Study of cores and cuttings of subsurface Precambrian rocks encountered in more than 500 wells in Texas and southeast New Mexico has resulted in the following subdivision of the basement in this area: (1) a central stable area composed mostly of granitic rocks--the Texas craton--extending from south-central Texas northwestward into New Mexico; (2) the Van Horn mobile belt--a prism of deformed and metamorphosed sedimentary and igneous rocks that borders the craton on the southwest; (3) the Red River mobile belt--an east-west belt in north- and west-central Texas and southwest Oklahoma composed predominantly of metasedimentary rocks and probably marginal to the craton; (4) an arcuate belt of low-grade metasedimentary rocks--the Fisher metasedimentary arc--in west-central Te as south of the Red River belt; (5) the Panhandle volcanic terrane--an extensive area of volcanic rocks, chiefly undeformed flows of rhyolite porphyry, that underlies a large part of the Panhandle and south plains of Texas and extends west into New Mexico; (6) the Swisher gabbroic terrane--an area of gabbro and diabase comprising the basement beneath part of the south plains of Texas; and (7) the Wichita igneous province which includes Precambrian rocks exposed in the Arbuckle and Wichita mountains of Oklahoma and the subsurface extension of the trend into the buried Amarillo Mountains of the Texas Panhandle.

The fundamental basement element is the Texas craton which apparently came into existence about 1 billion years ago in middle Precambrian time. The metasedimentary and volcanic units are late Precambrian and are marginal to or lie on the craton. The intrusive rocks of the Swisher terrane and the Wichita province are likewise late Precambrian.