The Rim Rock country, on the western edge of a vast Trans-Pecos Texas lava field, received thousands of feet of volcanic material during the Tertiary Period. Although pyroclastic rocks predominate, several flow rocks form the most distinctive stratigraphic markers. Dikes, sills, and laccoliths were emplaced following the cessation of extrusive igneous activity. The Vieja Group, comprising the oldest volcanic strata, has been tentatively assigned to the Chadronian Stage (Lower Oligocene). Late Tertiary block-faulting created an intermontane area which subsequently received thick bolson deposits. Recent climatic fluctuations have resulted in the development of gravel-capped terraces along the Rio Grande. / text

Fault scarps cutting alluvial deposits define the present-day eastern boundary of Basin and Range faulting in Trans-Pecos Texas. These faults are found in the Salt Basin graben north of Van Horn, Texas, and its southerly extensions along Lobo Valley and Michigan Draw. Discontinuous and en echelon Quaternary fault scarps quickly die out to the south on the eastern side of the graben but become more continuous and develop larger displacements (up to 6 meters) along the west side of Salt Basin at the base of Sierra Diablo. The orientation of the more than one hundred Quaternary down-to-the-basin fault scarps and photolineaments appears to be controlled by pre-existing structural zones of weakness. The preferential alignment of Quaternary (Holocene?) fault scarps along the western side of the graben, the westward shifting of playa lakes and the preferred orientation of giant desiccation polygons show that the graben floor is subsiding more rapidly along the western margin. These faults also separate fresh from saline ground water. Four transverse structural lineaments crossing the northern portion of Salt Basin graben were mapped by P.B. King (1948, 1965) and M. Wiley (1970). A fifth zone, trending east-west between the Babb flexure and Bitterwell Mountain is proposed in this thesis. A tectonic origin is suggested by the orientation of the scarps and the proximity of the Mayfield fault scarps to the estimated epicenters of the 1931 Valentine earthquake. The frequency of small earth tremors felt by the residents of the basin and recorded by temporary seismograph stations indicate tectonic adjustments are presently occurring. / text