Abstract
K-Ar age of the Am-4 Tuff in the Miocene marine sequence of the Kanigawa section in the Boso Peninsula, central Japan, was determined. The obtained age is 11.73±0.24Ma (1σ error), which represents an age of the last occurrence (LO) horizon of Cyclicargolithus floridanus, because this biohorizon locates only two meters below the dated tuff (Mita and Takahashi, 1998). As the LO horizon of C. floridanus almost coincides with the first occurrence (FO) level of Discoaster kugleri, defining the CN5a/CN5b boundary of calcareous nannofossil zone of Okada and Bukry (1980), the age of the Am-4 can be regarded as an age of the CN5a/CN5b boundary. Precisely established age for the CN5a/CN5b boundary in central Japan is well concordant with previous estimation of Berggren et al. (1995).
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