Abstract
An estuary mouth shoal is a submarine topographic ridge at or outside of an estuary or bay mouth, and is found within tide-dominated elongated estuaries. During the last deglacial sea level rise, elongated estuaries were present in coastal lowlands along the Japanese Islands. However, few estuary mouth shoal deposits have been found in the transgressive estuary system of the latest Pleistocene to Holocene incised valley fills (Alluvium) of the Japanese Islands. On the basis of sedimentary facies analysis and radiocarbon dating of four sediment cores obtained from the Tokyo Lowland along Tokyo Bay, we found shell scattered and bioturbated sand at depths of T.P. −25 – −30 m within the Alluvium. This sand was deposited in the Tokyo Lowland between 5 and 9 ka. The ridge topography, as reconstructed from isochrons, indicates that these sands are an estuary mouth shoal deposit formed by ebb tide currents in the bay mouth of the Paleo-Tokyo River and Urayasu incised valleys in the Tokyo Lowland. The estuary mouth shoal deposits at the top of the system are overlain by prodelta mud associated with a prograding delta system. Similar estuary mouth shoal deposits are expected within the Alluvium of the Japanese Islands; these deposits are expected to have a similar basal topography to that of the Alluvium in the Tokyo Lowland.
