Geological Society, London, Memoirs Jurassic to Paleogene: Part 2 Paleogene geochronology and chronostratigraphy W. A. Berggren, Dennis V. Kent and John J. Flynn Geological Society, London, Memoirs 1985; v. 10; p. 141-195 doi: 10.1144/GSL.MEM.1985.010.01.15 Email alerting click here to receive free e-mail alerts when new articles cite this article service Permission click here to seek permission to re-use all or part of this article request Subscribe click here to subscribe to Geological Society, London, Memoirs or the Lyell Collection Notes Downloaded by on January 17, 2012 © 1985 The Geological Society. Authorization to photocopy items for internal or personal use, or the internal or personal use of specific clients, is granted by The Geological Society for libraries and other users registered with the Copyright Clearance Center (CCC) Transactional Reporting Service, provided that a base of $02.00 per copy is paid directly to CCC, 21 Congress Street, Salem, MA 01970, USA. Jurassic to Paleogene: Part 2 Paleogene geochronology and chronostratigraphy W. A. Berggren, Dennis V. Kent and John J. Flynn S U M M A R Y : We present a revised Paleogene geochronology based upon a best fit to selected high temperature radiometric dates on a number of identified magnetic polarity chrons (within the late Cretaceous, Paleogene and Neogene) which minimizes apparent accelerations in sea-floor spreading. An assessment of first order correlations of calcareous plankton biostratigraphic datum events to magnetic polarity stratigraphy yields the following estimated magnetobiochronology of major chrono- stratigraphic boundaries: Cretaceous-Tertiary boundary (Chron C29R), 66.4 Ma; Paleocene-Eocene (Chron C24R), 57.8 Ma; Eocene-Oligocene (Chron C13R), 36.6 Ma; Oligocene-Miocene (Chron C6CN), 23.7 Ma. The Eocene is seen to have expanded chronologically (- 21 m.y.) at the expense of the Paleocene (- 9 m.y.) and is indeed the longest of the Cenozoic epochs. In addition, magnetobio- stratigraphic correlations require adjustments in apparent correlations with standard marine stage boundaries in some cases (particularly in the Oligocene). Finally, we present a correlation between standard Paleogene marine and terrestrial stratigraphies. It is nearly 20 years since Brian Funnell prepared the first biostratigraphy and radiochronology. The magnetobio- relatively precise Cenozoic time-scale based on an assess- chronology of the calcareous plankton (and by extension, the ment of palaeontologically controlled radiometric data in age estimate of the standard epoch and age boundaries) is connection with the symposium on the Phanerozoic time- based on a compilation of first order correlations between scale sponsored by the London Geological Society, and 10 biostratigraphic datum levels and magnetic stratigraphy in years since one of us (WAB) presented the first in a series of continental, marine, and deep sea core material. These data attempts to further refine Cenozoic geochronology. During are present in tabular form in the appendix. the past decade several revisions to the Cenozoic time-scale have appeared and here, at this, the second symposium on the Phanerozoic time-scale sponsored by the Geological Paleogene geomagnetic polarity time-scale Society of London, it is appropriate to present an updated and, hopefully, improved version of the Cenozoic time-scale. The basis for a geomagnetic polarity reversal chronology for It is opportune that over the past decade direct correlation the late Jurassic to Recent is the polarity sequence inferred has been achieved between plankton biostratigraphy in some from analysis of marine magnetic anomalies. Although the of the standard European continental marine sections and Paleogene portion of geomagnetic reversal history is of North American terrestrial vertebrate biochronology and interest here, it is best considered in the context of the magnetic polarity stratigraphy over much of the Cenozoic magnetic anomaly sequence extending from the present sea- Era. The recent improvement in deep sea coring techniques floor spreading axis to the younger limit of the Cretaceous has further extended these correlations on a global scale. It is Long Normal or Quiet Zone. Because of the lack of cor- now possible to make age estimates of epoch boundaries and relatable features in the Cretaceous Quiet Zone, the older the extent of time-stratigraphic (standard ages) units in terms (late Jurassic and early Cretaceous) set of anomalies, referred of plankton biostratigraphy and magnetic polarity chrons to as the M-sequence (Larson & Hilde 1975), can be treated and/or anomalies. separately. Finally a critical evaluation must be made within a geo- The first extended geomagnetic reversal time-scale was historical context of biostratigraphically controlled radio- presented by Heirtzler et al. (1968) who chose a magnetic metric dates and radiometrically dated polarity stratigraphy in profile from the South Atlantic Ocean as representative of order to provide constraints on an internally consistent geomagnetic reversal history for about the past 80 Ma. Their geologic time-scale. chronology, hereafter referred to as HDHPL68, was derived The revised Cenozoic geochronology has been prepared in by a correlation of the axial anomalies to the 0 to 4 Ma, two parts: (a) Paleogene; (b) Neogene. In this paper dealing radiometrically-dated magnetic reversal time-scale (Cox et al. with the Paleogene we first discuss the development of 1965) and by extrapolation to the oldest then recognized geomagnetic polarity history of the late Cretaceous and polarity interval (anomaly 32). This twenty-plus fold extra- Cenozoic. A revised geochronology is then presented which is polation assumed that the rate of sea-floor spreading in this based upon a best fit to selected high temperature radio- area of the South Atlantic was constant over about 1400 km metric dates on a number of identified magnetic polarity or 80 Ma, at the value calculated from 0 to 3.35 Ma (anomaly chrons (in the late Neogene, early Oligocene, middle 2A). Despite the severe extrapolation required, HDHPL68 Eocene, and late Cretaceous) which minimizes apparent has proved its utility in description of sea-floor spreading acceleration in sea-floor spreading. This is followed by a histories in the world ocean and continues in large part to be discussion of the biostratigraphy of the major Paleogene the basis for all subsequent revised geomagnetic reversal epochs and their boundaries beginning with the Cretaceous- time-scales (see review by Ness et al. 1980). It is now Tertiary boundary. Our revised Paleogene geochronology is apparent that HDHPL68 generally comes within 10% of presented in a series of figures and reflects our assessment of currently accepted ages for this reversal sequence, a remark- presently available data from the fields of magneto- and able achievement and an indication that the assumption of 14I I42 W. A. Berggren, D. V. Kent & J. J. Flynn sea-floor spreading at a constant rate over prolonged time potentially used for calibration. Radiometric dates are intervals is a valid approximation. sometimes available from the same section investigated for Although recent magnetostratigraphic investigations have magnetostratigraphy. A notable example is the work on identified large portions of essentially the same magnetic Icelandic lavas (McDougall et al. 1976) where it has been reversal pattern in marine sedimentary sections (e.g. Lowrie possible to directly estimate ages of polarity reversal levels et al. 1982; Poore et al. 1982) and in volcanic sequences with from the stratigraphic distribution of numerous radiometric radiometric-date control (McDougall et al. 1976), the marine dates. Much more commonly, however, sedimentary sections magnetic anomaly record continues to be the standard for which have not been dated directly are studied and numerical determining the relative position of polarity intervals and age control is derived by biostratigraphic correlation to a hence for correlation. This is largely due to the great wealth geologic time-scale. The accuracy of such ages depends on of marine magnetic anomaly data which can be used to both the precision of the correlation and the quality of the demonstrate that the interpreted record of geomagnetic age estimates for the standard geological stage boundaries. reversals is relatively smooth and continuous, that is, the An appraisal of such correlations and age estimates for the same sequence of anomalies (polarity reversals) can be found Paleogene is presented elsewhere in this paper. everywhere, differing over appreciable intervals only by some Given a set of ages tied by various correlations to the proportionality factor that reflects formation at different standard' magnetic reversal sequence, several approaches can spreading rates. The large number of profiles available also be used to calibrate it. One method is to fix one or more makes possible averaging or stacking of profiles to reduce points in the polarity reversal sequence to the corresponding noise, resulting in a better representation of the true age estimates obtained by correlation and calculate the ages geomagnetic reversal sequence. In contrast, there are few of other reversals by interpolation or extrapolation. Besides long magnetostratigraphic sections to adequately allow the origin, only a single calibration point was used in separation of changes in accumulation rates from differences HDHPL68, whereas in the time-scale of LaBrecque et al. in duration of polarity