The thesis area includes 54 square miles of the southeastern
Cuddy Mountains in west-central Idaho. The oldest rocks within the
area are plutonic phases of the Jurassic Cuddy Mountain complex.
These and Cretaceous (?) intrusive and associated (?) extrusive rocks
and nearby Triassic to Early Jurassic metasedimentary and metavolcanic
rocks were covered by late Tertiary basalt flows. They have
been exhumed by Quaternary glacial and stream erosion.
The Cuddy Mountain intrusive complex consists of gabbro, quartz
diorite, porphyritc granodiorite, and associated mafic and silicic dikes,
in order of decreasing age. Hydrothermal alteration is apparently related
to late residual fluids derived from the porphyritic granodiorite
intrusive and is 'primarily of the propylitic (epidote, chlorite, and
calcite) and silicious (quartz) types.
Country rocks and the intrusive complex were regionally metamorphosed
to the greenschist facies in Middle Jurassic time.
A porphyritic rhyolite dike post-dates the regional metamorphism
and is suggested to be related to the Cretaceous Idaho Batholith. A
welded ash-flow tuff unconformably overlies a part of the intrusive
complex and is postulated to be the extrusive equivalent of the porphyritic
Columbia River Basalts were erupted during Middle Miocene to
Early Pliocene time and unconformably rest on all pre-Tertiary rocks.
Both Picture Gorge and Yakima members of the Columbia River
Basalt are present and their combined thickness exceeds 1,000 feet.
The lowest flow of the Picture Gorge member is chemically similar to
an older series of basalt flows (High-Titania Alkali-Olivine Basalts)
in north-central Oregon.
The Cuddy Mountains probably existed as a topographic high
prior to extrusion of the Columbia River Basalt. However their present
expression, although modified by erosion, has resulted from Late
Tertiary and Quaternary uplift by vertical to steeply dipping normal
faults that were accompanied by minor folding.