LATE PALEOZOIC ZONES OF FAULTING AND CRYPTOVOLCANIC OR METEORITE IMPACT STRUCTURES 



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southeast and 290 feet per mile to the west. The relief of this rather 

 sharp, small dome is at least 1500 feet. Surrounding the crystalline core 

 is a "disturbed area" 20 miles in diameter in which the sedimentary rocks 

 are severely deformed. Mississippian limestone and Lower ( ? ) Cretaceous 

 shale have been sampled in drill cores in the disturbed area, and there- 

 fore the structure was formed in post-Early Cretaceous time. The upper 

 200 feet of the Precambrian rock beneath the drift is badly shattered. 



The writers believe that the crystalline rock was forced upward into 

 the limestone and shale, and in the process was badly shattered. The 

 mechanism responsible is postulated to be a hidden igneous intrusion. 



Magnet Cove 



The Magnet Cove structure is included by Rucher in his resume of 

 cryptovolcanic structures, although it consists of an elliptical intrusive 

 complex about 3 miles across and is within the compressional structures 

 of the Ouachita Mountains. See map of Fig. 16.4. The igneous rocks are 

 alkaline, and for the most part belong to the nephelite-syenite group 

 (Landes, 1931). The peripheral intrusives, which are more resistant to 

 erosion than those toward the center of the complex, form a circular ring. 

 Metamorphosed sandstone and shale border the intrusions in places. 



Some time after the folding of the Ouachitas, a stock of highly alkalic 

 magma was intruded into the Paleozoic rocks, and then either by differ- 

 entiation or through separate intrusions several rock types were formed 

 and an unusual suite of minerals was emplaced. Compounds of titanium 

 are especially abundant. 



Upheaval Dome 



The Upheaval dome is in the flat-lying red-beds of the Colorado 

 Plateau and is sharply conical with a surrounding ring-like syncline. From 



the axis of the syncline on one side to the axis of the syncline on tin- 

 other is only 2 miles, and the diameter of the entire affected area is 3 

 miles. The White Rim sandstone member of the Cutler formation appears 

 as huge, up-ended blocks the size of a house in the highly disturbed 

 central area (McKnight, 1940), and the cliff-making Wingate sandstone 

 rings a spectacular crater about a mile in diameter. 



Roth aeromagnetic and gravity surveys have been made of the area. 

 The magnetic contours resolve two strong and symmetrical highs, one 

 directly over the Upheaval dome and the other about 7 miles to the 

 southeast. The gravity survey also indicates two structures in about the 

 same places but not so distinctly. Joesting and Plouff (1958) conclude 

 that the broad magnetic and gravity highs each require the rise of a 

 mass of Precambrian crystalline rock about 5 miles in diameter 2000 

 feet above its normal position. Salt flow emphasized the one dome 

 ( Upheaval ) but failed to materialize for some unknown reason in the 

 other. Lastly, because the gravity anomalies are not entirely satisfied by 

 the salt plug, a small igneous intrusion into the salt of the dome is postu- 

 lated. The process took place in several steps from Permian to the 

 Miocene. Refer to "salt anticlines" in Chapter 26 on the Colorado Plateau. 



Meteorite Impact Origin 



With the space age has come increased interest in terrestrial meteorite 

 impact craters, and Dietz (1960) has called attention to this theory of 

 origin, especially for such structures as Serpent Mound (Fig. 16.3) and 

 the Wells Creek basin (Fig. 16.2). Evidence for the impact theory comes 

 from the presence of shatter cones (small percussion fractures in conical 

 shape) and coesite powder, a high pressure crystalline form of silica, 

 supposedly generated at the time of impact. According to some geologists, 

 the theory is gaining much favor. 



