EVIDENCE FOR ERRORS IN THE GEOMAGNETIC POLARITY TIME-SCALE AT 17-15 MA - 40AR/39AR DATING OF BASALTS FROM THE PACIFIC-NORTHWEST, USA

40Ar/39Ar incremental heating studies have been carried out on whole‐rock basalt samples from the Columbia River Basalt (CRB) and the Steens Basalt, to obtain crystallization ages with a standard error <1%. These high precision, accurate, ages are utilized with the known magnetostratigraphy of the CRB, to test the accuracy of the Early to Middle Miocene section of the geomagnetic polarity time‐scales currently in use. We conclude that these time‐scales underestimate the ages of various chrons within this time range by ∼0.9 m.y. © 1990 by the Chinese Geophysical Society