ADDITIONAL NOTES ON THE IGNEOUS HISTORY OF BIG BEND NATIONAL PARK WITH EMPHASIS ON THE SIERRA QUEMADA AND THE MULE EAR SPRING TUFF

Observations over the decade or two have focused on the igneous history of Big Bend. Detailed mapping has shown that the Sierra Quemada is a resurgent caldera that erupted the Mule Ear Spring Tuff Member of the Chisos Formation. The caldera produced significant materials that can be grouped into those that formed before, during, or after collapse. Rocks that were formed prior to the main eruptive event include minor rhyolitic lavas in and near the Sierra Quemada and volcaniclastic products that become progressively thinner away from the caldera margins. Dikes that radiate northwest of the Sierra Quemada were apparently formed at this time also.

The major eruptive event that created the caldera produced the ash-flow tuff that has been mapped as the Mule Ear Spring Tuff in many areas of the Park south and west of the Sierra Quemada. The tuff extends into Mexico near Santa Helena but otherwise is probably not found outside of the Park. The tuff's thickness within the caldera exceeds 130 m (400 ft) and in many areas south and west of the caldera it is 25 m (80 ft) or greater. Post caldera materials include ring-fracture intrusions and dikes that extend northeast of the caldera. Additional related intrusive bodies may be located in the subsurface to the east of the caldera perhaps extending to Tortuga Mountain.

Other possible eruptive centers include the following: a small igneous complex on the north end of Burro Mesa; feeder dikes for the Tule Mountain Trachyandesite on the south-facing side of Punta de la Sierra; a mafic complex near Croton Spring Road; and a complex intrusive/vent sequence near Cerro Castellan. Volcanic centers are apparently quite common within the Park.