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

在这个项目中，我明确了晚更新世古东京湾堰洲岛的存在，并提出了新的堰洲岛沉积相模式。巴纳群岛形成于古东京湾东口。堰洲岛向陆地方向形成了潮水三角洲，现在在茨城县的玉果里。沙楔状沉积物逐渐变成泻湖泥，由潮洪三角洲和冲积沙组成。潮三角洲沉积可划分为底端、三角洲前缘和上端3大相。底部沉积物显示出非常细到细粒度的砂和泥的小规模交替，其中一些显示出泥幔或潮汐带。三角洲前缘沉积物的特征是由沙丘迁移或沿坡流下的水流移动的沙波所形成的倾斜交错地层。上层沉积物在浅水条件下沉积在三角洲平面上。它们表现为三角洲前向递进作用形成的向上抬升序列。冲积砂位于潮洪三角洲砂体之上，直接分布在泻湖泥上。冲积砂由两种主要相组成。冲覆砂近端相具有平行层状和明显的卷曲构造特征。远端相以大规模跨层和同沉积开缝为特征。这表明大量冲刷砂的快速沉积。一些冲积砂单元在沉积过程中含有地震活动的证据。造成冲积的一些强大海浪可能是地震引发的海啸波。

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.