Geology of the northwest one-quarter of the Prineville Quadrangle, central Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/tb09j767b

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  • The northwest one-quarter of the Prineville Quadrangle is underlain by Tertiary and Quaternary volcanic and volcaniclastic rocks of the Columbia River Basalt Group, and the Clarno, John Day, Rattlesnake and Deschutes Formations. The Clarno Formation is dominated by pyroxene-bearing andesites, but also contains olivine-bearing basalts, oxyhornblende-bearing dacite and rhyodacite flows and intrusives. Many of these rocks are deeply weathered and some have been strongly silicified. The john Day Formation in the area consists of large rhyolite domes and flows, thick tuffaceous deposits, minor trachyandesite flows and welded ash-flow tuffs. The stratigraphy of these john Day rocks is similar to the section exposed in the Ashwood area but lacks some of the upper ash-flow tuff units. Fossil bearing tuffs found within the area are similar to tuffs in the John Day and Crooked River basins and contain fossil leaves that are similar to the Bridge Creek flora. A single flow of the Columbia River Basalt Group is found in the southwest part of the thesis area. This flow has normal magnetic polarity and is similar to the Prineville Chemical type basalt. The entablature of this flow is glassy and very thick. It resembles exposures found at Butte Creek and along the Deschutes River at Pelton Dam and near Gateway. Exposures of the Rattlesnake Ignimbrite Tongue in the northwest part of the thesis area are the western most recognized outcrops of the ignimbrite. Two other exposures of the Rattlesnake ignimbrite were found to the south in Swartz Canyon and near Little Bear Creek. These exposures indicate a previously unrecognized channel for the ignimbrite, trending northwest from its source, entering the Crooked River drainage, and traveling at least as far northwest as Grizzly. The Deschutes Formation is represented by a diktytaxitic basalt flow, epiclastic tuffaceous sediments and air-fall pumice. These deposits lie along the eastern margin of the Deschutes Basin. Structural upwarping along the Blue Mountains Anticline has caused local tilting and folding of the rocks in the area. Most of the Clarno and John Day rocks dip gently to the south. The Deschutes Formation appears to be undeformed. Hydrothermal activity led to the formation of several mineralized breccias which contain abundant silica, lesser amounts of goethite and manganite, and traces of silver and mercury.
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