Abstract The Columbia Plateau is a basin-like subprovince of the Columbia Intermontane Physiographic Province (Freeman and others, 1945; Thornbury, 1965). The Blue Mountains area of southeastern Washington and northeastern Oregon is the southern upwarped part of the province, whereas the region north of the Blue Mountains, east of the Cascade Mountains, south of the Okanogan Highlands, and west of the Idaho Rockies makes up the less deformed part of the province. The entire province is characterized by great late Cenozoic outpourings of basaltic lava. Because of gentle dips on the lava flows in the northern and eastern sections of the plain, the term “Columbia Plateau” has been applied to this region. Waitt and Swanson (1987) propose that the area be named “Columbia Plain” to indicate its analogy to the Snake River Plain, which is also characterized by basaltic lava flows. The Miocene tholeiitic flood basalt that characterizes the Columbia Plateau is named the Columbia River Basalt Group (Swanson and others, 1979). Its volume is approximately 1.7 × 105 km 3 , and it extends over an area of approximately 1.6 × 105 km 2 (Tolan and others, 1987). Most of the flows date between 17.5 and 14.5 Ma, but basalt eruptions from linear vents in the eastern part of the province continued to 6 Ma on a reduced scale (Swanson and others, 1979). Sedimentary interbeds occur between some basalt flows, especially near the margins of the basalt plain (Waters, 1955;

The previously accepted estimates for the areal extent (200,000 km 2 ) and volume (325,000 to 382,000 km 3 ) of the Columbia River Basalt Group (CRBG) have, upon reevaluation, been found to be too large. New area and volume estimates for 38 units that compose most of the CRBG indicate that it once covered an area of approximately 163,700 ± 5,000 km 2 and has a volume of approximately 174,300 ± 31,000 km 3 . Our work further suggests that the volume of individual flows is huge, on average exceeding hundreds of cubic kilometers. The maximum known volume of an individual flow exceeds 2,000 km 3 , and some flows may have volumes on the order of 3,000 km 3 . Typically such huge-volume flows (here termed “great flows”) were able to travel hundreds of kilometers from their vents, with some flows known to have advanced more than 750 km. The eruption of great flows generally ceased with the end of Wanapum volcanism. The extent and volume of great flows qualifies them as the largest known terrestrial lava flows.

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.

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.