LATE PALEOZOIC ZONES OF FAULTING AND CRYPTOVOLCANIC OR METEORITE IMPACT STRUCTURES 



257 



depression with irregular and local faulting. Including the marginal ring, 

 they range in diameter from 2 to 8 miles. The inner intensely deranged 

 core may be only part of a mile across in some, but in others up to 2 

 miles. 



The faults make both an approximate concentric pattern and a radial 

 pattern. In some, the radial pattern is resolved strongly into a northwest- 

 southeast direction. Examine the representative illustrations of Figs. 16.2 

 and 16.3. 



i Distribution 



The map of Fig. 16.1 shows the distribution of the cryptovolcanic 

 structures. Numbers 1 to 6, and 9 are in the general arch and dome area 

 of the central Mississippi Valley. Numbers 1 to 4 are in the Cincinnati 

 and Nashville domes, number 5 in the Ozark dome, and number 6 in the 

 | Kankakee arch. They avoid the Illinois basin fairly well. Number 7 is 

 I in the orogenic belt of the Ouachita Mountains, but it is dissimilar to 

 the rest in having igneous rocks exposed in the core, in being free of 

 faults, and in being the site of considerable mineralization. See Fig. 14.2. 

 Number 8 is in the Colorado Plateau and is complexly associated with 

 salt dome upheaval. 



Origin and Age 



No volcanic rocks are associated, at least at the surface, with the small 

 circular structures of the Central Stable Region, yet their circular shape, 

 their upheaved, broken, and in places brecciated condition, and the 



J presence of a number of dikes cutting the near horizontal Paleozoic rocks 

 in surrounding areas, lead Rucher to imagine an explosive volcanic 



i origin. 



These cryptovolcanic structures are thought to be the result of a sudden lib- 

 eration of pent-up volcanic gases, which had accumulated near the surface, the 

 ; explosion having been too weak to produce a shallow crater such as formed in 

 ] the Ries Basin, southern Germany (Bucher, 1933). 



These unique structures in the United States have been eroded more 

 than those of Tertiary age in Germany, and so Rucher regards them as 

 older and of probable late Paleozoic or Mesozoic age. 



Fig. 16.3. Structure contour map of Serpent Mound, O. The length of each square is about 

 2200 feet. Reproduced from Bucher, 1933. 



Recently a new cryptovolcanic (?) structure has been found near 

 Manson, Iowa. It is number 9 on Fig. 16.1. Unlike the others it has a 

 Precambrian crystalline core about lM square miles in area which lay 

 unknown because of a cover of glacial drift until discovered by core 

 drilling (Hoppin and Dryden, 1958). In this area a thin Paleozoic veneer 

 of sedimentary rocks plus a cover of Cretaceous shale is the normal 

 expectation under the drift. The contact of the crystalline rock with the 

 surrounding sedimentary rocks dips outward 350 feet per mile to the 



