Pg. 22. San Carlos Tuff. Ash-flow tuff exposed in Big Bend area, Brewster County, and southern Presidio County, southwestern Texas, but was erupted from a caldera in the Sierra Rica area, Chihuahua, Mexico. Rhyolite samples from Chihuahua, Mexico, yielded K-Ar ages of 30.6 +/-0.7 Ma and 30.2 +/-0.7 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.