The so-called Yayoi regression in Japan is known as a relative sea-level lowering to m below T.P. (Tokyo Peil; Mean Sea Level of Tokyo Bay) during -cal kyr BP. However, the regions where this event occurred in the Japanese Islands and its continuous sea-level index points from verified regions have yet to be clarified. Lake sediments beneath the Innermost Tonegawa Lowland, deposited in response to a sea-level rise during -cal kyr BP, indicate a paleo-water depth of ca. -m. The sea-level index points of the lake sediments indicate that the sea level dropped to . m T.P. at . cal kyr BP, while the paleo-water depth estimated from plant macrofossils in the sediments is m. The amount of sea-level lowering is greater than the sediment compaction effect; moreover, no large tectonic movements have affected the region over the past years. These results demonstrate that the Yayoi regression occurred in the Innermost Tonegawa Lowland and that isostatic movements triggered by sediment loading might have brought about the observed lowering of sea level.

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-ka; however, this Yayoi regression is not a eustatic phenomenon. Sedimentary facies analysis and radiocarbon dating in the Tokyo and Nakagawa Lowlands indicate that: () flood plain sediments of the Uppermost Alluvium are located at depths > m below the present sea level, and () the morphology of fluvial channel sediments of the Uppermost Alluvium changes from sheet-like (-m below present sea level, deposited at-ka) to anastomosing (< m below present sea level, deposited at-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-ka, before rising in the Tokyo and Nakagawa Lowlands to current levels between ka and the present day.

Abstract:The sedimentary facies, diatom assemblages, physical properties and AMS radiocarbon ages were determined for the core sediment (GS-MHI-1) of the latest Pleistocene to Holocene incised valley filled deposits in the southern Nakagawa Lowland, Kanto Plain, Japan. In the core sediment, we recognized seven sedimentary facies, that is, braided river channel fill, meandering river floodplain sediments, tidal-influenced channel fill, tide-influenced transgressive shallow marine sediments, upward shallowing marine sediments, tide-influenced upward shallowing marine sediments, and modern river channel fill to floodplain sediments, in ascending order. These sedimentary facies and the radiocarbon dates are correlated to the stratigraphy of the other core that was obtained at the western side of the incised valley. These lithologies are also correlated to the existing borehole log columns drilled for engineering purposes. Judging from these correlation, we idenfied four sedimentary systems to account for the spatio-temporal distribution of the sedimentary facies across the axis of the main incised valley in this area; braided river, meandering river, estuary, and upward-shallowing delta, in ascending order. The meandering river system indicates almost a horizontal attitude across the valley; however the estuary and delta systems have an asymmetric depositional attitude dipping eastward. The thick soft marine mud is included in these systems. This asymmetric distribution of the muddy system should be especially remarked for geological hazards under the activating urbanization area. Keywords