ABSTRACT In late Wisconsin time, the Purcell Trench lobe of the Cordilleran ice sheet dammed the Clark Fork of the Columbia River in western Montana, creating glacial Lake Missoula. During part of this epoch, the Okanogan lobe also dammed the Columbia River downstream, creating glacial Lake Columbia in northeast Washington. Repeated failure of the Purcell Trench ice dam released glacial Lake Missoula, causing dozens of catastrophic floods in eastern Washington that can be distinguished by the geologic record they left behind. These floods removed tens of meters of pale loess from dark basalt substrate, forming scars along flowpaths visible from space. Different positions of the Okanogan lobe are required for modeled Missoula floods to inundate the diverse channels that show field evidence for flooding, as shown by accurate dam-break flood modeling using a roughly 185 m digital terrain model of existing topography (with control points dynamically varied using automatic mesh refinement). The maximum extent of the Okanogan lobe, which blocked inundation of the upper Grand Coulee and the Columbia River valley, is required to flood all channels in the Telford scablands and to produce highest flood stages in Pasco Basin. Alternatively, the Columbia River valley must have been open and the upper Grand Coulee blocked to nearly match evidence for high water on Pangborn bar near Wenatchee, Washington, and to flood Quincy Basin from the west. Finally, if the Columbia River valley and upper Grand Coulee were both open, Quincy Basin would have flooded from the northeast. In all these scenarios, the discrepancy between modeled flood stages and field evidence for maximum flood stages increases in all channels downstream, from Spokane to Umatilla Basin. The pattern of discrepancies indicates that bulking of floods by loess increased flow volume across the scablands, but this alone does not explain low ­modeled flow stages along the Columbia River valley near Wenatchee. This latter discrepancy between modeled flood stages and field data requires either additional bulking of flow by sediment along the Columbia reach downstream of glacial Lake Columbia, or coincident dam failures of glacial Lake Columbia and glacial Lake Missoula.

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.

Abstract Like nowhere else on Earth, repeated cataclysmic floods—first of molten lava, then of water from Ice Age floods—decimated southeastern Washington during the late Cenozoic. Beginning ca. 17 Ma, successive outpourings of Columbia River basalt spread for hundreds of kilometers from volcanic vents located in the southern and eastern Columbia Plateau. Up to 300 separate basalt flows have been identified, reaching cumulative thicknesses of 5 km in the Pasco Basin. With the close of basalt volcanism ca. 6 Ma, only a few million years elapsed before the Pacific Northwest succumbed to a new era of flooding. Outburst floods are associated with regular glacial cycles that have occurred periodically over the past 1–2 m.y. from one or more Pleistocene, ice-marginal lakes. During the last glacial cycle (15,000–20,000 calendar yr) alone, as many as 100 separate flood events, mostly from glacial Lake Missoula, are postulated. In the Channeled Scabland, after removing a blanket of loess, differential erosion through hundreds of meters of layered basalt with widely contrasting variations in fracture patterns and structure resulted in a unique assemblage of erosional landforms including multi-tiered cataract canyons, buttes, mesas, and rock basins. A number of depositional features, including huge flood bars blanketed with giant current ripples, as well as ice-rafted erratics and bergmounds, are also prevalent.

Grande Ronde Basalt (GRB) flows from 135 surface stratigraphic sections and 34 boreholes throughout Washington, Oregon, and Idaho, were examined to determine which chemical and physical properties would allow the recognition and mapping of GRB on a regional scale. At least 120 major GRB flows, with individual volumes ranging from 90 km 3 to more than 2,500 km 3 , produced a total volume of 148,600 km 3 , which erupted between 17.0 and 15.6 Ma. Although all known GRB feeder dikes and vents occur in the eastern and southeastern part of the Columbia Plateau, the thickest and most complete basalt sections (>3.2 km) occur in the Pasco Basin. The number of flows and section thickness decrease outward from the central Columbia Plateau so that a consistent stratigraphy exists in the interior, but an incomplete and variable stratigraphy exists along the margins. The distribution of some flows suggests that their vents lie buried in the northern part of the Columbia Plateau, far north of the known vent area. The GRB has a narrow range of chemical compositions and a relatively uniform lithology. Many flows have similar chemical compositions, and few flows have distinctive lithologies that can be mapped with confidence across the Columbia Plateau. When chemical compositions are combined with paleomagnetic polarity, lithology, and stratigraphic position, we are able to subdivide the 4 GRB magnetostratigraphic units into 17 informal units that are mapped and recognizable across the Columbia Plateau. These new informal units incorporate and expand on previously defined units using proven techniques for identifying Columbia River basalt flows. The informal stratigraphy proposed here provides a framework for correlation and resolution of local stratigraphies across the Columbia Plateau.

Stratigraphy of individual basalt flows in the N 2 magnetostratigraphic unit of the Grande Ronde Basalt (GRB) within the central Columbia Plateau has been developed using data from seven surface sections and fifteen boreholes. Twenty-one individual flows have been identified and grouped into eight flow packages. The flow correlations were developed based on chemical composition, paleomagnetic vector direction, stratigraphic position, and thickness of individual flows. A multivariate statistical procedure, discriminant analysis, was used to test the validity of using chemical composition alone to define the flow packages. Results of the test show that samples can be correctly classified within one adjacent flow package in 94 percent of the cases. Application of discriminant analysis to chemical composition data indicates that within the Pasco Basin the upper two-thirds of the N 2 GRB contains 17 individual flows, of which only 9 to 15 may be present at any one location. Seven of these flows are present throughout the portion of the Pasco Basin studied. Correlation of flows between boreholes or surface sections means that units with the same stratigraphic position and diagnostic characteristics have been identified. In most cases this means that a single flow formed from one eruption. However, some flows may not be continuous, or some correlated flows may represent eruptions separated by as much as several thousand years but which gave rise to flows with identical stratigraphic positions and similar characteristics. The greatest number of flows occurs in the southeastern part of the basin, but the thickest total section occurs in the western and northwestern parts of the basin. The detailed flow correlations provide evidence of deformation during emplacement of the N 2 GRB. Variations of the thickness of individual flows and packages of flows disclose that subsidence was greatest in the western portion of the basin and that growth of the Yakima Ridges began at least by late GRB time. This timing of deformation of the Yakima Ridges is consistent with previous interpretations. We conclude that discriminant analysis applied to chemical composition of GRB flows provides a means of quantitatively defining correlation of flow packages. The method yields estimates of uncertainty in correlations but must be applied in the context of an appropriate stratigraphic framework. Discriminant analysis should be useful in a variety of volcanic terranes where subtle but consistent differences in composition occur between individual flows or flow packages. Successful application of the technique requires that variations in the chemistry of individual flows or packages of flows be generally less than chemical differences among flow packages and that a relatively large number of samples be available for classification.

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.

Tectonic development and evolution of the central Columbia Plateau since middle Miocene time is a product of dynamic interplay among (1) the eruption and emplacement of the Columbia River Basalt Group (CRBG), (2) the subsidence of the area encompassing the Yakima fold belt subprovince, (3) the growth of the Yakima folds, and (4) the influence of regional structures transecting the fold belt, specifically the Hog Ranch-Naneum Ridge anticline and the Cle Elum–Wallula disturbed zone. Subsidence of the Yakima fold belt subprovince began prior to the eruption of the CRBG and has continued from Miocene time to the present. The rate of subsidence kept pace with the rate of CRBG flow emplacement, decreasing as CRBG volcanism waned. Simultaneously, anticlinal fold growth within the Yakima fold belt occurred under north–south compression and also decreased as the rates of subsidence and eruptions of lava declined. Paleomagnetic data indicate fold growth was accompanied by local clockwise rotation of basalt within the anticlines. The tectonic and volcanic histories of the central Columbia Plateau are interrelated and imply a common cause. The structural rotation and north-south compression, and thus fold growth, are interpreted to result from oblique subduction along a converging plate margin. The coincidence of the timing and rates of fold growth, subsidence of the central Columbia Plateau, and basalt production rates suggest that CRBG volcanism is primarily a product of oblique subduction off western North America.

Umtanum Ridge is one of the best-exposed Yakima ridges formed by folded basalt flows in south-central Washington. An analysis was made of the structural geometry and strain distribution in the deformed basalt layers exposed on the ridge at Priest Rapids Dam. The purpose of the analysis was to gain an understanding of the distribution and orientations of the small-scale structures (faults, breccias, joints) around the anticlinal structure. From this we can assess the relative strain intensity and distribution around the fold, and use this information, along with the mapped profile shape of the fold and associated faults, to construct a balanced section leading to constraints on the tectonic models of the Columbia Plateau. The strain distributions and structural geometries within Umtanum Ridge accord well with an asymmetric kink-fold geometry with predominantly flexural strains in the steep limb; however, the internal cataclastic flow is not penetrative at field-observation scale. Discrete flexural slip has occurred, both within and along flow contacts, as well as some internal shatter brecciation and faulting between and across the flow-parallel faults. The Umtanum fault, a large reverse fault, is associated with the anticline and, on the basis of the reconstructed section, is conjectured to have formed out of the kink-like fold at depth. Slickenside striae orientations on faults developed during folding are generally perpendicular to the fold axis. This is interpreted to mean that the dominant movement of basalt layering during folding was perpendicular to the fold axis. The mechanical continuity between the anticline and the adjacent syncline to the north is interpreted to have not been disrupted until late in the fold history. Because of this hypothesized continuity and because the dominant relative movement direction of displacement was perpendicular to the fold axis, movement on the Umtanum fault is intepreted to have been predominantly dip-slip. It is further inferred that if any regional strike-slip component was present in the Pasco Basin, it does not manifest itself obviously in the Umtanum fold kinematics.