16. 



THE LATE PALEOZOIC 

 ZONES OF FAULTING 

 AND CRYPTOVOLCANIC 

 OR METEORITE 

 IMPACT STRUCTURES 



FORELAND ARCUATE FAULT ZONE 



An arcuate zone of faults extends from the Llano dome in Texas north- 

 ward to Oklahoma, northeastward to the Ozark dome and eastward across 

 Kentucky to West Virginia. The faults are subparallel with the zone for 

 the most part, but some are divergent, especially two long faults in Mis- 

 souri which strike northwestward directly athwart the zone. See Tectonic 

 Map of the United States, 1944. The zone crosses domes and basins alike, 

 and therefore does not seem to be controlled by them. On the other hand, 

 the fault zone wraps around the Ouachita arc of the marginal orogenic 

 belt of the continent, and although the fault zone has a lesser curvature 



than the Ouachita arc and departs from it a considerable distance on the 

 north, the subparallelism may mean that a genetic relationship exists. 



Most of the faults are known to have originated in Pennsylvanian time 

 or immediately thereafter. A few others are post-Devonian. Thus the time 

 relation as well as the spatial indicates that the zone of faults is a single 

 tectonic element. 



The faults in the Llano uplift are known from surface mapping. Oil 

 wells and geophysical prospecting have extended the known length of 

 some of the faults more than 100 miles to the north-northeast into the 

 Strawn basin (Cheney, 1940). The faults are probably of the high-angle, 

 normal variety, and have blocked out narrow grabens and horsts. The 

 high blocks have been named from west to east, the Richland Springs, 

 Pontotoc, San Saba, and Lampasas axes. According to geophysical work 

 in the Fort Worth basin, some of these faults have displaced the Smith- 

 wick formation 1100 feet, so the movement occurred in post-Smithwick 

 (Early Pennsylvanian) time. However, beds only slightly younger than 

 Smithwick, namely middle Strawn, are only slightly disturbed along a 

 major fault near Regency in the Colorado River area, and the faults are 

 not known in still younger Pennsylvanian beds. Cheney, therefore, con- 

 cludes that the faulting in the Llano dome and Strawn basin occurred 

 in early Pennsylvanian time. 



The Stonewall fault in the Hunton arch area of southern Oklahoma is 

 said to have occurred in about middle Strawn time and to have a displace- 

 ment of about 3500 feet (Morgan, 1924), but from Dott's (1934) discus- 

 sion the fault may be one of the Arbuckle group of thrust faults and not a 

 part of the arcuate fault zone. 



A group of faults in the northeastern corner of Oklahoma bound six 

 small crustal blocks, each about 6 miles wide (Wilson, 1937). The faults 

 trend in general northeastward, and some have been traced for 50 miles. 

 One block is tilted to the north; two are tilted to the south, and the re- 

 maining three are about level. Their throws range from 90 to 600 feet, but 

 these figures apply only to surface offsets. The faults, as well as the folds 

 in this region, become more pronounced in the older underlying forma- 

 tions. Where it is possible to trace the stratigraphy at depth by oil wells, 

 the structural relief decreases upward through the conformable sand- 



253 



