228 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Fig. 14.5. Cross section of the Black Knob Ridge area of the western end of the Ouachita Mountains. 

 After Hendricks, 1943. Formations may be identified by reference to chart, Fig. 14.3. 



crystalline Piedmont are exposed on the south flank of the folded and 

 thrust-faulted Ouachitas. These tectonic units have been looked for in 

 numerous wells which have penetrated the Cretaceous and Jurassic cover, 

 but the wells are apparently not sufficiently far enough down dip and 

 seaward to discern the units. 



/Aetamorphism 



The pre-Mississippian formations of the central anticlinorium or 

 "core" of the Ouachita Mountains in both Oklahoma and Arkansas are 

 slightly metamorphosed. The shales are dynamically altered to argillites, 

 meta-argillites, and in places to phyllites (Goldstein and Reno, 1952; 

 Flawn, personal communication and 1956). The novaculite and chert 

 units are most metamorphosed at the eastern end of the anticlinorium 

 near Little Rock and at the southwestern end in McCurtain County, 



Oklahoma (Miser, 1943). In McCurtain County the fissility of the 

 Cambro-Ordovician shales is parallel or subparallel with the bedding 

 (Pitt, 1955). The small folds around the central core are overturned 

 southward and slaty cleavage has developed which dips generally steeply 

 north. 



The position of the Ouachita front under the Cretaceous and Tertiary 

 cover is recognized on the basis of metamorphism and high dips in 

 contrast to the lack of metamorphism and very low dips of the beds 

 of the foreland. See Fig. 14.6. The siliceous nature of the Devonian to 

 Cambrian rocks of the Ouachitas is an additional guide. 



Structural Problems 



The Geological Map of Oklahoma (Miser, 1954) shows the Hendricks 

 version of the multiple thrust structure as well as two windows, the 



