The Paleocene/Eocene boundary Global Standard Stratotype-section and Point (GSSP): Criteria for Characterisation and Correlation

Abstract

The choice of a Paleocene/Eocene (P/E) Global Standard Stratotype-section and Point (GSSP) is complicated by the fact that there exists confusion on the exact denotation of the Paleocene and Eocene Series and their constituent lower rank (stage) units. While we can now resolve this problem by recourse to rigorous historical analysis, actual placement of the GSSP is further exacerbated by an embarrassment of riches (in regards to 7 criteria suitable for characterising and correlating a PIE GSSP but which span a temporal interval of>2 my). Following the precept that the boundaries between higher level chronostratigraphic units are to be founded upon the boundaries of their lowest constituent stages in a nested hierarchy, we note that one of the criteria providing global correlation potential (a stable isotope excursion in marine and terrestrial stratigraphies) lies at a stratigraphic level more than !my older than the base of the stratotypic Ypresian Stage to which the base of the Eocene Series has been subordinated until now. Lowering a chronostratigraphic unit by this extent risks a significant modification to the original geohistorical denotation of the Ypresian Stage and the Eocene Series. We discuss here four options that are open to Voting Members of the Paleogene Subcommission. One solution consists in adjusting slightly the base of the Ypresian Stage (and, thus, the Eocene Series) so as to be correlatable on the basis of the lowest occurrence/First Appearance Datum (LO/FAD) of the calcareous nannofossil species Tribrachiatus.digita/is. Another solution would be to decouple series and stages so that the Ypresian Stage remains essentially unaltered but the base oftbe Eocene is relocated so as to be correlated on the basis of the Carbon Isotope Excursion (CIE). Two (compromise) solutions consist in erecting a new stage for the upper/terminal Paleocene (between the Thanetian (sensu Dollfus) and Ypresian Stages) characterised at its base by the global stable isotope excursion. The P/E GSSP may then be placed at the base of the stratotypic Ypresian Stage (thus preserving historical continuity and conceptual denotation and stability) or at the base of the newly erected stage (facilitating correlation of the base of the Eocene series, at least in principle). Both GSSPs should be placed in suitable marine stratigraphic sections yet to be determined but upon which there is considerable current investigative activity.