Abstract

The Ouachita system includes a long sinuous belt of sedimentary rocks of distinctive facies and of generally congruent Appalachian-type structures. However, foreland facies rocks occur within the structural belt in the two major salients and locally along the structural front. Two major tectonic zones—a frontal or exterior zone marginal to the foreland and an interior zone—have been mapped. The course of the belt is known from borehole data and geophysical evidence from southwest Alabama to the U.S.-Mexico border. Geophysical evidence suggests a southeast extension beneath southern Florida. In Mexico, scattered outcrops and boreholes indicate that the system strikes south for some hundreds of miles, but data are insufficient to map tectonic divisions. Deformation occurred later or lasted longer in the southern part of the system.

One hypothesis to explain the deep foundering of the Ouachita system below the Gulf Coastal Plain is based on the apparent lack of large volumes of stabilizing granite in this part of the crust, which in turn may be due to a relatively small volume of clastic sediments deposited in the interior part of the pre-existing geosyncline. Following this hypothesis, large volumes of clastic sediments were restricted to the Ouachita Mountains and Marathon salients and to other concealed frontal basins. In Mexico, where evidence indicates granitic terranes in the interior part of the belt, the interior part of the system has not subsided as deeply as it has to the north.