Named as member of Starlight Formation (revised); previously called tuff of Arbon Valley of Starlight by Trimble, Carr (1976, USGS Bull. 1399) for exposures in Arbon Valley, Arbon quad, Power Co, southeast ID. Type section designated in The Cove, SE/4 sec 18, T2S, R38W, Bingham Co, ID, Snake River basin. Outcrops for 125 km along south margin of Snake River Plain from Rockland Valley to Blackfoot Range; also occurs in southern Lemhi Range. At type consists of 20-25 m of poorly to moderately welded, poorly sorted, pumiceous, massive, sparsely crystal-bearing, very pale tan air-fall tuff grading upward into pale-tan, planar-bedded base-surge deposit in lower part which disconformably underlies upper part consisting of 35-40 m of massive, more strongly welded, crystal-rich tuff. Maximum thickness is 60 m. Overlies and underlies unnamed or informal parts of Starlight (fig. 6, diagrammatic cross section). Source of Arbon Valley may be from a now-buried caldera (Tabor caldera of Kuntz, Covington, 1979, EOS, v. 60, p. 945) north and (or) west of Blackfoot, ID, which is part of proposed Picabo volcanic field of Pierce, Morgan (1990, GSA Memoir, p. 1-54). Geochemistry. K-Ar and 40Ar/39Ar age determinations yield weighted mean age of 10.2 +/-0.06 Ma (tables 1, 2); had been considered 7.9 Ma in earlier reports. Age is oldest Miocene isotopic age recorded east of Sublette Range along southeastern margin of Snake River Plain.

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