OUACHITA, MARATHON, AND COAHUILA SYSTEMS 



22') 



Fig. 14.6. Cross section of Ouachita Mountains in Arkansas. After cross section on Geo/ogic Mop 

 of Arkansas, 1929. Gc, Collier shale; Owe, Womble shale, Blakely sandstone, Mazarn shale, and 



Crystal Mountain sandstone; DSO, Arkansas novaculite, etc; Cs, Stanley shale; Cj, Jackfork sand- 

 stone; Ca, Atoka formation; Csh, Savanna, Paris, Fort Smith, Spadra, and Hartshorne formations. 



Potato Hills and the McCurtain County core area (also called the 

 Choctaw anticlinorium ) . These have been reproduced in Fig. 14.2. 

 Hendricks' synthesis of the thrust structure involves translation of rocks 

 considerable distances, a seeming requisite of the Ouachita overthrusting 

 of the Arbuckles. See Figs. 14.1 and 14.6. Hendricks postulates that a 

 deep-seated thrust plane exists, the Powers, along which rocks of 

 "Arbuckle facies" were thrust southeastward, and then, slightly later, the 

 strata involving the thick Carboniferous clastic sequences were thrust 

 northward to rest as allochthonous sheets on a foreign ( Arbuckle) founda- 

 tion. 



The Tectonic Map of Oklahoma (Arbenz, 1956) shows the thrust 

 complex of the Geologic Map including Potato Hills window, but not 

 the core window. The core area was remapped and reported on by Pitt 

 in 1955, and he concluded that a normal sequence of formations exists 

 on and around a rather simple dome — that no klippe is indicated; the 

 previous need for a fault was due to erroneous reading of bedding and 

 an inadequate understanding of the stratigraphic succession. 



In 1957 Misch and Oles took issue with Hendricks on the basis of their 

 own detailed mapping of the Ouachitas. They concur with Pitt on the 

 structure of the "core" and also recognize no window in the Potato 

 Hills. They conclude that Potato Hills is an anticlinorium of closely 

 spaced, steep, and partly overturned folds. 



The overturning is both to north and, against the direction of the supposed 

 overthrusting, to south. Some overturned anticlinal limbs have ruptured, and 

 steep reverse faults have developed. Some of these faults yield to the north; 

 others yield to the south. All of these reverse faults die out along the strike, 

 generally in the steep limbs of anticlines. 



The Arkansas anticlinorium displays the same fold pattern as that seen in 



the Potato Hills. Steep northward and southward overturning of folds are about 

 equal. The greatest stratigraphic and structural depth is exposed in the core 

 of the western part of the anticlinorium (south of Mt. Ida), and there is the 

 same continuous change in tectonic style as that found in the core of the 

 Choctaw anticlinorium. 



Misch and Oles contend that the mapped overthrusts, both major and 

 minor, are partly steep reverse faults and partly no faults at all. The large 

 exotic boulders of Arbuckle rocks in the Johns Valley shale are considered 

 evidence of thrusting by Hendricks, but Misch and Oles believe they are 

 of "deposition origin" — apparently not associated with an advancing 

 thrust front. 



Misch and Oles also believe that the differences between the "Ouachita 

 facies" and the "Arbuckle facies" have been overemphasized. 



Some units are indentical, as for example, the upper Arkansas novaculite of 

 the Ouachitas and the Woodford chert of the Arbuckle region. Others differ 

 relatively litde, as the Bigfork "chert" and the major part of the Viola lime- 

 stone, or the Stanley shale and the Caney shale. Others differ more strongly, 

 as the Ouachita Mountains correlatives of the Simpson group. And some units 

 differ very strongly, as the Missouri Mountain shale and the lower Hunton 

 limestone. However, contrasted facies are not disconnected as the hypothesis 

 of overthrusting requires. Most of the contrasted facies have transitional re- 

 lationships. Some of the transitions are very gradual; others are pronounced 

 and also have been accentuated by the intense shortening resulting from folding 

 and faulting. None of these changes, however, exceeds those often encountered 

 in adjacent and connecting basins, or different parts of the same basin. More- 

 over, the fact is often overlooked that there are marked facies changes within 

 the Arbuckle region itself, as well as within the Ouachita Mountains. 



For a review of the problems in the Ouachita Mountains see Tomlin- 

 son (1959). 



