Tectonic style and deformational environment in the Eagle-Southern Quitman Mountains, Western Trans-Pecos Texas
— D. F. Reaser and James R. Underwood Jr.

In the exposed rocks in the Eagle-southern Quitman Mountains region there is evidence of several major tectonic episodes:

1. Geosynclinal subsidence, deposition and metamorphism within the old Texas Craton late in the Precambrian (Flawn, 1956).

2. Development of the Marathon geosyncline, the sediments in which were deformed by late Paleozoic orogeny that ended early in the Permian Period. This orogeny uplifted and tilted the foreland area, created the Van Horn uplift as well as the larger Diablo Platform, and erected the present structural framework of Texas, indeed of the central United States (King, 1942).

3. Geosynclinal subsidence of, and deposition in, the Chihuahua Trough and episodic advance northward and eastward onto the Diablo Platform of the Mexican sea during the Jurassic and Cretaceous Periods. The alternating advance and retreat of the Mexican sea resulted in the deposition of couplets of calciclastic and siliciclastic rocks. The geosynclinal cycle culminated in the late Mesozoic-early Cenozoic orogeny (Laramide), which produced open to nearly recumbent folds and created thrust blocks, most of which moved northeastward.

4. Intrusion and widespread extrusion of alkaline igneous rocks in late Eocene-early Oligocene times (Jones and Reaser, 1970).

5. Development of the gross outline of present-day ranges in the region by late Tertiary normal faulting that created a series of debris-filled grabens (bolsons) and eroded horsts (mountain masses).

In rocks exposed in the Eagle Mountains-Indio Mountains-Devil Ridge complex there is evidence of all five of the episodes; in the Quitman Mountains, there is evidence only of the last three. The Eagle-southern Quitman Mountains region lies, therefore, within and near the margins of two superimposed structural provinces: near the eastern margin of the Basin and Range Province and near the northeastern margin of the Chihuahua Tectonic Belt (fig. 1).