Sedimentary rocks of Ouachita facies are here defined as rocks lithologically similar and stratigraphically equivalent to sedimentary and low-grade meta-sedimentary rocks exposed in the Ouachita Mountains of Oklahoma and Arkansas. Data obtained from numerous well borings show that rocks of Ouachita facies form a long, folded, geosynclinal belt that passes beneath the Cretaceous overlap in southern Oklahoma, bends around the Central Mineral Region of Texas in a buried structural belt, strikes almost due west, is exposed at the surface again in the Marathon and Solitario uplifts of extreme southwestern Texas, and crosses the border into Mexico. In the outcrop areas, and throughout the length of the Ouachita structural belt, these rocks show strong similarities in lithologic character, fabric, and metamorphic grade, which may be used to differentiate them from sediments of foreland or Arbuckle facies.

In the Ouachita Mountains, rocks of late Mississippian and Pennsylvanian age reach a maximum thickness of at least 22,000 feet and are divided into two major groups. These are (1) strata widely exposed in the central part of the Ouachita Mountains, and (2) strata of frontal zone Ouachita facies exposed only in small slivers in the western part of the Ouachita Mountains in Oklahoma. The former comprise, from oldest to youngest, the Stanley shale, Jackfork sandstone, Johns Valley shale, and Atoka formation. In the frontal zone they comprise, from oldest to youngest, the Caney shale, Springer formation, Wapanucka limestone and Chickachoc chert, and the Atoka formation.

The rocks of late Mississippian and Pennsylvanian age in the Marathon uplift in southwest Texas attain a maximum thickness of 12,800 feet and consist of four formations. From oldest to youngest, these are the Tesnus formation, Dimple limestone, Haymond formation, and Gaptank formation.

In both outcrop areas and in the buried structural belt, sediments of Ouachita facies were downwarped in an active geosyncline and subsequently deformed and uplifted. Most of the beds were laid down as sediment flows or deposits from turbidity currents in deep water, but the Wapanucka, Chickachoc, and Dimple formations are shallow-water, shelf-type deposits formed mostly in the zone of wave action. The Caney shale and dark siliceous shales in several formations were deposited in deep unaerated basins with part of the sediments derived from ash falls. The boulder beds in the Jackfork, Johns Valley, and Haymond formations apparently were formed by submarine landslip and mudflows.

The Caney shale of the frontal Ouachitas is of late Mississippian age and is the stratigraphic equivalent of the Stanley, Jackfork, and lower part of the Johns Valley formations of the central Ouachitas. The upper part of the Johns Valley is of early Pennsylvanian (Morrow) age and is equivalent to the Springer, Wapanucka, and Chickachoc formations of the frontal Ouachitas. The lower part of the Atoka is also of Morrow age, but the upper limit of its age has not been established.

The Tesnus formation is of late Mississippian and early Pennsylvanian (lower Morrow or Springer) age. The Dimple limestone is of Morrow age; the Haymond formation is early to middle Pennsylvanian (Morrow and Atoka) in age; and the overlying Gaptank formation is of middle and late Pennsylvanian (Des Moines to Virgil) age.

During the last stages of the deformation of the geosynclinal belt, rocks of Ouachita facies were thrust over rocks of foreland or Arbuckle facies for considerable distances. The foreland rocks underlying the thrust sheets are potentially petroliferous; exploration for petroleum production in these buried shelf sediments should be more rewarding than in sediments of Ouachita facies.