw (1938), is the northwest 

 branch of the Cincinnati dome, and passes in a northwest direction across 

 Indiana and Illinois, connecting with the Wisconsin dome. Kankakee is 

 preferred to Wabash, a name sometimes used. The earliest significant 

 uplift preceded the deposition of the St. Peter sandstone, as in the Wis- 

 consin dome. The St. Peter sandstone rests on Cambrian beds at Oregon, 

 Illinois, indicating arching above sea level and removal of 500 to 600 feet 

 of rock in this early movement. The Cambrian and Prairie du Chien ( pre- 

 St. Peter) beds are believed to be about 4000 feet thick, both on the 

 Kankakee arch and in the Illinois basin, and therefore the arch was evi- 

 dently an area of subsidence just as much as the basin until Early Ordo- 

 vician time. 



Oil wells show that the structural relief at present, if measured on the 

 top of the Trenton limestone, is about 6000 or more feet in relation to 

 the Illinois basin and 10,000 feet in relation to the Michigan basin. As the 

 Trenton is above the St. Peter, the arch has acquired this much additional 

 structural relief since the pre-St. Peter uplift. It is clear that the large part 

 of this structural relief is a result of subsidence of the basins on either 

 side of the arch, and that the upward movements of the arch itself, suffi- 

 cient to cause it to be eroded, contributed only in small part to the relief. 

 See Figs. 5.4 and 5.5 for pre-St. Peter structural relations. 



The only reflection of the Middle and Late Devonian uplifts of the 

 nearby Cincinnati and Nashville domes is the conspicuous thinning of one 

 of the zones of the Traverse group in the Michigan basin toward the 

 arch (Cohee, personal communication). The greater subsidence of the 

 basin area than the arch area, as indicated by this zone in the Traverse, 

 occurred in late mid-Devonian. The basin had previously sunk rapidly, 

 and a thick evaporite series was deposited during the Silurian and pre- 



Traverse Devonian. These thick salt, gypsum, limestone, and dolomite 

 beds are represented by thinner nonevaporite series in the Illinois basin, 

 and hence the structural relief of the arch is not so great to the southwest 

 as to the northeast. 



The early Mississippian seas probably spread over the arch even though 

 Lower Mississippian rocks are not there today. This is concluded because 

 the beds do not display any characteristics of overlap on a land area. The 

 Upper Mississippian (Chester) beds of Illinois are not represented in 

 the Michigan basin, nor anywhere north of the Kankakee arch, and it 

 therefore seems that in late Mississippian time the arch and the area to 

 the northeast were gently emergent, and from this region and still farther 

 north the Chester sands were derived. Since the present structure dis- 

 plays the geologic pattern of a broad anticline with Silurian rocks in the 

 core and Devonian and Mississippian successively away on either side, it 

 follows that in addition to regional uplift over the Great Lakes region in 

 late Mississippian time there must also have been local uplift along the 

 arch. This movement occurred at the same time as the one described in 

 the Cincinnati dome with which the Kankakee arch merges. 



The deposition of Pennsylvanian sediments across the Cincinnati dome 

 on a surface of appreciable relief corresponds to the well-known Pennsyl- 

 vanian overlap in Illinois south of the Kankakee arch and over the La 

 Salle anticlinal belt. The Upper Mississippian and pre-Pennsylvanian up- 

 lift along the arch was probably a movement of only a few hundred feet. 

 Again it was the appreciable subsidence of the adjacent basins that con- 

 tributed most to the arch structure. 



Recause the Pennsylvanian strata were gently arched and eroded back 

 from the Cincinnati arch, a post-Pennsylvanian uplift of gentle but broad 

 dimensions is indicated. It appears that the uplift spread northward so as 

 to embrace the Kankakee arch, the Wisconsin dome, the Michigan basin, 

 and the southern part of the Canadian shield. 



In summary, the Kankakee arch acquired its structural relief chiefly by 

 greater subsidence of the basins on its sides than by actual uplift. It was 

 lifted out of water in early Ordovician time, and in one place it suffered 

 600 feet of erosion. Again it rose out of water in late Mississippian time, 

 and finally participated in a regional uplift of the Great Lakes region in 

 the late Pennsylvanian. 



