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Grande Ronde Basalt (GRB) flows from 135 surface stratigraphic sections and 34 boreholes throughout Washington, Oregon, and Idaho, were examined to determine which chemical and physical properties would allow the recognition and mapping of GRB on a regional scale. At least 120 major GRB flows, with individual volumes ranging from 90 km3 to more than 2,500 km3, produced a total volume of 148,600 km3, which erupted between 17.0 and 15.6 Ma.

Although all known GRB feeder dikes and vents occur in the eastern and southeastern part of the Columbia Plateau, the thickest and most complete basalt sections (>3.2 km) occur in the Pasco Basin. The number of flows and section thickness decrease outward from the central Columbia Plateau so that a consistent stratigraphy exists in the interior, but an incomplete and variable stratigraphy exists along the margins. The distribution of some flows suggests that their vents lie buried in the northern part of the Columbia Plateau, far north of the known vent area.

The GRB has a narrow range of chemical compositions and a relatively uniform lithology. Many flows have similar chemical compositions, and few flows have distinctive lithologies that can be mapped with confidence across the Columbia Plateau. When chemical compositions are combined with paleomagnetic polarity, lithology, and stratigraphic position, we are able to subdivide the 4 GRB magnetostratigraphic units into 17 informal units that are mapped and recognizable across the Columbia Plateau. These new informal units incorporate and expand on previously defined units using proven techniques for identifying Columbia River basalt flows. The informal stratigraphy proposed here provides a framework for correlation and resolution of local stratigraphies across the Columbia Plateau.

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