U-Pb age of the tuff bed in the Urahoro Group, eastern Hokkaido, northern Japan

Abstract

The Paleogene Urahoro Group crops out in the Kushiro area of eastern Hokkaido, Japan. The depositional history of the Urahoro Group probably has significant implications for interpreting Paleogene arc-arc collisional processes that are considered to have occurred on Hokkaido Island. However, the depositional age of the Urahoro Group remains uncertain. Here, we present the U-Pb zircon depositional age of an acidic tuff bed of the Tenneru Formation in the middle of the Urahoro Group in the coastal Kushiro area. Using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), we obtained a weighted mean age of zircon grains in the tuff bed of 39.06 ± 0.23 Ma (late Eocene). The tuff bed is stratigraphically higher than the horizon where provenance of gravels in the Urahoro Group have changed. This indicates that the tectonic event that caused uplift of the sedimentary provenance area of eastern Hokkaido occurred in the late Eocene.