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[HQR04-01] Distribution and classification of buried flat surfaces beneath the post-LGM deposits in central Tokyo
Keywords:Tokyo Lowland, post-LGM deposits, buried flat surfaces, basal conglomerates, borehole logs, 3D geomodelling
The Tokyo Lowland in the eastern part of Tokyo metropolitan area is underlain by the thick post-Last Glacial Maximum (LGM) deposits. Many studies have been conducted on the post-LGM deposits in the Tokyo Lowland since the 1923 Kanto earthquake. Based on those previous works and borehole logs of the Civil Engineering Support and Training Center, Tokyo Metropolitan Government, the most detailed three-dimensional geological map, 3D Urban Geological Map of Central Tokyo has been published by Geological Survey of Japan, AIST. As a part of this achievement, we introduce a classification result of buried flat surfaces (BFSs) beneath the post-LGM deposits. In this study, a base of the post-LGM deposits is defined as the bottom of conglomerate around the incised valley areas and top of conglomerate in buried terraces areas.
BFSs are classified into one valley floor and four other higher flat surfaces. The valley floor is located along the axis of incised valley formed during LGM. It has an altitude of −50 to −80 m and is covered with thick conglomerate. BFSs 1–4 are 0 to −10 m, −18 to −36 m, −21 to −25 m, −45 to −54 m altitude, respectively. BFS 1 and BFS 3 lack conglomerate. BFS 2 and BFS 4 are accompanied by widely traceable conglomerate on them.
BFS 1 is interpreted mostly as Holocene wave-cut platform, partly as remains of MIS 5a marine terrace. BFS 2, known as Honjo buried terrace, is interpreted MIS 3 fluvial terrace and correlated with Tachikawa II surface in Tamagawa area, western Tokyo. The age and origin of BFS 3 remain unknown, and BFS 4 is interpreted as a fluvial terrace formed during MIS 3 or older age.
BFS 1 and BFS 2 have a potential to be subdivided due to their wide range of altitudes. Drilling core survey on those surfaces for volcanic ash soil and/or tephrostratigraphy may provide clues for subdivision.
BFSs are classified into one valley floor and four other higher flat surfaces. The valley floor is located along the axis of incised valley formed during LGM. It has an altitude of −50 to −80 m and is covered with thick conglomerate. BFSs 1–4 are 0 to −10 m, −18 to −36 m, −21 to −25 m, −45 to −54 m altitude, respectively. BFS 1 and BFS 3 lack conglomerate. BFS 2 and BFS 4 are accompanied by widely traceable conglomerate on them.
BFS 1 is interpreted mostly as Holocene wave-cut platform, partly as remains of MIS 5a marine terrace. BFS 2, known as Honjo buried terrace, is interpreted MIS 3 fluvial terrace and correlated with Tachikawa II surface in Tamagawa area, western Tokyo. The age and origin of BFS 3 remain unknown, and BFS 4 is interpreted as a fluvial terrace formed during MIS 3 or older age.
BFS 1 and BFS 2 have a potential to be subdivided due to their wide range of altitudes. Drilling core survey on those surfaces for volcanic ash soil and/or tephrostratigraphy may provide clues for subdivision.