Precise ages for flood basalts obtained from 40Ar/39Ar feldspar ages for intercalated silicic tuffs. Estimated eruptive interval of Steens Basalt is 16.64±0.0516.64±0.05 to 16.43±0.03 Ma16.43±0.03 Ma. Initiation of Miocene Climatic Optimum appears to precede eruption of Steens Basalt. Steens Mountain geomagnetic reversal occurred at 16.496±0.028 Ma16.496±0.028 Ma (±0.18 Ma total error).