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The nature and chronostratigraphic position of the Rupelian-Chattian boundary (Early-Late Oligocene) unconformity in its historical type region (Belgium) is examined using biostratigraphy, strontium isotope dating of benthic foraminifera and K-Ar dating of glauconites. The duration of this unconformity is derived from the absence of the globally synchronous Svalbardella dinocyst event associated with the important mid-Oligocene Oi2b cooling that occurred in the middle and upper part of chron C9n. This hiatus represents a gap in the rock record of about 500,000 years. Two 87 Sr/ 86 Sr dates from the upper Rupelian Boom Formation and two from the lower part of the Chattian Voort Formation suggest that the boundary lies between 28.6 Ma and 27.2 Ma. Although there is some inconsistency in the K-Ar glauconite dates, those considered correct from the upper part of the Boom Formation and from lower part of the Voort Formation suggest that the boundary lies between 29.2 Ma and 27.0 Ma. Sr and K-Ar dating indicate the top of the Rupelian not to be older than 29 Ma and the basal Chattian not younger than about 27 Ma. The recently proposed GSSP in the Apennines with an estimated date of 28.2±0.2 Ma therefore honours the position of the historically defined Rupelian-Chattian boundary.

On the nature and chronostratigraphic position of the Rupelian and Chattian stratotypes in the southern North Sea basin