Named as middle member of Salt Lake Formation for locality in the Jim Sage Mountains, Cassia Co, ID in the Great Basin province. Type locality established at about 42 deg, 05 min to 42 deg, 08 min Lat; 113 deg, 28 min to 113 deg, 32 min Long. Composed of rhyolite flows and domes. Divided into: lower rhyolite flows; middle unit of breccia (mostly a distinctive of angular to subrounded, gray, glassy rhyolite ranging from coarse sand to 1 m blocks in a matrix of glass or tuff); sandstone, siltstone, and fine-grained lacustrine units; and upper rhyolite flows. Maximum thickness 1,200 m. Overlies lower tuffaceous member; underlies upper tuffaceous member, both of Salt Lake Formation. Radiometric dates of 9.2 +/-0.5 K-Ar, and 9.4 +/-1.6 Zr-F-T m.y. Ranges from 7 to 11 m.y., or Miocene age.

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

Pg. 11, geologic time scale (inside front cover). Jim Sage Volcanic Member of Salt Lake Formation of Williams and others (1982). Sample from rhyolite flow in upper unit of volcanic member at Jim Sage Mountain (now Jim Sage Volcanic Member), in Jim Sage Mountains, Cassia County, Idaho (exact location unknown). Previously published age of 9.4 Ma (Williams and others, 1976, 2nd United Nations Symp. Geotherm. Res.) is recalculated using decay constants of Steiger and Jager, 1977 (Earth Planet. Sci. Letters, v. 36, p. 359-362). K-Ar age on zircon is 9.65 +/-1.7 Ma. [Age considered Miocene (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.