[H-QR04] Quaternary, Diachronic dynamics of human-environment interactions

[HQR04-02] Trace of Lower Pleistocene Kazusa Group from the Tama Hills to the south part of the Musashino Uplands, NE Japan

*Takehiko Suzuki¹, Junichi Sato¹, Tatsuki Watanabe¹, Kuniki Kokubun², Shinichi Kawashima², Masabumi Kawai², Toshio Nakayama² (1.Faculty of Urban Environmental Sciences, Tokyo Metropolitan University, 2.Civil Engineering Support and Training Center, Tokyo Metropolitan Government)

Keywords:Musashino Uplands, Kazusa Group, tephra

The Kazusa Group distributed in the Kanto Plain is a key to reconstruct the development history of this plain. The Tama Hills and the Musashino Uplands in the west part of the Kanto Plain is located in the area between the east part of the Kanto Plain where the Kazusa Trough stretched, and the Kanto Mountains. This implies that these areas are significant field area to construct development histories of emergence of the early stage of the Kanto Plains and the Kanto Mountains. The Kazusa Group in the west part of the Tama Hills comprises alternations of gravel, mud and sand reflecting glacial eustacy sea level (Takano, 1994), suggesting shelf environment. On the other hand, the Kazusa Group on the area from the east part of the Tama Hills to the east part of the Musashino Uplands partly shows continuous marine environment in this area. Tephras in the Kazusa Group have been correlated to those in Choshi and the Boso Penisula, providing datum planes (Suzuki and Murata, 2011).
The Kazusa Group in the west part of the Tama Hills consist of the Oyabe, Komaiya, Fukushima, Hirayama, Oyamada, Renkoji and Inagi Formations in ascending order (Ueki et al., 2013). They generally dip 1-2°E to ENE, suggesting they occur under the Musashino Uplands. However, depths of each formation under the uplands are unclear. Under the Musashino Uplands, the Higashi-kurume and Toneri Formations composed of gravel and sand in the Kazusa Group occur (Endo, 1978). Relationship of these formation with sediments cropped out in the west part of the Tama Hills has not yet well. This study discusses this relationship tephras in six boring cores obtained in the south part of the Musashino Uplands, emphasizing description of Fuchu and Chofu Cores.
All boring cores including the Fuchu and Chofu Cores observed have been drilled by Tokyo Metropolitan Government, named the Tachikawa, Fuchu, Koganei, Koganei South, Chofu and Mitaka Cores (west to east) due to site names for drilling. Tephras in all horizon have been carefully observed for Fuchu and Chofu Cores, exploring tephra identification. In Fuchu Core, Suzuki et al. (2021) reported three tephras, that is, Sgn-Kd44 (2.0-1.8 Ma), Nyg (1.75 Ma), Ebs-Fkd (1.70 Ma). In addition, we identified Yarimizu in the Hirayama Formation, Om-SK110 (= HU2; 1.6 Ma) and Ob4b-1 (= HU1) in the Oyamada Formation in the Renkoji Formaion, TN in the Renkoji Formaion, KK in the Inagi Formation. Om-SK110 was also identified in the Chofu Core. In other cores, Om-SK110 and Ob4b-1 have been broadly identified with exception of the former in the Tachikawa Core and the latter in the Koganei Core.
In the Koganei, Fuchu and Tachikawa Cores, Om-SK110 accompanies with gravel bed in the stratigraphic position just below Om-SK110. This gravel bed was named the Fuchu Sand and Gravel Bed (FSGB), and it divides upper and lower parts of the Higashi-kurume Formation. In the Fuchu Core, Yarimizu was detected within the lower part of the Higashi-kurume Formation below FSGB, and Om-SK110, TN and KK within the upper part of the Higashi-kurume Formation above FSGB. These positions show the Hirayama Formation can be correlated to the lower part of the Higashi-kurume Formation, and the Oyamada, Renkoji and Inagi Formations to the upper part of the Higashi-kurume Formation. FSGB was not detected in the Chofu and Mitaka Cores above Om-SK110. This indicated the east area including these sites were under marine environment during the low-stand of sea level causing deposition FSGB.