The tholeiitic flood basalts of the Columbia River Basalt Group of middle and late Miocene age cover more than 200,000 km2 in Washington, Oregon, and Idaho. The most voluminous formation of the Group, the Grande Ronde Basalt, erupted for 2 m.y. from north-northwest-trending fissure systems concentrated in southeast Washington and adjacent Oregon and Idaho. Four magnetostratigraphic units (designated R1, N1, R2, and N2 from oldest to youngest) are recognized on the basis of polarity in the Grande Ronde and provide the broad stratigraphic framework for the formation. In this study, major-element chemistry and relative stratigraphic position within the polarity intervals are used to identify and correlate individual flows and sequences of flows within the Grande Ronde Basalt on a regional scale.

Systematic examination of more than 350 analyses from 47 stratigraphic sections show that most flows fall into one of five major chemical groupings, which are distinguished primarily by small but significant variations in MgO, TiO2, and P2O5 content. In addition, four minor chemical types local to the eastern part of the province have been identified. Feeder dikes of each chemical type have also been located.

Flows or packets of flows of each chemical type can be correlated between field sections to define specific chemical-stratigraphic subunits. These subunits consist of several flows collectively 30–150 m thick. Subunits of most chemical types are repeated at irregular intervals throughout the formation; no progressive chemical trend occurs within the Grande Ronde.

Many of the chemical-stratigraphic subunits extend to the margins of the province, although most are confined to the source region in eastern Washington. Although the total number of subunits is less in the west away from the fissure systems, the total thicknesses of the N2 and R2 magnetostratigraphic units are each as thick or thicker than the corresponding units in eastern Washington. The greatest thicknesses occur in the central part of the province within the Pasco basin.

The distribution of basalt relative to the location of vents, as well as the relative east-west thicknesses, suggests that basalt flowed hundreds of kilometres westward during the most voluminous Grande Ronde eruptions, ponding against the irregular margin of the Cascade Range and being diverted through the ancestral Columbia Gorge toward the Washington-Oregon coast. Between these huge sheetflood events, smaller eruptions blanketed areas within the source region, and ongoing regional subsidence created a shallow westward-draining basin in the center of the province.

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