Ar-40/Ar-39 geochronology of Middle Miocene calcareous nannofossil biohorizons in Central Japan

The Middle Miocene marine sequence in the Karasuyama area, Central Japan, i s interbedded with numerous volcaniclastic layers and contains calcareous n annofossils and planktonic foraminifera. This succession is thus of great i nterest for both age calibration of biostratigraphic events and for refinem ents to the geologic time scale. The volcaniclastic layers contain various minerals suitable for isotopic dating and six purified mineral separates of biotite, hornblende, sanidine, and plagioclase were analysed by the conven tional step heating Ar-40/Ar-39 technique. Geochronological results for the calcareous nannofossil zonal boundaries CN4/CN5a (last occurrence [LO] of Sphenolithus heteromorphus) and CN5a/CN5b (LO of Cyclicargolithus floridanu s) have been determined at 13.7 (13.74 +/- 0.16, 2 sigma) and 12.0 Ma (11.9 9 +/- 0.18, 2 sigma), respectively, which are valid for Central Japan. The age estimate for the LO of S. heteromorphus of 13.7 Ma is analytically indi stinguishable to results determined for the same biohorizon in Italy, thus, indicating a potentially significant biological time horizon occurring on a global scale. (C) 2001 Elsevier Science B.V. All rights reserved.