Diatom biostratigraphy of the Neogene strata in the Akan area, the eastern Hokkaido, northern Japan

Abstract

Diatom biostratigraphy of the Neogene strata in the Akan area, eastern Hokkaido is established by this study. The Neogene strata in the study area have been subdivided into the Tonokita, Chichappu and Kotan Formations in ascending order. The diatom assemblages from the upper part of the Tonokita Formation correspond to the Denticulopsis lauta Zone (15.9-14.9 Ma). The lower part of the Chichappu Formation correlates to the Thalassiosira praefraga and T. fraga Zones (23.0-18.4 Ma). The middle part of the Chichappu Formation is correlative to the D. lauta (15.9-14.9Ma), D. hyaline (14.9-13.1Ma), Crucidenticula nicobarica (13.1-12.9Ma), D. praedimorpha (12.9-11.5Ma), and Thalassiosira yabei Zones (11.5-10.1Ma). The upper part of the Chichappu Formation corresponds to the Rouxia californica (7.6-6.4Ma) and Neodenticula kamtschatica Zones (6.4-3.9/3.5 Ma). The diatom assemblages from the Kotan Formation are correlative with the R. californica and N. kamtschatica Zones, or indicate younger than the age of the N. kamtschatica Zone. These results lead to the following conclusions: 1) The Chichappu Formation includes three different horizons with hiatuses between them. 2) The lower part of the Chichappu Formation is the lower Miocene and indicates older age than that of the Tonokita Formation, which has been considered to underlie it. 3) The middle part of the Chichappu Formation is correlative with the upper part of the Tonokita Formation. These formations are correlative with the middle Miocene. 4) The upper part of the Chichappu Formation is correlative with the Kotan Formation. These formations correlate to the uppermost Miocene to Pliocene. The Miocene stratigraphy in the Akan area should be revised and redefined, because the current lithostratigraphic division is inconsistent with the age of diatom biostratigraphy of this study.