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

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Title Geology of the northwest one-quarter of the Prineville Quadrangle, central Oregon
Names Thormahlen, David J. (creator)
Taylor, Edward M. (advisor)
Date Issued 1984-04-25 (iso8601)
Note Graduation date: 1984
Abstract 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.
Genre Thesis/Dissertation
Topic Geology -- Oregon -- Prineville
Identifier http://hdl.handle.net/1957/18842

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