In this study, we identifi ed the basement of the latest Pleistocene to Holocene incised-valley fi lls (Alluvium) from 7,021 borehole logs in the northern area of the Tokyo Lowland and Nakagawa Lowland. We constructed the dataset, which consists of the latitude and longitude of the borehole site and depth of the Alluvium basement. The depth-distribution map of the Alluvium was illustrated based of this dataset and interpolation of the Alluvium depth distribution by the inverse distance weighting method. We assumed the top of the Basal Gravel of the Alluvium as the basement of the Alluvium where the Basal Gravel exists. This depth-distribution map illustrates a detailed topography of the Nakagawa, Moto-arakawa, Ayasegawa, Arakawa and Paleo-Tokyo River Valleys, buried terraces and buried wave-cut benches. The comparison between the thickness of the Alluvium and surface elevation in the northern area of the Tokyo Lowland and Nakagawa Lowland indicates that there are major relationships. The elevation is less than T.P. +2 m in the area where the Alluvium thickness is more than 40 m, and the elevation is more than T.P. +2 m in the area where the Alluvium thickness is less than 25 m.
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 filled 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.