The Yakima Basalt and Ellensburg Formation of south-central Washington

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The uppermost part of the Yakima Basalt herein assigned to the Columbia River Group has the following largely conformable units (from oldest to youngest) : lower basalt flows; the Vantage Sandstone Member; the Frenchman Springs Member, which locally includes the Squaw Creek Diatomite Bed; the Roza Member; the Priest Rapids Member, which locally includes the Quincy Diatomite Bed; and the Saddle Mountains Member. Sedimentary deposits of the Ellensburg Formation interflnger with basalt flows of the youngest member of the Yakima Basalt. The maximum thickness of the Yakima Basalt is not known, but the formation is about 4,500 feet thick at a test well drilled in the Odessa area, and is probably much thicker toward the center of the Columbia Plateau. The part described herein has a composite thickness of about 1,200 feet above the Vantage Sandstone Member. The overlying and interflngering Ellensburg Formation is as thick as 2,000 feet in the type area, and the Beverly Member, which represents the lower part of the Ellensburg in the Sentinel Gap area, is as thick as 300 feet. The Yakima Basalt is late Miocene and early Pliocene in age, as determined from the flora and fauna in the underlying Mascall Formation in Oregon, the intrabedded sedimentary rocks and the interbedded and overlying Ellensburg Formation. INTRODUCTION The stratigraphy of a thick series of outwardly monotonous basalt flows in the Columbia Plateau has been studied in the past to determine the geologic structure as it affects engineering and ground-water projects. Whether the project is an areal ground-water study or a relatively localized engineering investigation, the presence or absence of anticlines, synclines, faults, unconformities, interbedded sedimentary deposits, and other features must be known before sound geologic interpretations can be made. U.S. Bureau of Reclamation geologists and consultants noted about 1940 that some basalt flows in the Columbia Basin Irrigation Project area had distinctive characteristics which