The middle and south forks of the Malheur River provide a unique mapping corridor connecting two flood-basalt successions—Steens basalt to the south and the basalt of Malheur Gorge to the north. Each contains chemically defined subtypes, which merge stratigraphically across the north-south length of the map area. The lowermost flows of Steens basalt are stratigraphically equivalent to the lowermost flows of the basalt of Malheur Gorge. The uppermost flows of Steens basalt pinch out in the map area, but are partly interbedded with the middle and uppermost flows of the basalt of Malheur Gorge, which continue to thicken northward. The upper part of the tholeiitic succession is interbedded with a group of previously unrecognized lavas—the Venator Ranch basalt flows. Tholeiitic volcanism ceased at ca. 15.3 Ma; the last tholeiitic unit that erupted is the Hunter Creek basalt, which also thickens northward.

Subsequent (younger than 15.3 Ma), more localized eruptions were dominated by calc-alkaline to mildly alkaline lavas associated with Basin and Range extension. Local uplift generated deep canyons, which were filled by andesitic lavas of the Keeney sequence (ca. 13–10 Ma). The final eruptive products include the Devine Canyon tuff (ca. 9.7 Ma), the Drinkwater basalt (ca. 6.9 Ma), and the Voltage flow (older than 32,000 yr B.P.).

Major and trace element analyses demonstrate that (1) crystal fractionation was a universal process in the derivation of the tholeiitic lavas, (2) the diverse Steens basalt and basalt of Malheur Gorge chemical subtypes were further modified by variations in the degree of partial melting, mantle-source composition, crustal contamination, and/or magma mixing, (3) the Keeney sequence lavas show little evidence of crystal fractionation, but instead were derived from the mixing of basaltic melts with high-silica, low-Fe, granitic sources, and (4) the felsic rock types were derived from the anatexis of heterogeneous crustal sources.

Stratigraphic correlations demonstrate that the main phase of middle Miocene flood-basalt volcanism generated ∼220,500 km3 of basalt over an interval of ∼1.3 m.y., which equates to a magma supply rate of 0.17 km3/yr. The rapid accumulation of lava appears to have been contemporaneous with a markedly consistent, northward propagation of regional uplift, basalt regression, and vent migration across the breadth of eastern Oregon and into southeastern Washington. The northward advancement of volcanism and uplift may represent the surface expression of a spreading mantle plume head, deflected against the thick cratonic margin of North America and channelized beneath a thin oceanic lithosphere of accreted terranes.

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