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.

The La Grande–Owyhee eruptive axis in eastern Oregon is an ~300-km-long, north-northwest–trending, middle Miocene to Pliocene volcanic belt located along the eastern margin of the Columbia River flood basalt province. The eruptive axis extends from Elgin on the north to Jordan Valley on the south and is juxtaposed between the Chief Joseph dike swarm on the east and the Monument dike swarm and the middle Miocene Strawberry volcanics on the west. Numerous volcanic vents, from which a diverse assemblage of tholeiitic, silicic, calc-alkaline, and alkalic lavas erupted, are contained within or directly adjacent to the La Grande, Baker, and Oregon-Idaho grabens along the length of the eruptive axis. The volcanic rocks that erupted from and are preserved within the eruptive axis form a stratigraphic link between the flood basalt–dominated Columbia Plateau on the north and bimodal basalt-rhyolite vent complexes of the Owyhee Plateau on the south. Volcanism along the La Grande–Owyhee eruptive axis progressed through six stages beginning in the middle Miocene and continuing through the Pliocene. Stage 1 (16.1–15.5 Ma) was characterized by fissure eruptions that produced the Grande Ronde Basalt. Stage 2 (15.5 Ma) was marked by fissure eruptions of highly evolved, tholeiitic lavas (icelandites) and rhyolites. Stage 3 (15.5–14.7 Ma) was distinguished by caldera-forming eruptions of ashflow tuffs and high-temperature rhyolite lavas. Stage 4 (14.7–13.7 Ma) was marked by fissure eruptions that produced olivine basalts. Stage 5 (13.5–10.0 Ma) was characterized by the eruption of calc-alkaline basaltic andesite, andesite, and dacite lavas. Stage 6 (7–1 Ma) was marked by small-volume alkalic eruptions. Mapped field relations and similar timing between emplacement of the Columbia River Basalt Group and volcanic rocks erupted within the La Grande–Owyhee eruptive axis verify a common temporal link between the two successions. This link indicates that chemically diverse volcanic strata exposed along the La Grande–Owyhee axis need to be considered when developing further detailed petrologic and volcano-tectonic models for the Pacific Northwest during the middle Miocene.