Difference in timing of maximum flooding in two adjacent lowlands in the Tokyo area caused by the difference in sediment supply rate

Abstract

The Arakawa incised-valley fill beneath the Tokyo area, Japan, has formed by delta progradation during Holocene transgression and highstand. This paper presents the integrated stratigraphy of this valley fill as deduced from three continuous sediment cores with rich radiocarbon age control and abundant data from the surrounding, and compares the filling history with the adjacent Nakagawa valley.Six depositional units make up the valley fill, distinguished based on sedimentary facies. In a sequence-stratigraphic framework, these units span from lowstand fluvial deposits (latest Pleistocene) through retrogradational to transgressive marine deposits (early and middle Holocene), to highstand fluvio-deltaic deposits (middle and late Holocene). The isochron and stacking pattern of sedimentary facies indicate that around 8 cal ka BP the shoreline trajectory flipped from landwards to seawards and maximum flooding occurred in the Arakawa valley. This is considerably earlier than in the neighboring Nakagawa valley, where this happened between 7 and 6 cal ka BP. Differences in the sediment supply at the time of transgression explain the diachronicity. The findings identify the Tone River, which was at that time feeding the Arakawa valley, as a system of very large sediment supply, compared to average rivers worldwide.