Mitchell Mesa Welded Tuff mapped as one of seven formations of Vieja Group in the Sierra Vieja area, Presidio Co, west TX in Permian basin. Vieja Group is shown on the 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. In the southeastern part of the mapped area, Mitchell Mesa is mapped as a formation of Buck Hill Group, which consists of (ascending): Duff and Pruett Formations (undivided) and Duff Formation, Mitchell Mesa Welded Tuff, and Tascotal Formation. Shown below Tascotal Formation in both areas (Tascotal mapped alone in Sierra Vieja area). Mitchell Mesa is a cliff-forming ash flow, generally nonwelded to slightly welded; where more than 30 ft thick pronounced foliation in a broad zone about midway between base and center. Is porphyritic in type area, with phenocrysts of quartz and chatoyant sanidine as much as .2 inches in diameter in an aphanitic, pink to reddish-gray groundmass. Weathers dark reddish gray to black. Thickness as much as 255 ft; averages about 45 ft (includes Brite Ignimbrite of Rim Rock Country). [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.] K-Ar age of 31.5 +/-0.6 m.y. obtained (18 determinations). Oligocene age. Geologic map.

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

Pg. 14 (Mitchell Mesa Rhyolite of Vieja Group); pg. 16-17 (Mitchell Mesa Rhyolite). Rhyolite samples from Agua Fria Mountain, Cactus Flat, Capote Falls, Capote Peak, Cuesta Del Burro NW, Duff Springs, Goat Mountain, and Ruidosa Hot Springs quadrangles, Presidio and Brewster Counties, southwestern Texas, yielded K-Ar ages from 36.0 +/-0.8 Ma to 31.2 +/-1.5 Ma (alkali feldspar, amphibole). Accepted age is 32.3 +/-0.7 Ma. Data from E.R. Burt, 1970 (Univ. Texas-Austin, PhD dissert., 153 p.), J.C. Cepeda and C.D. Henry, 1983 (Univ. Texas-Austin Bur. Econ. Geol. Rpt. Inv. No. 135, 32 p.), R.K. DeFord, 1958 (Texas Jour. Sci., v. 10, p. 1-37), J.F. Evernden and others, 1964 (Amer. Jour. Sci., v. 262, p. 145-198), and J.A. Wilson and others, 1968 (Amer. Jour. Sci., v. 266, p. 590-604); ages recalculated using decay constants of Steiger and Jager, 1977 (Earth Planet. Sci. Letters, v. 36, p. 359-362).
[Age considered 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.