USING PXRF AND LITHOGEOCHEMISTRY TO BETTER DEFINE THE VOLCANIC STRATIGRAPHY IN BIG BEND NATIONAL PARK

Big Bend National Park has complex volcanic stratigraphy due to multiple tectonic episodes that were punctuated with periods of volcanism. Recent efforts by the USGS to map the Big Bend National park included mapping the South Rim Formation in the Chisos Mountains which contains several well defined members (e.g. Lost Mine Member, Boot Rock Member, Pine Canyon Rhyolite Member), and some poorly defined units that could not be correlated to others. For example, Tesre includes undifferentiated rhyolite flows and ignimbrites that are discontinuous on the map. The Tesre unit may be equivalent to one of the members in the South Rim Formation or a member of the Punta De La Sierra (Tetps) (Bohannon,R.G., 2011). When using only traditional field methods, these units are difficult to distinguish from trachyte flows of Punta De La Sierra (Tetps) (Bohannon, R.G., 2011).

We are using a handheld X-ray fluorescence (pXRF) spectrometer, to determine the lithogeochemistry to correlate and map these undifferentiated volcanic units of the South Rim Formation. While the handheld XRF does not have the full analytical capabilities of traditional bench top models, recent advances produce excellent results for certain analytes that can chemically differentiate rhyolitic tuffs based on silicon, aluminum, iron, zirconium, niobium, yttrium, hafnium and many others. We are using the pXRF and Cernuschi’s et al. (2013) rhyolite calibrations in the field to quantitatively correlate rhyolitic tuffs using lithogeochemistry. Additional samples will be collected and analyzed with the handheld XRF in “bench top mode” which can provide further detailed chemical analysis. Optical petrography of thin sections to determine mineralogy combined with the chemical analyses will complete the tuff correlation. The pXRF has already been applied to mapping basalts in the field (Cernuschi et al., 2013) but not used to correlate rhyolites.

Lithogeochemistry in the field coupled with more traditional methods (“bench top mode", geochemistry and petrography) will result in a better understanding of the spatial distribution of the individual volcanic eruptions of the South Rim Formation flows (Tesre) and to a refined geologic map. This will lead to a better understanding of Eocene volcanism that was occurring during the Rio-Grande rifting episode.