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Bofecillos volcano volcanic rocks, including units 1-8 of Rawls Formation and lava flows in upper part of Fresno Formation

Bofecillos volcano volcanic rocks, including units 1-8 of Rawls Formation and lava flows in upper part of Fresno Formation
StateTexas
NameBofecillos volcano volcanic rocks, including units 1-8 of Rawls Formation and lava flows in upper part of Fresno Formation
Geologic agePhanerozoic | Cenozoic | Tertiary | Oligocene
Original map labelOr
CommentsLava flows in upper part of Fresno Fm is the 29-Ma old Campo Javelina Basalt Member (Henry and others, 1998), which is mostly trachybasalts ranging to basaltic trachyandesite having SiO2 contents 48-51 percent. For the Rawls Formation, informal units 1-9 were set up by Dietrich (1966) and McKnight (1970). Henry and others (1998) modified these units and redefined the Rawls Fm, of which units 1-8 is the older part that erupted in at least two pulses at 27.3 and 27.1 Ma. In Bofecillos Mountains area (Brewster Co, southwest TX), fr. top down (from Emory Peak-Presidio Sheet, 1979), conglomerate, sandstone, and tuff, basalt; bolson fill interbedded with above; trachyte; trachyandesite; latite porphyry; trachybasalt porphyry; nonwelded to thoroughly welded crystal-vitric to lithic-vitric, ash-flow tuff; latite porphyry; basalt; trachyandesite; latite porphyry; volcanic mudflow; latite; basalt, tuff, sandstone, and conglomerate; and trachybasalt porphyry. Diorite and olivine syenite. Max. thickness of formation about 1,200 ft. K-Ar ages upper Rawls 23.0 +-0.4, 22.2+_0.4, middle Rawls 26.2+-0.5 m.y. Henry and others (1998, p. 33) report that "latite porphyries" are not present, as previous interperted by McKnight (1970) and are instead trachytes and prophyritic trachytes.
Primary rock typebasalt
Secondary rock typetrachyte
Other rock typesfelsic volcanic rock; rhyolite
Lithologic constituents
Major
Igneous > Volcanic > Felsic-volcanic > Rhyolite (Pyroclastic, ash-flow)
Igneous > Volcanic > Mafic-volcanic > Basalt (Flow)
Igneous > Volcanic > Felsic-volcanic > Trachyte (Flow)also quartz trachyte and porphyritic trachytes
Incidental
Igneous > Plutonic > Syenitic > Syenite (Flow)
Igneous > Plutonic > Dioritic > Diorite (Flow)
Sedimentary > Clastic > Mixed-clastic > Conglomerate-sandstone (Flow)lahars, tuffaceous debris-flow deposits
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
Henry, C.D., McDowell, F.W., Price, J.G., and Smyth, R.E., 1986, Geochronology of Tertiary igneous rocks, Trans-Pecos, Texas, in Price, J.G. and others, eds., Igneous geology of Trans-Pecos, Texas: The University of Texas at Austin, Bureau of Economic Ge
Bureau of Economic Geology, 1979, Emory Peak--Presidio Sheet, Geologic Atlas of Texas, University of Texas at Austin.
Dietrich, J.W., 1966, Geology of Presidio area, Presidio County,Texas: University of Texas, Austin, Bureau of Economic Geology Geologic Quadrangle Map 28, scale 1:48,000 and text, 45 p.
McKnight, J.F., 1970, Geology of Bofecillos Mountains area,Trans-Pecos Texas: The University of Texas Bureau of Economic Geology Geologic Quadrangle Map 37, scale 1:48,000, text 36 p.
Henry, C.D., Davis, L.L., Kunk, M.J., and McIntosh, W.C., 1998, Tertiary volcanism of the Bofecillos Mountains and Big Bend Ranch State Park, Texas--revised stratigraphy and 40Ar/39Ar geochronology: The University of Texas at Austin, Bureau of Economic
Geographic coverageBrewster - Presidio

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