Abstract

The John Day Formation in Oregon consists largely of silicic to intermediate pyroclastic material ranging in age from about 19 to 37 m.y. Stratigraphic and lithologic variations within the formation indicate that the bulk of this material was derived from vents west of the 121st meridian, in or near the present-day Cascade Range. Voluminous dacitic to andesite air-fall material was probably derived from volcanoes within the western Cascade Range, whereas rhyolitie ash-flow tuffs and lava flows were erupted from vents farther east. Sparse alkali basalt and trachyandesite flows, compositionally distinct from Cascade Range lavas, were erupted from local vents within the John Day outcrop area. Initiation of John Day volcanism about 37 m.y. ago signified a shift in the locus of calc-alkaline volcanic activity from the Blue Mountains to the Cascade Range and marked the emergence of the Cascade Range as a major volcanic feature.

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