Abstract The Middle Miocene Columbia River Basalt Group (CRBG) is the youngest and smallest continental flood basalt province on Earth, covering over 210,000 km 2 of Oregon, Washington, and Idaho and having a volume of 210,000 km 3 . A well-established regional stratigraphic framework built upon seven formations, and using physical and compositional characteristics of the flows, has allowed the areal extent and volume of the individual flows and groups of flows to be calculated and correlated with their respective dikes and vents. CRBG flows can be subdivided into either compound flows or sheet flows, and are marked by a set of well-defined physical features that originated during their emplacement and solidification. This field trip focuses on the Lewiston Basin, in southeastern Washington, western Idaho, and northeastern Oregon, which contains the Chief Joseph dike swarm, where classic features of both flows and dikes can be easily observed, as well as tectonic features typical of those found elsewhere in the flood basalt province.

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.

The radiometric dating evidence for the timing and duration of volcanism for the Steens through Wanapum Basalt of the Columbia River Basalt Group is critically reviewed here. K-Ar dates generally underestimate the age of crystallization, though one important exception is detected, where excess argon led to dates that were too old. The 40 Ar/ 39 Ar results on whole-rock basalts from 1980 through 2010 are examined for statistical validity of plateau sections, as well as alteration state of the material dated. In most instances, listed ages are shown to be invalid. The 40 Ar/ 39 Ar total gas (fusion) ages are, in general, not accurate estimates of the time of formation of these rocks. The 40 Ar/ 39 Ar ages on plagioclase separates from basalts yield good estimates of the extrusion age of the lavas. New 40 Ar/ 39 Ar ages on whole-rock basalts are presented that are in good agreement with the plagioclase ages. Various forms of the geomagnetic polarity time scale for mid-Miocene time are examined, along with the ages of lavas and their magnetic polarity. The main sections of the Columbia River Basalt Group (Imnaha through Wanapum Basalt) were formed in ~0.5 m.y. between 16.3 and 15.8 Ma. Steens Basalt extrusion occurred about ~0.1 m.y. before the Imnaha Basalt and appears to have been a precursor to the more voluminous volcanism noted in the Columbia River Basalt Group.

The Frenchman Springs Member is the most voluminous member of the Wanapum Basalt, Columbia River Basalt Group. We have revised the distribution maps and estimates of the areas and volumes for the units of the Frenchman Springs based on mapping and fieldwork that have been conducted since the previous compilation in 1989. The revised estimates indicate that many of the Frenchman Springs flows are significantly larger than previously believed. The total area underlain by Frenchman Springs flows is now estimated to be ~72,595 km 2 ; the revised volume is estimated to be ~7628 km 3 . Based on the new data, the Basalts of Palouse Falls, Ginkgo, Sand Hollow, and Sentinel Gap are significantly more voluminous than previously indicated, while the Basalt of Silver Falls is significantly smaller. We have also reduced the number of units within the Frenchman Springs Member to five. The Basalt of Lyons Ferry has been combined with the Basalt of Sentinel Gap based on the similarity of their geochemistries and paleomagnetism and the lack of an identified dike/vent system. Geochemical variations in Frenchman Springs Member flows (as well as those in the Roza and Priest Rapids Members, Wanapum Basalt) are consistent with being produced by open magma system processes.

The Columbia River system is one of the great river systems of North America, draining much of the Pacific Northwest, as well as parts of the western United States and British Columbia. The river system has had a long and complex history, slowly evolving over the past 17 m.y. The Columbia River and its tributaries have been shaped by flood basalt volcanism, Cascade volcanism, regional tectonism, and finally outburst floods from Glacial Lake Missoula. The most complex part of river development has been in the northern part, the Columbia Basin, where the Columbia River and its tributaries were controlled by a subsiding Columbia Basin with subtle anticlinal ridges and synclinal valleys superimposed on a flood basalt landscape. After negotiating this landscape, the course to the Pacific Ocean led through the Cascade Range via the Columbia Trans-Arc Lowland, an ancient crustal weakness zone that separates Washington and Oregon. The peak of flood basalt volcanism obliterated the river paths, but as flood basalt volcanism waned, the rivers were able to establish courses within the growing fold belt. As the folds grew larger, the major pathways of the rivers moved toward the center of the Columbia Basin where subsidence was greatest. The finishing touches to the river system, however, were added during the Pleistocene by the Missoula floods, which caused local repositioning of river channels.

Newly generated and previously published strontium isotope signatures of plagioclase phenocrysts in Columbia River Basalt Group lavas exhibit heterogeneity largely imposed by mantle-derived magmas assimilating variable crustal rocks. Steens basalts assimilated accreted terrane crust with 87 Sr/ 86 Sr ratios of <0.7040. In contrast, Imnaha, Grande Ronde, Wanapum, and Saddle Mountains basalts likely assimilated crust with more radiogenic Sr (>0.7040) including a cratonic component, perhaps as a result of residence in magma chambers partly located east of the 87 Sr/ 86 Sr = 0.7060 line. Strontium isotope ratios in plagioclase phenocrysts from early- erupted Imnaha basalts anticipate whole-rock signatures of later-erupted Grande Ronde basalts consistent with a geochemical continuum between the two formations, which is also seen in whole-rock trace element abundances, undermining the notion of an abrupt change in the magma sources generating Imnaha and Grande Ronde basalts.

Studies of groundwater flow within the Mosier syncline, part of the Yakima Fold Belt, have shown complex flow system boundaries that result from the interaction of Columbia River Basalt Group flows with the paleoenvironment. A developing Mosier syncline provided low areas that hosted drainages and accumulated sediment and controlled the distribution of Saddle Mountains and Wanapum Basalt lava flows of the Columbia River Basalt Group. Those flows interacted with water and water-saturated sediment within the developing syncline to form permeable zones at the flow contacts. Aquifers have been identified in the Pomona Member of the Saddle Mountains Basalt, and in the Priest Rapids and Frenchman Springs Members of the Wanapum Basalt. Units stratigraphically equivalent to the Ellensburg Formation locally form low-permeability sedimentary interbeds within the Columbia River Basalt Group section. Boundary conditions of present groundwater flow systems within Columbia River Basalt Group terrains reflect the stratigraphy and permeability distribution resulting from the depositional environment, and the influences of postdeposition landscape development. Low-permeability boundaries tend to be controlled by stratigraphy and structure. Recharge and discharge boundaries tend to result from postdepositional landscape development such as changes in topography and drainage systems. Analysis of hydrologic information provides insights into the influences of stresses and boundaries on Columbia River Basalt Group groundwater systems. Hydraulic head data have shown that the main stresses to the flow system near Mosier are pumping, interaquifer flow through uncased wells, and climate fluctuations. Long-term groundwater declines are the result of overpumping in the Pomona aquifer and depressurization of other aquifers connected to the Pomona aquifer through uncased wells. The groundwater system discharges to Mosier Creek, and the elevation of the discharge point appears to control the lower limit of observed heads throughout the aquifer system.

ABSTRACT Miocene flood basalts of the Columbia River Basalt Group inundated eastern Washington, Oregon, and adjacent Idaho between 17 and 6 Ma. Some of the more voluminous flows followed the ancestral Columbia River across the Cascade arc, Puget-Willamette trough, and the Coast Range to the Pacific Ocean. We have used field mapping, chemistry, and paleomagnetic directions to trace individual flows and flow packages from the Columbia River Gorge westward into the Astoria Basin, where they form pillow palagonite complexes and mega-invasive bodies into older marine sedimentary rocks. Flows of the Grande Ronde, Wanapum, and Saddle Mountains Basalts all made it to the ocean; at least 33 flows are recognized in the western Columbia River Gorge, 50 in the Willamette Valley, 16 in the lower Columbia River Valley, and at least 12 on the Oregon side of the Astoria Basin. In the Astoria Basin, the basalt flows loaded and invaded the wet marine sediments, producing peperite breccias, soft sediment deformation, and complex invasive relations. Mega-invasive sills up to 500 m thick were emplaced into strata as old as Eocene, and invasive dikes up to 90 m thick can be traced continuously for 25 km near the basin margin. Mega-pillow complexes up to a kilometer thick are interpreted as the remains of lava deltas that prograded onto the shelf and a filled submarine canyon southeast of Astoria, possibly providing the hydraulic head for injection of invasive sills and dikes at depth.

Nearly twenty flows of the Columbia River Basalt Group (CRBG) can be paleomagnetically and chemically correlated westward as far as 500 km from the Columbia Plateau in Washington, through the Columbia Gorge, to the Coast Range of Oregon and Washington. In the Coast Range near Cathlamet, Washington, the CRBG flow stratigraphy includes 10 flows of Grande Ronde Basalt (1 low-MgO R 2 flow, 6 low-MgO N 2 flows, 3 high-MgO N 2 flows), 2 flows of Wanapum Basalt (both flows of Sand Hollow from the Frenchman Springs Member), and the Pomona Member of the Saddle Mountains Basalt. Elsewhere in the Coast Range, additional Grande Ronde Basalt flows, including flows of Winterwater or Umtanum, and additional Wanapum flows, including the flows of Ginkgo, have been reported. Thus at least 18 to 20 CRBG flows reached the coast region. Several of these distal flows have distinctive chemical and magnetic characteristics that are shared by nearby isolated intrusions in Coast Range sedimentary rocks, thus strongly supporting recent suggestions that these intrusions are invasive bodies fed by CRBG flows. Magnetization directions from several flows indicate 16 to 30° of clockwise rotation of the coast with respect to the plateau since middle Miocene time.

The Miocene Columbia River Basalt Group within the Troy basin and adjacent Blue Mountains of northeastern Oregon and southeastern Washington consists of Grande Ronde, Wanapum, and Saddle Mountains Basalts erupted during uplift and basin formation. By Wanapum and Saddle Mountains times (15.6 to 6.0 Ma) the Troy Basin was sufficiently developed to begin receiving thick accumulations of lacustrine and alluvial sediments along with a wide variety of relatively thick basalt flows. As subsidence, sedimentation, and volcanism proceeded, stratigraphic relationships became more complex, with some Saddle Mountains flows forming invasive and intracanyon phases and a possible sill, in addition to the subaerial sheet phases mapped by Ross (1978). This chapter reviews and revises the stratigraphy of the Saddle Mountains Basalt in view of recent mapping and additional major- and trace-element analyses. Principal revisions are: the Elephant Mountain Member consists of the Wenaha flow represented by subaerial sheet, intracanyon, and invasive phases; the Buford Member consists of 1 (Buford) or 2 (Buford and Mountain View) flows represented by subaerial sheet, intracanyon, invasive phases, and possibly a sill, interfingered with the Wenaha flow and sedimentary interbeds; the Eden flow consists of subaerial and invasive phases and is elevated to member status within the Saddle Mountains Basalt; the Umatilla Member consists of the Sillusi flow overlain by the Bear Creek flow, the latter being a new Umatilla Member flow. An andesite breccia that may pre-date the cessation of Columbia River Group volcanism in the area is also recognized. Also addressed is the problem of determining if the Wenaha and Buford Member phases repeated within the section represent discrete eruptions of identical lavas or invasive and subaerial phases of single lavas. The Wenaha phases are believed to have resulted from a single subaerial eruption that spread as a sheet flow until encountering thick, wet sediments, which it invaded. Locally the invasive phase extruded as intracanyon lavas into shallow valleys eroded into the invaded sediments. The subaerial sheet, intracanyon, and invasive phases of the Buford Member may also have resulted from 1 eruption (possibly with local sills to the east of the study area) or they may represent 2 discrete lavas.

Miocene tholeiitic basalt flows and intrusions crop out along the Pacific Coast from Seal Rock, Oregon, to Grays Harbor, Washington. Based on extensive mapping of dikes, sills, and lava flows, previous workers proposed that these coastal basalts erupted from local vents. However, based on field associations and petrogenetic considerations, others have suggested that the coastal basalts represent the distal ends of plateau-derived flows of the Columbia River Basalt Group that flowed into estuarine and deltaic environments, invading and deforming soft sediment. To what depth the coastal basalt dikes extend is a critical test of the alternative hypotheses regarding their origin. Dikes marking former vents would likely be connected at depth with a magma source or conduit, i.e., these dikes would be “rooted” at depth. Invasive flows, on the other hand, would be “rootless.” Gravity data provide geophysical constraints on the depths of coastal basalt intrusions. Our gravity profiles across coastal basalt dikes near Astoria, Oregon, are consistent with shallow near-surface basalt masses. All of the intrusions investigated (including the linear dikes at Fishhawk Falls and Denver Point, the arcuate segments of the “ring dike” on the Klaskanine River, the U-shaped dike at Voungs River Falls) can be interpreted as extending less than 300 m below sea level; many continue for only about 100 m below the surface. The gravity data do not match the profiles expected from deep vertical dikes. Additional evidence supporting the plateau origin for the coastal basalts includes stratigraphic correlations and areal distributions. Mapping, geochemical analyses, and magnetic polarity determinations by us and by other workers demonstrate that all the coastal basalt units have Columbia River Basalt Group counterparts in the Willamette Valley.

Thick (>30 m) flows of Columbia River basalt contain internal vesicular zones within otherwise dense, sparsely vesicular basalt. These zones are continuous over distances ranging from 0.5 km to 30 km; most are characterized by an abrupt transition from vesicle-rich to vesicle-poor rock above and by a gradational lower margin. The zones were formed by post-emplacement migration, coalescence, and entrapment of aqueous vapor bubbles. Under appropriate physicochemical conditions, bubbles nucleate at the lower solidification front and rise buoyantly until retarded by the higher viscosities below the upper solidification front. In some cases, ponding of bubbles against this ceiling occurs before freezing-in of the vesicles by the downward passage of the upper front. We have developed a one-dimensional dynamic model of the process of vesicular zone emplacement by starting with the measured vesicle distribution in a lava flow and calculating movement of vesicles according to Stokes Law as the flow is melted, or “uncrystallized.” Data for the solidification history of the Cohassett flow are based on a previously developed cooling model. Melting of the flow was accomplished by reversing movement of the upper and lower solidification fronts; when a solidification front passes a vesicle, the vesicle is free to move in the reverse direction since time is moving in reverse. The model clearly illustrates the ponding effect of the upper solidification front and shows that nucleation of vesicles on the lower solidification front is required in trials where the model approximates a reasonable distribution of bubbles at T = 0. Internal vesicular zones consist of layers representing variations in number and size of vesicles. The regular layer spacing suggests a cyclic enrichment and depletion of bubble nuclei. If the production of nuclei is controlled by the state of the system at the lower solidification front, then the layers can be explained by oscillations between periods of crystallization, during which the system moves toward higher oversaturation pressure and periods of vapor exsolution tending to reestablish equilibrium. The layered distribution of vesicles in Columbia River basalt flows, for example, the McCoy Canyon and Rocky Coulee flows, can be explained by such a mechanism.

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.