Geology of the Stockade Mountain 15' Quadrangle, Malheur and Harney Counties, Oregon

The Stockade Mountain 15' Quadrangle lies in Harney and Malheur counties of southeastern Oregon. The study area has twelve mappable units that range in age from Early Miocene 10 Recent and are mostly volcanic or lacustrine in origin. The oldest unit is an Early Miocene unnamed igneous complex composed of basaltic and andesitic lows. This unit forms the basement rock in the region and is unconformably overlain by the Early to Middle Miocene Littlefield Rhyolite. The Littlefield Rhyolite is a laterally extensive unit that had more than one point of origin. Faulting along the eastern flank of Stockade Mountain has exposed the most extensive section of the rhyolite. The Middle Miocene Juntura Formation, a lacustrine unit, unconformably overlies the rhyolite. During deposition of the Juntura Formation, the Wildcat Creek Ash-Flow Tuff was erupted. Volcanic activity in the Harney Basin to the west produced the very extensive. Late Miocene Devine Canyon Ash-flow Tuff, which separates the Juntura Formation and the overlying Late Miocene Drewsey Formation, also a lacustrine unit. At approximately the same time that the Devine Canyon Ash-flow Tuff was erupted, an unnamed basalt southwest of Crowley was also erupted from an area to the south of the study area. The unnamed brown volcanic siltstone at Roger's Valley indicates a period of continued erosion of previous units and deposition of the siltstone. The unconformable Late Miocene Drinkwater Basalt is a prominent ledge-fomer in the area. The youngest basalt found within the area is an unnamed one at Buck Mountain. This pile of basalts flowed into the area from the north or northwest. Quaternary colluvium, alluvium, and playa deposits are the youngest units that are present. Structurally, the area is dominated by the Basin and Range tectonics. But the juxtaposition of the Owyhee Plateau has complicated the structures, stratigraphy, and correlation of the units.