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.

Abstract The Grande Ronde-Joseph Canyon area is located in southeast Washington near the common boundaries of Washington, Oregon, and Idaho (Fig.1). The locality lies within the canyon country which forms the southeast part ofthe Columbia Plateau Province near the eastern termination of the Blue Mountains and the southern boundary of the Lewiston Basin. The site can be reached by following Washington SR 129 south from Clarkston to Asotin, Washington, and then by following the Snake River road (lst street in Asotin) south from Asotin for 25 mi (40 km) to the confluence of the Snake and Grande Ronde rivers. The Snake River road is paved for the first 13 mi (21 km) but then becomes a well-maintained gravel road that serves the area. All sites are easily reached from this road. Land within the area either is owned by the Washington Department of Game, Fish, and Wildlife Service or is private. Information about current ownership can be obtained from the headquarters of the Chief Joseph Fish and Wildlife area in Joseph Canyon (H, Fig.2). Permission must be obtained to visit private land, but landowners understand the significance of the area and have always granted access graciously.

Abstract The Grande Ronde-Joseph Canyon area is located in southeast Washington near the common boundaries of Washington, Oregon, and Idaho (Fig.1). The locality lies within the canyon country which forms the southeast part ofthe Columbia Plateau Province near the eastern termination of the Blue Mountains and the southern boundary of the Lewiston Basin. The site can be reached by following Washington SR 129 south from Clarkston to Asotin, Washington, and then by following the Snake River road (lst street in Asotin) south from Asotin for 25 mi (40 km) to the confluence of the Snake and Grande Ronde rivers. The Snake River road is paved for the first 13 mi (21 km) but then becomes a well-maintained gravel road that serves the area. All sites are easily reached from this road. Land within the area either is owned by the Washington Department of Game, Fish, and Wildlife Service or is private. Information about current ownership can be obtained from the headquarters of the Chief Joseph Fish and Wildlife area in Joseph Canyon (H, Fig.2). Permission must be obtained to visit private land, but landowners understand the significance of the area and have always granted access graciously.