The thesis area is located southwest of Portland, Oregon, in the
Western Cascades. In the area are volcaniclastic rocks and lava flows
that range in composition from basaltic andesite to dacite. The dominant
volcaniclastic rock type is thin intracanyon ash-flow tuff. The
dominant lava flows are a thick sequence of basaltic andesites to
dacites that account for 60 percent of those found in the thesis area.
Within this group is an intracanyon flow of basaltic andesite that contains
double pheriocrystic stages of clinopyroxene and plagioclase.
Quartz is also present and surrounded by a reaction rim of clinopyroxene.
This unit is an example of magmatic mixing. The most distinctive
unit is a cliff-forming horublende ignimbrite found in two isolated
locations. This unit contains large hornblende crystals, collapsed
pumice lumps, and sanidine granite inclusions. There are four andesite
intrusions in the thesis area; two are dikes and two are irregular
discordant bodies. The most distinctive is Pin Creek andesite with
large embayed quartz phenocrysts.
Variation diagrams based on chemical data follow normal trends
as expected for calc-alkaline or calcic rocks. Rocks from the thesis
area are calcic, due to abundant calcic plagioclase phenocrysts and
The thesis area lies along the western flank of a broad northeast-plunging
anticline. All units consistently dip 8 to 11° in a westerly
direction or northerly direction depending upon their location with
respect to the nose of the anticline. Two distinct groups of faults
occur. One group trending north-northwest, the other east-west. It is
the northern fault of the east-west-trending group that has served as a
zone of crustal weakness for the intrusion of a dike system and the
conduit of hot water supplying Bagby Hot Springs.
Bagby Hot Springs lies in a broad, deeply-glaciated valley used
primarily for recreational activities. The projected geothermal reservoir
temperature of 127°C and the low flow rate do not make this area a
good potential reservoir for generation of electricity.