14. 



OUACHITA, MARATHON, 

 AND COAHUILA SYSTEMS 



OUACHITA SYSTEM 



Location and Topography 



The Ouachita Mountains occupy a belt 50 to 60 miles broad and 

 200 miles long in southeastern Oklahoma and western Arkansas. See 

 maps, Figs. 14.1 and 14.2. They are somewhat like the Appalachians in 

 topographic appearance, although not generally so high. Their level- 

 topped subparallel east-west ridges reflect structure and dissection of 

 erosion surfaces. The ridges rise scarcely 250 feet above the valley west 

 of Little Rock but gradually increase in height toward the Oklahoma- 

 Arkansas border, where the highest point is 2900 feet above sea level 



and nearly 2000 feet above the valley floors. Their eastern, western, 

 and southern margins are blanketed by the Gulf Coastal Plain sediments. 



Stratigraphy 



The oldest rocks of the Ouachita Mountains are Cambrian, and these 

 are exposed in the central anticlinorium. The section of the anticlinorium 

 or "core area" of southeastern Oklahoma in McCurtain County as 

 measured by Pitt (1955) is as follows: 



Bigfork chert 



p 



Womble shale 



66+ ft 



Mazarn shale 



600 ft 



Crystal Mountain sandstone 



50-100 ft 



Collier shale 



180 ft 



Lukfata sandstone 



150 + ft 



Northwestward each thrust sheet has elements of its stratigraphy, and 

 these are given by Hendricks ( 1943 ) in Fig. 14.3. 



The Arkansas novaculite is a conspicuous formation of the pre- 

 Mississippian sequence. It has a counterpart in the Marathon uplift of 

 west Texas, the Caballos chert, but is not present in the southern Appa- 

 lachians. The Bigfork chert, Pinetop chert, and Woodford chert, as well 

 as the siliceous nature of the limestones and shales indicate that a 

 dominant characteristic of these formations is silica. Pitt (1955) thinks 

 that much of the silica is secondary, having been introduced by ground- 

 water after extensive fracturing. 



The combined thickness of the Cambrian, Ordovician, Silurian, and 

 Devonian rocks is hardly 3000 feet, and they are regarded as a shelf or 

 platform type of deposit, although the high silica content is unusual in 

 such a setting. The Mississippian and Pennsylvania!) strata are almost 

 entirely clastic — shale and sandstone — and are very thick. A measure- 

 ment of 18,950 feet for the Ouachita Mountains sequence of Stanley, 

 Jackfork, and Johns Valley formations is given by Cline and Moretti 

 (1956), and 17,000 feet for the foredeep sequence of Atoka (Hendricks 

 et al, 1936). 



The terms Ouachita facies and Arbuckle facies have been widely used 

 to compare or contrast the sequences of the Ouachita Mountains and 



223 



