Abstract
We carried out fossil diatom analysis and fission-track dating of the upper Koetoi Formation, which is distributed in the western limb of the Sarobetsu anticline in the western part of the Horonobe area, northern Hokkaido.
A mudstone sample collected from the upper Koetoi Formation contains abundant autochthonous diatoms which can clearly be correlated with the Neodenticula koizumii Zone. The assemblage includes N. seminae, which has a first appearance age of ~2.4 Ma.
A zircon FT age obtained from an interbeded volcanic ash is 2.3±0.1 Ma. This FT result gives a precise depositional age for the upper Koetoi Formation in the area surveyed, and is consistent with the results of the diatom fossil analysis. These results indicate that the depositional age of the upper Koetoi Formation is about 2.3 Ma.
Considering previous chronological work and our results, the age of the boundary between the Koetoi and the Yuchi Formations in the western part of the Horonobe area is younger than about 2.3 Ma, which is at least 1 million years younger than that of the same boundary in the eastern part. This implies westward depositional migration of the Koetoi and Yuchi Formations, and a diachronous relationship between the eastern and the western parts. These geochronological constraints from diatom fossils and FT data further suggest that the period of incipient active tectonism, which formed complex anticlinal and synclinal structures in this area is younger than about 2.3 Ma.
The volcanic ash layer in the upper Koetoi Formation contains characteristic zircon crystals with unusually high uranium contents (>1000 ppm), and may become a useful marker bed in this area.
