The Carboniferous rocks of the Ouachita Mountains are a great succession of sedimentary strata, chiefly shale and sandstone. They present two different sequences or facies: one of these is revealed in the belt lying between the Ti Valley and Choctaw faults along the northwest margin of the Oklahoma structural salient and the other is revealed in all portions of the Ouachita region south and east of the Ti Valley-Choctaw belt. The only rocks here assigned a Mississippian age are included in the Caney shale. They measure a few hundred feet in thickness and all their exposures in the Ouachita Mountains are confined to the Ti Valley-Choctaw belt.

The Pennsylvanian rocks are 18,000-20,000 feet in thickness in most parts of the Ouachita Mountains and if the thickness of the younger Pennsylvanian rocks in the adjoining Arkansas Valley be added, the total reaches about 25,000 feet. Thus, the thickest known section of Pennsylvanian rocks in the United States is revealed in the Ouachita Mountains and the Arkansas Valley.

A portion of the Carboniferous sequence that is exposed south and east of the Ti Valley-Choctaw belt has been assigned various ages in the past. The units, so variously classified, are as follows, the oldest being named first: the Hot Springs sandstone, the Stanley shale, the Jackfork sandstone, and the boulder-bearing shale (formerly designated Caney shale but now designated Johns Valley shale). These units measure altogether about 12,000 feet in thickness. The faunal and floral evidence obtained by the writer and other geologists in recent years indicates that the formations just mentioned are Pennsylvanian in age. This age assignment makes it possible for the writer to reach a more satisfactory conclusion that he has been able to reach hitherto concerning the nature and origin of t e boulder-bearing shale. Available evidence appears to indicate that the boulders were transported by submarine land slips from fault scarps on an early Pennsylvanian uplift in southeastern Oklahoma. This uplift is now largely, if not entirely, concealed underneath the northwestwardly overthrust western portion of the Ouachita Mountains.

End_Page 971------------------------------