Two major positive elements south and east of the foreland basins marginal to the Ouachita tectonic belt in Texas have been confirmed by wells drilled by Shell Oil Company and by seismic data in and adjacent to the concealed structural belt.

Foreland facies sediments (dominantly carbonates) of early and possibly middle Paleozoic age were drilled on the Devils River uplift in southwest Texas. In addition, much of the Devils River uplift is apparently underlain by a Precambrian intrabasement basaltic flow(?) which has been mapped from seismic reflections and penetrated in Shell No. 1 Stewart.

The presence of a second major linear positive element covered by lower Paleozoic foreland rocks has been established in east-central Texas by seismic records and by Shell No. 1 Barrett. This faulted anticlinorial structure, herein named the Navarro uplift, is at least 25 mi behind the leading edge of overthrust Ouachita facies rocks. It appears to be similar in structural position and history to the Devils River uplift but the data are insufficient to determine definitely the structural style of either.

Foreland strata are metamorphosed to greenschist facies. Some formation fluids and minor hydrocarbon shows were recovered but the quality of the reservoir was generally poor. The carbonates on the Navarro uplift contain extensive shear and flowage structures and resemble the marbles in wells on the south flank of the Devils River uplift.

Isotopic age dates from both Ouachita facies and foreland rocks record multiple Paleozoic thermal events and probable accompanying deformation along the tectonic belt. The isotopic data also suggest the presence of a post-Precambrian, pre-Late Cambrian volcanic and metasedimentary province in the area of the Devils River uplift.

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