New age constraints and tectonic significance of the late Oligocene marine biosiliceous mudstone in the Hidaka Belt, northeastern Hokkaido, Japan

The Tatsuushi Formation in the Kitami-Monbetsu area of northeastern Hokkaido, northern Japan, consists mainly of siliceous mudstone and has previously been considered part of the Hidaka Supergroup along with the adjacent Uenkotan and Rurochi formations. In this study, the depositional age of the Tatsuushi Formation was constrained using two methods. A laser ablation-inductively coupled plasma-mass spectrometry U-Pb age of 26.9 ± 0.2 Ma was obtained for the maximum depositional age of the formation based on the minimum age of detrital zircon grains from turbiditic sandstone. In addition, the presence of dinoflagellate fossils in the formation indicates a late Oligocene (Williamsidinium sp. C and Spinidinium? sp. A zones) depositional age for the formation. Given that the depositional age of the Hidaka Supergroup is Paleocene to early Eocene, the Tatsuushi Formation should be excluded from the Hidaka Supergroup. Instead, it can be correlated with the lower units of the Tsubetsu and Kawakami groups on the basis of their depositional age and lithology. Similar siliceous mudstone crops out in the southeastern part of the Tatsuushi Formation across the Abashiri tectonic line, which has been an active right-lateral strike-slip fault since the late Oligocene. These three correlated bodies of rock may owe their origin to upwelling of nutrient-rich deep water from the Sea of Okhotsk after the opening of the Kuril basin.