Geology of Eagle Mountains and vicinity, Trans-Pecos Texas
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The Eagle Mountains and adjacent highlands, Devil Ridge to the northwest and the Indio Mountains to the south, rise steeply above flanking intermontane lowlands in southeastern Hudspeth County, Texas, to an elevation almost 7,500 feet above sea level. The Precambrian Carrizo Mountain Formation, about 5,000 feet of steeply dipping metamorphosed sedimentary rocks, crops out on the northeast flank of the Eagles, Rock ranging in age from Ordovician through Pennsylvanian has been eroded, but the basal siliciclastic Powwow Conglomerate and the overlying Hueco Limestone record the transgression of the Permian sea over the Diablo platform during the Wolfcamp Epoch. Erosion has removed younger Permian rocks. The halting transgression of the Cretaceous Mexican sea over the Diablo platform is recorded by about 7,000 feet of alternating siliciclastic and carbonate rock of the Comanche and Gulf series that were deposited on the erosional surface of Permian rock along the western margin of the platform and the adjacent Chihuahua trough. The Comanche Series consists of, from oldest to youngest, the Yucca Formation, Bluff Formation, Cox Sandstone, Finlay Limestone, Benevides Formation, Espy Limestone, Eagle Mountains Sandstone, and Buda Limestone. The conformably overlying Chispa Summit Formation represents the Gulf Series. Some of the major structural elements of the Eagle Mountains and vicinity were created during the Laramide orogeny when sediments that had been deposited in the Chihuahua trough were asymmetrically folded and overthrust northeastward. Widespread mid-Tertiary vulcanism spread a blanket of flow and pyroclastic rock over Trans-Pecos Texas. In the Eagle Mountains rhyolitic and trachytic flow rock, volcanic breccia, and flow breccia overlie centroclinally dipping Cretaceous strata. The high, interior part of the Eagles is a crescent-shaped stock composed of the Eagle Peak Syenite. Rhyolitic and trachytic fine-grained tuff alternating with welded tuff and trachyte cover part of the tilted Cretaceous rock of the southern Indio Mountains. Cretaceous rock in the northern Indio Mountains and in Devil Ridge has been intruded by rhyolite. Regional uplift that followed the vulcanism was accompanied by block faulting that created the mountains, or horsts, and the intermontane basins, or grabens. Subsequent erosion has partly filled the grabens to form bolsons and has reduced the mountains to their present form. The newly established Rio Grande drainage system is forming a series of stream terraces as it incises the bolson fill.