Hooper et al. (2002) present stratigraphic, chemical, and field relationships for Tertiary volcanic rocks of eastern Oregon and conclude that (1) the Imnaha Basalt are ∼15.5 m.y. old and (2) the Steens Basalt (ca. 16.6 Ma) represents the earliest eruptions of the Columbia River Basalt flood-basalt province. Critical scrutiny of published geochronological data (Baksi, 1989; Baksi and Farrar, 1990) indicates the Imnaha Basalt ca. 17.5 Ma is older than the Steens Basalt and the latter is equivalent in age to lower sections of the Grande Ronde Basalt. Herein, 40Ar/39Ar ages are reported relative to...

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