THE GEOLOGY OF THE PINE CANYON CALDERA SYSTEM, BIG BEND NATIONAL PARK, TEXAS

An Oligocene caldera measuring about 6 km in diameter occupies the Pine Canyon area of Big Bend National Park, Texas. The Pine Canyon caldera and related extracaldera vents erupted a series of ash-flow tuffs, lava flows, and lava domes that comprise the South Rim Formation in the Chisos Mountains.

The initial mapping of the area by Maxwell and others (1967) was done without the benefit of modern techniques and theories, particularly regarding caldera eruption mechanisms, and subsequent investigations have updated those early interpretations. Ogley (1978) and Barker and others (1986) suggested the area was a caldera and refined the stratigraphic nomenclature. The Pine Canyon Rhyolite Member of the South Rim Formation represents the original caldera forming eruption, followed by the Boot Rock, Lost Mine, Wasp Spring and Burro Mesa Members. Interpretation of the geology at that time was hampered by the belief that igneous rocks in Pine Canyon were correlative with igneous rocks at Burro Mesa and other localities some 15 to 25 km away. However, newer interpretations depict the Burro Mesa rocks as erupting from multiple small vents and being distinct and non-correlative with the Pine Canyon eruptive center, rendering obsolete the use of the terms Wasp Spring and Burro Mesa when referring to the eruptive products of the Pine Canyon caldera. Urbanczyk and White (2000) proposed the existence of maar-type surge deposits located along a semicircular series of post caldera ring vents along the margin of the caldera and suggested a similar eruptive mechanism as seen in the Burro Mesa area (gas rich surge deposits followed by the emplacement of lava domes). Our new mapping supports this hypothesis, but we suggest that the Pine Canyon eruptions were more gas rich than those documented at Burro Mesa, with most eruptions being exclusively pyroclastic.

Clearly the Pine Canyon caldera needs to be re-mapped and re-interpreted in light of changing methods and theories. Our poster will present the latest status of our re-mapping efforts in the Pine Canyon caldera, with preliminary interpretations of the eruptive history of the caldera and associated South Rim Formation deposits.