Up to 1700 feet of the upper Miocene-lower Pliocene Yakima Basalt of the Columbia River Group underlie much of the eastern flank of the Cascade Range in the Tieton River area, Yakima County, Washington. Local prebasalt relief was more than 1700 feet, so thicknesses of each of the 15 exposed flows vary widely. Single flows can be traced for many miles, and terminate only against local topographic highs. The flows show typical colonnade-entablature jointing, and commonly overlie thin pillow-palagonite complexes.

The basalts have Yakima-type chemistry. Plagioclase varies chiefly from An65 (microphenocrysts) to An45 (microlites). Clinopyroxenes range from Ca-rich pigeonite to augite, with subcalcic augite most abundant. Complex continuous zoning with respect to optic angles occurs between all the clinopyroxene phases. Olivine is sparse, and phenocrysts of all minerals are rare.

Plots of 40 modal analyses indicate that plagioclase and pyroxene began crystallizing at about the same time, and crystallized at the same rate until the flows were at least 75 per cent crystalline. Individual flows differ in their plagioclase/pyroxene ratios and, to a lesser degree, in other microscopic characteristics. Therefore they can be correlated between the measured sections. The stratigraphy thus defined indicates that the youngest flow in the area is slightly older than the Vantage Sandstone Member (about 13.5 m.y.), a prominent sedimentary interbed farther east on the Columbia River Plateau.

Flow directions show that the basalts advanced into the area from the east and southeast. Sedimentary interbeds between some of the flows contain a heavy mineral suite indicative of a northern or northeastern provenance, and crossbedding measurements suggest westward and southwestward current directions. The regional paleoslope, therefore, sloped westward or southwestward; and the basalts probably extended somewhat beyond the present Cascade Crest before being dammed by the ancient Western Cascades. Floods of pyrogenic hornblende in the interbeds and overlapping K/A ages suggest sporadic explosive activity of Tatoosh-type plutons in the Cascades contemporaneous with Yakima extrusions.

The basalts are warped into five nearly west-trending folds and an eastward-sloping homocline. The homocline is related directly to Cascade uplift, which may have begun at about the time that Yakima-type flows ceased flooding the area.

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