GEOLOGY OF THE SOUTHERN CHISOS MOUNTAINS: INSIGHTS FROM NEW GEOLOGIC MAPPING IN BIG BEND NATIONAL PARK, TEXAS

The southern Chisos Mountains are rugged and inaccessible by vehicle. The only published geologic map covering this part of the park is Maxwell and others (1967) and that map is highly generalized in this area. The Chisos Formation, of Oligocene and Eocene age, is intruded by small stocks, dikes, and sills of varied composition. Internal Chisos stratigraphy is undifferentiated on the Maxwell map except for rare areas of contact metamorphism and a few lava flows, and the intrusive rocks are all depicted uniformly. The Chisos is mapped as depositional above Late Cretaceous Javelina Formation in the southwestern part of the mountains where a locally continuous lava flow, the Alamo Creek Basalt Member, is exposed at the base of the formation. The nature of the Chisos-Cretaceous contact is enigmatic in the southeastern part of the mountains where the lava flow is absent and the Chisos rests on the older, Late Cretaceous Aguja Formation. New geologic mapping, conducted east of Sierra Quemada volcanic center has been directed at resolving 1) the internal Chisos stratigraphy, 2) the intrusive contact relations and their varied lithologies, and 3) the character of the Chisos/Cretaceous contact. Subtle lithologic differences within the Chisos have allowed the differentiation of sixteen facies. Also, numerous flows and sills have been mapped where none had been identified before. The Chisos is known primarily from exposures in Blue Canyon, which is north of Sierra Quemada. Most of the area to the east of that volcanic center is comprised of Chisos beds that are older than the described facies and are not present elsewhere. These older beds apparently fill a deep semi-circular depression that may have pre-dated the Sierra Quemada center. The enigmatic contact between Chisos and the Cretaceous marks the edge of this depression.