The Upper pliocene diatom zone and fission-track age of the upper Koetoi Formation around the Sarobetsu anticline, northern Hokkaido

Yasue, Kenichi; Akiba, Fumio*; Ohira, Hiroto*; Ishii, Eiichi   

The Upper Pliocene Neodenticula koizumii Zone was recognized near the boundary between the Koetoi and Yuchi Formations in the western part of the Horonobe area, northern Hokkaido. In previous studies, numerous reworked fossil diatoms from underlying strata have obscured the recognition of the zone. However, this study samples include commonly both N. seminae and N. koizumii as probable autochthonous diatoms and almost no reworked fossil diatoms. Therefore, the samples belong obviously to the Neodenticula koizumii Zone. In addition, fission track ages of zircon crystals from a tuff between the diatom samples show 2.20.1 Ma and 2.30.1 Ma (2.50.1 Ma:). The diatoms and FT age data indicate that the depositional age around the boundary between the Koetoi and Yuchi Formations in the study area is 2.1-2.4 Ma. The age is younger than that in the eastern part of the Horonobe area for about one million years, which is concordant with the previously suggested westward migration of the Tenpoku Basin after Pliocene.