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 .. 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.

Collisional bending of the western Paleo‐Kuril Arc deduced from paleomagnetic analysis and U–Pb age determination

“…Radiometric dating conducted in this study indicated that tuffaceous layers in the Urahoro Group were deposited at about 40 Ma. The calculated depositional age of Ht1 and Ht2 were (39.54 ±0.17) Ma and (40.8 ±1.1) Ma, respectively, and these are concordant to the depositional age of the tuff layer Tn1 (39.06 ±0.23) Ma in the Tenneru Formation (Figure ; Katagiri et al, ). The depositional ages of the layers were calculated as the weighted mean values of the radiometric ages of zircons except for the grains that were considered to be detrital in origin.…”

“…Stratigraphic relationships among the tuffaceous layers with Tn1 and layers with magnetic polarities are shown in Figure . The normal polarities from Sites 1501, 1506, 1507, 1608, 1612, 1616, and 1617 were correlated to the polarity chron C18n.1n, ranging from 39.6 Ma to 38.6 Ma (Ogg, ; Figures 3 and 11) because the Tn1 layer with an age of (39.06 ±0.23) Ma was stratigraphically intercalated in the intervals of these sites (Katagiri et al, ; Ogg, ; Figures and ). The lower reversed polarities of Site 1615 were correlated with C19r, C18r, or C18n.1r based on the ages of Ht1 (39.54 ±0.27) Ma and Ht2 (40.8 ±1.1) Ma.…”

“…When two or more concordant data were obtained from a grain and there was no significant difference between the data, the datum with the smallest error range was selected as the representative age of the grain. Error ranges of individual grains were determined according to Sakata, Hattori, and Iwano (2014) and Katagiri et al (2016).…”

“…Grains showing very old ages (1 746 Ma and 1 880 Ma) were clearly different from the other grains in roundness of grain outlines and colors in CL images (Figure 9). (Figure 10; Katagiri et al, 2016). The depositional ages of the layers were calculated as the weighted mean values of the radiometric ages of zircons except for the grains that were considered to be detrital in origin.…”

“…K. Kimura and Tsuji () demonstrated that the depositional age of the bentonite tuff bed intercalated in the Rushin Formation is 38.0 Ma, based on a determination of the zircon fission track age. Recently, Katagiri, Naruse, Hirata, and Hattori () reported the depositional age of an acidic tuff bed intercalated in the Tenneru Formation as (39.06 ±0.23) Ma by using the U–Pb radiometric dating method.…”