ABSTRACT Emplacement models for voluminous sheet flows of the Columbia River flood basalts vary significantly in style and duration, with the latter ranging from as little as one week to decades and even centuries. Testing the efficacy of such models requires detailed field studies and close examination of each stratigraphic unit. The Steens Basalt, the oldest formation of the Columbia River flood basalts, differs from the later formations in that it is composed of stacked successions of thin, commonly inflated flow lobes combined into thicker compound flows, or flow fields. These flow lobes are of limited geographic extent, with relatively high emplacement rates, but they are otherwise similar to modern examples. Evidence for flow inflation in the much larger sheet flows of the Grande Ronde Basalt, Wanapum Basalt, and Saddle Mountains Basalt is also apparent, but with more variable rates of emplacement. For example, the Asotin and Umatilla Members (Saddle Mountains Basalt) and Sentinel Bluffs Member flows (Grande Ronde Basalt) erupted distinct compositions along their linear vent systems, but over 200 km west of their vents, these flows are no longer distinct. Instead, they exist as compositional zones of a single, moderately mixed lava flow. Such flows must have been emplaced rapidly, in perhaps weeks to months, while others have been shown to erupt over much longer time periods. We conclude that emplacement rates may be quite variable throughout the Columbia River flood basalt province, with thin flow units of Steens Basalt erupting continuously and rapidly, and larger inflated sheet flows erupting over variable time spans, some from a few weeks to months, and others over a duration of years.

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.

Abstract The Moscow-Pullman basin, located on the eastern margin of the Columbia River flood basalt province, consists of a subsurface mosaic of interlayered Miocene sediments and lava flows of the Imnaha, Grande Ronde, Wanapum, and Saddle Mountains Basalts of the Columbia River Basalt Group. This sequence is ~1800 ft (550 m) thick in the east around Moscow, Idaho, and exceeds 2300 ft (700 m) in the west at Pullman, Washington. Most flows entered from the west into a topographic low, partially surrounded by steep mountainous terrain. These flows caused a rapid rise in base level and deposition of immature sediments. This field guide focuses on the upper Grande Ronde Basalt, Wanapum Basalt, and sediments of the Latah Formation. Late Grande Ronde flows terminated midway into the basin to begin the formation of a topographic high that now separates a thick sediment wedge of the Vantage Member to the east of the high from a thin layer to the west. Disrupted by lava flows, streams were pushed from a west-flowing direction to a north-northwest orientation and drained the basin through a gap between steptoes toward Palouse, Washington. Emplacement of the Roza flow of the Wanapum Basalt against the western side of the topographic high was instrumental in this process, plugging west-flowing drainages and increasing deposition of Vantage sediments east of the high. The overlying basalt of Lolo covered both the Roza flow and Vantage sediments, blocking all drainages, and was in turn covered by sediments interlayered with local Saddle Mountains Basalt flows. Reestablishment of west-flowing drainages has been slow. The uppermost Grande Ronde, the Vantage, and the Wanapum contain what is known as the upper aquifer. The water supply is controlled, in part, by thickness, composition, and distribution of the Vantage sediments. A buried channel of the Vantage likely connects the upper aquifer to Palouse, Washington, outside the basin. This field guide locates outcrops; relates them to stratigraphic well data; outlines paleogeographic basin evolution from late Grande Ronde to the present time; and notes structures, basin margin differences, and features that influence upper aquifer water supply.

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 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.

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.

Sedimentary interbeds preserved between flows of the Columbia River Basalt Group provide a record of the depositional and erosional conditions that characterized the Columbia Plateau between eruptions of basalt. Examination of the sedimentary, stratigraphic, and petrologic character of the Sweetwater Creek interbed from within the Lewiston basin of southeastern Washington and north-central Idaho allows insight into the paleogeographic conditions that existed following eruption of the Priest Rapids Member of the Wanapum Basalt, ca. 14.5 Ma. The Sweetwater Creek interbed is composed of generally unconsolidated and inter-stratified beds of clay, silt, sand (with local thin gravel stringers), and volcanic ash-rich sediment. Three broadly defined sedimentary facies are identified on the basis of lithology and texture. The spatial distribution of these facies, abundance of clay- and silt-rich sediment, and internal sedimentary structures suggest that deposition of the interbed resulted primarily from fluvial and mixed fluvial-lacustrine sedimentation. Fluvial drainages that headed in the ancestral Clearwater Mountains entered the Lewiston basin on the east and exited to the northwest. Basin streams appear to have been primarily of the meandering, mixed-load type. Channel sands deposited by these streams were concentrated east and north of the basin center, and transported extrabasinal sediments are characterized by plutonic and metamorphic sand- and gravel-sized clasts. Fine-grained silt- and clay-rich flood-plain and associated lacustrine deposits extend across the basin, but are thickest near the basin center. The Umatilla basalt flow entered the Lewiston basin during deposition of the Sweetwater Creek interbed and locally invaded fine-grained lacustrine sediments. A later flow, the Wilbur Creek basalt, partially buried the interbed. Complete burial of the Sweetwater Creek interbed sediments followed eruption of the Asotin flow.

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.