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 km 2 and 31,800 km 3 , respectively. We integrate petrochemical, paleomagnetic, and 40 Ar- 39 Ar 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 km 3 /yr. Lava flows of primitive, homogeneous tholeiite initially erupted over a wide expanse of eastern Oregon during a reversed polarity interval (R 0 ). 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 (N 0 ). 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.

The radiometric dating evidence for the timing and duration of volcanism for the Steens through Wanapum Basalt of the Columbia River Basalt Group is critically reviewed here. K-Ar dates generally underestimate the age of crystallization, though one important exception is detected, where excess argon led to dates that were too old. The 40 Ar/ 39 Ar results on whole-rock basalts from 1980 through 2010 are examined for statistical validity of plateau sections, as well as alteration state of the material dated. In most instances, listed ages are shown to be invalid. The 40 Ar/ 39 Ar total gas (fusion) ages are, in general, not accurate estimates of the time of formation of these rocks. The 40 Ar/ 39 Ar ages on plagioclase separates from basalts yield good estimates of the extrusion age of the lavas. New 40 Ar/ 39 Ar ages on whole-rock basalts are presented that are in good agreement with the plagioclase ages. Various forms of the geomagnetic polarity time scale for mid-Miocene time are examined, along with the ages of lavas and their magnetic polarity. The main sections of the Columbia River Basalt Group (Imnaha through Wanapum Basalt) were formed in ~0.5 m.y. between 16.3 and 15.8 Ma. Steens Basalt extrusion occurred about ~0.1 m.y. before the Imnaha Basalt and appears to have been a precursor to the more voluminous volcanism noted in the Columbia River Basalt Group.