Abstract
The age and depth of the Uppermost Alluvium in the coastal lowlands of Japan indicates that minor sea-level drop, termed the “Yayoi regression”, occurred at 3–2 ka; however, this “Yayoi regression” is not a eustatic phenomenon. Sedimentary facies analysis and radio-carbon dating in the Tokyo and Nakagawa Lowlands indicate that: (1) flood plain sediments of the Uppermost Alluvium are located at depths >1 m below the present sea level, and (2) the morphology of fluvial channel sediments of the Uppermost Alluvium changes from sheet-like (7–3 m below present sea level, deposited at 3–2 ka) to anastomosing (<3 m below present sea level, deposited at 2–0 ka). Sheet-like and anastomosing sand bodies typically form during lowstand and transgressive fluvial systems, respectively. This suggests that relative sea level was lower than the present at 3–2 ka, before rising in the Tokyo and Nakagawa Lowlands to current levels between 2 ka and the present day.
