Stratigraphy and Petrography of Four Upper Yakima Basalt Flows in South-Central Washington

Abstract

Field and laboratory investigations of the Yakima Basalt in south-central Washington show that the upper flows consist of four widespread sheets and interbedded sediments. From oldest to youngest, they are: Umatilla Basalt, Selah Member of the Ellensburg Formation, Pomona Basalt, Rattlesnake Ridge Member of the Ellensburg Formation, Elephant Mountain Basalt, and Ward Gap Basalt. Compared with two other local occurrences, the Pomona Basalt is identical with the Selah Butte flow and correlated with it, but is only similar to and probably older than the Huntzinger flow. The basalts are tholeiites and, except for Pomona Basalt, aphyric. During flowage, they were more than 95 percent liquid and crystallization began with olivine, followed by magnetite, plagioclase, and clinopyroxene. Intraflow variations in textures, modes, and mineral composition are determined by the crystallinity, which depends on the cooling history and varies widely within a flow. In general, the flows are glassiest in their interiors, whereas the lower and upper colonnades are hypocrystalline. Glass contents, for example, vary more than 60–70 modal percent within the Elephant Mountain and Umatilla flows. Vertical or lateral variations unrelated to crystallinity were not found. Characteristic structural and petrographic interflow differences, which are present despite internal variations, depend mainly on primary differences in chemical composition. Pomona Basalt is distinguished from the other flows by its magnesium- and calcium-rich composition. Umatilla Basalt differs from the Elephant Mountain flow in having lower total iron, magnesium, and calcium but higher alkalies. Ward Gap Basalt resembles the crystalline Elephant Mountain Basalt. The approximate average thicknesses (in meters), minimum extents (in square kilometers), and volumes (in cubic kilometers) of the four flows are: Umatilla Basalt—45 m, 5180 km 2 , 233 km 3 ; Pamona Basalt—30 m, 18,000 km 2 , 540 km 3 ; Elephant Mountain Basalt—10 m, 10,350 km 2 , 103 km 3 ; and Ward Gap Basalt—10 m, 5120 km 2 , 51 km 3 .