The middle Miocene Columbia River Basalt Group is the youngest and smallest continental flood basalt province on Earth, covering over 210,000 km 2 of mainly Oregon, Washington, and Idaho, with an estimated basalt volume of ~210,000 km 3 . A well-established regional stratigraphic framework built upon six formations contains numerous flows and groups of flows that can be readily distinguished by their physical and compositional characteristics, thus producing mappable units, the areal extent and volume of which can be calculated and correlated with their respective feeder dikes. The distinct physical features that help to define these units originated during their emplacement and solidification, as the result of variations in cooling rates, degassing, thermal contraction, and interaction with their paleoenvironment. Columbia River Basalt Group flows can be subdivided into two basic flow geometries. Sheet flows dominate the basalt pile, but the earliest flows comprising the Steens Basalt and some of the Saddle Mountains Basalt flows are compound flows with elongated bodies composed of numerous, local, discontinuous, and relatively thin lobes of basalt lava. The internal physical characteristics of the voluminous sheet flows are recognizable throughout their extent, thus allowing mechanistic models to be developed for their emplacement. The emplacement and distribution of individual Columbia River Basalt Group flows resulted from the interplay among the regional structure, contemporaneous deformation, eruption rate, preexisting topography, and the development of paleodrainage systems. These processes and their associated erosional and structural features also influenced the nature of late Neogene sedimentation during and after the Columbia River Basalt Group eruptions. In this paper, we describe and revise the stratigraphic framework of the province, provide current estimates on the areal extent and volume of the flows, and summarize their physical features and compositional characteristics.

The Columbia River flood basalt province, United States, is likely the most well-studied, radiometrically well-dated large igneous province on Earth. Compared with older, more-altered basalt in flood basalt provinces elsewhere, the Columbia River Basalt Group presents an opportunity for precise, accurate ages, and the opportunity to study relationships of volcanism with climatic excursions. We critically assess the available 40 Ar/ 39 Ar data for the Columbia River Basalt Group, along with K-Ar data, to establish an up-to-date picture of the timing of emplacement of the major formations that compose the lava stratigraphy. Combining robust Ar-Ar data with field constraints and paleomagnetic information leads to the following recommendations for the age of emplacement of the constituent formations: Steens Basalt, ca. 16.9 to ca. 16.6 Ma; Imnaha Basalt, ca. 16.7 to ca. 16 Ma; Grande Ronde Basalt, ca. 16 Ma to ca. 15.6 Ma; Wanapum Basalt, ca. 15.6 to ca. 15 Ma; and Saddle Mountains Basalt from ca. 15 Ma to ca. 6 Ma. The results underline the previously held observation that Columbia River Basalt activity was dominated by a brief, voluminous pulse of lava production during Grande Ronde Basalt emplacement. Under scrutiny, the data highlight areas of complexity and uncertainty in the timing of eruption phases, and demonstrate that even here in the youngest large igneous province, argon dating cannot resolve intervals and durations of eruptions.

We present a reappraisal of Columbia River basalt petrogenesis based on an internally consistent X-ray fluorescence and inductively coupled plasma–mass spectrometry data set for major and trace elements plus new and existing isotopic analyses of the Imnaha, Steens, Picture Gorge, and Grande Ronde Basalts. Source materials for the main-phase Columbia River Basalt Group are upwelling ocean-island basalt source–like mantle, depleted mantle variably fluxed by slab-derived fluids, Phanerozoic arc crust, and ancient North American cratonic crust. The mantle upwelling may be a deep-seated plume or material displaced and mobilized by fragmented sinking slabs. We endorse the conclusions of earlier workers that the Imnaha, Steens, and Picture Gorge Basalts represent different mixtures of upwelling mantle, depleted mantle, slab-derived fluids, and crust. Cratonic crust of the Idaho batholith has a minor role as a contaminant of Imnaha basaltic magma and a major role in the petrogenesis of the Grande Ronde basaltic andesites, which we model as contaminated Imnaha basalt. We emphasize the geochemical continuity of the Imnaha and Grande Ronde Basalts and propose that they derive from a single central crustal magma system (or chamber) that lasted from ca. 16.7 Ma to 16.0 Ma. The Imnaha–Grande Ronde magma system was centered beneath the location where the western Snake River Plain, Oregon-Idaho graben, and Chief Joseph dike swarm converge, and probably transgressed the cratonic boundary to the east. Magma from this system was injected into dikes and traveled hundreds of kilometers northward to erupt and feed the gigantic Grande Ronde lava flows. In contrast to previous studies, we question the idea that the dike swarm provides a geographic map of the magma source regions.

The Picture Gorge Basalt Subgroup was mapped over an area of 1,600 km 2 in the John Day Basin in north-central Oregon. Lateral continuity of the Twickenham, Monument Mountain, and Dayville Basalts is demonstrated, and each is elevated to formation status within the Picture Gorge Basalt Subgroup of the Columbia River Basalt Group. Seventeen members are defined based on field and compositional characteristics. Sixty-one flows are identified, of which 39 belong to recognized members. The remaining 22 flows that are unassigned are of limited areal extent. Compositional data and field characteristics were used with the assistance of discriminant analysis to extend correlations of members beyond their type localities. The earliest Twickenham Basalt flows filled areas of moderate relief in the John Day Basin. The resultant flat topography was covered by numerous extensive (and some less extensive) flows of the Monument Mountain and Dayville Basalts, creating a crude shield-like structure in east–west profile. Poor correlation of the Picture Gorge type section with sections farther north suggests at least two loci of eruptions within the John Day Basin, one centered near Picture Gorge, the other between Kimberly and Monument. Compositional variation within flows of Twickenham Basalt is commonly great, due to in situ fractionation. Monument Mountain Basalt flows are comparatively homogeneous, and the formation as a whole displays minor compositional variation. Flows within the Dayville Basalt are also homogeneous, and the formation is characterized by an abundance of areally restricted and compositionally diverse flows.

The K-Ar data for various sections of the Columbia River basalt (Imnaha, Picture Gorge, and Grande Ronde Basalt formations) have been reevaluated, utilizing the atmospheric argon contents of the rocks to identify the least altered samples. These ages, together with the results of paleomagnetic studies on these same sections, were then fitted into the known magnetostratigraphy of the Columbia River basalt. Finally, these findings are integrated with the geomagnetic polarity time-scale for mid-Miocene times. The Imnaha Basalt was formed ~17.2 Ma, and the Picture Gorge Basalt ~ 16.0 Ma. The Grande Ronde Basalt was extruded between ~16.9 and 15.6 Ma, with >50 percent of the total volume (magnetostratigraphic units R 2 –N 2 ) formed within ~300,000 yr, around 15.8 Ma.

Neogene sedimentation on and adjacent to the Columbia Plateau in Oregon, Washington, and Idaho was related to volcanism and tectonism. During emplacement of the largest volume of middle Miocene flood basalts (Grande Ronde, Picture Gorge, and Wanapum Basalts), local drainage disruption and gradient diminishment caused deposition in lakes and by sluggish mixed-load streams at or near the flow margins (e.g., Latah, lower Ellensburg, and Simtustus Formations). The Pasco basin was the principal subsiding feature at this time, but because of its central position on the basalt plateau, it received only minor accumulations of detrital and organic-rich sediments. The Mascall and Payette Formations (and equivalents) were deposited in subsiding basins along the southern and southeastern plateau margins. As basalt eruptive frequency and volume diminished in late Miocene time (Saddle Mountains Basalt), deposition occurred primarily in response to intrabasin tectonism and Cascade volcanism. A well-integrated through-flowing river system transported detritus from the surrounding highlands across the plateau. Late Miocene sedimentation along the western plateau margin was strongly influenced by large volcaniclastic sediment loads from the Cascade Range (upper Ellensburg, Dalles, and Deschutes Formations). Elsewhere, fluvial and lacustrine deposition occurred in response to basin subsidence (e.g., Ringold and Idaho Formations) or influx of coarse clastics into shallow basins (e.g., Alkali Canyon and McKay Formations, Thorp Gravel). Widespread unconformities and provenances indicative of drainage reversals in the Blue Mountains region may reflect a transition from primarily compressional to extensional deformation along the southern margin of the plateau between 12 and 10 Ma.

Almost all Columbia River Basalt Group (CRBG) flows contain a residuum that consists of two phases, one chlorophaeite-rich and one a granite-glass. The chlorophaeite includes types that vary from irregular aggregates to polycrystalline spherules and drop-shaped inclusions of two major types occurring totally within the unaltered granite-glass. The granite-glass is usually isotropic but may be cryptocrystalline. The average analysis for our samples from Wanapum Basalt flows is SiO 2 , 76.1 percent; Al 2 O 3 , 12.5 percent; FeO(T), 1.7 percent; MgO, 0.6 percent; CaO, 0.4 percent; Na 2 O, 2 percent; K 2 O, 6 percent; and TiO 2 , 0.7 percent. The modal abundance by volume varies from a trace in the Picture Gorge Basalt and 1 percent in some Imnaha Basalt flows to 10 percent in the Grande Ronde Basalt and as much as 24 percent in the Wanapum Basalt. If segregated and accumulated, this glass could yield a potential potassic granite batholith of about 10,000 km 3 , comparable in size to the Idaho Batholith to ~1 km depth. This glass is compositionally very similar to some of the Tertiary granites of Syke and to Tertiary rhyolites from east Iceland. Separated glasses contain rare-earth elements (REE) that mimic the whole-rock REE except for a substantial negative Eu anomaly. Cs, Rb, Ba, Hf, and Ta are greatly enriched over the whole-rock composition. The glasses must represent the residual liquid from which the fayalitic olivine, augite, andesine, and magnetite of the Wanapum Basalt ferrobasalts have crystallized. The current petrogenetic theory for North Atlantic Tertiary granite occurrences is by derivation from ferrobasalt by fractional crystallization. The CRBG glasses fit this model, except that in this case the rapid eruption of CRBG has precluded the physical separation of the rhyolite component. Our present theory of direct derivation of CRBG from the mantle without significant crustal contamination thus has the corollary that it is also possible to derive large volumes of granite from that same source, given a suitable fractionation process.

A plausible parental magma for many flows of Picture Gorge Basalt Subgroup (PGBS) is represented by two high-Mg dikes in the Monument dike swarm. The dikes bear strong resemblance to primitive aluminous Mid-ocean Ridge Basalts (MORB), which have been interpreted as near-primary mantle melts. However, enrichment of large-ion-lithophile (LIL) elements in the dikes relative to MORB and the occurrence of a mixed phenocryst assemblage imply magma mixing and/or contamination. Alternatively, the LIL enrichment may be explained by an enriched mantle source. Six members and chemical types of Dayville Basalt, consisting of nine flows, were selected for this study for the overall similarity of their spider diagram signatures to that of the high-Mg dike average and for their wide range in composition, which spans much of the variation seen in the PGBS. Other flows of PGBS may display slightly different spider diagram patterns, an observation that suggests a different evolutionary history for them. Major-element modeling of the compositional variation between the high-Mg dikes and evolved flows requires a phenocryst assemblage consisting of 55 percent plagioclase, 25 percent clinopyroxene, and 20 percent olivine. Clinopyroxene is not a common phenocryst phase, nor would it be expected as one in these magmas if crystallization were at low pressure. However, the local occurrence of high-Al clinopyroxene phenocrysts is consistent with crystallization at higher pressure and resorption of entrained phenocrysts during storage at lower pressure. The occurrence of plagioclase and olivine showing evidence of disequilibrium in the flows suggests that magma mixing also played a role. Variable enrichment of most trace elements, relative to the high-Mg dikes, is closely related to bulk distribution coefficient and is interpreted as due to open-system differentiation involving magma recharge. “Over-enrichment” of the LIL elements necessitates an additional process, probably assimilation of crustal or granitic rocks that are isotopically similar to PGBS magmas. Trace-element concentrations in the Johnny Cake Member can be achieved using a magma recharge model with the high-Mg dikes as parental magma and a composite sample from the Wallowa batholith as assimilant. The other members and chemical types presumably reflect intermediate stages in the magma recharge system. For the chosen parameters of the model, approximately 23 recharge cycles are required to arrive at trace-element concentrations in the Johnny Cake Member.