48 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



smaller valleys. The geologic map at this time would have appeared as in 

 Fig. 5.11, when formations from the Middle Ordovician St. Peter sand- 

 stone to the Upper Mississippian Kinkaid limestone cropped out. The 

 southeast border of the basin sank progressively and resulted in a regular 

 increase in thickness of the uppermost Pennsylvanian strata in that direc- 

 tion (Wanless, 1955). 



The assemblage of faults in southern Illinois, shown on the map of 

 Fig. 5.9, consists principally of the following trends: (1) the east- west 

 trend of the Rough Creek-Shawneetown system which extends west into 

 Illinois as the Cottage Grove and associated faults; (2) a prominent 

 northeast-southwest system of faults which is dominant in the fluorspar 

 district; and (3) the Wabash Valley fault system with north-northeast 

 trend, a few of which cross and offset the Rough Creek fault. These 

 faults are post-Pennsylvanian in age and the maximum throw is about 

 800 feet along the Rough Creek fault. 



Studies of crossbedding and stratigraphic relations indicate that the 

 late Mississippian Chester sands as well as those of the Pennsylvanian 

 came mostly from the northeast (Potter et al., 1958, Potter and Siever, 

 1956, and Wanless, 1955), and some were probably carried by streams 

 from the site of the Michigan basin across the site of the previous Kanka- 

 kee arch. A minor amount of sand came from the Transcontinental Arch. 

 See Fig. 5.1. 



Nashville Dome 



The Nashville dome is at present the site of a topographic basin, with 

 surrounding escarpments of successively younger rocks. Ordovician strata 

 are the oldest rocks exposed in the core, and the escarpments are in the 

 overlapping Mississippian and Pennsylvanian formations. The dome ex- 

 perienced several movements in pre-Chattanooga (early Mississippian) 

 time, synchronous with those of the hinterland of the Appalachian geo- 

 syncline, according to Wilson ( 1935 ) . The dome was below sea level dur- 

 ing several epochs of various lengths of time, and during other times the 

 central part was above sea level but probably so slightly emergent that 

 little erosion occurred. The structure is a broad, gentle arch, less because 

 of uplift than because of greater subsidence of the adjacent basins. Its 



domal structure was acquired by gentle sags between it and the Ozark 

 dome on the west (Wilson, 1939) and the Cincinnati dome on the north 

 (MacFarlan, 1943). See cross section of Fig. 5.12. 



The first major uplift in which considerable truncation of the beds oc- 

 curred was in late Devonian time. The Chattanooga shale rests on the 

 Trenton (Ordovician), showing that about 500 feet of beds had been 

 eroded away in the central part of the dome consequent to this pre- 

 Mississippian doming (Wilson and Born, 1943). 



The second major uplift was in late Mississippian and early Pennsyl- 

 vanian time, when its associated domes, the Ozark and Cincinnati, were 

 also elevated (Plate 5). The Chattanooga shale was domed gently, pro- 

 ducing regional dips of 16 feet per mile on the flanks, and along the axis, 

 both northeast and southwest, of about 8 feet per mile. A structural re- 

 lief of 700 feet was acquired by the dome above the saddle separating it 

 from the Cincinnati dome on the north. The structural relief of the dome 

 over the flanking basins was at least twice as much (Wilson and Spain, 

 1936). 



Detailed structure contour maps reveal many local irregularities in the 

 Nashville dome. A conspicuous "grain" to the northwest is noted by Wil- 

 son and Born (1943), and axes of folds may be drawn in a few places. A 

 structure contour map of the Pencil Cave ( Ordovician ) formation shows 

 the grain equally as well as one drawn on the Chattanooga shale ( Missis- 

 sippian), but the local structures are not closely superposed. It may, 

 therefore, be inferred that part of them originated in pre-Chattanooga 

 time, and part in post-Chattanooga. 



Cincinnati Dome 



The Cincinnati dome is much like the Nashville dome, and is separated 

 from it by a shallow structural saddle. Several writers refer to the two 

 domes together as the Cincinnati arch, with the central part of the 

 northern structure, the Jessamine dome, and the Nashville dome as ele- 

 ments of it. The Cincinnati dome splits into two branches on the north, 

 one extending to the west-northwest and the other to the north-northeast, 

 which are known, respectively, as the Kankakee arch and the Findlay 

 arch. 



