Geology of Pinto Canyon area, Presidio County, Texas

Pinto Canyon area includes the southern Tierra Vieja Mountains and the northern Chinati Mountains. Lower and middle Permian siltstone, limestone, and chert underlie Comanche limestone and sandstone; Gulf rocks were deposited in the area but were eroded from the crest of a north-trending Laramide anticline. Normal faults that trend N30E cut the anticline before burial by silicic middle Tertiary volcanic strata from several sources. Emplacement of silicic intrusions that range from 1-inch dikes to a 27-square-mile granite-syenite complex followed. Large-scale Basin-and-Range normal faults that trend N5W and N85E displaced the volcanic strata to form the Tierra Vieja, Cuesta del Burro, and Presidio bolson. Gravel, sand, and clay deposited in the bolson are being dissected by tributaries to the Rio Grande. In ascending order the lithostratigraphic sequence comprises the Wolfcamp(?) Alta formation and the Leonard and lower Guadalupe Pinto Canyon formation (new name), both in the Permian system; the Yucca formation, Bluff formation (lower Albian), Cox formation, Finlay formation, Bienevides formation (middle Albian; new name), Loma Plata limestone (upper Albian; new name), Del Rio formation, and Buda formation (Cenomanian?), all in the Comanche series of the Cretaceous system; and the Jeff conglomerate, Vieja formation (basal Oligocene), Shely group (Oligocene?; new name), Moonstone rhyolite (new name), Petan trachyte (new name), and Chinati Mountain group (emended), all in the Tertiary system. Six terrace gravels were mapped in the dissected Presidio bolson, plus alluvium in present arroyos and the Rio Grande floodplain.Geological Science