During the Paleozoic, the southern margin of the North American craton underwent one major ocean margin cycle (“Wilson cycle”) lasting from the Late Proterozoic into the Permian. Breakup of a preexisting continent may have begun in the Late Proterozoic and lasted to the Middle Cambrian along a series of divergent rifts and transforms, which gave the margin between southern Alabama and northern Mexico an angular appearance. The breakup was accompanied by a system of rift arms and fault systems that reached far into the cratonic interior. The cycle ended with the formation of a collisional orogenic belt known as the Ouachita system.
The Ouachita Mountains of Oklahoma and Arkansas, the Marathon uplift of west Texas, and the Solitario (a small outcrop of Paleozoic rocks west of the Marathon region) are the only outcrop areas of the Ouachita system in the United States (Fig. 1). In northern Mexico, a number of small outcrops of Ouachita-facies rocks appear as inliers of the eastern Cordilleran orogen. The remainder of the frontal elements of the Ouachita system and the entire hinterland are covered by postorogenic Permian, Mesozoic, and Tertiary rocks of the Gulf Coastal Plain. Thus, some 90 percent of this orogenic belt cannot be inspected at the surface and must be mapped from subsurface borehole and geophysical data. Regional tectonic interpretations remain therefore rather tentative, especially in the deep hinterland, where boreholes are sparse.