Buried River Terrace Surfaces and Paleo-Environmental History since the Later Stadial of the Last Glacial Age in the Kuromegawa Valley Plain, Central Japan

Abstract

The buried topographies and paleo-environments were investigated in and around the Kuromegawa valley plain facing to the Arakawa lowland, in the central part of the Kanto Plain. Stratigraphic examination shows that two river terrace surfaces had been constructed before 20, 000-30, 000y. B. P., in the lower drainage of the Kuromegawa river incising into the Musashino upland. Although the Arakawa lowland was deeply dissected at the later maximum stage of the Last Glacial Age, the Kuromegawa valley was not eroded to that level.It is clear that the post-glacial Jomon transgression invaded these areas and subsequently filled them with younger deposits, based on 14C datings and vertical changes of fossil diatom assemblages. The sea-level reached to its maximum height about 7, 000y. B. P. In the early stage of the transgression, fresh water sediments were deposited mainly in the lowland. As the sea-level continued to arose, an inner bay was formed extending about 1.7-2.0km into the valley at the maximum stage. Because the Arakawa river could transport larger amount of materials, natural levees and/or delta developed over the marine deposits at the mouth of the valley. However, it does not seem like that the valley was dammed by these fluvial deposits and a deep lake was created. The inner valley bottom had been dominated by maddy tidal flat. The marine regression resulted in marsh deposits resting on these sediments at most places.