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The Tieton andesite lavas of the south-central Washington Cascades have lengths of 74 km and 52 km, ranking them among the longest known andesite flows in the world. These two Pleistocene intracanyon flows occupy ancestral canyons of the Tieton River and its tributaries that drained the Goat Rocks volcanic complex, and part of the Naches River valley from its confluence with the Tieton River to Cowiche Creek. Don Swanson was the first to identify the existence of two Tieton andesite flows near Rimrock Lake in his Ph.D. thesis. Our geologic mapping, supported by geochemistry and age dating, confirms the existence of two Tieton andesite flows separated by ~250,000 yr there and in Tieton Canyon.

Tieton andesite lavas are high-potassium, calc-alkaline pyroxene andesite to trachyandesite and have excellent columnar jointing, easily viewed along U.S. Highway 12 in Tieton Canyon. The older, longer Tieton andesite flow, Qta1 has ~62 wt% SiO2 and a 40Ar/39Ar date of 1.64 ± 0.07 Ma. It traveled from the east flank of Bear Creek Mountain to Cowiche Canyon. The shorter, younger flow, Qta2, has ~60 wt% SiO2 and a 40Ar/39Ar date of 1.39 ± 0.10 Ma. It reached only the Oak Creek Wildlife Area Headquarters from its vent on the ridge north of the summit of Bear Creek Mountain. The volumes of Qta1 and Qta2 are estimated to be at least 6.6 km3 and 2.5 km3, respectively. A relatively high effusion rate, estimated at 11–18 m3/s from a large volume of available magma, combined with confinement in preexisting stream channels contributed to the development of these long andesite flows. Emplacement by flow inflation is suggested based on similarities in physical characteristics observed in Columbia River basalt flows. Emplacement of the Tieton andesite flows lasted on the order of a decade, based on a constant effusion rate.

The flows were erupted from Bear Creek Mountain, ~6 km north of the core of the Goat Rocks volcanic complex, and they are not from Black Thumb, as previously inferred; Black Thumb is a mafic dacite plug that is chemically and age distinct from Tieton andesite. The vents at Bear Creek Mountain are characterized by oxidized andesitic agglomeratic debris formed around remnant fins and plugs of a lava dome complex. The composition of the rocks there matches Tieton andesite well. Other andesite flows in the area were likely erupted from the same vent as Qta2 and partially buried the vent associated with Qta1.

Olivine basalt, Qob1, and basaltic andesite, Qob2, flows are in contact with or adjacent to Tieton andesite at several localities and appear to occupy valleys cut in or alongside Tieton andesite lavas. Qob1 is older than Qta2 based on field relationships between the flows, although their radiometric ages are indistinguishable (Qob1 at 1.38 ± 0.25 Ma vs. Qta2 at 1.39 ± 0.10 Ma). Qob2 (1.31 ± 0.04 Ma) may have a similar length as that for Qta2, suggested by an exposure of Qob2 recently identified near the confluence of the Naches River and Tieton River.

In Tieton Canyon and at other localities near Rimrock Lake, the older flow, Qta1, is perched higher on the slope than Qta2, indicating a period of erosion and downcutting between emplacements of the two flows. At two locations where the flows are in close proximity to one another, the base of the upper, older flow is ~40 m above the base of the lower flow. Comparisons of paleodrainage stream gradients, as estimated from the elevation of the distal to proximal flow bases of Qta1, Qta2, and Qob2, with those of the modern-day Naches and Tieton Rivers suggest uplift or tilting of the Cascades of almost 7 m since emplacement of the first Tieton andesite flow.

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