Abstract
K-Ar ages were determined for four volcaniclastic layers, interbedded in a middle Miocene marine sequence of the Arakawa Group in the Karasuyama area, central Japan. Combining with the previously reported K-Ar ages of four other tuff layers, the numerical ages of some important biohorizons of calcareous nannofossils and planktonic foraminifera, defined by previous studies, were geochronologically estimated. The CN 4/CN 5a and CN 5a/CN 5b calcareous nannofossil zone/subzone boundaries (Okada and Bukry, 1980), defined by the last occurrences (LOs) of Sphenolithus heteromorphus and that of Cyclicargolithus floridanus, were estimated as 13.7 Ma and 11.8 Ma, respectively. Both ages coincide with previous estimations (e. g. Berggren et al., 1995; Montanari et al., 1997; Odin et al., 1997). On the other hand, the N.13/N.14 zone boundary of planktonic foraminifera (Blow, 1969), defined by the first occurrence (FO) of Globigerina nepenthes, was estimated as 11.6 Ma. This age almost coincides with the results of the Tomioka sequence in Japan (11.5 Ma; Odin et al., 1995 recalculated) and the Italian section (11.5Ma; Odin et al., 1995). The integrated stratigraphic results of the Karasuyama sequence should constitute data sets in the refinement of a global geologic time scale.
