11:00 〜 13:00
[HCG25-P02] 内陸盆地における上部更新統～完新統の堆積シーケンス: 中部日本諏訪盆地の堆積物コアの例
キーワード：後期更新世–完新世、堆積シーケンス、内陸盆地、砕屑物供給、諏訪湖
Introduction: The subsurface geology and sedimentary characteristics of alluvial deposits have been researched to elucidate the underlying sedimentary processes and their controlling factors [1, 2]. Numerous studies have reported the depositional sequence of alluvial deposits in coastal plains. In contrast, the sedimentary systems and depositional sequence of alluvial deposits in inland basins that are distant from the coastline remain unclear. In general, the sedimentary system in inland basins is controlled by equilibrium surface, which is determined by climate change, tectonics, lake level, and sediment input [3]. Regional settings, such as climatic conditions, tectonic settings, and geology in the source region, can result in individual sedimentary systems and depositional sequence in each inland basin. In this study, the sedimentary facies analysis and paleosol descriptions were carried out for the upper Pleistocene–Holocene sediment cores in Suwa Basin, central Japan, to clarify the sedimentary systems and depositional sequence in the inland basin.
Geomorphological setting: Suwa Basin, located at a high altitude in central Japan, is a structural basin formed by left-lateral motion on the Itoigawa-Shizuoka Tectonic Line active fault system [4]. This basin has large catchments including high mountains, such as Mt. Yatsugatake, the highland of Kirigamine, and the northern part of Mts. Akaishi, with heavy sediment supply.
Methods: Two sediment cores (ST2020, SK2021; 30.0 m depth) were drilled at two sites on the southern coast of Lake Suwa. Grain size, sedimentary structures, color, petrographic components, and soil micro-structures in thin sections were visually described. The age-depth model of these cores was constructed using radiocarbon dating (AMS 14C) of plant fragments. Spatial distributions of the sedimentary facies were described considering previous borehole logs [5, 6], to recognize systems tracts.
Sedimentary systems and systems tract: The sedimentary systems in the upper Pleistocene and Holocene sediment cores can be divided into meandering-river (ca. 26.7–13.9 kyr BP), lacustrine (ca. 13.9–5.7 kyr BP), and delta systems (ca. 5.7–2.2 kyr BP), in an ascending order. The boundary between the meandering-river and lacustrine systems corresponds to a transgressive surface (TS) and is dated at ca. 13.9 kyr BP. The upper part of the muddy sediments in the lacustrine system contains abundant diatom fossils and shows the lowest C/N ratios and fine grain size, representing the deepest water facies. The facies horizontally extends approximately 4.5 km from the lakeshore to the south–southeast. Therefore, the top surface of the deepest water facies, showing the boundary between lacustrine and delta systems, corresponds to a maximum flooding surface (MFS) dated at ca. 5.7 kyr BP. The timing of the transgression and the decrease of the sedimentation rate in Suwa Basin approximately coincided with the intensification of chemical weathering [7], the plant succession, and the ascending of the alpine tree line [8] in the catchments. Overall, sediment input, which is affected by the soil and plant covers in the large catchments and climate change, and fault motion on the Itoigawa-Shizuoka Tectonic Line, can be major factors involved in controlling the sedimentary systems and depositional sequences of the upper Pleistocene–Holocene sediments in Suwa Basin.
References: [1] Umitsu, M., 1994, Kokon-shoin Publ., 270p. [2] Tanabe, S., 2019, Jour. Geol. Soc. Japan, 125, 55–72. [3] Shanley, K.W. and McCabe, P.J., 1994, Am. Assoc. Petrol. Geol. Bull., 78, 544–568. [4] Fujimori, T., 1991, Geogr. Rev. Japan Ser. B 64, 665–696. [5] Suwa-ko Lake River-basin Sewerage Office, 1983, An Omnibus of the Soil Survey (The Ground Map in the Suwa Basin). Nagano Prefecture, 92p. [6] Anma, K. et al., 1990, Mem. Geol. Soc. Japan 36, 179–194. [7] Kawano, R. et al., 2021, Ann. Environ. Sci., Shinshu Univ. 43, 53–56. [8] Yoshida, A. et al., 2015, Veg. Hist. Archaeob., 25, 45–55.
Geomorphological setting: Suwa Basin, located at a high altitude in central Japan, is a structural basin formed by left-lateral motion on the Itoigawa-Shizuoka Tectonic Line active fault system [4]. This basin has large catchments including high mountains, such as Mt. Yatsugatake, the highland of Kirigamine, and the northern part of Mts. Akaishi, with heavy sediment supply.
Methods: Two sediment cores (ST2020, SK2021; 30.0 m depth) were drilled at two sites on the southern coast of Lake Suwa. Grain size, sedimentary structures, color, petrographic components, and soil micro-structures in thin sections were visually described. The age-depth model of these cores was constructed using radiocarbon dating (AMS 14C) of plant fragments. Spatial distributions of the sedimentary facies were described considering previous borehole logs [5, 6], to recognize systems tracts.
Sedimentary systems and systems tract: The sedimentary systems in the upper Pleistocene and Holocene sediment cores can be divided into meandering-river (ca. 26.7–13.9 kyr BP), lacustrine (ca. 13.9–5.7 kyr BP), and delta systems (ca. 5.7–2.2 kyr BP), in an ascending order. The boundary between the meandering-river and lacustrine systems corresponds to a transgressive surface (TS) and is dated at ca. 13.9 kyr BP. The upper part of the muddy sediments in the lacustrine system contains abundant diatom fossils and shows the lowest C/N ratios and fine grain size, representing the deepest water facies. The facies horizontally extends approximately 4.5 km from the lakeshore to the south–southeast. Therefore, the top surface of the deepest water facies, showing the boundary between lacustrine and delta systems, corresponds to a maximum flooding surface (MFS) dated at ca. 5.7 kyr BP. The timing of the transgression and the decrease of the sedimentation rate in Suwa Basin approximately coincided with the intensification of chemical weathering [7], the plant succession, and the ascending of the alpine tree line [8] in the catchments. Overall, sediment input, which is affected by the soil and plant covers in the large catchments and climate change, and fault motion on the Itoigawa-Shizuoka Tectonic Line, can be major factors involved in controlling the sedimentary systems and depositional sequences of the upper Pleistocene–Holocene sediments in Suwa Basin.
References: [1] Umitsu, M., 1994, Kokon-shoin Publ., 270p. [2] Tanabe, S., 2019, Jour. Geol. Soc. Japan, 125, 55–72. [3] Shanley, K.W. and McCabe, P.J., 1994, Am. Assoc. Petrol. Geol. Bull., 78, 544–568. [4] Fujimori, T., 1991, Geogr. Rev. Japan Ser. B 64, 665–696. [5] Suwa-ko Lake River-basin Sewerage Office, 1983, An Omnibus of the Soil Survey (The Ground Map in the Suwa Basin). Nagano Prefecture, 92p. [6] Anma, K. et al., 1990, Mem. Geol. Soc. Japan 36, 179–194. [7] Kawano, R. et al., 2021, Ann. Environ. Sci., Shinshu Univ. 43, 53–56. [8] Yoshida, A. et al., 2015, Veg. Hist. Archaeob., 25, 45–55.