Abstract

Tieton volcano, a large andesitic cone of Miocene age in the Fifes Peak Formation, is exposed in Tieton River Canyon about 20 miles west of Yakima, Washington. Gently dipping andesite flows at its base lap against the deformed flanks of an older cone, and form a shield about 700 feet thick. Surmounting the shield—with an abruptly transitional contact—are more than 5000 feet of bedded tuffs and breccias, whose steep radial dips define a pyroclastic cone. The reconstructed volcano had a basal diameter of at least 6.5 miles, and probably towered 8000–10,000 feet above its base before erosion and partial inundation by the Yakima Basalt.

On Bethel Ridge, 4–6 miles west of the center of the volcano, at least 1000 feet of gently dipping slurry-flood breccias, lava flows, and rhyolitic pumice flows can be traced nearly continuously into the pyroclastic cone. Some breccias thin and pinch out to the west, and one andesite flow ends with westward-inclined foreset lenses of breccia and lava at its snout. The Bethel Ridge rocks probably formed part of a lowland apron at the foot of the volcano.

Tieton volcano erupted chiefly block lavas, coarse-to fine-grained bedded tuffs, and chaotic breccias of multiple origins. Intruding them are more than 200 dikes, which form the southern half of a radial swarm (Tieton dike swarm) that centers on the volcano. Single dikes are 5–20 feet thick, and dip 70–90 degrees. They decrease in abundance with increased elevation, but many probably did not feed lava flows or breccias. Vertical lineations indicate the dike magma was injected upward, not laterally.

The dikes and lava flows are orogenic andesites containing phenocrysts of plagioclase, hypersthene, and augite. Mafic mineral compositions are nearly constant in the same rock, intratelluric corrosion and marked reverse zoning in plagioclase are minor, and whole-rock chemical analyses better represent the parent magma than phenocryst-free groundmass compositions. Hence, conditions in the magma chamber were in general rather calm, with little migration of crystals relative to enclosing melt. Both the pigeonitic and hypersthenic series of ground-mass mafic minerals occur, and were probably derived from the same magma without sialic contamination. The dikes are younger than the flows and contain slightly more SiO2, Al2O3, and alkalies, and less CaO, MgO, and total iron, indicating weak differentiation.

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