Sediment accumulation pattern of the muddy Alluvium in the Nakagawa Incised Valley, Saitama Prefecture, central Japan

Tanabe Susumu, Nakanishi Toshimichi, Nakashima Rei, Ishihara Yoshiro, Uchida Masao, Shibata Yasuyuki

doi:10.5575/geosoc.116.252

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.

Sediment accumulation pattern of the muddy Alluvium in the Nakagawa Incised Valley, Saitama Prefecture, central Japan

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