DEVELOPMENT OF THE OUACHITA EMBAYMENT 183 



lachian folds plunge beneath the Cretaceous overlap. The exact stratal 

 connections between these two troughs may never be known because of 

 the very wide Mesozoic-Cenozoic covering of the Mississippi Valley. On 

 the other hand, the Paleozoic faunal connections of the Ouachita embay- 

 ment with those of the Mississippian seas are more intimate than they 

 are with those of Alabama. In other words, the seaway connections on 

 the basis of the faunas are more intimate with the Mississippian sea than 

 with the southern end of the Appalachian trough. 



The Ouachita embayment appeared early in Upper Cambrian time and 

 its waters remained more or less continuously as a recording seaway into 

 the early ^Iiddle Ordovician. The deposits are mainly dolomites, and 

 vary in thickness from 1,500 feet in eastern Arkansas to 7,000 feet in 

 the Arbuckle Mountains of eastern Oklahoma, and to 5,000 feet in the 

 Wichita Mountains, farther west. 



During Middle Ordovician time the southern end of the neutral area 

 Siouia was warped above sealevel, blotting out the medial reach of the 

 long transverse seaway formerly uniting the Ouachita embayment with 

 the Sonoran embayment, and hence from Mohawkian time onward the 

 former trough is restricted to Oklahoma and Arkansas. From Middle 

 Ordovician time up to the close of the Mississippian, the Ouachita em- 

 bayment was only intermittently filled with marine waters, and therefore 

 during this long interval the deposits are not in great volume. They 

 vary from about 1,200 feet in central Arkansas to about 2,400 feet in the 

 Arbuckles. Then a most marked change took place in the land to the 

 south of the Ouachita embayment, and Llanoria must have been elevated 

 most decidedly, since the Pennsylvanian strata in Arkansas attain, ac- 

 cording to Branner, to a depth of over 23,000 feet. The total maximum 

 thickness of all Pennsylvanian formations in southeastern Oklahoma 

 appears to be about 37,000 feet, though the general average in any place 

 may not exceed 28,000 feet. In other words, the whole of the eastern 

 two-thirds of the Ouachita embayment sank during the Paleozoic no- 

 where less than 25,000 feet, and to the east of the Arbuckle Mountains, 

 the region of greatest depression, it appears to have gone down about 

 35,000 feet. Shortly after Pottsville time all of the trough in southern 

 Arkansas began to fold into the Massern Mountains. In Oklahoma, how- 

 ever, the folding took place later, and according to McCoy^"* it came at 

 the close of Monongahela or just before Cisco time. The thrusting is 

 toward the north and northwest. 



3* A. W. McCoy : A short sketch of the paleogeography and historical geology of the 

 Mid-Continent oil district and its importance to petroleum geology. Bull. Amer. Assoc. 

 Petrol. Geologists, vol. 5, 1921, pp. 541-584. 



XIII— Bull. Geol. Soc. Am., Vol. 34, 1922 



