Magnetostratigraphy, radiometric dating, and carbon isotope analysis of continental strata in western Canada: implications for the age of the Paleocene-Eocene boundary
|Location||International Geological Congress,oslo 2008|
|Author||Evans, M.E.; Lerbekmo, J. F.; Heaman, L. M.; Muehlenbachs, K|
|Holding Date||07 October 2008|
We have studied a 110 m-thick late Paleocene to early Eocene sequence of flat-lying continental strata in west-central Alberta, Canada. The sequence consists of interbedded fluvial channel sandstones and overbank mudrocks containing five 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 25n and the first half of 24r. The latter was found to contain six normal polarity subzones that we correlate to tiny wiggles 6-11 in marine magnetic profiles. Carbon isotope analysis of 14 samples from two cores revealed a negative shift of about 2 per mil 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. 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.