2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 3
Presentation Time: 2:00 PM


URBANCZYK, Kevin M., Sul Ross State Univ, SRSU Box C-143, Alpine, TX 79832-0001, kevinu@sulross.edu

The Lost Mine trail is among the most popular trails in Big Bend National Park (BBNP). It is located in the Chisos Mountains, and begins at an elevation of 1756 m (5760 ft), and traverses 3.2 km to an elevation of 2089 m (6850 ft). The geology of the area covered by the trail is a complex mix of Tertiary volcaniclastic and volcanic rocks belonging to the Chisos and South Rim Formations. The trail is of significant geologic importance in the BBNP area because it terminates on the western rim of the 31.7 to 32.9 Ma Pine Canyon caldera. The geology at the trailhead is within the eastern facies of the Chisos Formation, which includes the distinctive Crown conglomerate first recognized by Udden (1907). The trail rapidly crests a divide between Green Gulch to the northwest and Juniper canyon to the southeast. This divide is supported by a southwest trending dike that continues up to the base of Casa Grande. A series of switchbacks begin beyond this divide. Here, the trail progresses in and out of another dike with each switchback. The trail remains in a combination of Chisos Formation and dike until near the top, where it progresses into the South Rim Formation, which here represents the eruptive products of the Pine Canyon caldera. The caldera is interpreted to be the result of two episodes of volcanism. First, the caldera forming eruption produced the Pine Canyon rhyolite, which is the first member of the South Rim Formation passed along the trail. Second, a sequence of eruptions occurred that were located along a semi-circular series of post caldera ring vents with similar eruptive histories as the nearby Burro Mesa, Goat Mountain, and Cerro Castellan localities. These eruptions produced maar-type surge deposits with concentric inward dips toward lava domes. Lost Mine Peak near the end (top) of the Lost Mine trail is located along the western part of this sequence. The bulk of the topography in the Pine Canyon area is the result of the differential erosion of the generally resistant, caldera related lavas and tuffs of the South Rim Formation, and the less resistant sediments of the Chisos Formation. The Lost Mine trail provides excellent access into the Pine Canyon caldera. It provides one of the few opportunities in the Trans-Pecos region to get a glimpse of the field relations that represent an eroded caldera system.