Bulletin of the Mizunami Fossil Museum

Bulletin of the Mizunami Fossil Museum Vol. 50, No. 1

The chronostratigraphy of the Kazusa Group in the Tokorozawa observatory well, Saitama Prefecture, was determined by Sr isotope age and calcareous nannofossil biostratigraphy. This group is lithlogically divided into three parts; lower (depth 873–1,027 m), middle (depth 573–873 m) and upper (depth 66–573 m), and the upper part is interleaved with two pumice-bearing layers.

The top of the group is estimated to be 1.5–1.4 Ma, which is in harmony with the uppermost age of the Bushi Formation. The base of the group is also estimated to be 2.2–2.1 Ma, which is slightly younger than the base of the Kazusa Group in the Boso Peninsula. The group is composed of a sedimentary cycle with marine transgression and regression whose maximum transgression is about 2.0 Ma