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STRUCTURAL GEOLOGY OF NORTH AMERICA 



stones and shales of the Atoka, Hartshorne, McAlester, Savanna, and 

 Boggy formations. Bloesch (1919) believed that this decrease is due to 

 recurring deformation during the deposition of the above formations, 

 which are of the late Early Pennsylvanian age. 



North of the six fault blocks and parallel with them is the hundred- 

 mile-long Seneca fault. It extends into the southwest corner of Missouri. 

 Surface evidence for the fault is not conclusive everywhere along the 

 structure, and it may be a syncline rather than a fault in several places 

 (Weidman, 1939). 



Several rows of small faults are well known in Creek and Osage coun- 

 ties, Oklahoma, just west of the previously mentioned Seneca fault. The 

 individual faults are arranged en echelon and trend northwest. The rows 

 trend nearly north and make an angle of about 45 degrees with the in- 

 dividual faults. On Plate 8 the rows are indicated by dashed lines. The 

 largest stratigraphic throw at the surface is about 130 feet, and the great- 

 est length is 3/4 miles; yet the length of one of the rows is 80 miles. They 

 are all normal faults. 



Fath (1920) analyzed the en echelon faults as follows. The Precam- 

 brian crystalline rocks were cut by a system of faults before the Paleozoic 

 veneer of sediments was laid down. In fact, peneplanation had removed 

 most of the relief incident to the faulting before the Paleozoic beds started 

 to accumulate. Beginning in Early Pennsylvanian time, the faults in the 

 basement complex again became active, this time, however, with hori- 

 zontal (strike-slip) movement. The overlying veneer was shear-strained 

 along the underlying faults and broke in rows of small faults arranged 

 en echelon. The east side of each fault in the Precambrian moved north- 

 ward. The movement recurred several times during the Pennsylvanian, so 

 that the throw of the faults is greater at depth. Some rows of en echelon 

 faults may not even show in the Pennsylvanian beds at the surface today, 

 and others are reflected in small asymmetrical anticlinal flexures over the 

 faults. Several such rows of anticlines farther west in Oklahoma and north 

 in Kansas may belong to the same system. 



The postulate that the great arcuate fault zone is related to the Ouach- 

 ita lobe of the marginal orogenic belt is supported by Fath's mechanical 

 analysis. As would be expected in this theory, the foreland block directly 



in front of the lobe would be moved horizontally ahead of its left-hand 

 neighbor, which is the direction of shear indicated by the en echelon 

 faults. 



In eastern Missouri, two stages of faulting are recognized (Weller and 

 St. Clair, 1928), one in late Devonian and one in post-Mississippian. The 

 faults form a complex system, and the total displacement in the fault zone 

 ranges up to 1200 feet. 



The eastern Missouri faults continue eastward across the southern tip 

 of Illinois into Kentucky, where a region of widespread and intensive 

 faulting exists. Along the north side of this complex of faults is the Shaw- 

 neetown fault of southern Illinois, and its eastward continuation, the 

 Rough Creek fault zone of Kentucky. The Shawneetown-Rough Creek 

 fault zone is really an uplift that varies in structural relief and detail from 

 place to place (McFarlan, 1943). Most characteristic of the uplift is its 

 anticlinal structure. At places, a series of anticlines is developed, in part 

 asymmetrical to the north and broken to form reverse faults. Normal faults 

 arranged en echelon are also present. The structural relief of the uplift 

 ranges from a few hundred feet to 2500 feet, and Mississippian beds in 

 places are brought into outcrop. The complex structural zone divides the 

 Pennsylvanian coal basin into a northern and a southern division. 



Just south of the Shawneetown-Rough Creek structure in western Ken- 

 tucky is a cluster of high-angle faults, the main ones of which trend north- 

 east and east and have displacements up to 1500 feet. They are joined by 

 smaller cross faults. The area is semicircular, about 60 miles in diameter, 

 and is the most intensely faulted area in the interior lowlands. Along with 

 the faults, peridotite dikes and highly commercial veins of fluorspar occur. 

 The faults are post-Pennsylvanian and pre-Cretaceous. 



The Rough Creek fault zone is continued after a gap of a few miles to 

 central and eastern Kentucky by the Kentucky River fault and its asso- 

 ciates, the West Hickman fault, the Irvine-Point Creek fault, and other 

 smaller ones (McFarlan, 1943). Maximum displacement on the Kentucky 

 River fault is 600 feet. Some suggestion of pre-Pennsylvanian movement 

 and progressive movement has been made, but McFarlan believes the 

 faulting occurred in post-Pennsylvanian time. 



