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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

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The latest Pleistocene to Holocene deposits, filling incised valleys carved during the last glacial age, are distributed beneath the Tokyo Lowland, Tokyo Metropolis, central Japan. The GS-KM-1 cores (GS-KM-1A, GS-KM-1B and GS-KM-1C) were drilled at a central part of the Tokyo Lowland, and sedimentary facies, AMS radiocarbon dates and physical properties of the cores were analyzed in detail for clarifying stratigraphy, sedimentary environments and the relationship between sedimentary environment and sediment properties on incised valley fills in the Tokyo Lowland. The latest Pleistocene to Holocene deposits of GS-SK-1 cores consist of fluvial to shallow marine succession as follows, in ascending order: (1) fluvial channel fills of pebble to pebbly sand, (2) flood plain, fluvial channel fills influenced by tidal current and muddy tidal flat deposits, which are characterized by alternation of sand and mud, (3) sandy tidal flat to sand shoal deposits of sand containing blackish to marine shell fragments, (5) inner bay (prodelta to delta front) deposits of massive mud intercalated by thin ripple sand and abundant burrows, and (6) fluvial channels fills to flood plain deposits in deltaplain of alternation of sand and mud. The AMS radiocarbon dates of 21 numbers, covering 11,090 to 2,350 yrBP in radiometric 14 C ages, indicate that the incised valley fills were formed by a set of transgression and regression. Sediment properties, such as N-value, mud contents, bulk densities, magnetic susceptibility, water contents, luminosity and color contrasts tend to vary with horizon and sedimentary facies

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