OUACHITA, MARATHON, AND COAHUILA SYSTEMS 



231 



Fig. 14.7. Relation of Ouachita Mountains to southern Appalachians under the Coastal Plain 

 cover. The pre-Upper Cretaceous geology of Arkansas and Louisiana is by Fisher, Kirkland, and 

 Burroughs (1949). Fredericksburg, Paluxy, Mooringsport, Ferry Lake anhydrite, Lower Glen 

 Rose, and Hosston formations are Lower Cretaceous; the Smackover and Cotton Valley are 

 Upper Jurassic; the Eagle Mills is possibly Lower Jurassic (King, 1950a) or Permian (Philpott and 

 Hazzard, 1949). 



vanian clastic rocks derived from the hinterlands. See paleotectonic 

 maps, Plates 5 and 6. 



According to King ( 1950a ) : 



The sequence of Paleozoic deposits in the Ouachita Mountains resembles 

 that in the Valley and Ridge province in that it is composite, the older part 

 indicating quiet deposition, and the younger part deposition during a time of 

 considerable crustal mobility. It differs in that the boundary between the older 

 and younger parts is post-Devonian rather than Middle Ordovician as in the 

 Valley and Ridge province, so that there is no representation of the Taconian 

 orogeny. Moreover, the deposits of the older part are black graptolite shales, 

 bedded cherts, novaculites, and fine sandstones, rather than carbonates, and 

 hence are of "eugeosynclinal" facies, as contrasted with the "miogeosynclinal" 

 facies of the Valley and Ridge province to the east, and of the Arbuckle and 

 Wichita Mountains farther west in Oklahoma. Deposits of the younger part, 

 laid down under conditions of greater crustal mobility, are of early Pennsyl- 

 vanian (Springer) age, and probably formed in response to the Wichita period 

 of orogeny. They are similar to the thick late Mississippian and early Pennsyl- 

 vanian deposits of the Valley and Ridge province in Alabama. The deposits of 



the Ouachita geosynclinal were remarkably persistent in character, for nearly 

 the same units are present in the extension of the system in the Marathon 

 region, Texas, many hundreds of miles to the southwest. 



The Appalachian folds have been traced as far southwest as Marengo Counts . 

 Alabama, on line of strike from the exposed structures of the Valley and Ridge 

 province in the Birmingham district, where wells have encountered Ordovician 

 limestones and dolomites directly beneath the Mesozoic. 



The Ouachita folds have been traced southeastward from their outcrops in 

 the Ouachita Mountains, across the Mississippi Embayment and into central 

 Mississippi. Here, the boundary between Paleozoic rocks of Ouachita facies and 

 the foreland rock trends southeastward. That this is likewise the strike of the 

 folding is suggested by the fact that folds in the adjacent Black Warrior Basin 

 trend southeast. In Newton and Neshoba Counties, Mississippi, near the bound- 

 ary between the Ouachita area and the foreland, wells have encountered Ordo- 

 vician limestones and dolomites below the Mesozoic. These are of Appalachian 

 or Arbuckle facies, rather than Ouachita facies, which indicates the existence 

 of an intermediate slice between the Ouachita folds and the foreland. 



The Appalachian and Ouachita systems have thus been traced by drilling to 

 within about 60 miles of each other, and they seem to be approaching at an 

 acute angle. Southward, they pass beneath the thick Jurassic and Lower Cre- 

 taceous deposits of the Gulf Coastal Plain, so that their point of junction is 

 beyond the reach of the drill. 



Connection of Ouachitas and Marathons 



The Ouachita thrust sheets not only overlie the east end of the Wichita 

 system, but continue southward under the Cretaceous rocks of the Gulf 

 Coastal Plain. If not the thrust sheets, the deformed strata of the orogenic 

 belt wrap around the Llano uplift of Texas and connect with the Mara- 

 thon Mountains to the west. Miser and Sellards (1931) have traced the 

 Ouachita front under the Cretaceous strata by means of well records south 

 to the Llano uplift, and Sellards ( 1931 ) has traced the geosynclinal rocks 

 westward from the uplift to the Marathon exposures, also by means of 

 well records. Flawn (personal communication and 1956) more recently 

 has mapped this front in considerable detail. 



MARATHON SYSTEM 



Location and Principal Structures 



Paleozoic formations appear in the Marathon basin of trans-Pecos 

 Texas, and there reveal another great orogenic system. The Marathon 



