“Chuseki-so”, which is an incised-valley fill deposited since the last glacial maximum, is distributed under the Nakagawa Lowland, Saitama Prefecture, central Japan. In this study, sedimentary facies, radiocarbon dates and sediment properties of the GS-SK-1 cores (GS-SK-1A, GS-SK-1N and GS-SK-1T) drilled at a central part of the Nakagawa Lowland, were analyzed for clarifying stratigraphy and sedimentary environments of incised valley fills in the Nakagawa Lowland. The “Chusekiso” of the GS-SK-1A core is composed of fluvial to shallow marine sediments as follows, in ascending order: (1) fluvial channel fills of pebble to pebbly sand, (2) flood plain to salt marsh deposits of alternation of sand and mud, (3) muddy tidal flat deposits of massive mud containing molluskan fossils, (4) sandy tidal flat to sand shoal deposits of poorly-sorted silty sand containing shell fragments and granules, (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 of alternation of sand and mud. Nineteen radiocarbon dates, 48,350 to 1,270 yrBP in radiometric 14C ages,indicate that the incised valley fills are formed by a set of transgression and regression. Sediment properties, i.e. N-value, mud contents, bulk densities, magnetic susceptibility, water contents, luminosity and color contrasts of these deposits, vary stratigaraphically, corresponding to sedimentary facies or at boundaries of them.
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 GSKM-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 14C 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.
“Chuseki-so” in the Tokyo and Arakawa Lowlands, incised-valley fills of the uppermost Pleistocene-Holocene sequence, are composed of fluvial to brackish deposits of the Nanagochi Formation and overlying marine to fluvial deposits of the Yurakucho Formation. We clarified successions of sedimentary facies and radiocarbon dates of “Chuseki-so” by analysis of three borehole cores in the Tokyo and Arakawa Lowlands (DK, TN and HA cores). The results are as follows: (1) the sedimentary successions of the cores are composed mainly of gravel beds of fluvial channel fills, upwardfining sand and/or alternation of sand and mud regarded as meandering rivers or natural levee / flood plain deposits, mud rhythmically interbedded by ripple sand as tidal deposits, homogeneous mud with in-situ marine shells as deposits of prodelta to delta front and alternation of sand and mud as delta plain deposits, in ascending order; (2) AMS radiocarbon dates of 42 samples in total have been obtained from these cores (19 points in DK core, eight points in TN core and 15 points in HA core), and show the successive sedimentary accumulation curve during 12,000 to 1,000 yrBP; (3) facies successions of these cores denote that the incised-valley fills are divided into the river system, estuary system and delta system, in ascending order.
The latest Pleistocene to Holocene incised-valley fills distribute in the Tokyo and Nakagawa lowlands, Kanto Plain, central Japan. The deposits contain abundant molluskan fossils. We examined mollusks obtained from two sediment cores (GS-KM-1A and GS-SK-1) in the Tokyo and Nakagawa lowlands and recognized 6 species of gastropod, 15 species of bivalve, and 3 species of scaphopod from GS-KM-1A, and 6 species of gastropod, 15 species of bivalve, and 1 species of scaphopod from GS-SK-1. Based on the molluskan assemblage analysis, these sediment cores show environmental changes caused by a Holocene transgression-regression cycle, which comprises brackish intertidal/subtidal zone, marine subtidal zone, marine subtidal zone (inner-bay), marine-brackish subtidal zone,in ascending order.