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South Rim Formation from Pine Canyon Caldera

South Rim Formation from Pine Canyon Caldera
State Texas
Name South Rim Formation from Pine Canyon Caldera
Geologic age Phanerozoic | Cenozoic | Tertiary | Oligocene
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
Primary rock type rhyolite
Secondary rock type ash-flow tuff
Other rock types lava flow; volcanic breccia (agglomerate); vitrophyre
Lithologic constituents
Igneous > Volcanic > Felsic-volcanic > Rhyolite (Pyroclastic, ash-flow)
Igneous > Volcanic > Felsic-volcanic > Quartz-trachyte (Pyroclastic, ash-flow)
Map references
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
Unit references
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 o
Counties Brewster