ABSTRACT The Eocene Tyee Formation of west central Oregon, USA, records deposition in a forearc basin. With outcrop exposures of fluvial/deltaic to shelf and submarine fan depositional environments and known sediment sourcing constrained by detrital zircon dating and mineralogy linked to the Idaho Batholith, it is possible to place deposits of the Tyee Formation in a source-to-sink context. A research program carried out by the Department of Geological Sciences at The University of Texas at Austin and ExxonMobil Research Company’s Clastic Stratigraphy Group has reconstructed the Eocene continental margin from shelf to slope to basin floor using outcrop and subsurface data. This work allows us to put observations of individual outcrops into a basin-scale context. This field trip will visit examples of depositional environments across the entire preserved source-to-sink system, but it will focus on the deep-water deposits of the Tyee Formation that range from slope channels to proximal and distal basin-floor fans. High-quality roadcuts reveal the geometry of slope channel-fills in both depositional strike and dip orientations. Thick, sand-rich medial fan deposits show vertical amalgamation and a high degree of lateral continuity of sandstones and mudstones. Distal fan facies with both classic Bouma-type turbidites and combined flow or slurry deposits are well exposed along a series of new roadcuts east of Newport, Oregon. The larger basin-scale context of the Tyee Formation is illustrated at a quarry in the northern end of the basin where the contact between the oceanic crust of the underlying Siletzia terrane and submarine fan deposits of the Tyee Formation is exposed. The Tyee Formation provides an excellent opportunity to see the facies and three-dimensional geometry of deep-water deposits, and to show how these deposits can be used to help reconstruct ancient continental margins.

ABSTRACT The Oregon Coast Range is a dynamic landscape that is continually shaped by shallow and deep-seated landslides that can have disastrous consequences to infrastructure and human lives. Searching for evidence of potentially coseismic mass wasting is incredibly difficult, particularly when historical observations are limited. Landslide-dammed lakes with submerged “ghost forests” in the Oregon Coast Range present the unique opportunity to establish landslide chronologies with subannual accuracy when dendrochronology is applied. This field guide will visit the unique landslide-dammed Klickitat Lake and explore a drowned ‘ghost forest’ to discuss methods used to establish a prehistoric landslide chronology in western Oregon, USA. After exploring the lake and exposing its geomorphic secrets, the guide will end with a stop on Marys Peak, a mafic volcanic intrusion composed of gabbroic dikes and pillow basalt that forms the highest point in the Oregon Coast Range. With the landscape of western Oregon laid out before us, we will discuss short- and long-term geomorphic evolution of the Oregon Coast Range and Willamette Valley.

ABSTRACT The Klamath Mountains province and adjacent Franciscan subduction complex (northern California–southern Oregon) together contain a world-class archive of subduction-related growth and stabilization of continental lithosphere. These key elements of the North American Cordillera expanded significantly from Middle Jurassic to Early Cretaceous time, apparently by a combination of tectonic accretion and continental arc– plus rift-related magmatic additions. The purpose of this field trip is twofold: to showcase the rock record of continental growth in this region and to discuss unresolved regional geologic problems. The latter include: (1) the extent to which Mesozoic orogenesis (e.g., Siskiyou and Nevadan events plus the onset of Franciscan accretion) was driven by collision of continental or oceanic fragments versus changes in plate motion, (2) whether growth involved “accordion tectonics” whereby marginal basins (and associated fringing arcs) repeatedly opened and closed or was driven by the accretion of significant volumes of material exotic to North America, and (3) the origin of the Condrey Mountain schist, a composite low-grade unit occupying an enigmatic structural window in the central Klamaths—at odds with the east-dipping thrust sheet regional structural “rule.” Respectively, we assert that (1) if collision drove orogenesis, the requisite exotic materials are missing (we cannot rule out the possibility that such materials were removed via subduction and/or strike slip faulting); (2) opening and closure of the Josephine ophiolite-floored and Galice Formation–filled basin demonstrably occurred adjacent to North America; and (3) the inner Condrey Mountain schist domain is equivalent to the oldest clastic Franciscan subunit (the South Fork Mountain schist) and therefore represents trench assemblages underplated >100 km inboard of the subduction margin, presumably during a previously unrecognized phase of shallow-angle subduction. In aggregate, these relations suggest that the Klamath Mountains and adjacent Franciscan complex represent telescoped arc and forearc upper plate domains of a dynamic Mesozoic subduction zone, wherein the downgoing oceanic plate took a variety of trajectories into the mantle. We speculate that the downgoing plate contained alternating tracts of smooth and dense versus rough and buoyant lithosphere—the former gliding into the mantle (facilitating slab rollback and upper plate extension) and the latter enhancing basal traction (driving upper plate compression and slab-shallowing). Modern snapshots of similarly complex convergent settings are abundant in the western Pacific Ocean, with subduction of the Australian plate beneath New Guinea and adjacent island groups providing perhaps the best analog.

ABSTRACT New findings about old puzzles occasion rethinking of the Grand Coulee, greatest of the scabland channels. Those puzzles begin with antecedents of current upper Grand Coulee. By a recent interpretation, the upper coulee exploited the former high-level valley of a preflood trunk stream that had drained to the southwest beside and across Coulee anticline or monocline. In any case, a constriction and sharp bend in nearby Columbia valley steered Missoula floods this direction. Completion of upper Grand Coulee by megaflood erosion captured flood drainage that would otherwise have continued to enlarge Moses Coulee. Upstream in the Sanpoil valley, deposits and shorelines of last-glacial Lake Columbia varied with the lake’s Grand Coulee outlet while also recording scores of Missoula floods. The Sanpoil evidence implies that upper Grand Coulee had approached its present intake depth early the last glaciation at latest, or more simply during a prior glaciation. An upper part of the Sanpoil section provides varve counts between the last tens of Missoula floods in a stratigraphic sequence that may now be linked to flood rhythmites of southern Washington by a set-S tephra from Mount St. Helens. On the floor of upper Grand Coulee itself, recently found striated rock and lodgement till confirm the long-held view, which Bretz and Flint had shared, that cutting of upper Grand Coulee preceded its last-glacial occupation by the Okanogan ice lobe. A dozen or more late Missoula floods registered as sand and silt in the lee of Steamboat Rock. Some of this field evidence about upper Grand Coulee may conflict with results of recent two-dimensional simulations for a maximum Lake Missoula. In these simulations only a barrier high above the present coulee intake enables floods to approach high-water marks near Wenatchee that predate stable blockage of Columbia valley by the Okanogan lobe. Above the walls of upper Grand Coulee, scabland limits provide high-water targets for two-dimensional simulations of watery floods. The recent models sharpen focus on water sources, prior coulee incision, and coulee’s occupation by the Okanogan ice lobe. Field reappraisal continues downstream from Grand Coulee on Ephrata fan. There, some of the floods exiting lower Grand Coulee had bulked up with fine sediment from glacial Lake Columbia, upper coulee till, and a lower coulee lake that the fan itself impounded. Floods thus of debris-flow consistency carried outsize boulders previously thought transported by watery floods. Below Ephrata fan, a backflooded reach of Columbia valley received Grand Coulee outflow of small, late Missoula floods. These late floods can—by varve counts in post-S-ash deposits of Sanpoil valley—be clocked now as a decade or less apart. Still farther downstream, Columbia River gorge choked the largest Missoula floods, passing peak discharge only one-third to one-half that released by the breached Lake Missoula ice dam.

ABSTRACT This field guide reviews 19 sites providing insight to four Cenozoic deformational phases of the Cascadia forearc basin that onlaps Siletzia, an oceanic basaltic terrane accreted onto the North American plate at 51–49 Ma. The field stops visit disrupted slope facies, prodelta-slope channel complexes, shoreface successions, and highly fossiliferous estuarine sandstones. New detrital zircon U-Pb age calibration of the Cenozoic formations in the Coos Bay area and the Tyee basin at-large, affirm most previous biostratigraphic correlations and support that some of the upper-middle Eocene to Oligocene strata of the Coos Bay stratigraphic record represents what was differentially eroded off the Coast Range crest during ca. 30–25 Ma and younger deformations. This suggests that the strata along Cape Arago are a western “remnant” of the Paleogene Tyee basin. Zircon ages and biostratigraphic data encourages the extension of the Paleogene Coos Bay and Tyee forearc basin westward beyond the Fulmar fault and offshore Pan American and Fulmar wells. Integration of outcrop paleocurrents with anisotropy of magnetic susceptibility data from the middle Eocene Coaledo Formation affirms south-southeast to north-northwest sediment transport in current geographic orientation. Preliminary detrital remanent magnetism data show antipodal directions that are rotated clockwise with respect to the expected Eocene field direction. The data suggest the Eocene paleo-shoreline was relatively north-south similar to the modern shoreline, and that middle Eocene sediment transport was to the west in the area of present-day Coos Bay. A new hypothesis is reviewed that links the geographic isolation of the Coos Bay area from rivers draining the ancestral Cascades arc to the onset of uplift of the southern Oregon Coast Range during the late Oligocene to early Miocene.

The eight field trips in this volume, associated with GSA Connects 2021 held in Portland, Oregon, USA, reflect the rich and varied geological legacy of the Pacific Northwest. The western margin of North America has had a complex subduction and transform history throughout the Phanerozoic, building a collage of terranes. The terrain has been modified by Cenozoic sedimentation, magmatism, and faulting related to Cascadia subduction, passage of the Yellowstone hot spot, and north and westward propagation of the Basin and Range province. The youngest flood basalt province on Earth also inundated the landscape, while the mighty Columbia watershed kept pace with arc construction and funneled epic ice-age floods from the craton to the coast. Additional erosive processes such as landslides continue to shape this dynamic geological wonderland.

ABSTRACT The Mount Hood fault zone is a N-trending, ~55-km-long zone of active faulting along the western margin of the Hood River graben in north-central Oregon. The Mount Hood fault zone occurs along the crest of the Cascade Range and consists of multiple active fault segments. It is presently unclear how much Hood River graben extension is actively accommodated on the fault zone, and how Cascade intra-arc extension accommodates regional patterns of clockwise rotation and northwest translation of crustal blocks in the Pacific Northwest region of the United States. Evidence for Holocene activity on the Mount Hood fault zone was discovered in 2009 after acquisition of high-resolution lidar topography of the area. This trip will visit sites displaying evidence of Holocene surface rupture on fault strands within the Mount Hood fault zone. Day 1 starts with a two-hour drive from Portland to Mount Hood, a 3429-m-high glaciated active volcano, where we will visit sites south of the summit along the Twin Lakes fault segment, including several fault scarps and two sites where dating of offset buried soils constrains the timing of the most recent surface-rupturing event to the Holocene. Day 1 includes two hikes of ~1 km and will be partly cross-country. The trip will overnight at the historic Timberline Lodge, an architectural masterpiece from the Civilian Conservation Corps (1933–1942) era, located at tree line on the southern flank of Mount Hood. Day 2 will visit sites north of the summit, stopping along the Blue Ridge fault segment to view the site of 2011 paleoseismic trenches and an offset glacial moraine. We will visit an unusual uphill-facing scarp in coarse talus along the Gate Creek fault segment near the north end of the Mount Hood fault zone. We will conclude Day 2 with a short hike into the Mark O. Hatfield Wilderness along the Gate Creek fault segment to view evidence of a surface-rupturing earthquake that occurred only a few centuries ago, illuminated by a nearby paleoseismic trench hand-dug in 2020. Our neotectonic and paleoseismic data are among the first efforts to document and characterize seismic sources within the Mount Hood fault zone. However, even with our new age data, fault slip rates and earthquake recurrence remain poorly constrained. With our limited earthquake timing data, it is not clear whether all segments of the Mount Hood fault zone rupture together as a ≥ M 7 earthquake, or alternatively, if the fault segments rupture independently in a sequence of smaller ~M 6–sized events.

ABSTRACT The Columbia River Gorge is the Columbia River’s long-held yet evolving passage through the volcanic arc of the Cascade Range. The globally unique setting of a continental-scale river bisecting an active volcanic arc at the leading edge of a major plate boundary creates a remarkable setting where dynamic volcanic and tectonic processes interact with diverse and energetic fluvial processes. This three-day field trip explores several elements of the gorge and its remarkable geologic history—cast here as a contest between regional tectonic and volcanic processes building and displacing landscapes, and the relentless power of the Columbia River striving to maintain a smooth passage to the sea. DEDICATION Dedicated to Russell C. Evarts (7 April 1947–11 July 2017) and his contributions to Pacific Northwest geology. Russ Evarts devoted most of his 30-year career with the U.S. Geological Survey to geologic mapping of Oregon and Washington. His thorough geologic mapping of the near-vertical terrain of the western Columbia River Gorge underpins much of what is reported in this guide and continues to inspire our studies of the geology of the Pacific Northwest.

ABSTRACT The Miocene Columbia River Basalt Group (CRBG) is the youngest and smallest continental flood basalt province on Earth. This flood basalt province is a succession of compositionally diverse volcanic rocks that record the passage of the Yellowstone plume beneath eastern Oregon. The compositionally and texturally varied suite of volcanic rocks are considered part of the La Grande–Owyhee eruptive axis (LOEA), an ~300-km-long, north-northwest–trending, Middle Miocene to Pliocene volcanic belt that extends along the eastern margin of the Columbia River flood basalt province. Volcanic rocks erupted from and preserved within the LOEA form an important regional stratigraphic link between the flood basalt–dominated Columbia Plateau to the north, the north and bimodal basalt-rhyolite volcanic fields of the Snake River Plain to the east, the Owyhee Plateau to the south, and the High Lava Plains to the south and east; the latter two have time transgressive rhyolite centers that young to the east and west, respectively. This field-trip guide details a four-day geologic excursion that will explore the stratigraphic and geochemical relationships among mafic rocks of the CRBG and coeval and compositionally diverse silicic rocks associated with the early trace of the Yellowstone plume and High Lava Plains in eastern Oregon. The trip on Day 1 begins in Portland then traverses across the western axis of the Blue Mountains, highlighting exposures of the widespread, Middle Miocene Dinner Creek Welded Tuff and aspects of the Picture Gorge Basalt lava flows and northwest-striking feeder dikes situated in the central part of the CRBG province. Travel on Day 2 progresses eastward toward the eastern margin of the LOEA, examining a transition linking the Columbia River Basalt province with a northwestward-younging magmatic trend of silicic volcanism of the High Lava Plains in eastern Oregon. Initial field stops on Day 2 focus on the volcanic stratigraphy northeast of the town of Burns, which includes regionally extensive Middle to Late Miocene ash-flow tuffs and lava flows assigned to the Strawberry Volcanics. Subsequent stops on Day 2 examine key outcrops demonstrating the intercalated nature of Middle Miocene tholeiitic CRBG flood basalts, temporally coeval prominent ash-flow tuffs, and “Snake River–type” large-volume rhyolite lava flows cropping out along the Malheur River. The Day 3 field route navigates to southern parts of the LOEA, where CRBG rocks are associated in space and time with lesser known and more complex silicic volcanic stratigraphy forming Middle Miocene, large-volume, bimodal basalt-rhyolite vent complexes. Key stops will provide a broad overview of the structure and stratigraphy of the Middle Miocene Mahogany Mountain caldera and of the significance of intercalated sedimentary beds and Middle to Late Miocene calc-alkaline lava flows of the Owyhee basalt. Initial stops on Day 4 will highlight exposures of Middle to Late Miocene silicic ash-flow tuffs, rhyolite domes, and calc-alkaline lava flows overlying the CRBG across the northern and central parts of the LOEA. The later stops on Day 4 examine more silicic lava flows and breccias that are overlain by early CRBG-related rhyolite eruptions. The return route to Portland on Day 4 traverses the Columbia River gorge westward from Baker City. The return route between Baker and Portland on Day 4 follows the Columbia River gorge and passes prominent basalt outcrops of large volume tholeiitic flood lavas of the Grande Ronde, Wanapum, and Saddle Mountains Formations of the CRBG. These sequences of basaltic and basaltic andesite lavas are typical of the well-studied flood basalt dominated Columbia Plateau, and interbedded silicic and calc-alkaline lavas are conspicuously absent. Correlation between the far-traveled CRBG lavas and calcalkaline and silicic lavas considered during the excursion relies on geochemical fingerprinting and dating of the mafic flows and dating of sparse intercalated ashes.

ABSTRACT The deeply eroded Goat Rocks volcanic complex was a major locus of andesitic volcanism in the Cascade arc in southwest Washington during the late Pliocene to Pleistocene. This volcanic complex includes the remnants of multiple andesitic edifices over an area of ~200 km 2 , centered ~35 km north of Mount Adams on the arc axis. New 40 Ar/ 39 Ar ages for seven samples and U/Pb zircon ages for nine samples indicate a 2.5–2.9 m.y. eruptive history at Goat Rocks. Four eruptive stages are delineated: Tieton Peak (potentially 3.0–2.6 Ma), Bear Creek Mountain (>1.6–1.3 Ma), Lake Creek (1.1–0.6 Ma), and Old Snowy Mountain (0.4–0.1 Ma), each named for the major vent that was active during that time. Lake Creek volcano was the most voluminous of these edifices and probably rose at least 3400 m above sea level with a volume of ~60 km 3 , comparable to nearby active composite volcanoes. Thirty new bulk composition X-ray fluorescence (XRF) and inductively coupled plasma–mass spectrometry analyses from the volcanic complex are presented, in addition to 54 previously unpublished XRF analyses for samples collected by Don Swanson. The compositional variability is greatest in the early and late stages, ranging from basaltic andesite to rhyolite, whereas the more voluminous middle stages are dominated by andesite to dacite. The middle eruptive stages are interpreted to have been a time of peak thermal energy with a mature subvolcanic plexus. In addition, compositions shift from high-K to medium-K compositions with time, which mimics variation across the arc; early eruptive products are similar in composition to those of Mount Adams, and Old Snowy Mountain stage compositions are more similar to those of Mount St. Helens. The life cycle of Goat Rocks volcanic complex provides new perspective on the longevity and evolution of major arc volcanoes, and on the complex distribution of magma in the Cascade arc at the latitudes of southern Washington and adjacent Oregon.