No abstract

No abstract

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

No 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.