Geologic mapping of Tertiary volcanic rocks in the Sierra Hechiceros, 50 to 80 km south of Big Bend National Park has delineated a 20 by 25 km Mid-Tertiary caldera, herein named the Hechiceros Caldera. This caldera was formed by the eruption of the Palomas tuff which has a minimum volume of approximately 170 km3, and is distributed radially around the caldera. The caldera was filled with more than 500 meters of postcaldera rhyolitic lavas which overflowed the caldera margin and caused additional subsidence. The rocks of the Sierra Hechiceros are subalkalic and include rhyolites and dacites. The chemistry of these rocks is transitional between the calc-alkalic province of the Sierra Madre Occidental and the alkalic rocks of the eastern Trans-Pecos magmatic province. The potassium argon ages both the Palomas tuff and the postcaldera El Jobero rhyolite are about 29¿0.6 Ma. ¿American Geophysical Union 1991