GEOLOGIC FEATURES OF A VOLCANIC VENT COMPLEX, NORTH BURRO MESA, BIG BEND NATIONAL PARK, TEXAS

A suite of volcanic rocks exposed on the northern end of Burro Mesa in Big Bend National Park provides insight into the origin of a central volcanic vent complex. Here a series of ignimbrites are wrapped around lava flows and breccias of various origins associated with a prominent central dome. Distributed around the flanks of the dome is a brecciated unit distributed as isolated blocks resting primarily above older ignimbrites and ash-fall deposits. These isolated blocks range in dimension from 35m x 52m and 10m high to as large as 180m x 365m and 30 m high. Many blocks are cored by dark gray, dense, slightly vesicular, aphanitic lava and rimmed with angular clasts of similar composition. The breccia is monolithic and clast supported with little interstitial fine-grained material; clasts are agglutinated and clearly welded. Rheomorphic structures in the breccia mass as well as individual breccia clasts show plastic deformation suggesting that the breccia formed whilst the unit was still hot. Minor clasts of the underlying ignimbrite have been incorporated into the base of the breccia. Higher around the flanks of the central dome, the breccia shows pervasive hydrothermal alteration and mineralization, including sulfides, prehnite (?), quartz, and calcite. Some of the larger breccia blocks closer to the central dome have very frothy vesicular cores , some of which are strongly jointed. The central dome rises more than 50 m above the surrounding terrain and fills a large, west-facing, bowl-shaped amphitheater. Generally, the breccia blocks are exposed to the west and southwest on the open side of the amphitheater. Wrapping around the south, east, and north side of the dome and rimming the amphitheater is a thick (>30 m) massive, well-jointed lava flow of similar composition to the dome and breccia blocks. We envision these three units as part of a single volcanic complex where lava flows were emplaced on topographically steep slopes that caused flow fronts to collapse. This resulted in welded and deformed blocky lava breccias that are now exposed on the lower flanks of the dome. This mechanism may be similar to Merapi-type hot avalanches off the face of an advancing lava flow/dome.