10:00 AM - 10:15 AM
[SGL23-05] Provenance of sandstones from the Eocene Urahoro Group, Hokkaido, Northern Japan: Tectonic Implications
Keywords:Eocene Urahoro Group, Provenance, Arkosic sandstone, Collision of the Proto-Kuril arc against Honshu arc, East Hokkaido magnetic anomaly belt, 40 Ma igneous activity in Hokkaido
In the southeastern Hokkaido, the Late Cretaceous to Oligocene sediments are widely distributed, and the Eocene sediments distributed in the Shiranuka hill is called the Urahoro Group. We have recently been investigating the Shitakara and Shakubetsu Formations of the Urahoro Group distributed to the east of the Urahoro-town. We have found deformation bands in the Shakubetsu Formation constituting a flexure, and analyzed microstructures in them (Kaji and Takeshita, this meeting, session S-IT19). On the other hand, we noted that sandstones from the Urahoro Group are characteristically rich in biotite and K-feldspar detrital grains, and discussed the provenance based on the mineral mode analysis of these sandstones. In this presentation, we will discuss the location of the provenance and its composition together with recently acquired U-Pb ages of detrital zircon grains from the Urahoro Group, and their implications.
The Urahoro Group is widely distributed to the east of Kushiro-city and Shiranuka hill to the west, which is covered by the Kushiro marsh in the middle. While the Urahoro Group shows moderately dipping structures with E-W strike in the east, the one distributed in the Shiranuka hill constitutes a fold and thrust belt trending N-S (Suzuki, 1958). This structure is inferred to have been formed due to the collision of the proto-Kuril arc against Honshu arc (Kimura and Tamaki, 1985). The Urahoro Group, which yields coal beds, consists of the Beppo, Harutori, Tenneru, Yuubetsu, Shitakara, and Shakubetsu Formations in ascending order (Sasa, 1940). Although the age of the Urahoro Group has been considered as the Late Eocene based on the fossils of shell and plant (Sasa, 1940) and foraminifera (Kaiho, 1983), Katagiri obtained the U-Pb age of 39.06±0.23 Ma in zircons from tuff beds of the Tenneru Formation of the Kushiro district. Further, Katagiri et al. (2020) obtained the youngest U-Pb ages of 39.54 ±0.27 Ma and 40.8 ±1.1 Ma in zircons in two samples from the Harutori Formation of the Shiranuka hill district, and confirmed that the sedimentary age of the Urahoro Group is in the Middle Eocence. However, it remains unknown where the igneous activity at c. 40 Ma which provided detrital grains for the Urahoro Group occurred. On the other hand, it has been accepted that sediments of the latest Ctretaceous to earliest Paleogene Kushiro Group which is unconformably overlain by the Urahoro Group are provided from the proto-Kuril arc, which was perhaps situated at the distribution area of the Kushiro Group. If this is true, it might be considered that the igneous activity at c. 40 Ma also occurred in the proto-Kuril arc. However, not only there is no evidence for this hypothesis, but al so it contradicts the paleocurrent direction. To resolve this discrepancy, since it is important to clarify the composition of the provenance of the Urahoro Group, we have carried out the mineral mode analysis of sandstone from one for each sample of the Shitakara and Shakubetsu Formations. Although the analyses are underway, it is safely concluded that the sandstones are a typical arkosic sandstone, and the provenance mostly consists of granite sensu stricto. Further, since the sandstone contain lithic fragments of rhyolite, it is inferred that the provenance could consist of a plutonic-volcanic rock complex of granite-rhyolite. Furthermore, biotite and K-feldspar grains are extremely fresh, and also granite conglomerates occur in the Urahoro Group, the provenance was located not far from the site of sedimentation. In this respect, one important finding is that it has been inferred that the detrital grains could have been provided from the ESE direction based on paleocurrent analysis. Therefore, the continental shelf off the Kushiro-city is the possible provenance, where the magnetic anomaly belt called “the East Hokkaido belt” is widely distributed. Nanayama et al. (1994) inferred that this magnetic anomaly belt is an extension of the Nikoro Group ophiolite of the Tokoro belt, based on the fact that chromian spinel was provided from it.
However, since the origin of magnetic anomaly could be equally considered as granitic rocks as the one in the “Ishikari-Kitakami magnetic anomaly belt”(Japanese nuclear fuel research, 2016), the detrital grains could have been provided from this belt mostly consisting of granitic rocks. Although the tectonics of large-scale felsic igneous activity in the continental shelf off the Kushiro city remains unknown, this problem could be argued in relation to c. 40 Ma granitic rocks (Maeda et al., 1986; Kawakami et al., 2006), and also recently found c. 40 Ma metamorphic ages (Takahashi, 2021, accepted) in the Hidaka belt.
The Urahoro Group is widely distributed to the east of Kushiro-city and Shiranuka hill to the west, which is covered by the Kushiro marsh in the middle. While the Urahoro Group shows moderately dipping structures with E-W strike in the east, the one distributed in the Shiranuka hill constitutes a fold and thrust belt trending N-S (Suzuki, 1958). This structure is inferred to have been formed due to the collision of the proto-Kuril arc against Honshu arc (Kimura and Tamaki, 1985). The Urahoro Group, which yields coal beds, consists of the Beppo, Harutori, Tenneru, Yuubetsu, Shitakara, and Shakubetsu Formations in ascending order (Sasa, 1940). Although the age of the Urahoro Group has been considered as the Late Eocene based on the fossils of shell and plant (Sasa, 1940) and foraminifera (Kaiho, 1983), Katagiri obtained the U-Pb age of 39.06±0.23 Ma in zircons from tuff beds of the Tenneru Formation of the Kushiro district. Further, Katagiri et al. (2020) obtained the youngest U-Pb ages of 39.54 ±0.27 Ma and 40.8 ±1.1 Ma in zircons in two samples from the Harutori Formation of the Shiranuka hill district, and confirmed that the sedimentary age of the Urahoro Group is in the Middle Eocence. However, it remains unknown where the igneous activity at c. 40 Ma which provided detrital grains for the Urahoro Group occurred. On the other hand, it has been accepted that sediments of the latest Ctretaceous to earliest Paleogene Kushiro Group which is unconformably overlain by the Urahoro Group are provided from the proto-Kuril arc, which was perhaps situated at the distribution area of the Kushiro Group. If this is true, it might be considered that the igneous activity at c. 40 Ma also occurred in the proto-Kuril arc. However, not only there is no evidence for this hypothesis, but al so it contradicts the paleocurrent direction. To resolve this discrepancy, since it is important to clarify the composition of the provenance of the Urahoro Group, we have carried out the mineral mode analysis of sandstone from one for each sample of the Shitakara and Shakubetsu Formations. Although the analyses are underway, it is safely concluded that the sandstones are a typical arkosic sandstone, and the provenance mostly consists of granite sensu stricto. Further, since the sandstone contain lithic fragments of rhyolite, it is inferred that the provenance could consist of a plutonic-volcanic rock complex of granite-rhyolite. Furthermore, biotite and K-feldspar grains are extremely fresh, and also granite conglomerates occur in the Urahoro Group, the provenance was located not far from the site of sedimentation. In this respect, one important finding is that it has been inferred that the detrital grains could have been provided from the ESE direction based on paleocurrent analysis. Therefore, the continental shelf off the Kushiro-city is the possible provenance, where the magnetic anomaly belt called “the East Hokkaido belt” is widely distributed. Nanayama et al. (1994) inferred that this magnetic anomaly belt is an extension of the Nikoro Group ophiolite of the Tokoro belt, based on the fact that chromian spinel was provided from it.
However, since the origin of magnetic anomaly could be equally considered as granitic rocks as the one in the “Ishikari-Kitakami magnetic anomaly belt”(Japanese nuclear fuel research, 2016), the detrital grains could have been provided from this belt mostly consisting of granitic rocks. Although the tectonics of large-scale felsic igneous activity in the continental shelf off the Kushiro city remains unknown, this problem could be argued in relation to c. 40 Ma granitic rocks (Maeda et al., 1986; Kawakami et al., 2006), and also recently found c. 40 Ma metamorphic ages (Takahashi, 2021, accepted) in the Hidaka belt.