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Data on biostratigraphy, biochronology, magnetostratigraphy, and K-Ar dating of basalts from the lower Eocene and upper Paleocene sequences drilled during Leg 81 are summarized in this chapter. The K-Ar results from Site 555 basalts indicate that the age of marine magnetic Anomaly 24 and adjacent anomalies represents a key problem in the Cenozoic time scale. Three short events of normal polarity in the reversed interval between Anomalies 24B and 25 are interpreted either to represent true physical properties that are detected for the first time in the Leg 81 sequences or to represent overprints of the modern magnetic field direction. In post-Anomaly 24 times, the sequences are condensed (except part of Site 552) or characterized by hiatuses. Rapid sediment accumulation rates (20-70 cm/1000 yr.) characterize the pre-Anomaly 24B sequences of Hole 553A and Site 555. INTRODUCTION: THE TIME SCALE PROBLEM Cenozoic and late Mesozoic time scales are derived from the patterns of marine magnetic anomalies. Absolute ages are determined from a few points that have been dated radiometrically, by assuming linear seafloor spreading rates between or beyond these control points. One of the most widely used time scales was published by Heirtzler et al. (1968). Heirtzler and his colleagues had a single radiometric control point at Anomaly 2 (Gauss/ Gilbert boundary; present estimate 3.40 m.y. ago) and used this to make a linear extrapolation well back into the late Cretaceous. Numerous efforts have since been made to refine this time scale, the most important of which are compared and discussed in the elegant study of Ness et al. (1980). The most recent of these efforts is the comprehensive presentation, partly based on new radiometric age data, of Berggren, Kent, and Flynn (in press). But before that, a major step toward improved radiometric age control was taken in 1979 when Mankinen and Dalrymple provided new K-Ar decay constants. It has long been assumed that seafloor spreading rates changed on a global scale at around Anomaly 24; conseRoberts, D. G., Schnitker, D., et al., Init. Repts. DSDP, 81: Washington (U.S. Govt. Printing Office). 2 Addresses: (Backman) Department of Geology, University of Stockholm, S-10691 Stockholm, Sweden; (Morton) British Geological Survey, Keyworth, Nottingham NG12 5GG, United Kingdom; (Roberts, present address): British Petroleum Co., Ltd., Britannic House, Moor Lane, London, United Kingdom; (Brown) Paleoservices Ltd., Paramount Ind. Est., Sandown Rd., Watford WD 24XA, England. (Krumsiek) Geologisches Institut, Friedrich-WilhelmsUniversitàt, Nussallee 8, 5300 Bonn 1, Federal Republic of Germany; (Macintyre) Isotope Geology Unit, Scottish Universities Research and Reactor Centre, East Kilbridge, Glasgow G 750Q4, Scotland and Department of Applied Geology, University of Strathclyde, Glasgow Gl1XJ, Scotland. quently, many workers suggested an inflection point at this anomaly during construction of time scales of marine magnetic anomalies (see discussion and review in Ness et al., 1980). Anomaly 24 was until recently bracketed by age-control points tens of millions of years away; thus the slope of the line being extrapolated to Anomaly 24 from the nearest younger age-control point could vary profoundly, depending on which suggested intermediate ages were considered most tenable. This resulted in highly varying ages of the inflection point at Anomaly 24 (see Ness et al., 1980). Hailwood et al. (1979) utilized several sources (Heirtzler et al., 1968; Talwani et al., 1971; Blakely, 1974; PremoliSilva et al., 1974; Hardenbol and Berggren, 1978) to construct a time scale used in Volume 48 of the Initial Reports of the Deep Sea Drilling Project. Their work was based on pre-1979 K-Ar decay constants. We had three reasons for using the Hailwood et al. time scale for Leg 81 work. The first was that comparisons between the important, near-identical lower Eocene sequences drilled during Leg 48 and Leg 81 would be more easily made if just one time scale was used. The second reason was that there still existed large uncertainties about the chronostratigraphic position of Anomaly 24 and thus the slope of the time line between Anomaly 24 and nearest younger radiometric age-control point. Finally, the calibrations shown by Hailwood et al. between the biostratigraphic and magnetostratigraphic indications were considered comparatively reliable. On the basis of these arguments, and particularly in view of the fact that the first had such weight during Leg 81 shipboard work, we chose to adhere to the Hailwood et al. time scale although we were aware of its weak-

Geochronology of the Lower Eocene and Upper Paleocene Sequences of Leg 81