Figure 31. Paleomagnetic-polarity column showing ages of polarity reversals and durations of polarity chrons for latest Cretaceous strata based on data from San Juan Basin, New Mexico. Polarity-chron durations and reversal ages for Cretaceous magnetochrons were determined by proportionalizing chron durations of Cande and Kent (1992, 1995) between ages of 65.51 Ma for the Cretaceous-Tertiary (K-T) boundary (Hicks et al. 2002) and 73.50 Ma for the C33n-C32r reversal (Fassett and Steiner 1997; Fassett 2000). Polarity-chron ages above K-T boundary are from Gradstein et al. (2004). Ages of ash beds J and H in polarity chron C32r from Fassett and Steiner (1997).