1:45 PM - 3:15 PM
[HQR03-P04] Tama Hills replaced by Musashino Uplands due to erosion of the Paleo Tama River - Focusing on MIS6 and after -
Keywords:Tama Hills, Musashino Uplands, Erosion of the Paleo Tama River, Buried Valley, MIS 6 - 5
The Tama Hills are a hilly area mainly composed of the Kazusa Formation. The northeast side is bordered by the Tama River, and the Musashino Uplands extend on the other side of the Tama River.
Recently, a new topographic classification of the Musashino Uplands were conducted (Endo et al., 2019). In the process and recent studies have revealed the existence of buried valleys carving the Kazusa Formation in the subsurface of the southeastern part of the Musashino Uplands.
Although marine strata were known to be deposited in the Setagaya Buried Valley (Nakazawa et al., 2019; Naya et al., 2021; Uemura et al., 2020), shell fragments and benthic foraminifera from the inner bay depths were also found in a clay layer around 32.5 m elevation in a drilling core collected at Shinkawa in southern Mitaka City, located northwest of this valley (Endo et al., 2023). After tracing the valley topography again, including neighboring columnar map data, we found that it roughly follows from Todoroki to Mitaka, and thus the authors changed the name to Mitaka - Setagaya Buried Valley. This valley becomes narrower and shallower toward Mitaka.
The Tokyo Formation, which fills the Mitaka-Setagaya valley, is also called the Setagaya Formation (Endo et al., 1996), and is known to be approximately 120,000 years old (MIS 5.5) marine mud layer.
On the other hand, the Yoyogi-Takanawa buried valley has been reported that the marine sediments and basal gravels corresponding to the MIS 5.5 transgressive stage and the early transgressive stage were deposited in a valley-filling manner while narrowing two erosional surfaces (Nakazawa et al., 2020). Following this valley in an upstream, we found that the sediments filling the valley gradually became mainly basal gravels while continuing through Nakano to the Musashiseki area located just north of Mitaka City. Since this series of gravel beds are considered to be fan/river gravels, the Yoyogi-Takanawa buried valley is presumed to have been fed by the Paleo-Tama River around MIS 6-5.5.
Cross-sectional study of the Yoyogi area, which crosses this valley from east to west, revealed that on the west side of the valley, there are two gravel beds with sand and mud beds in between. This is considered to be pre-MIS6 sediments that escaped erosion during MIS6, and thus the Paleo-Tama River may have been flowing through this valley since at least MIS8.
Thus, there is a big difference between the abovementioned buried valleys: in the southern part of Mitaka City, this valley is filled by soft mud layers, while in the northern part, this is filled by fan-shaped gravels. This difference may be attributed to the presence or absence of upstream rivers such as the Tama River that supply fan-shaped gravels. Similar differences can be seen in the relationship between the present-day Tama River and rivers originating in the Tama Hills, such as the Tsurumi River. The present Tama River flows along the northeastern edge of the Tama Hills, and because the Tama Hills act as a barrier, the flow of the Tama River does not extend into the upper reaches of the Tsurumi River.
From the above, it is possible that the northeastern margin of the Tama Hills at least in MIS 6-5 extended about 8 km northeastward to include Mitaka City. Furthermore, this extended area was eroded by the Musashino Uplands during the 90,000 years after MIS4.
References
Endo et al., (2019) New division of landforms in Musashino Uplands, Tokyo. The Quaternary Research, 58 (6), 353-375.
Endo et al., (2023) Middle to Late Pleistocene Geology and Paleoenvironment Based on Drilling Cores in the Musashino Upland, Tokyo, Japan: Chiefly on the Index Cores. Proceedings of the Institute of Natural Sciences, Nihon University, 58, 153-183.
Endo, Suginaka (2021) What can we learn from New division of landforms in Musashino Uplands?. Tama-no-Ayumi, 184, 92-99.
Endo et al., (1996) Subsurface Geology in Wards District, Tokyo.
Nakazawa et al., (2019) Stratigraphy, distribution patterns, and ground motion characteristics of the Pleistocene Setagaya and Tokyo formations beneath the Musashino Upland, Setagaya, Tokyo, central Japan. J. Geol. Soc. Japan, 125, 5, 367–385.
Nakazawa et al., (2020) Re-examination of the stratigraphy of the Tokyo Formation at the type core section in the Yoyogi Park, Tokyo, central Japan. Bulletin of the Geological Survey of Japan, 71 (1), 19–32.
Naya et al., (2021) Explanatory Text of the Urban Geological Map of Central Tokyo (Special Wards Area). Geological Survey of Japan, AIST, 15-38.
Uemura et al., (2020) Tephras from NU-SKG-1 Core of the Tokyo (Setagaya) Formation in Setagaya, Tokyo. Proceedings of the Institute of Natural Sciences, Nihon University, 55, 155-164.
Recently, a new topographic classification of the Musashino Uplands were conducted (Endo et al., 2019). In the process and recent studies have revealed the existence of buried valleys carving the Kazusa Formation in the subsurface of the southeastern part of the Musashino Uplands.
Although marine strata were known to be deposited in the Setagaya Buried Valley (Nakazawa et al., 2019; Naya et al., 2021; Uemura et al., 2020), shell fragments and benthic foraminifera from the inner bay depths were also found in a clay layer around 32.5 m elevation in a drilling core collected at Shinkawa in southern Mitaka City, located northwest of this valley (Endo et al., 2023). After tracing the valley topography again, including neighboring columnar map data, we found that it roughly follows from Todoroki to Mitaka, and thus the authors changed the name to Mitaka - Setagaya Buried Valley. This valley becomes narrower and shallower toward Mitaka.
The Tokyo Formation, which fills the Mitaka-Setagaya valley, is also called the Setagaya Formation (Endo et al., 1996), and is known to be approximately 120,000 years old (MIS 5.5) marine mud layer.
On the other hand, the Yoyogi-Takanawa buried valley has been reported that the marine sediments and basal gravels corresponding to the MIS 5.5 transgressive stage and the early transgressive stage were deposited in a valley-filling manner while narrowing two erosional surfaces (Nakazawa et al., 2020). Following this valley in an upstream, we found that the sediments filling the valley gradually became mainly basal gravels while continuing through Nakano to the Musashiseki area located just north of Mitaka City. Since this series of gravel beds are considered to be fan/river gravels, the Yoyogi-Takanawa buried valley is presumed to have been fed by the Paleo-Tama River around MIS 6-5.5.
Cross-sectional study of the Yoyogi area, which crosses this valley from east to west, revealed that on the west side of the valley, there are two gravel beds with sand and mud beds in between. This is considered to be pre-MIS6 sediments that escaped erosion during MIS6, and thus the Paleo-Tama River may have been flowing through this valley since at least MIS8.
Thus, there is a big difference between the abovementioned buried valleys: in the southern part of Mitaka City, this valley is filled by soft mud layers, while in the northern part, this is filled by fan-shaped gravels. This difference may be attributed to the presence or absence of upstream rivers such as the Tama River that supply fan-shaped gravels. Similar differences can be seen in the relationship between the present-day Tama River and rivers originating in the Tama Hills, such as the Tsurumi River. The present Tama River flows along the northeastern edge of the Tama Hills, and because the Tama Hills act as a barrier, the flow of the Tama River does not extend into the upper reaches of the Tsurumi River.
From the above, it is possible that the northeastern margin of the Tama Hills at least in MIS 6-5 extended about 8 km northeastward to include Mitaka City. Furthermore, this extended area was eroded by the Musashino Uplands during the 90,000 years after MIS4.
References
Endo et al., (2019) New division of landforms in Musashino Uplands, Tokyo. The Quaternary Research, 58 (6), 353-375.
Endo et al., (2023) Middle to Late Pleistocene Geology and Paleoenvironment Based on Drilling Cores in the Musashino Upland, Tokyo, Japan: Chiefly on the Index Cores. Proceedings of the Institute of Natural Sciences, Nihon University, 58, 153-183.
Endo, Suginaka (2021) What can we learn from New division of landforms in Musashino Uplands?. Tama-no-Ayumi, 184, 92-99.
Endo et al., (1996) Subsurface Geology in Wards District, Tokyo.
Nakazawa et al., (2019) Stratigraphy, distribution patterns, and ground motion characteristics of the Pleistocene Setagaya and Tokyo formations beneath the Musashino Upland, Setagaya, Tokyo, central Japan. J. Geol. Soc. Japan, 125, 5, 367–385.
Nakazawa et al., (2020) Re-examination of the stratigraphy of the Tokyo Formation at the type core section in the Yoyogi Park, Tokyo, central Japan. Bulletin of the Geological Survey of Japan, 71 (1), 19–32.
Naya et al., (2021) Explanatory Text of the Urban Geological Map of Central Tokyo (Special Wards Area). Geological Survey of Japan, AIST, 15-38.
Uemura et al., (2020) Tephras from NU-SKG-1 Core of the Tokyo (Setagaya) Formation in Setagaya, Tokyo. Proceedings of the Institute of Natural Sciences, Nihon University, 55, 155-164.