DELINEATION OF ADDITIONAL VOLCANIC CENTERS AND STRATIGRAPHIC VARIATIONS IN BIG BEND NATIONAL PARK, TEXAS

Several volcanic centers and their eruptive products have been identified in various areas of Big Bend National Park, Texas. The most notable is the Sierra Quemada Caldera that is located a few km south of the South Rim in the central part of the park. Seven units have been identified within the caldera itself including an intra-caldera tuff that is over 120 m thick. Although it is a relatively small caldera, about 5 km in diameter, it erupted materials that are found in many parts of the western half of the park. The main unit formed by this caldera is the 34 Ma Mule Ear Spring Tuff Member (Tmet) of the Chisos Formation. Changes to Ttmt outcrop patterns include areas where Tmet was originally mapped as “undifferentiated lavas” and other areas where Tmet is present but was not previously mapped as a formal unit. These include exposures on both sides of Blue Creek Canyon near the overlook, along the northeast side of Burro Mesa, and outcrops south of the Sierra Quemada toward Punta de la Sierra.

Additional volcanic vents and volcanic materials have been remapped as summarized below. Both intrusive and extrusive materials have been noted around Cerro Castellan that possibly are related to a lava dome. A small volcanic structure on the north end of Burro Mesa contains materials that are, at least in part, younger than the Tule Mountain Trachyandesite Member (Ttmt) of the Chisos Formation. Feeder dikes and numerous layers of Ttmt are found along Punta de la Sierra and hence are likely related to a volcanic source. A small mafic plug at Croton Spring Road contains both intrusive and extrusive components that may be a source for mafic lavas within the Chisos Formation. A few other stratigraphic variations have been documented. In the northern part of Burro Mesa there occurs at least one previously-unmapped mafic lava flow above Tmet. In several areas around the Sierra Quemada, additional pyroclastic flow deposits apparently not related to Tmet are found below and slightly above the Bee Mountain Basalt Member of the Chisos Formation. Finally, faults, mostly minor, are more abundant than previously mapped.