Sequence stratigraphy and paleogeography of the Alluvium under the northern area of the Tokyo Lowland, central Japan

Abstract

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.

Fluvial response to sea-level changes since the latest Pleistocene in the near-coastal lowland, central Kanto Plain, Japan

“…Kimura et al (2006) and Tanabe et al (2008) recognized sedimentary facies representing braided river, meandering river, estuary, spit, and delta systems, and correlated them with the successions identified by Matsuda (1974) and Kaizuka et al (1977). On the whole, the stratigraphy of the incised valley fill sequence represents a cycle of transgression and regression in response to post-glacial sea-level changes.…”

“…7.6-8.3 ka (Fig. 6), a period that corresponds to the time of the development of estuarine systems in the Tokyo Lowland during 5.9-11.8 ka (Tanabe et al, 2008). It can be inferred that unit C represents the marginal part of an estuary system at the time of maximum transgression.…”

“…In the Tokyo Lowland, braided river sediments (equal to BG) were overlain by meandering river sediments after ca. 14 ka in response to rapid sea-level rise; the boundary between them is defined as the transgressive surface (Tanabe et al, 2008). Hence, a succession from unit A to unit B indicates that retrogressive aggradation by postglacial sea-level rise has occurred in the middle Arakawa Lowland.…”

“…A Holocene sea-level curve, which was constructed by Endo et al (1989) and then was partly modified by Tanabe et al (2008) in the Tokyo Lowland, indicates that a rapid rise of the sea level to about 3 m above present sea level had occurred by ca. 7.5 ka, and was followed by a slow fall to present sea level.…”

“…In the coastal area of the Kanto Plain, the largest plain in Japan, many studies have clarified the stratigraphy of post-LGM incised valley fill sequences and determined the relationship of those sedimentary successions to post-glacial sea-level changes (e.g., Ikeda, 1964;Matsuda, 1974;Kaizuka et al, 1977;Endo et al, 1982Endo et al, , 1983Saito, 1995;Tanabe et al, 2008). In contrast, few studies of the entirely fluvial sequences of incised valley fill sequences in the near-coastal area of the coast exist.…”