K-Ar age of the Kn-1 Tuff in the Miocene marine sequence in the Boso Peninsula, central Japan.

A K-Ar age was determined for the Kn-1 Tuff in the Miocene marine sequence of the Kawadani section in the Boso Peninsula, central Japan. The weighted-mean age of 14.95±0.24Ma (±σ error) was calculated from two ages on Kn-1 biotite (14.96±0.34Ma and 14.93±0.34Ma). Combining with previously reported radiometric ages of other tuff layers, the established chronostratigraphy based on the radiometric dating indicates decline of the sedimentation rate between the Kn-3 Tuff in the upper part of the Kinone Formation and the Am-4 Tuff in the lowest part of the Amatsu Formation. The previously established biostratigraphy of microfossils is almost consistent with the chronostratigraphy of radiometric ages, while only the age of the last occurrence of Eucyrtidium asanoi (radiolaria) is suggested to be younger (ca. 12.2-12.7Ma) than the previous estimation (13.56-13.85Ma) by Morley and Nigrini (1995).