福山平野沖積層の堆積環境
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概要
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Fukuyama plain is one of the alluvial plains along the northern coast of Seto Inland Sea. The alluvium of this plain is comparatively thin and its base lies only twelve meters in depth, because the plain is located near the watershed of the previous river system during the Last Glacial Age. The purpose of this paper is to clarify the formative process of Fukuyama plain in detail. From this point of view, component analysis and diatom analysis were carried out at four boring core samples collected in this area. The deposit of Fukuyama plain is divided into three units; Basal layer, Middle Silt layer and Upper Sand layer (Iseki 1982). Nature of each unit of this plain is briefly sammarized as follows; 1) Basal layer lies under the alluvial deposits of the deltaic plain. This layer is divided into Basal layer I and Basal layer II, in ascending order. (1) Basal layer I consists of rather consolidated sand and gravel, and yields many calcareous nanno fossils. (2) Basal layer II mainly consists of clay with 3-10 N value. This layer contains many fresh water diatoms, suggesting fluvial in origin. 2) Middle Silt layer is divided into three sub-units. (1) Judging from the composition of the diatom assembleges, Middle Silt layer I seems to have been deposited near the coast. (2) In Middle Silt layer II, marine diatoms, faecal pellets and calcareous nanno fossils appear remarkably. This shows that the depositional environment was affected greatly by marine water. (3) Middle Silt layer III contains less marine components than those of Middle Silt layer II. 3) Upper Sand layer can be divided into two sub-units. (1) In Upper Sand layer I, calcareous nanno fossils and marine diatoms gradually decrease upward but fresh water diatoms increase. (2) The analyses of Upper Sand layer II show that this layer was deposited in marine condition because the sites of all boring cores are located on the reclaimed land. From the results of these analyses, we can estimate how the depositional environment has changed. Before the Last Glacial maximum, Basal layer I was deposited in beach environment. Basal layer II was accumulated on the land mainly in the Last Glacial Age. However, the upper part of this layer may have been formed even in Holocene, due to delayed transgression in the former watershed area. When Jomon transgression intruded into this region, Middle Silt layer I and II were formed. During high sea level of Jomon transgression, the most part of this plain was submerged and Middle Silt layer II was deposited. The 14C date from the middle part of this layer is about 6600 y.B.P. It sugests that Middle Silt layer III and Upper Sand layer I are sediments which accumulated during the regression stage after Jomon transgression and the depositional environment changed from inner bay to the foreset bed of the delta.
- 地理科学学会の論文
- 1988-04-28