Sedimentary facies, radiocarbon dates and physical properties of GS-AMG-1 core from Motogi district, Adachi-ku, western area of the Tokyo Lowland, central Japan.

Abstract

Latest Pleistocene to Holocene incised-valley fills (the Chuseki-so) under the lower reach of the Arakawa River comprise relatively thick fluvial-sand compared with those under the Nakagawa and Paleo-Tokyo Rivers. On the basis of sedimentary facies analysis, radiocarbon dating and measurement on physical properties of core sediments (GS-AMG-1) obtained from Motogi district, Adachiku, Tokyo Metropolitan, it has been revealed that the Chuseki-so in the core sediments (>T.P.-55.7 m) consists of (1) Gravel (braided-river channel fills), (2) Sand and peaty silt (meandering-river floodplain sediments), (3) Sand (tidal-current influenced channel fills), (4) Sand-mud alternation containing shells (tide-influenced deepening-upward shallow-marine sediments), (5) Silt to sandy silt containing shells (prodelta to deltafront sediments), (6) Sand-mud alternation containing burrows (tidalflat sediments), (7) Peaty silt (modern meandering-river floodplain sediments) and artificial soils, in ascending order. Upward-deepening facies succession of (2) to (4) (12,940±100-8,030±110 cal BP) and upward-shallowing facies succession of (5) to (7) (6,660±180-2,245±95 cal BP), respectively, deposited in the core site in relation to sea-level rise since after the Last Glacial Maximum and sealevel stillstand/gentle lowering during the middle to late Holocene. Previously reported thick fluvialsand is correlative to (3) in Motogi district. Density, water content, and N-value of the Chuseki-so in GS-AMG-1 are correlative to those of the sand-mud contents.