TECTONIC DEVELOPMENT AND MINERALOGY OF A RHYOLITE CAVE IN BIG BEND NATIONAL PARK, TEXAS, USA

Mount Emory Cave is tectonically formed in the upper flanks of the highest mountain in Big Bend National Park, Texas, at an elevation of 2,149 m. Formed in the Oligocene-age Burro Mesa Riebeckite Rhyolite (BMRR), the cave is a complex series of rifts along stress-release fractures that parallel the topographic contours of a mountaintop ridge. Meter and decimeter scale undulations in the passages and walls occur along variable-scale columnar joints. The cave has a known vertical range of 79.6 m; its lower limits coincide with the base of the jointing. The cave’s passages total 280 m in horizontal length and extend about halfway through the ridge. Cold airflow from the apparent continuation of the stress-release fractures on the opposite side of the ridge, and shadowed areas above, high in a cliff, suggest the cave extends through the ridge. While tectonic caves are usually short and morphologically simple, Mount Emory Cave is proving extensive and may possibly be the second deepest cave in Texas.

X-ray diffraction and scanning electron microscopy was used to quantify the BMRR as about 45% (by weight) quartz and 55% albite feldspar with small amounts of weathered clay and hematite. Analyses were also performed on three types of secondary deposits:

1) A sub-millimeter thick water-deposited white coating on many cave walls and breakdown is kaolinite, a weathering product of the feldspar.

2) Pale brown rimstone-like features, generally about 1 cm in length and <1 mm high were mostly quartz, weathered feldspar, and kaolinite. They were water-deposited but their geometry is not always perpendicular to overall water flow patterns probably due to adhesion by the clays.

3) Dark bluish-gray and pale brown coatings along fractures were predominantly weathered feldspar and clay; the darker color is consistent with organic staining.

No unusual minerals were found for the geologic setting.