|Name||Vieja Group, including Bracks Rhyolite, Chambers and Colmena Tuffs and Gill Breccia|
|Geologic age||Eocene to Oligocene|
Igneous > Volcanic > Felsic-volcanic > Rhyolite (Pyroclastic-tuff)
Sedimentary > Clastic > Conglomerate (Bed)
Igneous > Volcanic > Felsic-volcanic (Pyroclastic-tuff)
Sedimentary > Clastic > Sandstone (Bed)
|Comments||Brachs Rhyolite--sl. porphyritic with rhombic anothoclase phenocrysts up to 3 mm in diameter; matrix alkalic feldspar, quartz, and mafic minerals; dk reddish-brown to grayish olive; thickness as much as 360 ft. K-Ar ages 36.5+-1.2 m.y., 36.8 m.y. Chambers Tuff--fine grained, crystal-vitric tuff, mod. to well bedded, pale red, grayish-pink, grayish green, pale purple, and grayish orange pink; a persistent layer of coarse sandst. in southern area (of Marfa Sheet, 1979) with some conglo 130 ft above base; thickness 105-750 ft. contains Oligocene vertebrate fossils. Colmena Tuff-- fine-grained, thin bedded tuffaceous sandst. and limestone-and-sandstone conglomerate; some nonmarine limest., silty claystone, and glassy flow-rock, pale red to white; mostly tuffaceous sandst. and limest. on south , mostly conglo. on north; includes Jeff Conglomerate which is not separately mapped; thickness as much as 450 ft. contains Eocene vertebrate fossils. Gill Breccia (Eocene) consists of flow breccias of three types: a) med-gray fragments in grayish-red matrix, b) variegated mottled fragments in dk-greenish-gray to orange-pink matrix, c) brecciated to massive lt-olive-green to greenish-gray fine-grained rock. Thickness about 360 ft. K-Ar age on volcanic rock from Gill Breccia 40.0+-2.0 Ma.|
|Stratigraphic units||Jeff Conglomerate|
Bureau of Economic Geology, 1979, Marfa Sheet, Geologic Atlas of Texas: Bureau of Economic Geology and The University of Texas at Austin, scale 1:250,000.
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||Culberson - Jeff Davis - Presidio|