Stratigraphy of the latest Pleistocene to Holocene postglacial deposits beneath the Wakasu area, Koto-ku, Tokyo, central Japan: Sedimentary facies and depositional ages of core GS-KWS-1

Abstract

A buried valley incised during the Last Glacial Maximum beneath the Tokyo Lowland is the largest such valley in the Kanto District, central Japan. This paper presents the stratigraphy of valley fills and infilling processes as deduced from a continuous 85-m-long sediment core (core GS-KWS-1) from the Wakasu area, in Koto-ku, Tokyo, the deepest part of the incised valley that is accessible on land.Core GS-KWS-1 is composed of, in stratigrahpic ascending order, Pleistocene basement, postglacial fill, and artificial fill. Five depositional units make up the postglacial fill, distinguished on the basis of sedimentary facies: gravelly river deposits; sandy river deposits (15-11 ka); estuarine deposits or a transgressive lag; prodelta deposits (4-1 ka), and delta-front deposits (＜1 ka).In core GS-KWS-1, sediments from 10-4 ka are missing, indicating erosion and a hiatus at that time. The prodelta to delta-front deposits become younger seaward, and the rate of progradation is calculated as 2.7 km/kyr.