Normal polarity magnetosubchrons in 24r and the age of the Paleocene–Eocene boundary

Abstract

A late Paleocene to early Eocene sequence of flat-lying continental strata occurs in an area known as Obed Mountain, west-central Alberta. The upper 110 m consist of interbedded fluvial channel sandstones and overbank mudrocks containing five back swamp coal seams. Two coreholes, 3.5 km apart, that extend through the entire coal zone were sampled for magnetostratigraphy and 13C isotope analysis. Bentonites in the No. 1 (lowest) and No. 5 coal seams and a tuff in the No. 3 coal seam were sampled for U–Pb and (or) Rb–Sr dating of zircon and biotite, respectively. Magnetostratigraphic analysis of 520 samples identified the younger part of chron 25r, the whole of chron 25n and the older half of chron 24r. We find six normal polarity subzones in this part of chron 24r, which we correlate to tiny wiggles 6 to11 in marine magnetic profiles. Carbon isotope analysis of 14 samples from two cores revealed a negative shift of about 2‰ peaking near the base of 24r.8r. We interpret this as the carbon isotope excursion (CIE), the base of which is now accepted as defining the Paleocene–Eocene boundary. The thickness of the CIE in the Obed Mountain section implies that it lasted between 210 000 and 254 000 years. Radiometric dates of 58.4 ± 0.2, 57.7 ± 0.3, and 56.9 ± 0.8 Ma are obtained for the No. 1, No. 3, and No. 5 coals, respectively. Combining these with magnetostratigraphy and cyclostratigraphy yields an age of 57.1 ± 0.1 Ma for the Paleocene–Eocene boundary.