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ABSTRACT

Emplacement models for voluminous sheet flows of the Columbia River flood basalts vary significantly in style and duration, with the latter ranging from as little as one week to decades and even centuries. Testing the efficacy of such models requires detailed field studies and close examination of each stratigraphic unit. The Steens Basalt, the oldest formation of the Columbia River flood basalts, differs from the later formations in that it is composed of stacked successions of thin, commonly inflated flow lobes combined into thicker compound flows, or flow fields. These flow lobes are of limited geographic extent, with relatively high emplacement rates, but they are otherwise similar to modern examples. Evidence for flow inflation in the much larger sheet flows of the Grande Ronde Basalt, Wanapum Basalt, and Saddle Mountains Basalt is also apparent, but with more variable rates of emplacement. For example, the Asotin and Umatilla Members (Saddle Mountains Basalt) and Sentinel Bluffs Member flows (Grande Ronde Basalt) erupted distinct compositions along their linear vent systems, but over 200 km west of their vents, these flows are no longer distinct. Instead, they exist as compositional zones of a single, moderately mixed lava flow. Such flows must have been emplaced rapidly, in perhaps weeks to months, while others have been shown to erupt over much longer time periods. We conclude that emplacement rates may be quite variable throughout the Columbia River flood basalt province, with thin flow units of Steens Basalt erupting continuously and rapidly, and larger inflated sheet flows erupting over variable time spans, some from a few weeks to months, and others over a duration of years.

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