Regional Geologic Setting of the Fort HancockStudy Area, Hudspeth County, Texas

The Fort Hancock study area is located within 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, the Fort Hancock study area can be placed within a larger context, and the significance of the site-specific investigations can be more properly understood. The oldest rocks present in the region are Precambrian crystalline rocks, although none crop out within about 20 mi (32 km) of the proposed repository site. Precambrian rocks are present in the Hueco Mountains to the west and in isolated occurrences on the Diablo Plateau to the north of the study area. In northeastern Chihuahua, Precambrian rocks are known primarily from deep exploratory drilling. The Precambrian rocks show evidence of sedimentation, magmatism, metamorphism, and deformation prior to deposition of overlying Paleozoic strata. The greater depth of burial of Precambrian rocks in Chihuahua is interpreted to be a manifestation of Precambrian faulting and subsidence southwest of a structural zone that parallels the Rio Grande. This structural zone, which projects close to the study area, is also coincident with younger geologic structures and has been termed the Texas Lineament.