An area of unmapped intrusions lies within the Bull-of-the-Woods Roadless Area of Mount Hood National Forest. A variety of andesites, dacites,and diorites intruding units of tuff and andesite lava flows crop out over.an area of 52 sq km. These intrusions do not appear on the Geologic Map of Oregon West of the 121st Meridian (Hells and Peck, 1961). The purpose of this investigation is to map and describe these intrusions, with attention to the following questions: What is the petrographic and geochemical nature of these intrusions? What is the order of emplacement of these intrusions? What is the probable mechanism of intrusion? What relation do these intrusions bear to a possible underlying batholith? Contact relations in the field, petrographic studies, and major and trace element trends were used to address these questions.

Graduation date: 1983 / Best scan available for p.26, 111. Original is a copy of a copy. / For master (tiff) digital images of maps contained in this document contact scholarsarchive@oregonstate.edu

Topics in geology, which inevitably excite professional discussion and even tempered debate, often present challenging targets for the exploratory jabs of Master's degree candidates. The Portland Hills Silt and the controversy concerning its genesis provides just such an object. Although the physical descriptions of the silt are generally in agreement, they remain somewhat generalized and, at present, no single definition is generally accepted. Moreover, incongruities concerning structural and textural details--notably, the presence of minor stratification and/or scattered pebbles in the silt--have resulted in considerable disagreement regarding its mode of origin.

Within the thesis study area Spencer Formation arkosic/arkosic lithic sandstone lithofacies of Narizian age crop out in a sinuous north-northwesterly band from the Corvallis area into the Henry Hagg Lake vicinity ten kilometers southwest of Forest Grove, Oregon.

The Dinner Creek Tuff is a mid-Miocene rhyolitic to dacitic ignimbrite, consisting of four cooling units with 40Ar/39Ar ages 16--15 Ma. Previous geologists have suspected that the source of the tuff is located in northwestern Malheur County, eastern Oregon. This broad area is called the Dinner Creek Tuff Eruptive Center. This thesis summarizes field work, XRF/ICP-MS geochemistry, thin section petrography, and SEM feldspar analysis from the summers of 2015 and 2016. The main purpose of this study is to identify sources for the Dinner Creek Tuff units within the Dinner Creek Tuff Eruptive Center. The secondary purpose is to map lava flows that pre-date and post-date the Dinner Creek Tuff, and correlate them with regionally extensive volcanic units. Two volcanic centers related to the Dinner Creek Tuff were identified. The southern volcanic center, centered at Castle Rock, is a caldera and source of the Dinner Creek Tuff unit 1 (DIT1). Rheomorphic, densely welded DIT1 is over 300 m thick along the east side of Castle Rock. The northwestern margin of the caldera has been uplifted along faults, showing vertically foliated tuff dikes and associated mega-breccia deposits. Up to 200 m of incipiently welded tuffs, and fluvial volcanoclastic sediments were deposited on the caldera floor, which has been uplifted due to resurgence and regional extension, creating the complex structural relationships between the volcanic units. The northern volcanic center is located at Ironside Mountain, where densely welded rheomorphic Dinner Creek Tuff unit 2 (DIT2) is exposed in outcrops over 600 m thick. The top of the DIT2 consists of glassy, moderately welded tuff. Sources for the DIT2 are tuff dikes along the south and western flanks of Ironside Mountain. The thick deposits of DIT2 at Ironside Mountain indicate that the mountain is an uplifted caldera, herein named the Ironside Mountain caldera. Uplift may have been due to resurgence, but it is most likely due to normal faulting along the Border Fault, a major regional normal fault that strikes across the northern margin of the caldera. Pre-Dinner Creek Tuff lava flows occur throughout the study area, and can be correlated with the Strawberry Volcanics and the Basalt of Malheur Gorge. A distinct lava flow, herein called the Ring Butte trachy-basalt occurs within the center of the study area, and is distinct from regional lava flows. Following the eruptions of the Dinner Creek Tuff units 1 & 2, aphyric basaltic-andesite and icelandite intrude into, and overlie the intra-caldera tuffs and caldera floor sediments at both calderas. These aphyric lavas are similar in appearance and stratigraphic position with the regionally extensive Hunter Creek basalt. Porphyritic olivine basalt overlies the aphyric Hunter Creek basalt at the Castle Rock caldera. This porphyritic lava is similar in appearance and major/trace element geochemistry to the regional Tim's Peak basalt.

Large-volume silicic ignimbrites erupt from reservoirs that vary in composition, temperature, volatile content and crystallinity. The 9.7 Ma Devine Canyon Tuff (DCT) of eastern Oregon is a large-volume (>250 km3), compositionally zoned and variably welded ignimbrite. The ignimbrite exhibits heterogeneous trace element compositions, variable volatile content and crystallinity. These observations were utilized in the investigation into the generation, accumulation and evolution of the magmas composing the DCT. Building off previous research, pumices were selected from the range of trace element compositions and analyzed with respect to crystallinity, mineral abundances and assemblages. The DCT displays a gradational trace element enrichment and decrease in crystallinity from least evolved, dacite, at ~22% crystals to most evolved high-silica rhyolite at 3% crystals. Two distinct mineral populations of feldspar and clinopyroxene were identified in previous work, one belonging to the rhyolitic magma and the other to the dacitic magma. Volatile content derived from melt inclusion Fourier Transform Infrared (FTIR) spectrometer analysis revealed an increase in water content from 1.2 to 3.7 wt.% in the most evolved rhyolite. The DCT exhibits low and variable δ18O signatures, 4.52‰ to 5.76‰ , based on δ18O values measured on quartz and sanidine. Low δ18O signatures of all DCT rhyolites suggest the incorporation of hydrothermally altered crust into the melt. Furthermore, quartz phenocrysts from all high-silica rhyolite groups display dark oscillatory zoned cores and Ti-rich bright rims. These data provide insight into how these magmas were generated and subsequently stored in the crust. Commonalities of petrographic and compositional features among rhyolites, especially the zoning characteristics of quartz phenocrysts, exclude the possibility of storage and evolution in multiple reservoirs. Envisioning a scenario where all magmas are stored within a single reservoir prior to eruption and assuming rhyolites A and D are the product of partial melting. The mixing of A and D rhyolites produced rhyolite B, and subsequent mixing of intermediate rhyolite B and end-member rhyolite D generated rhyolite C. However, some trace element inconsistencies, between mixing model and observed intermediate rhyolites suggest a secondary process. Post mixing, rhyolites B and C require some modification by fractional crystallization to account for LREE and other inconsistencies between mixed models and observed rhyolites. Finally, the origin of the dacite is likely through mixing of group D rhyolite and an intrusive fractionated basalt, which could have led to the eruption of the Devine Canyon Tuff.

Rhyolite lava flows are found at two dome complexes at Graveyard Butte and Gordon Butte, Mount Hood Quadrangle, Oregon. At Graveyard Butte, the White River has cut a winding canyon 150 m deep, exposing at its base, a 40-meter-thick outcrop of flow-banded rhyolite (73 wt.% SiO2, 3.67±0.01 Ma) that laterally extends along the canyon wall for about 1 km. Stratigraphically above the flow-banded rhyolite is locally-erupted iron-rich andesites (lava flows, agglutinate and other pyroclastic rocks as well as clastic debris), a rhyolitic ash-flow tuff (74 wt.% SiO2), and the 2.77±0.36 Ma tholeiitic basalt lava flows of Juniper Flat (Sherrod and Scott, 1995). Roughly 2 km downstream, a phenocryst-poor, maroon-colored rhyolite (3.65±0.01 Ma) is visible again, forming steep canyon walls for about 1.6 km. A compositionally similar silicic unit is found 18 km to the northwest of Graveyard Butte at Gordon Butte. Exposed units along Gordon Butte's Badger Creek (3.64±0.03 Ma) and the southeastern upper slopes of Gordon Butte include rhyolite flows (69.6-72.1 wt.% SiO 2). The rhyolite lava flows at Graveyard Butte and Gordon Butte's Badger Creek are nearly chemically indistinguishable and both contrast with the younger rhyolitic ash-flow tuff at Graveyard Butte and lava flows on Gordon Butte's Upper Slopes. The rhyolites of Graveyard Butte and Badger Creek are richer in Nb and Zr (30-40 ppm and 487-530 ppm, respectively) than the younger rhyolitic tuff and Upper Slopes flows (13-19 ppm and 235-364 ppm, respectively) and share characteristics with A-type granitoids. The rhyolite lavas are porphyritic (~7%) with the porphyroclasts comprising primarily individual feldspars (250-500 µm in length) with ragged margins, oscillatory zoning and less commonly, spongy cores. Other phenocrystic phases include fayalitic olivine, Fe-rich clinopyroxene, and Fe-Ti oxides. A-type-like incompatible trace-element-enriched compositions as well as mineralogical indicators suggest rhyolite lava flows at Graveyard Butte and Gordon Butte's Badger Creek are likely generated in an extensional tectonic setting. A possible geotectonic framework for generation of these rhyolite lavas is the northward propagating intra-arc rift of the Oregon Cascades.

Glass Buttes complex lies at the northern margin of the Basin and Range province in central Oregon and is cut by the northwest-trending Brothers fault zone. An older acrystalline volcanic sequence of high-silica rhyolites (>75% SiO2) forms a broad platform composed of domes and flows with minor pyroclastic deposits. The high-silica rhyolite sequence is divided on the basis of texture into 1) zoned flows and domes, 2) obsidian flows, 3) felsite flows, and 4) biotite-phyric flows and domes.