Topography and ground conditions were important factors in controlling the distribution of individual Columbia River Basalt Group (CRBG) flows in western Oregon. The Columbia trans-arc lowland, the Yakima fold belt, the Portland Hills–Clackamas River structural zone, and Cascadian volcanism largely controlled the distribution of CRBG flows across the Miocene Cascade Range. The first flows to cross the Miocene Cascades into the Willamette Valley encroached onto a low-relief topography generally consisting of eroded Tertiary-age marine sedimentary rocks deformed along northwest-trending structural zones, volcanic highs, and estuaries. No north-south trough affected the distribution and thickness of the CRBG in the Willamette Valley, but an incipient Coast Range acted as a leaky barrier to the Oregon coast. Water-saturated sediments rapidly extracted heat from advancing CRBG lava flows, producing narrow, abnormally thick lobes extending along existing topographic lows. Deformation along the northwest-trending Portland Hills–Clackamas River structural zone produced a major topographic barrier early and late in the incursion of CRBG flows. The CRBG thins across this zone from 600 to 150 m. This zone diverted the earliest Grande Ronde flows into and through the Portland Basin. Some of the succeeding R 2 and N 2 Grande Ronde flows were able to cross this zone and followed another structural low, the Sherwood trough, to the Oregon coast. The total thickness of CRBG along the Sherwood trough is approximately 300 m, about twice that on either side. Paleodrainage developed during time intervals between emplacement of CRBG flows. The positions of these drainage courses were influenced by the position of the CRBG flow margins and/or structural lows. A longer hiatus between flows (> 100,000 yr) enabled rivers to develop major canyons by headward erosion, which served to channelize subsequent CRBG flows.

ABSTRACT Basalt flows of the Columbia River Basalt Group (CRBG) host a series of regionally extensive aquifers between western Idaho and the Pacific Ocean that serve as an important source for domestic, municipal, agricultural, and industrial water supply throughout much of this area, and are the sole source for some communities in the Willamette Valley. Rapid growth and increased pumping have resulted in significant water level declines in some locales in the Willamette Valley, forcing some communities to develop other water sources, and/or develop aquifer storage and recovery projects to store water in CRBG aquifers. The CRBG generally consists of multiple concordant, tabular sheet flows. The primary water-bearing horizons within the CRBG are associated the vesicular and/ or brecciated flow top and flow bottom (pillow/hyaloclastite) structures that form the interflow zone between two flows. The interiors of the CRBG flows typically have limited vertical permeability and act as aquitards, creating a series of layered confined aquifers. The dominant groundwater flow pathway in the CRBG aquifer system is along these individual, laterally extensive, interflow zones. Tectonic structures may modify the dominant flow regime in the CRBG by offsetting or otherwise disturbing originally laterally continuous interflow zones. Faults result in a wide spectrum of effects on flow in the CRBG aquifers depending on the nature of the fault. The hydraulic properties inherent to CRBG aquifers, including high degree of confinement, low bulk permeability and limited recharge have led to overdraft conditions in many areas. Conversely, these characteristics create favorable conditions for aquifer storage and recovery system development in the central Willamette Valley and Tualatin Basin.

Abstract This field trip guide describes a one-day loop from Portland eastward around Mount Hood and returning through the Columbia River Gorge. The purpose is to visit a SNOTEL (SNOwpack TELemetry) site to observe processes and instrumentation applied in automated snowpack data collection, as well as observe geomorphic features related to snowmelt in the western United States. Annual snow accumulation in the higher elevations in the western United States provides a critical source of water for irrigation, hydroelectric power generation, municipal water supplies, and recreation. Snowmelt, however, also can cause various hydrogeologic hazards, such as floods and debris flows.

Abstract More than 80 small volcanoes are scattered throughout the Portland-Vancouver metropolitan area of northwestern Oregon and southwestern Washington. These volcanoes constitute the Boring Volcanic Field, which is centered in the Neogene Portland Basin and merges to the east with coeval volcanic centers of the High Cascade volcanic arc. Although the character of volcanic activity is typical of many monogenetic volcanic fields, its tectonic setting is not, being located in the forearc of the Cascadia subduction system well trenchward of the volcanic-arc axis. The history and petrology of this anomalous volcanic field have been elucidated by a comprehensive program of geologic mapping, geochemistry, 40 Ar/ 39 Ar geochronology, and paleomag-netic studies. Volcanism began at 2.6 Ma with eruption of low-K tholeiite and related lavas in the southern part of the Portland Basin. At 1.6 Ma, following a hiatus of ~0.8 m.y., similar lavas erupted a few kilometers to the north, after which volcanism became widely dispersed, compositionally variable, and more or less continuous, with an average recurrence interval of 15,000 yr. The youngest centers, 50-130 ka, are found in the northern part of the field. Boring centers are generally monogenetic and mafic but a few larger edifices, ranging from basalt to low-SiO 2 andesite, were also constructed. Low-K to high-K calc-alkaline compositions similar to those of the nearby volcanic arc dominate the field, but many centers erupted magmas that exhibit little influence of fluids derived from the subducting slab. The timing and compositional characteristics of Boring volcanism suggest a genetic relationship with late Neogene intra-arc rifting.

ABSTRACT This field guide is for a three-day trip from Portland to Klamath Falls, Oregon, and back, traversing many of the physiographic provinces in Oregon, including the Columbia River, the Willamette Valley, the Western and High Cascade Mountains, the High Lava Plains, and the Basin and Range. Geologic field stops on Day 1 will be made along the drive to Klamath Falls at Salt Creek Falls and Crater Lake, and will include short discussions of Oregon's geological history and a brief introduction to geothermal resources in the state. Day 2 includes an extensive introduction to geothermal resources in Oregon and how these relate to Oregon geology, with tours to examine how the Oregon Institute of Technology, the City of Klamath Falls, and several local businesses in the Klamath Falls area use a local geothermal resource. Day 3 is devoted to the return trip to Portland, and will include geological stops at Newberry Crater, Crooked River Bridge, and Mount Hood, with more discussions of geology, geothermal resources, and geothermal exploration in Oregon.

ABSTRACT The Miocene Columbia River Basalt Group (CRBG) covers a large part of Oregon, Washington, and Idaho and is one of the youngest and perhaps the best studied flood-basalt province on Earth. Decades of study have established a regional strati-graphic framework for the CRBG, have demonstrated the CBRG flows can be correlated with dikes and vents, have documented a wide variety of physical features within the CRBG flows, and have demonstrated that many characteristics of the CRBG are recognizable throughout its extent. Detailed studies of individual flows and their feeder dikes have allowed the development of models for the emplacement of voluminous basaltic lava flows. The interplay between the regional structure, contemporaneous deformation, preexisting topography, and paleodrainage systems helped to control the emplacement of individual CRBG flows. These features have also affected the nature of late Neogene sedimentation in the region covered by basalt flows. Finally, the distribution of sediments within the CRBG and the character of the intraflow and interflow structures have played a significant role in the development of aquifers within the CRBG. In this paper we present an overview of the regional aspects of the stratigraphy, structural geology, tectonics, and hydrogeology of the CRBG.