The age of the Steens reversal and the Columbia River Basalt Group

The Columbia River Basalt Group (CRBG) eruptions have a well-defined relative magnetostratigraphy but have not been definitively correlated to the geomagnetic polarity time scale. 40Ar/39Ar ages are presented from lavas erupted in the R-0 through N-1 magnetozones of the CRBG and in the transition between R-0 and N-0. Four ages from transitionally magnetized lava flows at Steens Mountain, Catlow Peak, and Poker Jim Ridge with a weighted mean age 16.58 +/- 0.10 Ma(1) and the more precise age 16.654 +/- 0.025 Ma of the normally magnetized Oregon Canyon tuff at the top of the Catlow Peak section show that the oldest CRBG magnetozone (R-0) correlates with the C5Cr chron. Bayesian statistical analysis applied to data from four flows at Catlow Peak (using the mean age of the Steens reversal) gives a best and preferred age of the Steens reversal of 16.73 + 0.13/-0.08 Ma (95% confidence). Depending on the geomagnetic polarity time scale model, the eruption rate from N-0 through R-2 (0.34-0.45 Ma in the middle and the bulk of the CRBG emplacement) averaged 0.30-0.41 km(3)/a and peaked at a rate 1 1/2 to 4 1/2 times higher during R-2. (C) 2010 Elsevier B.V. All rights reserved.