The thesis area is located in the Blue Mountains of northeast Oregon. Pre-Tertiary and Cenozoic rocks, divided on the basis of age and tectonic history, are exposed along the eroded crest of the Blue Mountain anticline. Permian (?) metasedimentary rocks probably correlate with the Elkhorn Ridge Argillite, and consist mainly of phyllite and argillaceous chert, with minor amounts of biotite schist, biotite gneiss and marble. Metasedimentary rocks may have accumulated in a marginal basin to the east of a late Paleozoic island-arc. Interbedded with the metasedimentary rocks is a Permo-Triassic amphibolite, which may represent either metamorphosed volcanic rocks of Permian (?) age or a Triassic (?) metamorphosed sill-like equivalent of the metagabbro. Metagabbro and related metatroctolite, the serpentinite of
Copple Creek, and the trondhjemite of Willow Creek are of Triassic (?) age. These rocks petrographically and chemically resemble the Canyon Mountain ophiolite complex as well as Other ophiolites throughout the world. The metagabrro and associated rocks are considered to represent part of a dismembered ophiolite assemblage emplaced. into the metasedimentary rocks during an Early to Middle Triassic orogeny. Mesozoic deformation is represented by ill defined northwest trending open folds in the metasedimentary rocks. The trondhjemite of Coalmine Hill is similar to intrusives associated with the Bald Mountain batholith and is considered to be Jurassic-Cretaceous in age. Unconformably overlying pre-Tertiary rocks in a nonmarine,
Eocene sedimentary sequence here named the Herren Formation. It consists of interbedded arkose and mudstone with minor carbonaceous shale and thin coal seams. These sedimentary rocks were deposited by westward to northwestward flowing streams. Overlying and possibly concordant with the Herren Formation is a sequence of andesite and basalt flows interbedded with tuffs, and mudflow breccias of the Eocene-Oligocene Clarno Formation. Upwarping and faulting of the Herren and Clarno Formations, forming the modern Blue Mountain anticline, began during early Oligocene and culminated in dips up to 50 degrees. Unconformably overlying the Clarno Formation are middle Miocene flows of low-Mg, Yakima-type, Columbia River Basalt, which onlapped and flowed southward over the crest of the anticline. Gentle folding of the anticline continued after extrusion of the basalts, possibly during the early middle Pliocene. Post-Pliocene erosion has resulted in exposure of pre-Tertiary and early Tertiary rocks.
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