Abstract

Stratigraphic and chemical correlations of Tertiary volcanic units in eastern Oregon confirm that the Steens Basalt represents the earliest eruptions of the Columbia River flood-basalt province. Field correlations are supported by major and trace element analyses and confirmed by 40Ar/39Ar dates. Within the basalt of Malheur Gorge, situated between Steens Mountain and the southernmost extent of the previously mapped Columbia River Basalt Group, the lowest unit correlates with the Steens Basalt, and the conformably overlying middle and upper units correlate with the Imnaha and Grande Ronde Basalt Formations of the Columbia River Basalt Group. New dates indicate that Imnaha and Grande Ronde Basalt Formations on the Columbia Plateau (>90% of the Columbia River Basalt Group) erupted between 16.1 and 15.0 Ma. These were immediately preceded by the Steens Basalt, a plagioclase-phyric tholeiite that erupted above the calculated position of the Yellowstone hotspot at 16.6 Ma. In eastern Oregon, the flood-basalt tholeiites of Steens Mountain and Malheur Gorge form a voluminous but brief interlude (16.6–15.3 Ma) superimposed on the low-volume, calc-alkalic to mildly alkalic, volcanism associated with continuing Eocene to present east-west extension.

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