Vieja Group mapped in report area, west TX in Permian basin. Mapped undivided in Presidio Co, west of Sierra Vieja, along the Rio Grande River, where Bracks Rhyolite of Vieja is absent. Also mapped undivided in the northern Oak Hills. In Sierra Vieja area, group is shown on Correlation of Map Units as consisting of (ascending): Gill Breccia, Colmena Tuff, Buckshot Ignimbrite, Chambers Tuff, Bracks Rhyolite, Capote Mountain Tuff, and Mitchell Mesa Welded Tuff [Mitchell Mesa is a formation of Buck Hill Group in the southeastern part of the Marfa sheet]. Shown below Tascotal Formation in Sierra Vieja area. [Unclear if following formations in Davis Mountains area belong to Vieja Group (ascending): Huelster Formation, Gomez Tuff, Adobe Canyon, Frazier Canyon, Sheep Pasture, and Merrill Formations; Barrel Springs, Mount Locke, Eppenauer Ranch, and Wild Cherry Formations; Wild Cherry-Barrel Springs (undivided); Medley, Goat Canyon, and Brooks Mountain Formations.] [Author states that correlation of volcanic rocks from area to area on Marfa sheet is uncertain--little attempt is made to correlate between areas. The rocks are described by area.] Age ranges from Eocene to Oligocene. Geologic map.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Pg. 14-15. Vieja Group. Includes (ascending) Gill Breccia, Buckshot Ignimbrite, Chambers Tuff, Pantera Trachyte (High Lonesome Tuff), Bracks Rhyolite, and Mitchell Mesa Rhyolite. Probable K-Ar ages from about 38 to 32 Ma. Data from R.K. DeFord, 1958 (Texas Jour. Sci., v. 10, p. 1-37), J.A. Wilson and others, 1968 (Amer. Jour. Sci., v. 266, p. 590-604), J.F. Evernden and others, 1964 (Amer. Jour. Sci., v. 262, p. 145-198), P.C. Twiss, 1970 (West Texas Geol. Soc. Pub. 71-59, p. 139-155), E.R. Burt, 1970 (Univ. Texas-Austin, PhD dissert., 153 p.), and J.C. Cepeda and C.D. Henry, 1983 (Univ. Texas-Austin Bur. Econ. Geol. Rpt. Inv. No. 135, 32 p.); ages recalculated using decay constants of Steiger and Jager, 1977 (Earth Planet. Sci. Letters, v. 36, p. 359-362). [See also entries under individual formations.]
[Age considered Eocene to Oligocene; Eocene-Oligocene boundary 36.6 (38-34) Ma (from Geologic Names Committee, USGS, 1983 ed. geol. time scale, with additions from N.J. Snelling, 1985, The Geol. Soc. Mem., no. 10).]

Source: Publication.