NEW EVIDENCE FOR STRATIGRAPHIC REVISION OF THE SOUTH RIM FORMATION, BIG BEND NATIONAL PARK, TEXAS

The South Rim Formation was initially thought to be the extensive peralkaline byproducts of the Pine Canyon caldera and was divided into 5 members: 1) Pine Canyon Rhyolite, 2) Boot Rock Member, 3) Lost Mine Member, 4) Wasp Spring Tuff Member, and 5) Burro Mesa Rhyolite. However, recent field observations, geochemical, and geochronological data show numerous discrepancies suggesting the need for revision of original stratigraphy. Previous field mapping indicated multiple vents for South Rim Formation in addition to the Pine Canyon caldera in both the high Chisos Mountains and the Castolon Graben. However, recent field studies focusing on the high Chisos Mountains note the absence of Wasp Spring Member amidst other members of the South Rim Formation. New 40Ar/39Ar ages show geochronologic dissimilarities between rhyolite capping Emory Peak in the high Chisos Mountains dated at 32.2 Ma and numerous samples of Burro Mesa Rhyolite Member from the Castolon Graben dated at 29.4 Ma Furthermore, while numerous researchers document petrographic similarities between Burro Mesa Rhyolite Member in the Castolon Graben and rhyolite capping Emory Peak in the high Chisos Mountains, recent geochemical data demonstrates that the Burro Mesa Rhyolite Member is generally less silicic and more enriched in Nb, Th, Zr, Y, Hf, and Rb than Emory Peak rhyolite. Based on current samples, Lost Mine Member may be geochemically and petrographically indistinguishable from Boot Rock Member and the term may be unnecessary. Here a stratigraphic revision is suggested for the South Rim Formation. Where the South Rim Formation was originally described as five members, we suggest limiting it to only three ~32 Ma members within the high Chisos Mountains: Pine Canyon Rhyolite Member, Boot Rock Member (trachytic composition), and the newly termed Emory Peak Rhyolite Member (high silica rhyolite capping Emory Peak). Due to its absence within the high Chisos Mountains, we also suggest that the 29.4 Ma Wasp Spring Member and the Burro Mesa Rhyolite Member be isolated from the South Rim Formation and upgraded to the Burro Mesa Formation.