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College of Liberal Arts and Sciences, Tokyo Medical and Dental University, 2-8-30 Kohnodai, Ichikawa-shi, Chiba 272-0827, Japan (e-mail: yajima.michiko{at}gupi.jp or pxi02070{at}nifty.com)
Tokyo is situated in the southern part of the Kanto Plain, the largest plain in Japan. It is filled with thick marine Quaternary sediments deposited in the Palaeo-Tokyo Bay. The concept of a Palaeo-Tokyo Bay was proposed by Hisakatsu Yabe in 1913 and 1914, based on molluscan fossils, geography and tectonics. Palaeo-Tokyo Bay used to open to the east and, at the time of the high sea-level phase, perhaps also to the south, whereas the modern Tokyo Bay opens to the south. The Paleao-Tokyo Bay concept is supported by recent sedimentological evidence, sequence stratigraphy and tectonic studies, and is considered to have formed in a forearc basin near the triple junction of plates (the Pacific, Asian and Philippine plates) and trenches (Japan Trench, Sagami Trough and Izu-Ogasawara Trench) in the Quaternary. The bay is unique and shows evidence of repeated differential vertical movements during its formation.
