New data and a reinterpretation of existing data are the bases for this emended Paleogene numerical time scale. Biostratigraphy, radiometric dating, stratotypes, and paleomagnetic stratigraphy have been independently evaluated and subsequently integrated into a single numerical time scale framework.
In the three Paleogene series, Oligocene, Eocene, and Paleocene, we have recognized eight commonly used stages. The corresponding age-units vary in duration from 3 to 8 million years (average 5 Ma). As a result of the improvement in establishing the relationships between the stratotypes of these stages and plankton biostratigraphy, the position of the stages within the series has been modified from earlier integrated chronostratigraphic schemes.
Stratotypes (type sections) represent the formal basis for relating rock and time. However, the stratotypes of the western European Paleogene stages were not originally established on the basis of planktonic microfossils, and it was therefore not possible to recognize these stages worldwide. Subsequently, independent plankton zonations were established that could not be properly related to the stratotypes of the standard European chronostratigraphic units. Nevertheless, tentative relationships between these planktonic zonations and the relative chronostratigraphic units have been suggested and have been followed for practical reasons by nearly all plankton biostratigraphers.
Calcareous nannoplankton have recently provided an indirect means of relating the stratotypes of the northwestern European Paleogene stages to the planktonic foraminiferal biostratigraphy and have revealed discrepancies between these two stratigraphic systems. The most significant of these is in the Eocene, where the type section of the traditional late Eocene Bartonian contains planktonic microfossils generally considered indicative of middle Eocene age according to most biostratigraphers. This situation is corrected by placing the middle-late Eocene boundary between the Priabonian and the Bartonian. The alternative solution, moving planktonic foraminiferal zones P13 and P14 from middle to late Eocene, would cause far greater confusion in the worldwide correlation framework.
An evaluation of published radiometric dates provided us with the following age ranges for the Paleogene geochronologic units:
Oligocene: 24 to 37 Ma Late: 24 to 32 Ma (Chattian)
Early: 32 to 37 Ma (Rupelian)