Millennial-scale stratigraphy of a tide-dominated incised valley during the last 14kyr: Spatial and quantitative reconstruction in the Tokyo Lowland, central Japan

摘要

Spatial and quantitative analysis of infilling processes of the tide-dominated incised valleys beneath the Tokyo Lowland during the last 14kyr 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 8ka in the Arakawa Valley and 7ka 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 5ka, and suggests that the widely held view that the global initiation of deltas coincided with the abrupt rise of sea-level at 9 to 8ka 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 7mmyr(-1). Before migration of the Tone River at 5ka, 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.