9:15 AM - 9:30 AM
[HQR04-02] 240 ka Ata-Th tephra layer found in Tokyo and its geological significance
Keywords:Ata-Th tephra, Drilled core of Akabane-dai, Stratigraphic sequence of Tokyo, Musashino upland, Middle Pleistocene
1. Introduction
Our recent study (Endo et al., 2020) shows that drilled cores and their geological information are extremely useful tools for understanding hidden underground, like densely populated Tokyo where almost no exposure exists nowadays. Here, we propose new geomorphic/geologic division of the Tokyo area together with the underground geology and geological history of Musashino Upland interpreted from the numerous drilled cores. Many of the cores included several tephra layers, and they provided the temporal geological evolution of the area very well. Also, we will discuss on the correlation of two major tephra layers that were found in the several cores, Ata-Torihama ash layer (Ata-Th ash) from Southern Kyushu and TCu-1 (Tm-2) sourced Hakone Caldera.
2. Tephra occurrence and correlation
Two clearly defined tephra layers were found in the NU-AKD-1 core, drilled site at Akabane-dai NE Musashino Upland, at 24 m below the surface, specifically from 24.15 m to 24.35 m. These two layers seem deposited almost no erosional interval, the upper (“24.2 m ash”) consist from very fine ash almost entirely of bubble-wall glass shards, and 15 cm thick lower unit (“24.3 m pumice”) consist from pale yellowish coarse-grained pumice lapillus. Similar ash and pumice were also found at Hongo and Kami-Nakazato site and probable correlation to Ata-Th ash and TCu-1(Tm-2) pumice by using mainly refractive index analyses of the glass shards and minerals (Suzuki et al., 2020).
3. Major element analyses of the tephra layers
According to the major elements chemical analyses of glass shards by EDS and EPMA, a) “24.2 m ash”, ash from Hongo and Kami-Nakazato site were similar to those of the matrix glass of Ata-Torihama pyroclastic flow deposits (240 ka; Machida and Arai, 2004) sourced Ata Caldera, southern Kyushu, b) “24.3 m pumice” , and pumice samples of Hongo and Kami-Nakazato site were similar to TCu-1 (Tm-2) pumice and pyroclastic flows in Oiso Hills, main Hakone tephra type locality 15 km away from Hakone volcano.
The other tephra lenses (pumice) were found southern Tokyo in the NU-SKG-1 core, from the basal gravel layer of the Tokyo Formation of MIS 5.5 transgression. These pumice were chemically similar to Tu-1, Tu-7 to -8 pumice fall layers from the Hakone volcano, however, it is possible to distinguish from TCu-1 (Tm-2) by SiO2 vs. K2O plot.
4. Geological significance
Ata-Th tephra correspond to Tm-4 in Oiso Hills, which is separated by two tephra units above the TCu-1 (Tm-2). As the interval between Tm-4 (= Ata-Th) and TCu-1 (Tm-2) decrease towards to the east, these two units became thinner and more closely overlain each other farther east in Tokyo. This unique unit combination would help for define the broad stratigraphical correlations. Thus, Ata-Th and Tm-2 became one of the most useful marker horizons of 240 ka and/or MIS7 in the middle Pleistocene geological sequences in Musashino Upland areas in Tokyo and the surroundings.
References
Danhara, T. et al. (2010) The Quat. Res., 19, 101-119.
Endo, K. and Uesugi,Y. (1972) The Quat. Res., 11, 15-28.
Endo, K. et al. (2019) The Quat. Res., 58, 353-375.
Endo, K. et al. (2020) 2020 Annu. Meet. Japan Assoc. Quat. Res.
Machida, H. and Arai, F. (2004) Tokyo Univ. Press.
Nagahashi ,Y. et al. (2004) The Quat. Res., 43, 15-35.
Naya, T. et al. (2020) Jour. Geol. Soc. Japan, 126, 575-587.
Oka, S. (1991) Bull. Geol. Surv. Japan, 42, 553-653.
Suzuki M. et al. (2020) 2020 Annu. Meet. Japan Assoc. Quat. Res.
Uemura, K. et al. (2020) Proc. Inst. Nat. Sci., Nihon Univ. 55, 155-164.
Uesugi, Y. (1976) Bull.Assoc. Kanto Quat. Res., 3, 28-38.
Our recent study (Endo et al., 2020) shows that drilled cores and their geological information are extremely useful tools for understanding hidden underground, like densely populated Tokyo where almost no exposure exists nowadays. Here, we propose new geomorphic/geologic division of the Tokyo area together with the underground geology and geological history of Musashino Upland interpreted from the numerous drilled cores. Many of the cores included several tephra layers, and they provided the temporal geological evolution of the area very well. Also, we will discuss on the correlation of two major tephra layers that were found in the several cores, Ata-Torihama ash layer (Ata-Th ash) from Southern Kyushu and TCu-1 (Tm-2) sourced Hakone Caldera.
2. Tephra occurrence and correlation
Two clearly defined tephra layers were found in the NU-AKD-1 core, drilled site at Akabane-dai NE Musashino Upland, at 24 m below the surface, specifically from 24.15 m to 24.35 m. These two layers seem deposited almost no erosional interval, the upper (“24.2 m ash”) consist from very fine ash almost entirely of bubble-wall glass shards, and 15 cm thick lower unit (“24.3 m pumice”) consist from pale yellowish coarse-grained pumice lapillus. Similar ash and pumice were also found at Hongo and Kami-Nakazato site and probable correlation to Ata-Th ash and TCu-1(Tm-2) pumice by using mainly refractive index analyses of the glass shards and minerals (Suzuki et al., 2020).
3. Major element analyses of the tephra layers
According to the major elements chemical analyses of glass shards by EDS and EPMA, a) “24.2 m ash”, ash from Hongo and Kami-Nakazato site were similar to those of the matrix glass of Ata-Torihama pyroclastic flow deposits (240 ka; Machida and Arai, 2004) sourced Ata Caldera, southern Kyushu, b) “24.3 m pumice” , and pumice samples of Hongo and Kami-Nakazato site were similar to TCu-1 (Tm-2) pumice and pyroclastic flows in Oiso Hills, main Hakone tephra type locality 15 km away from Hakone volcano.
The other tephra lenses (pumice) were found southern Tokyo in the NU-SKG-1 core, from the basal gravel layer of the Tokyo Formation of MIS 5.5 transgression. These pumice were chemically similar to Tu-1, Tu-7 to -8 pumice fall layers from the Hakone volcano, however, it is possible to distinguish from TCu-1 (Tm-2) by SiO2 vs. K2O plot.
4. Geological significance
Ata-Th tephra correspond to Tm-4 in Oiso Hills, which is separated by two tephra units above the TCu-1 (Tm-2). As the interval between Tm-4 (= Ata-Th) and TCu-1 (Tm-2) decrease towards to the east, these two units became thinner and more closely overlain each other farther east in Tokyo. This unique unit combination would help for define the broad stratigraphical correlations. Thus, Ata-Th and Tm-2 became one of the most useful marker horizons of 240 ka and/or MIS7 in the middle Pleistocene geological sequences in Musashino Upland areas in Tokyo and the surroundings.
References
Danhara, T. et al. (2010) The Quat. Res., 19, 101-119.
Endo, K. and Uesugi,Y. (1972) The Quat. Res., 11, 15-28.
Endo, K. et al. (2019) The Quat. Res., 58, 353-375.
Endo, K. et al. (2020) 2020 Annu. Meet. Japan Assoc. Quat. Res.
Machida, H. and Arai, F. (2004) Tokyo Univ. Press.
Nagahashi ,Y. et al. (2004) The Quat. Res., 43, 15-35.
Naya, T. et al. (2020) Jour. Geol. Soc. Japan, 126, 575-587.
Oka, S. (1991) Bull. Geol. Surv. Japan, 42, 553-653.
Suzuki M. et al. (2020) 2020 Annu. Meet. Japan Assoc. Quat. Res.
Uemura, K. et al. (2020) Proc. Inst. Nat. Sci., Nihon Univ. 55, 155-164.
Uesugi, Y. (1976) Bull.Assoc. Kanto Quat. Res., 3, 28-38.