Spatial and quantitative analysis of infilling processes of the tide-dominated incised valleys beneath the Tokyo Lowland during the last 14 kyr was undertaken by using data from 18 sediment cores, 467 radiocarbon dates and 6100 borehole logs. The post-Last Glacial Maximum valley fills consist of braided river, meandering river, estuary, spit and delta systems in ascending order. The boundary between the estuary and delta systems is regarded as the maximum flooding surface. The maximum flooding surface beneath the Tokyo Lowland is dated at 8 ka in the Arakawa Valley and 7 ka in the Nakagawa Valley. This age difference is due to the migration of the Tone River from the Arakawa Valley to the Nakagawa Valley at 5 ka, and suggests that the widely held view that the global initiation of deltas coincided with the abrupt rise of sea-level at 9 to 8 ka is true only where there has been steady sediment supply from major rivers. The meandering river system is dominated by sheet-like sands that were deposited during lateral migration of channels during the Younger Dryas and isolated vertical sands within muds that reflect vertical aggradation of channels before and after the Younger Dryas. The transition between these channel geometries is controlled by a threshold sea-level rise of 4 to 7 mm yr À1 . Before migration of the Tone River at 5 ka, the tide-dominated bay in the Nakagawa Valley was filled by upward-fining laterally accreting muds. The muds accreted from the margin to the axis of the bay. Such lateral accretion of suspended particles derived from outside the bay has been documented in other tide-dominated coastal environments and is probably common in other similar settings. After the migration of the Tone River, the bay was filled by upward-coarsening deltaic sediments.

The Tokyo Lowland, surrounded by the Shimosa, Omiya and Musashino Uplands, is a coastal lowland mainly formed by the Tonegawa. Latest Pleistocene to Holocene incised-valley fi lls (the Alluvium) in the northern area of the Tokyo Lowland comprise complicated lithological distribution considerably affected by the basal topography and river shift of the Tonegawa. To clarify this complicated lithological distribution, we constructed a three-dimensional sequence stratigraphy and paleogeography of the northern area of the Tokyo Lowland based on sedimentary facies and radiocarbon dates of eight sediment cores and 2,308 borehole logs. The Alluvium, which fi lled the incised valleys of the Nakagawa, Arakawa and Paleo-Tokyogawa, consists of gravelly braided river sediments, meandering river sediments of sand-mud alternation with rootlets, estuary sediments of sand and mud bed with shells, spit sediments of sand with shells and shelly delta sediments, in which the contents of sand and wood fragments increase upward, in ascending order. By applying sequence stratigraphic concepts, a transgressive surface dated >14,100 cal BP and maximum fl ooding surface dated 8,100-5,900 cal BP can be identifi ed at the braided river-meandering river sediments boundary and estuary-delta sediments boundary, respectively. Spit sediments form a local sediment body in the eastern margin of the Tokyo Lowland's northern area. The paleogeography of the northern area of the Tokyo lowland changed from a braided river to a meandering river, tidal fl at and bay due to the sea-level rise since after the Last Glacial Maximum. Sand bar and delta sediments accumulated in the Arakawa Valley in response to the Tonegawa sediment discharge in the Arakawa Valley during 10,000-5,000 cal BP. On the other hand, a tidal river was distributed in the Nakagawa Valley because of the lack of sediment at that time. The Nakagawa Valley was fi lled with the Tonegawa sediments since the river shift from the Arakawa Valley to the Nakagawa Valley around 5,000 cal BP. Clastics eroded from Pleistocene terraces formed a spit in the bay mouth portion of the Paleo-Okutokyo Bay during 8,000-4,000 cal BP.

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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 s. 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 Abstract

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