|Name||Bofecillos volcano volcanic rocks, including units 1-8 of Rawls Formation and lava flows in upper part of Fresno Formation|
Igneous > Volcanic > Felsic-volcanic > Trachyte (Flow)also quartz trachyte and porphyritic trachytes
Igneous > Volcanic > Felsic-volcanic > Rhyolite (Pyroclastic-ash flow)
Igneous > Volcanic > Mafic-volcanic > Basalt (Flow)
Igneous > Plutonic > Dioritic > Diorite (Flow)
Igneous > Plutonic > Syenitic > Syenite (Flow)
Sedimentary > Clastic > Mixed-clastic > Conglomerate-sandstone (Tuffaceous)lahars, tuffaceous debris-flow deposits
|Comments||Lava 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.|
|Stratigraphic units||Campo Javelina Basalt Member of Fresno Formation (Henry and others, 1998)|
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 Geology Circular 86-2, 34 p.
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 Geology, Report of Investigations no. 253, 74 p.
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.
|Counties||Brewster - Presidio|