Gill Breccia mapped as one of seven formations of Vieja Group in the Sierra Vieja area, Presidio Co, west TX in the Permian basin, where it is restricted to the southern third of the area. 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. Gill consists of flow breccias of three types: (a) medium-gray fragments in a grayish-red matrix; (b) variegated mottled fragments in a dark-greenish-gray to orange-pink matrix; and (c) brecciated to massive light-olive-green to greenish-gray, fine-grained rock. Deposited on an irregular surface. Thickness to about 360 ft. K-Ar age obtained on volcanic rock from breccia of 40.0 +/-2.0 m.y. [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.] Gill is of Eocene age.

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

Pg. 15. Gill Breccia of Vieja Group. Flow breccia samples from Candalaria quadrangle, Presidio County, southwestern Texas, yielded K-Ar ages of 78.0 +/-3.1 Ma (plagioclase), 58.1 +/-3.9 Ma (plagioclase), and 41.4 +/-2.0 Ma (whole-rock). On the basis of stratigraphic relations, the 3 ages are contradictory; true age may be about 38 Ma. Data from R.K. DeFord, 1958 (Texas Jour. Sci., v. 10, p. 1-37), P.C. Twiss, 1970 (West Texas Geol. Soc. Pub. 71-59, p. 139-155), 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 Eocene; 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.