Abstract
The latest Pleistocene to Holocene incised-valley fill (Alluvium) beneath the Nakagawa Lowland upon the Kanto Plain consists of thick, muddy marine sediments. It has previously been proposed that the marine sediments were deposited as a result of delta progradation from the bayhead of the paleo-Okutokyo Bay to the bay mouth, following the mid-Holocene sea-level highstand. Based on an analysis of sedimentary facies and radiocarbon dates obtained for three sediment cores recovered from the Nakagawa Lowland, we determined the accumulation pattern of incised-valley fill along a transect oriented across to the valley. The period of sedimentation can be divided into three stages based on the obtained patterns of accumulation: Stage I (~10 cal kyr BP): aggradation of a meandering river system; Stage II (10-6 cal kyr BP): large aggradation of the estuary system at the axis of the valley and minor aggradation at the valley margin; and Stage III (6-0 cal kyr BP): progradation of the delta system from the bayhead to bay mouth and from the western margin of the valley to the eastern margin. During Stages II and III, a strong tidal current was active along the valley axis, possibly explaining the eastward migration of the delta.
