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.

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.

Topography and ground conditions were important factors in controlling the distribution of individual Columbia River Basalt Group (CRBG) flows in western Oregon. The Columbia trans-arc lowland, the Yakima fold belt, the Portland Hills–Clackamas River structural zone, and Cascadian volcanism largely controlled the distribution of CRBG flows across the Miocene Cascade Range. The first flows to cross the Miocene Cascades into the Willamette Valley encroached onto a low-relief topography generally consisting of eroded Tertiary-age marine sedimentary rocks deformed along northwest-trending structural zones, volcanic highs, and estuaries. No north-south trough affected the distribution and thickness of the CRBG in the Willamette Valley, but an incipient Coast Range acted as a leaky barrier to the Oregon coast. Water-saturated sediments rapidly extracted heat from advancing CRBG lava flows, producing narrow, abnormally thick lobes extending along existing topographic lows. Deformation along the northwest-trending Portland Hills–Clackamas River structural zone produced a major topographic barrier early and late in the incursion of CRBG flows. The CRBG thins across this zone from 600 to 150 m. This zone diverted the earliest Grande Ronde flows into and through the Portland Basin. Some of the succeeding R 2 and N 2 Grande Ronde flows were able to cross this zone and followed another structural low, the Sherwood trough, to the Oregon coast. The total thickness of CRBG along the Sherwood trough is approximately 300 m, about twice that on either side. Paleodrainage developed during time intervals between emplacement of CRBG flows. The positions of these drainage courses were influenced by the position of the CRBG flow margins and/or structural lows. A longer hiatus between flows (> 100,000 yr) enabled rivers to develop major canyons by headward erosion, which served to channelize subsequent CRBG flows.

Abstract The Columbia River Gorge is located astride the Cascade Range along a 72 mi (120 km)stretch of the Oregon/Washington border 1). The western half of the gorge is covered by the Camas and Bridal Veil 15-minute quadrangles. Primary access to the gorge is by two major highways which traverse its length-the two-lane State Highway 14 along the Washington side and the four-lane Interstate 84 freeway along the Oregon side. Also along the Oregon side are segments of the old Columbia River Scenic Highway, which when completed in 1917 was the first paved highway to extend the length of the gorge. All sites to be visited on the Oregon side are along the old Columbia River Scenic Highway (Fig).

Abstract The Columbia River Gorge is located astride the Cascade Range along a 72 mi (120 km)stretch of the Oregon/Washington border 1). The western half of the gorge is covered by the Camas and Bridal Veil 15-minute quadrangles. Primary access to the gorge is by two major highways which traverse its length-the two-lane State Highway 14 along the Washington side and the four-lane Interstate 84 freeway along the Oregon side. Also along the Oregon side are segments of the old Columbia River Scenic Highway, which when completed in 1917 was the first paved highway to extend the length of the gorge. All sites to be visited on the Oregon side are along the old Columbia River Scenic Highway (Fig).