The alluvial deposits in the hirasaku valley central part of the miura peninsula japan
Kanie, Y.; Ishikawa, S.
Science Report of the Yokosuka City Museum 23: 45-59
The Hirasaku River, about 10 km in length, flows northwest-southeast through the central part of the Miura Peninsula, and into Tokyo Bay. Under the Hirasaku Valley, thick Alluvial deposits were known. The data from shallow and deep borings and records of excavation for construction purposes were used. The deposits are divided into the Kurihama sand and gravel formation (20 m thick), Hirasaku mud member (30 m) and the Hirasaku sand member (15 m), in ascending order. These beds are inferred to be the fluviatile deposits of the late Wurm regression and early Jomon transgression, the deposits of the Jomon transgression (.apprx. 5000-6000 B.P.), and the deposits of the post-Jomon regression. The deposits buried the Paleo-Hirasaku River which had a steeper longitudinal profile than the recent Hirasaku River. Under the Hirasaku Valley, wave-cut benches are buried, Kubiri (0-5 m) in the upper, Kugo (-5 to -10 m) in the middle and Kurihama (-30 m) in the lower. The uppermost shell bed lies at 5 m above sea level. It was formed probably in the Jomon maximum transgression. These deposits, which yield molluscan fossils [Gastropoda, Scaphopoda, Pelecypoda], were laid down in the Paleo-Hirasaku Bay.