 One geomagnetic field reversal occurred during the Steens Basalt eruptions at approximately 16.7 Ma, as dated using 40Ar/39Ar ages and the geomagnetic polarity timescale.Steens Mountain and related sections of Oregon Plateau flood basalts at Catlow Peak and Poker Jim Ridge to the southeast and west of Steens Mountain, provide the most detailed magnetic field reversal data (reversed-to-normal polarity transition) yet reported in volcanic rocks.Virtually coeval with oldest of the flows, the Imnaha basalt flows welled up across northeastern Oregon.