Reconstruction of tidal delta and barrier islands in Paleo-Tokyo Bay

点击翻译按钮获取中文摘要

During this project, I clarify the existence of the barrier islands in the late Pleistocene Paleo-Tokyo Bay, and also proposed new sedimentary facies model of barrier islands. The barner islands were formed at the eastern mouth of Paleo-Tokyo Bay. A flood-tidal delta landward of barrier islands developed where now in Tamatukuri, Ibaraki. The sediments of sand wedge tapering into lagoonal mud, are composed of flood-tidal delta and washover sands. The tidal delta sediments are classified into three major facies of bottomset, delta-front and topset. The bottomset sediment shows a small-scale alternation of very fine- to fine-grained sand and mud, some of which show mud drapes or tidal bundles. The delta-front sediment is characterized by inclined cross-strata produced by migration of dunes or sand waves moved by currents flowing down the slopes. The topset sediments is deposited on the delta plane under shallow water condition. They show a upward-coasening sequence generated by the lagoonward progradation of delta. Overlying the flood-tidal delta sand body, the washover sands distribute directly on the lagoonal mud. The washover sands are composed of two major facies. The proximal facies of washover sand is characterized by parallel lamination and distinct convoluted structures. The distal facies is characterized by large-scale cross-strata and syndepositional open-cracks. These indicate rapid deposition of large quantities of washover sand. Some units of washover sands contain the evidence for earthquake activity during deposition. Some of the powerful waves responsible for the overwash may have been earthquake-generated tsunami waves.