Abstract
The latest Pleistocene to Holocene incised-valley fill (Alluvium) under the Tokyo and Nakagawa Lowlands has been divided into the underlying Nanagochi Formation and the overlying Yurakucho Formation since the 1960s. This division has been useful in the field of soil mechanics, because the Nanagochi Formation (deposited in a freshwater to brackish water setting) comprises stiff sediments with a high N value, whereas the Yurakucho Formation (deposited in a marine setting) comprises poorly consolidated sediments with a low N value. Based on the sedimentary facies identified in eight sediment cores recovered from these lowlands, in combination with radiocarbon age data, we divided the Alluvium into braided river, meandering river, estuary, spit, and delta systems, in ascending order. The fluvial and marine sedimentary systems yield high and low N values, respectively, and contain contrasting proportions of mud and water. Based on these results, we propose that the boundary between the Nanagochi and Yurakucho formations should be placed at the boundary between the meandering river and estuary systems. These systems are easily distinguished based on the presence of shells and bioturbation in the estuary
