Bofecillos volcano volcanic rocks, including units 1-8 of Rawls Formation and lava flows in upper part of Fresno Formation (TXOGr;0)

State	Texas
Name	Bofecillos volcano volcanic rocks, including units 1-8 of Rawls Formation and lava flows in upper part of Fresno Formation
Geologic age	Oligocene
Lithologic constituents	Major 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) Incidental 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)
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 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.
NGMDB product	NGMDB product page for 68390
Counties	Brewster - Presidio

State

Name

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

Geologic age

Oligocene

Lithologic constituents

Major

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)

Incidental

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)

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 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.

NGMDB product

NGMDB product page for 68390

Counties

Brewster - Presidio