Here formally named Teepee Butte Member and assigned to Miocene Grande Ronde Basalt in Miocene Columbia River Basalt Group. [Probably named for Teepee Butte, Teepee Butte 7.5 min quad, northeastern OR. Composite type locality at Limekiln Rapids (SW/NW1/4, sec 3, T6N, R46E, Black Butte 7.5 min quad, WA); at Joseph Creek (SW/NW1/4, sec 3, T6N, R46E, Black Butte 7.5 min quad, WA); and at Pruitt Draw (NE/NE1/4, sec 12, T30N, R4W, Wapshilla Creek 7.5 min quad, ID). Described are eight reference sections in WA and ID (Reidel, 1978; Reidel, 1983; Camp, 1976; Anderson, 1987). Unit occurs throughout much of Clearwater embayment, eastern part of Columbia Plateau (Reidel, 1978, 1983) and crops out near Hood River in Columbia River Gorge (Anderson, 1987; Reidel and others, 1989b). In central Columbia Plateau the Teepee Butte is covered by >3 km basalt and encountered only in deep hydrocarbon exploration boreholes (Reidel and others, 1989b). Compositionally homogeneous, typically fine grained to glassy and aphyric, the three informally named sheet flows of the Teepee Butte represent distinct eruptive units called (oldest to youngest) basalt of Limekiln (new), basalt of Joseph Creek and basalt of Pruitt Draw (was called high TiO2/high MgO unit and was called low TiO2/high MgO unit, respectively). The latter were first identified from chemical composition and stratigraphic position by Reidel (1983) and Reidel and others (1989b). Flows of the Teepee Butte show blocky to crude columnar jointing over much of eastern Columbia Plateau, poorly developed entablatures and colonnades at flow margins, and thick flow-top breccia composing up to 1/3 of each flow near vents. In contrast to Walker's Hawaiian samples of thin, overlapping, interconnected lobes of lava, Teepee Butte flows are extensive, thick, and easily mistaken as the total output of a single eruption. Total thickness of the Teepee Butte varies from minimum 30 m at the distal end to more than 200 m in Pasco basin, central Columbia Plateau. Volume of basalt erupted increased with each successive flow. Together, these mappable units cover >52,000 sq km with volume exceeding 5,000 cubic km. The underlying Miocene (16.5 Ma) Buckhorn Springs member (informal) is easily distinguished by chemistry of higher P2O5. Overlying, is Miocene Rogersburg member (informal). Age of entire Teepee Butte is Miocene, based on stratigraphic position, vent location, geochemical analysis, and paleomagnetic polarity. Well-documented flood-basalt volcanism on Columbia Plateau occurred between 17.0 and 6.0 Ma when Columbia River Basalt was erupted from fissures in eastern WA, northeast. OR, and western ID. Largest eruptions, in Grande Ronde time when 85 vol % of basalt was produced, date approximately 17.0 to 15.6 Ma. Report includes stratigraphic nomenclature chart, geologic sketch map, correlation diagram, and measured and cross sections.

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