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The Steens Formation, or Steens Basalt, is formally recognized as the oldest lithostratigraphic unit of the Columbia River Basalt Group, with an estimated areal extent and volume of 53,000 km2 and 31,800 km3, respectively. We integrate petrochemical, paleomagnetic, and 40Ar-39Ar age data on 13 collected sections to help evaluate stratigraphic and petrogenetic relationships through the Steens succession. We estimate that the overall duration of Steens Basalt volcanism from lingering eruptions could be as much as 300,000 yr, centered at ca. 16.7 Ma, but that the far greater volume erupted in <50,000 yr at an effusion rate ~0.67 km3/yr. Lava flows of primitive, homogeneous tholeiite initially erupted over a wide expanse of eastern Oregon during a reversed polarity interval (R0). Later eruptions became more focused at the presumed shield volcano at Steens Mountain, where dikes exploited a NNE-trending zone of crustal weakness related to the northeast extension of the mid-Cretaceous western Nevada shear zone. The Steens Mountain shield volcano generated increasingly more diverse flows of tholeiite, alkali basalt, and basaltic trachyandesite that erupted during a geomagnetic polarity transition culminating in upper flows of normal polarity (N0). The Steens sequence is dominated by compound flows (~10–50 m thick) produced by the rapid eruption of thin (<2 m) pahoehoe flow lobes. Analysis of these stacked sequences in the Catlow Peak section reveals periodic recharge of the magma chamber and ubiquitous fractional crystallization of plagioclase and olivine in each compound flow, accommodated by plagioclase accumulation and selective crustal contamination. The overall flood basalt stratigraphy records a rapid and progressive change in eruption style, from the early, near-continuous eruptions of small-volume Steens Basalt flows to later, more episodic eruptions of large-volume, tabular flows comprising the Imnaha, Grande Ronde, and Picture Gorge Basalts.

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