G25-G37. Wanapum Basalt of Yakima basalt subgroup [informal] of Columbia River Basalt Group. Medium-grained flows of olivine- and plagioclase-phyric basalt. Thickness 120 m. Includes (oldest to youngest) Eckler Mountain, Frenchman Springs, Roza, and Priest Rapids Members. Conformably and disconformably overlies Grande Ronde Basalt or overlies Vantage Member of Ellensburg Formation. Generally conformably underlies Saddle Mountains Basalt. Has normal magnetic polarity. Age is middle Miocene.
Type locality: exposures along east side of Columbia River near Wanapum Dam, from Sand Hollow, in sec. 28, T. 17 N., R. 23 E., south to top of section near Vantage Substation above Wanapum Dam, in sec. 16, T. 16 N., R. 23 E., Vantage and Beverly quadrangles, Grant Co., central WA.
[Yakima basalt subgroup considered informal and should not be capitalized. "Subgroup" not recognized as a formal stratigraphic rank term (CSN, 1933; ACSN, 1961, 1970; NACSN, 1983, 2005, 2021). Columbia River Basalt Group adopted by the ID, OR, and WA Geol. Surveys, and the USGS.]

Source: Publication; US geologic names lexicon (USGS Bull. 1564, p. 180-181); Changes in stratigraphic nomenclature, 1979 (USGS Bull. 1502-A, p. A82).

Wanapum Basalt of Columbia River Basalt Group. In Clearwater embayment, western Idaho, revised to include upper Miocene Onaway Member (new) at top. Underlies Saddle Mountains Basalt of Columbia River Basalt Group.

Source: US geologic names lexicon (USGS Bull. 1565, Onoway entry p. 230); GNU records (USGS DDS-6; Denver GNULEX).