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The Krumbo Reservoir 7.5-minute quadrangle encompasses parts of the Blitzen Valley and northwestern flank of Steens Mountain in the Basin and Range physiographic province of southeastern Oregon (fig. 1). The entire map area is underlain by a bimodal assemblage of middle and upper Miocene olivine basalt flows and rhyolite ash-flow tuffs (figs. 2 and 3). This assemblage is characteristic of volcanic rocks in the northern Basin and Range province and is thought to result from Cascade Range back-arc volcanic activity (Hart and Carlson, 1987). Locally derived Quaternary alluvium blankets part of the area. Faulting associated with Basin and Range extension has created the north-trending Blitzen Valley. In contrast, west-northwest-striking faults are parallel to and probably related to the Brothers fault zone, a northwest-trending zone of closely spaced, discontinuous fractures (Walker, 1969c; Lawrence, 1976).
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Geologic map of the Krumbo Reservoir Quadrangle, Harney County, Southeastern Oregon