2013 Volume 122 Issue 6 Pages 968-991
Chuseki-so is a valley-fill deposit that originated during the latest Pleistocene and Holocene, spanning the period from the last glacial maximum (LGM) to the present. This study describes its stratigraphic framework and sedimentary processes with particular reference to Holocene sea-level change, in order to understand its topographic and depositional evolution and geotechnical properties in the Tokyo and Nakagawa lowlands.
Chuseki-so has been subdivided into two formations: the Nanagochi Formation and the Yurakucho Formation in ascending order. These formations can be clearly distinguished on the basis of physical properties: the Yurakucho Formation is very soft with a high water content, while the Nanagochi Formation is slightly compacted with a lower water content. Previous studies have suggested a falling sea level at the final stage of the Nanagochi Formation, corresponding to the Younger Dryas cold event. This led to the emergence of the deposits and physical differences due to escaping of pore water. However, recent studies seem to suggest that the sea level did not fall during the Younger Dryas. During the main stage of the Nanagochi Formation, from the end of the LGM until the Younger Dryas Event, sea level rose quickly, particularly during the Bølling/Allerød interstadial (B/A), and rapid sedimentation characterized by alternations of sand and mud occurred under a warmer and wetter climate. A new idea proposes that at the stage of the Nanagochi Formation, pore water of mud layers escaped easily into sandy layers immediately after sedimentation (Tanaka et al., 2006).
The Yurakucho Formation, on the other hand, consists of very soft marine muddy and partly sandy sediments, and the main stage is thought to have been formed by rapid sedimentation in an inner bay environment during the Holocene. Topographic and paleogeographic changes closely related to the Holocene Jomon Transgression, have been investigated by reconstructing the migration of oyster reefs, which are good markers of paleoshorelines, throughout the wide inner bay during the rising stage of sea-level. After the highest stage of sea level, or thereabouts, the delta front began to prograde rapidly toward the south with the delta front migrating about 30-40km, reaching the present northern Tokyo Lowland 2000-3000 years ago.
The NE Japan Megaearthquake of March 11, 2011, damaged large coastal areas of eastern Japan due to liquefaction and related phenomena. The northeast coast of Tokyo Bay in the Tokyo Lowland was damaged severely. Liquefaction occurred throughout the reclaimed land, which is composed of soft sandy soil dredged from the bay. The reclaimed sandy soil is underlain by new sandy delta front deposits and very soft thick marine valley-fill clays. It is essential to clarify the mechanisms of liquefaction in relation to the properties of soft ground (Chuseki-so), and the effects of shaking caused by strong earthquakes and subsequent sloshing for a long period.
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