Abstract
The Chikapupetsu and Shii-Tokachigawa formations in the Tomuraushi area in central Hokkaido, northern Japan, consist mainly of alternating mudstone and discontinuous sandstone facies previously assigned to the Hidaka Supergroup. Here, we used detrital zircon U-Pb geochronology using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and microfossil occurrence to constrain the depositional ages of the formations. The youngest detrital zircon grains from a turbidite sandstone and an acidic tuff yielded maximum depositional ages of 22.5±0.7 and 19.5±0.1 Ma, respectively. In addition, the occurrence of diatom fossils indicates an early Miocene depositional age. Based on these depositional ages, the Chikapupetsu and Shii-Tokachigawa formations should be excluded from the Hidaka Supergroup, which ranges from Paleocene to early Eocene, and correlated instead with the upper members of the Tsubetsu and Kawakami groups. Previous research on the Tatsuushi and Erimo formations has shown that the Hidaka Belt has been active as a right-lateral strike-slip fault system since the late Oligocene. The Chikapupetsu and Shii-Tokachigawa formations are marine deposits that formed during the early Miocene between deposition of the late Oligocene Tatsuushi and Erimo formations and the middle Miocene Kamishiyubetsu Formation, while the right-lateral strike-slip fault system was active.
