Pg. 21. Santana Tuff. Rhyolite samples from Chihuahua, Mexico, and Redford SE quadrangle, Presidio and Brewster Counties, southwestern Texas, yielded K-Ar ages from 28.3 +/-0.6 Ma to 26.9 +/-0.6 Ma (alkali feldspar). In Chihuahua, unnamed lava underlying Santana Tuff dated at 28.0 +/-0.6 Ma (alkali feldspar). Data from R.L. Chuchla, 1981 (Univ. Texas-Austin, MS thesis, 199 p.), J.L. Gregory, 1981 (Univ. Texas-Austin, MS thesis, 78 p.), and J.F. McKnight, 1970 (Univ. Texas-Austin Bur. Econ. Geol., Geol. Map Quad. No. 37); ages recalculated using decay constants of Steiger and Jager, 1977 (Earth Planet. Sci. Letters, v. 36, p. 359-362).
[Age considered Oligocene (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.