13:45 〜 14:00
[HQR04-06] Formation process of lake sediment in Lake Tazawa, Akita, Japan
キーワード：田沢湖、湖底堆積物、シルト、第四紀
Lake Tazawa, located in the eastern part of Akita, is a caldera lake formed 1.7 Ma (Kano et al., 2008 & 2020). This lake has the largest depth in Japan (432 m in depth) and has a tray-like shape as a cross-section. The source area of lake sediments is limited only to the inner area of a caldera rim (catchment area) of the lake because there are no natural rivers flowing into the lake from outside the caldera rim. The geology of the catchment area is composed of basaltic, andesitic, and rhyolitic volcanic rocks (Usuda et al., 1985). Thus, the sediments of the lake are thought to be mainly composed of particles derived from these volcanic rocks in the catchment area. Comparison of the chemical composition of minerals in the lake sediments and the rocks in the catchment area is possible to understand the origin and formation process of the lake sediments. In this lake, four sediment cores were collected in 2015 and the age of sediments at the bottom of the cores was estimated to be around 7,000 BP (Matsuoka, 2016). In this study, the TZW15-4 core sample, taken at the point near the steep slope of the lake in the southwest part, was mainly analyzed. In addition, the TZW15-3 core sample, obtained at the deepest part of the lake, was also analyzed for comparison with TZW15-4. This study aims to clarify the sources of sediment particles in the silt and sand layers of the sediment and to consider the formation process of silt and sand layers in Lake Tazawa.
The geology in the catchment area mainly consists of (1) andesite volcanic rocks of the Tamagawa Formation, (2) rhyolitic volcanic products of Yoroihata caldera, (3) rhyolitic volcanic rocks (Kurasawa-yama welded tuff and volcanic products of Lake Tazawa caldera), (4) Mt. Omori and Mt. In’nai-dake andesite lava and (5) Mt. Takabachi basaltic lava.
The sediments in TZW15-4 are mainly composed of silt, sand, and diatom layers. The chemical composition of plagioclase in the silt layers shows a wide range of composition, which covers all plagioclase composition of basaltic to rhyolitic volcanic rocks in the catchment area. This result shows that the particles in the silt layers were derived from various volcanic rocks in the catchment area. The bulk chemical composition of major elements except SiO2 component of TZW15-4 shows that silt layers have similar chemical composition through the core (from lower to upper layers). This signature is also observed in the sediments of TZW15-3. These results suggest that the silt layers of the lake have similar chemical composition over a wide area. That is, the proto-materials of the particles in the silt layers (suspended materials that were supplied from the various parts of the catchment area into the lake) were well mixed and obtained a homogeneous chemical composition in the deposition process from the surface to the bottom of the lake. This sedimentological interpretation is supported by the hydrological data that shows homogeneous water quality throughout the lake (Katamura et al., 2021).
On the other hand, the chemical composition of plagioclase and the bulk chemistry of the sand layers indicate that the source of the sand layer might be a specific formation. The chemical compositions of plagioclase in the four sand layers are similar to the chemical compositions of plagioclase in the volcanic products of Yoroihata caldera that are distributed on the southwestern shore of the lake. Comparison of the compositions of major elements and rare earth elements of the sand layers and the volcanic products of Yoroihata caldera shows that the chemical compositions of the sand layers are similar to those of volcanic products of Yoroihata caldera.
In conclusion, the particles in the silt layers were supplied from the various volcanic rocks inside of the caldera rim with the well-mixed process in the lake water. On the other hand, the particles in the sand layers were supplied from a specific volcanic rock as an event such as turbidite.
The geology in the catchment area mainly consists of (1) andesite volcanic rocks of the Tamagawa Formation, (2) rhyolitic volcanic products of Yoroihata caldera, (3) rhyolitic volcanic rocks (Kurasawa-yama welded tuff and volcanic products of Lake Tazawa caldera), (4) Mt. Omori and Mt. In’nai-dake andesite lava and (5) Mt. Takabachi basaltic lava.
The sediments in TZW15-4 are mainly composed of silt, sand, and diatom layers. The chemical composition of plagioclase in the silt layers shows a wide range of composition, which covers all plagioclase composition of basaltic to rhyolitic volcanic rocks in the catchment area. This result shows that the particles in the silt layers were derived from various volcanic rocks in the catchment area. The bulk chemical composition of major elements except SiO2 component of TZW15-4 shows that silt layers have similar chemical composition through the core (from lower to upper layers). This signature is also observed in the sediments of TZW15-3. These results suggest that the silt layers of the lake have similar chemical composition over a wide area. That is, the proto-materials of the particles in the silt layers (suspended materials that were supplied from the various parts of the catchment area into the lake) were well mixed and obtained a homogeneous chemical composition in the deposition process from the surface to the bottom of the lake. This sedimentological interpretation is supported by the hydrological data that shows homogeneous water quality throughout the lake (Katamura et al., 2021).
On the other hand, the chemical composition of plagioclase and the bulk chemistry of the sand layers indicate that the source of the sand layer might be a specific formation. The chemical compositions of plagioclase in the four sand layers are similar to the chemical compositions of plagioclase in the volcanic products of Yoroihata caldera that are distributed on the southwestern shore of the lake. Comparison of the compositions of major elements and rare earth elements of the sand layers and the volcanic products of Yoroihata caldera shows that the chemical compositions of the sand layers are similar to those of volcanic products of Yoroihata caldera.
In conclusion, the particles in the silt layers were supplied from the various volcanic rocks inside of the caldera rim with the well-mixed process in the lake water. On the other hand, the particles in the sand layers were supplied from a specific volcanic rock as an event such as turbidite.