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

在本项目中，阐明了晚更新世古东京湾障壁岛的存在，并提出了新的障壁岛沉积相模式。巴纳群岛形成于古东京湾东口。在茨木的玉栗，一个由障壁岛向陆地发展的洪水-潮汐三角洲。砂楔逐渐变细，形成泻湖泥，由潮泛三角洲和冲覆砂组成。将潮汐三角洲沉积划分为底积、三角洲前缘和顶积三大相。底积沉积物显示出细粒至细粒沙和泥的小规模交替，其中一些显示出泥披覆或潮汐束。三角洲前缘沉积物的特征是由沙丘迁移或由沿斜坡流下的水流移动的沙波产生的倾斜交错地层。顶置沉积物是在浅水条件下沉积在三角洲平面上的。它们显示出由三角洲的泻湖向前冲积作用所产生的向上变粗的层序。冲砂直接分布在泻湖泥上，上覆于潮涨潮落三角洲砂体之上。冲刷砂主要由两种相组成。近相冲覆砂体以平行纹层和明显的褶曲构造为特征。远源相以大型穿层和同沉积开放裂缝为特征。这表明大量的冲刷砂快速沉积。一些单位的冲刷砂包含地震活动的证据，在沉积。一些造成过度冲刷的强大海浪可能是地震引发的海啸。

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.