1. TOWARD A REVISED GEOCHRONOLOGY by William A. Berggren, Dennis V. Kent John D. Obradovich, and Carl C. Swisher I11

ABSTRACT topes, magnetostratigraphy and sea-floor New information has become available that anomalies (Aubry et al., 1988), we believe that requires a revision of Paleogene chronology in- a thorough evaluation of each of these disci- corporated in most current time scales plines is required before any basic revision(s) (e.g. Berggren et al., 1985a, b). Age estimates can be made to the extant scale. Moreover revi- for the limits of the Paleogene (the sions to Paleogene geochronology are but part of / and /Paleogene a more comprehensive revision to the time scale boundaries) have not changed appreciably and for the Cenozoic Era in which we are currently remain at about 24 Ma and about 66 Ma, respec- engaged. Thus we view this contribution as an tively. However, new radioisotope data indi- interim step on the way to a fundamental revi- cate that boundaries of subdivisions within the sion of Cenozoic geochronology. Paleogene are generally younger than previ- We review below pertinent new Paleogene ously estimated, for example, the / radioisotopic age data with particular empha- and Eocene/Oligocene, by about 2 to 3 sis on those bearing on the Eocene/Oligocene m.y. We review the current status of mag- boundary and provide an assessment of the cur- netobiostratigraphic correlations and new ra- rent status of Paleogene geochronology. dioisotope data, with particular reference to late Eocene~earlyOligocene geochronology NEW RADIOISOTOPIC DATA and provide a reassessment of the age of the A compilation of (predominantly post-1985) Eocene/Oligocene boundary as 34 Ma. We an- radioisotopic data pertinent to Paleogene ticipate that with concurrent work on a funda- geochronology is presented in Tables 1.3,1.5-1.7 mental revision of the geomagnetic polarity se- and summarized and compared with the Paleo- quence, a comprehensive and detailed new time gene time-scale of Berggren et al. (1985a, b) in scale for the Cenozoic will soon be developed. Figure 1.1. A number of late Eocene - early Oligocene radioisotopic dates were generated INTRODUCTION from the Marche-Umbria Basin () in con- It is six since we published a time scale nection with the research for a potential for the Paleogene (Berggren et al., 1985a, b). Eocene/Oligocene boundary stratotype section All available first-order correlations between and point (Nocchi et al., 1986; Premoli-Silva et Paleogene calcareous plankton and magne- al., 1988). We have compiled an annotated list tostratigraphy were compiled in Berggren et al. of those dates relevant to the boundary (Table (1985a). Since that time there have been sev- 1.3) considered reliable by the responsible au- eral additions and refinements to this data set; thors (Montanari, 1988; Montanari et al., 1988; those relevant to late Eocene - early Oligocene Odin, et al. 1988). Of particular relevance to geochronology are shown in Tables 1.1 Paleogene geochronology is the fact that the (planktonic ) and 1.2 (calcareous ages obtained in the northeastern Apennines nannoplankton). Inasmuch as we view a time are from volcanic ashes in stratigraphic sec- scale to consist of a data set which integrates tions having (in general) both magneto- and information from biostratigraphy, radioiso- biostratigraphy. Comparison of the magnetic