Abstract
Two successive diatom zones, the Thalassiosira oestrupii Zone and the Neodenticula koizumii-Neodenticula kamtschatica Zone, are recognized, on the basis of 31 geologic samples collected from the Embetsu, Koetoi and Yuchi Formations, distributed in the Rubeshube River, the Kaminukanan River and the western part of Dannoppu in northern Hokkaido. It was clarified that the chronological gap up to one million years proposed by Akiba (2000) does not exist between the Embetsu Formation / Koetoi Formation and the Yuchi Formation in the surveyed area by the following reasons ; 1) existence of those two diatom zones, 2) fission track dating (3.8±0.4 Ma), 3) gradual change of lithofacies from siltstone to fine sandstone in ascending order, and 4) identification of almost unchanged geologic structure. Increasing numbers of fresh water species and Miocene extinct species in upper part of the Yuchi Formation indicate the increase of the supply of terrigenous sediments and the shallowing of the sedimentary basin.
