|Name||South Rim Formation from Pine Canyon Caldera|
Igneous > Volcanic > Felsic-volcanic > Quartz-trachyte (Pyroclastic-ash flow)
Igneous > Volcanic > Felsic-volcanic > Rhyolite (Pyroclastic-ash flow)
|Comments||Pine Canyon Rhyolite--lt. brown to gray, densely welded multiple cooling units of peralkaline rhyolitic ash-flow tuff, at least 300 m thick within the caldera. Boot Rock Mbr (informal unit of Barker, Henry, and McDowell, 1986) rocks lying between the Pine Cany. Rhyolite and Lost Mine Mbr in and north of South Rim Mesa--two ash-flow tuff facies, caldera fill (100 m thick) and outflow. Caldera fill is varicolored breccia having densly welded glassy matrix containing subangular fragments, 1-5 cm in dia. of quartz trachyte and rhyolite. The outflow facies spilled over rim of caldera and is poorly to moderately welded, 10-20 m thick and contains lithic and vitric clasts only in basal surge deposit. Lost Mine Mbr is package of quartz trachyte ash-flow tuffs and local lava flows. Wasp Spring Mbr. 33 m thick at Burro Mesa and is coarse, well-bedded tuff containing clasts up to 50 cm in dia, sag structures, low-angle crossbedding and pumic clasts. Burro Mesa Rhyolite has lower unit of highly welded blue-gray ash-flow tuff with rheomorphic folds, ramp structures. The upper unit, 130 m thick, has basal black vitrophyre, clasts of tuffaceous sediment and porphyritic quartz trachyte up to 1 cm in dia. and common pumice lumps|
|Stratigraphic units||(old to young) Pine Canyon Rhyolite, Boot Rock Member, Lost Mine Member, Burro Mesa Rhyolite|
Barker, D.S., Henry, C.D., and McDowell, F.W., 1986 Pine Canyon caldera, Big Bend National Park--a mildly peralkaline magmatic system: in Price, J.G, Henry, C.D., Parker, D.F., and Barker, D.S. eds., Igneous geology of Trans-Pecos Texas: The University of Texas at Austin, Bureau of Economic Geology, Guidebook 23, p. 266-285.
Bureau of Economic Geology, 1992, Geologic Map of Texas: University of Texas at Austin, Virgil E. Barnes, project supervisor, Hartmann, B.M. and Scranton, D.F., cartography, scale 1:500,000.