The basal part of Jaw Face section at the Wellsch Valley site, some 50 km north-northwest of Swift Current, Saskatchewan, contains mammalian fossils that are considered to be late Blancan to early Irvingtonian in age. It has been sampled for paleomagnetic studies through a thickness of 11 m. The section above 8.8 m has normal polarity; that below has reversed polarity. A tephra, which has yielded a minimum fission-track age of 0.69 ± 0.11 Ma, lies within the reversely magnetized part, near the top of the fossiliferous zone and just below the reversal at the 8.8 m level. Therefore, the change from normal to reversed polarity at the 8.8 m level is probably the Brunhes–Matuyama reversal at 0.73 Ma. Earlier studies have indicated that the Jaramillo (0.9–0.97 Ma) and Olduvai (1.67–1.87 Ma) Normal Polarity subzones occur within the lower predominantly reversed part of the Jaw Face section. In this more detailed study, we find no evidence of these normal subzones. We suggest that earlier results could have been artifacts of the procedures used. Several stratigraphic interpretations are possible. Our preferred interpretation is that the Jaw Face section extends from the upper part of the Matuyama Zone into the lower Brunhes Zone, that is, the mammalian faunas are entirely Irvingtonian in age. Alternatively, the section could contain a substantial hiatus, so the lower part would be early Matuyama in age and its faunas late Blancan – early Irvingtonisn, and the upper part could span the latest Matuyama and lower Brunhes zones. Other possibilities are discussed in the text.

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