A COMPARISON OF MITCHELL MESA RHYOLITE EXPOSURES ON THE DALQUEST DESERT RESEARCH STATION, PRESIDIO AND BREWSTER COUNTIES, TEXAS

The Mitchell Mesa Formation, an extensive rhyolite ignimbrite issued from the Oligocene Chinati Mountains Caldera Complex, serves as the focal point of this study. The ash-flow deposits, exposed at the Dalquest Desert Research Station (DDRS), exhibit varying textures and colors.

Situated 75 km to the ESE of the Chinati, the DDRS encapsulates the transition from the southern end of Bandera Mesa to the canyons of Alamo de Cesario Creek, including the most distal exposures of the Mitchell Mesa Formation. On Bandera Mesa, the Mitchell Mesa Formation is typically densely welded, displaying a beige-brown hue with feldspar and quartz phenocrysts.

From samples gathered at the DDRS, a subset representing the diverse facies within the rhyolite has been chosen for in-depth analysis. Ongoing research characterizes these variations using optical and electron microscopy, complemented by energy-dispersive microanalysis. Optical microscopy includes image analysis on high-resolution scans of thin sections, evaluating attributes such as quartz mode and porosity through ImageJ by thresholding raw images, refined by Weka Trainable Segmentation.

The study evaluates nine localities within the mapped Mitchell Mesa Formation, capturing a spectrum of variations that may result from local conditions during flow emplacement. Of particular interest are two localities on the ridge south of the "Red Canyon," displaying significant differences. The sample from site one, Blu 007, on the east end of the ridge, is a buff-colored welded rhyolite with streaks of pink-red and notable porosity. In Blu 007, quartz phenocrysts are subhedral and embayed, hosted within a matrix of silica-rich glass, fine skeletal quartz, intermediate alkali feldspar, and ilmenite. Image analysis indicates 26% quartz and 3% porosity.

The sample from site two, TMM-WBRdB, located in a ravine on the south side of the ridge near the contact with the overlying Tascotal Formation, is densely welded, and has a deep blue color. It features fewer and smaller quartz phenocrysts, dominated by a matrix of feldspathic glass, fine quartz, intermediate alkali feldspar, fine sodic pyroxene, and ilmenite. Image analysis reveals smaller vesicles, approximately 0.4% porosity, and a 21% quartz mode.