Sedimentary facies and physical properties of the Chuseki-so under Katsushika-ku, eastern margin of the Tokyo Lowland, central Japan:Time and spatial distribution of Holocene spit sediments.

Abstract

Latest Pleistocene to Holocene incised-valley fills (the Chuseki-so) under the eastern margin of the Tokyo Lowland comprise sandy sediments which stiff 20–50 in N-value in T.P. -40–0m. The sandy sediments have been interpreted as Pleistocene shallow marine deposits or as transgressive deltaic sediments deposited during the last deglaciation. However its distribution and depositional processes have not been well documented. On the basis of sedimentary facies analysis and radiocarbon dating of two core sediments obtained from the eastern margin of the Tokyo Lowland, we have clarified that the sandy sediments can be interpreted as Holocene spit sediments which wide 5km and elongate northwest-ward from the Shimosa Upland. The proximal and distal portions of the spit sediments, respectively, consist of shelly sand and sand-mud alternation. Large amount of the spit sediments deposited in the bay-mouth portion of the Paleo-Okutokyo Bay during the middle-Holocene sea-level highstand. Compared with a massive prodeltaic mud in the Tokyo and Nakagawa Lowlands,the spit sediments are relatively coarse and high-density. It is therefore predicted that the distribution of the spit sediments will be important to make earthquake simulation/city planning.