Revised Cenozoic polarity time scale D. H. Tarling, J. G. Mitchell Department of Geophysics & Planetary Physics The University, Newcastle upon Tyne NE1 7RU, England ABSTRACT The polarity sequence of Heirtzler and his co-workers for Neogene time is modi- fied by compromise solutions between the polarity record in deep-sea sediments and oceanic magnetic anomalies. We have also revised the entire chronology by use of more recent isotopic age determinations of the stratigraphic ages of sediments over- lying identified anomalies. The main fea- tures of the revised time scale are a general reduction of the length of polarity periods during Eocene and later times, and an expansion of Paleocene polarity periods. INTRODUCTION The original time scale for polarity changes of the Earth's magnetic field during Cenozoic time (Heirtzler and others, 1968) has proved extremely valuable in the development of plate tectonics models. It was based on magnetic profile V-20, in the South Atlantic, and was calibrated from the end of the Gilbert polarity epoch, then dated at 3.35 m.y. B.P., throughout Cenozoic time, assuming that the pre- Gilbert spreading rate of 1.9 cm yr" 1 re- mained unchanged. Heirtzler and others (1968) emphasized the unreliability of this calibration, and one of the early objectives of the Deep Sea Drilling Project was to verify this polarity scale by sampling the sediments overlying identified magnetic anomalies. The biostratigraphic age of these sediments could then be related to isotopically dated biostratigraphic levels elsewhere. Initial studies showed agreement within 5 percent of the Heirtzler and others estimates, and Tarling (1971) constructed a revised scale that depended largely on the validity of radiometric dates of glauco- «•I >> o « c o rsi « C o £L N E E * o ~o o c o ». o 0 0. < LL 3) c 5 "0 TJ / tJ o 1» -Ì — a u — <* 3 f— o / O 2 { n 1» / 1» 3 I •* C o -1 1» a r> » o c 1/1 Figure 1. Revised Cenozoic polarity scale. Biostratigraphic zones are largely after Berggren and Van Couvering (1974); isotopic ages are largely derived from Funnell (1964), Konechy and others (1969), Lambert (1971), Dalrymple (1972), McKenna and others (1973), Berggren and Van Couver- ing (1974), Odin (1975), and personal communications from G. S. Odin, R. H. Tedford, and J. Obradovich. GEOLOGY 133