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The K-Ar data for various sections of the Columbia River basalt (Imnaha, Picture Gorge, and Grande Ronde Basalt formations) have been reevaluated, utilizing the atmospheric argon contents of the rocks to identify the least altered samples. These ages, together with the results of paleomagnetic studies on these same sections, were then fitted into the known magnetostratigraphy of the Columbia River basalt. Finally, these findings are integrated with the geomagnetic polarity time-scale for mid-Miocene times. The Imnaha Basalt was formed ~17.2 Ma, and the Picture Gorge Basalt ~ 16.0 Ma. The Grande Ronde Basalt was extruded between ~16.9 and 15.6 Ma, with >50 percent of the total volume (magnetostratigraphic units R2–N2) formed within ~300,000 yr, around 15.8 Ma.

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