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

The Middle Miocene marine sequence in the Karasuyama area, Central Japan, is interbedded with numerous volcaniclastic layers and contains calcareous nannofossils and planktonic foraminifera. This succession is thus of great interest for both age calibration of biostratigraphic events and for refinements 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 conventional step heating 40Ar/39Ar technique. Geochronological results for the calcareous nannofossil zonal boundaries CN4/CN5a (last occurrence [LO] of Sphenolithus heteromorphus) and CN5a/CN5b (LO of Cyclicargolithus floridanus) have been determined at 13.7 (13.74±0.16, 2σ) and 12.0 Ma (11.99±0.18, 2σ), respectively, which are valid for Central Japan. The age estimate for the LO of S. heteromorphus of 13.7 Ma is analytically indistinguishable to results determined for the same biohorizon in Italy, thus, indicating a potentially significant biological time horizon occurring on a global scale.