Abstract
In the construction of geologic time scales and the related exercise of discussing the chronology of chronostratigraphic units and boundaries, it is important to clearly distinguish between radiochronologic, biochronologic, magnetochronologic, and magnetobiochronologic input to their formulation. Failure to do so has, in some places, led to the confusion which surrounds the discussion and uncritical acceptance of some of the variant scales now being used. A brief critical examination will be made of several of the currently used Cenozoic chronologies and their bearing on the age of the Eocene/Oligocene boundary. Current age estimates of the Eocene/Oligocene boundary range from about 32 to 38 Ma based on the assessment of various (predominantly glauconitic) radiometric dates, poleontological control of varying reliability, and quality and paleomagnetic chronologies employing different calibrations. High temperature radiometrically dated polarity stratigraphy in the middle Eocene (polarity chron 20-21 interval) and the latest Eocene-early Oligocene (chron 15/16-12 interval) in North American continental sections with mammalian faunas provide the framework for much needed calibration points in the mid-Cenozoic and for a revised Cenozoic time scale. This also provides constraints on age estimates of the magnetobiostratigraphically determined Eocene/Oligocene boundary in deep sea and continental m rine sections. The Eocene/Oligocene boundary (biostratigraphically linked with the LAD's of the Globorotalia cerroazulensis-cocoaensis group, Hantkenina and Globigerapsis, and the rosette-shaped discoasters, i.al. Discoaster barbadiensis and d. saipanensis), is situated within the reversed interval between marine magnetic anomalies 15 and 13 with younger and older boundary estimated age values of 37.24 and 35.87 Ma, respectively. Our best estimate of the age of the Eocene/Oligocene boundary (subject to minor changes as a result of further magnetobiostratigraphic studies) is 36.6 Ma. End_of_Article - Last_Page 423------------
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