COCORP deep crustal seismic profiles in southwestern Oklahoma show strong, persistent, continuous, and undeformed layering in the basement over an area probably very much greater than 2,500 km2. Such layering is very unusual, judging by COCORP experience with basement rocks elsewhere in the United States. The data can be interpreted as representing a Proterozoic basin filled with clastic sedimentary and felsic volcanic rocks 7 to 10 km thick, whose base lies 10 to 13 km deep. These rocks are believed, on the basis of sparse evidence from regional geology, to have been deposited or extruded about 1,200 to 1,400 m.y. ago, and some of them may now be metamorphosed. This basin lies on the south side of the Wichita Mountains, under the Paleozoic Hardeman Basin, and is similar in depth to the Paleozoic Anadarko Basin north of the mountains. The deep basement layering is truncated on the south side of the Wichita Mountains, probably by Precambrian faults in conjunction with granitic intrusions. Pennsylvanian compression probably reactivated these Precambrian trends. Extensive Precambrian basin deposits in this area were unexpected, on the basis of evidence from sparse well control, and reports of other layered basement reflections elsewhere in the southern Midcontinent suggest that Precambrian basins may be an important feature of this region. Simple models for the evolution of southwestern Oklahoma as an aulacogen must be reformulated in the light of these new data.