230 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Phases of Ouachita System 



Early Mississippian Phase. Elevations precursory to the late Paleozoic 

 orogeny seem to be indicated by an unconformity between the Arkansas 

 novaculite (Devonian) and the Upper Mississippian elastics (Chaney 

 shale). Chert conglomerates rest on the novaculite in the Potato Hills 

 section of the Ouachitas and they are found at the base of the Stanley 

 shale (Lower Pennsylvanian) in southern outcrops. In addition to this 

 suggested late Mississippian disturbance, the rise in the foreland of the 

 Ellis-Chautauqua-Ozark arch in late Devonian time may be mentioned. 



Late Mississippian Phase. The deposition of more than 17,000 feet of 

 clastic sediments of the Stanley, Jackfork, and Johns Valley formations 

 all within a very short time indicates a great and sudden uplift nearby, 

 which undoubtedly was one of active orogeny because a sedimentary 

 mass of the character and quantity noted requires actively rising moun- 

 tain chains. The elastics were deposited in a foredeep. 



Whereas van der Gracht and others before him postulated the orogeny 

 in the hinterland to the south, Hendricks (1943) believes that early 

 thrust sheets came from the north and pushed southward to form a land- 

 mass. The Stanley, Jackfork, and Johns Valley shales were deposited 

 in a basin to the south of this landmass, and the thrusting culminated in 

 Johns Valley time. The Atoka sediments were then spread thickly over the 

 sites of both facies. Van der Gracht believes the Atoka came from a 

 southern highland; Hendricks does not comment on the source. The Atoka 

 sediments reflect the second pulsation this time in the Early Pennsyl- 

 vanian. 



In eastern Texas, a foreland basin to the southward-trending chains 

 of the hinterland came into existence, and in the basin the Strawn and 

 Millsap formations were deposited, having been derived from an eastern 

 source. 



Mid-Pennsylvanian (?) Phase. The age of the major deformation of 

 the Ouachitas is believed by several authors to have occurred in post- 

 Atoka and pre-Boggy time. According to Fitts (1950); 



The unconformity at the base of the Boggy formation is the largest within 

 the Pennsylvanian of Oklahoma and is probably the most widespread. Along the 

 line of outcrop, it is progressively underlain by Pennsylvanian beds from 



Savanna to Atoka, locally in the Tri-State area upon Mississippian and in wes- 

 tern areas of Oklahoma all formations down to granite. 



The top of the Boggy is marked by another unconformity, this one of more 

 importance locally and to the westward in the Seminole region. The section of 

 beds above this unconformity is generally devoid of any angular discordance 

 and for the first time can be seen a relationship which will persist through the 

 rest of the Pennsylvanian and lower Permian; i.e., predominandy limestone in 

 the north grading to shales and elastics in the central to coarser elastics and red 

 beds as the Arbuckle Mountains are approached. 



The deformation of the Arbuckles in the Mid-Pennsylvanian influenced 

 the development of the red-bed facies in the upper Cisco and Lower 

 Permian, but later in Permian time much clastic material in the Wichita 

 system came from an eastern source (Cheney, 1929). 



Drilling operations have penetrated a formation, the Morehouse, 

 under the coastal plain sediments, in northern Louisiana, which contains 

 "late Paleozoic fossils" (Imlay and Williams, 1942). Its areal relations 

 have been worked out for a limited distance in southern Arkansas and 

 also, to some extent, its stratigraphic relations (Philpott and Hazzard, 

 1949; Fisher et al, 1949). See Fig. 14.6. It occurs above the Eagle Mills 

 formation and below the Louann salt and Werner formation. (Philpott 

 and Hazzard, 1949). According to the usage of Imlay and Williams, the 

 Louann sail and Werner formation make up the Eagle Mills. At any 

 rate, the Eagle Mills seems to overlie the folded Ouachita facies uncon- 

 formably, and if such is the case, the Ouachita thrusting predates the 

 Eagle Mills and Morehouse. When their age eventually is fixed, the age 

 of the Ouachita thrusting possibly will be fixed more definitely than is 

 now possible. 



Connection of Ouachitas and Appalachians 



Spatial Relations. The relation of the Ouachita system to the Appa- 

 lachian is hidden by the Cretaceous and Tertiary rocks of the Mississippi 

 embayment, but they have been traced by deep wells to within 60 miles 

 of each other. See map, Fig. 14.7. Both are strongly folded and faulted, 

 and in both there has been thrusting toward the central stable region of 

 the continent. In both areas there is a thick development of Early Pennsyl- 



