Regional Geologic Setting of the Eagle Flat Study Area, Hudspeth County, Texas

The Eagle Flat study area lies near the eastern limit of the Basin and Range geologic province in Trans-Pecos Texas. The geologic features of the region record a long history of geologic events. By describing the regional geologic setting, we can place the Eagle Flat study area within a larger context, and the significance of the site-specific investigations can be more properly understood. In general, the discussions are arranged chronologically, from oldest to youngest; each characterizes the regional geologic framework and describes how the proposed site at Faskin Ranch fits within that geologic setting.

The oldest rocks in the region are metamorphosed Precambrian sedimentary and igneous rocks. Precambrian rocks crop out about 6 miles (9 km) east of the proposed repository site and are extensively exposed in the hills in the northeast part of the study area and in the Carrizo Mountains east of the Eagle Flat study area. Precambrian rocks are also present in isolated occurrences on the northeast flank of the Eagle Mountains, southeast of the site, on the Diablo Plateau, and in the Franklin Mountains, and in New Mexico. In northeastern Chihuahua, Precambrian rocks are present in limited outcrops but are known primarily from deep exploratory drilling. The Precambrian rocks show evidence of sedimentation, magmatism, metamorphism, and deformation before deposition of overlying Paleozoic and younger strata. The greater depth of burial of Precambrian rocks in Chihuahua is interpreted to be partly a manifestation of Precambrian faulting and subsidence associated with a wide northwest-trending zone that parallels the Rio Grande. This zone, the Texas Lineament, broadly trends through the study area and locally coincides with younger geologic structures.