八郎潟の冲積層
スポンサーリンク
概要
- 論文の詳細を見る
Based upon the subsurface data obtained from drillings, the Holocene deposits of the Hachiro-gata (lagoon) are divided into three sedimentary facies as follows : a) Eastern lagoonal facies, developed in the eastern part of the lagoon, comprises gravel and sand in the lower, succeeded upwards with a peat layer, silt and clay beds. A volcanic ash bed lies at about 20 m. depth under the sea-level. b) Southwestern sand bar and lagoonal facies, comprises gravel and sand in the lower, succeeded upwards with a peat layer, lagoonal deposits and sand bed. The uppermost sand bed interfingers with the subjacent lagoonal deposits and also with the eastern lagoonal facies. c) Northwestern sand bar facies, comprises thick sand and thin silt beds. A volcanic ash bed lies at about 20 m. depth under the sea-level. The boundary between the Pleistocene and Holocene deposits may be drawn between the peat layer developed at about 50m. depth and superjacent lagoonal deposits. In the eastern lagoonal facies, molluscan remains are contained as localized assemblages (Fig. 2). The lowest part is characterized by an Ostrea bed which is succeeded upward by an accumulation of Macoma tokyoensis Makiyama and other marine molluscs. In the next younger horizon Raeta pulchella (Adams and Reeve) is dominant. The uppermost part yielded only Corbicula japonica Prime, a species which is now living in the lagoon. From the sequence of molluscs and facies the developmental stages of the lagoon may be respectively called Ostrea Bay, Macoma Bay, Raeta Bay and Corbicula Lake in upward order. The foraminifers from the deposits indicate that the sea water circulated freely into the lagoon of the early Raeta Bay. Subsequently the environment changed from shallow marine into brackish. This change, extending over the late Raeta Bay and Corbicula Lake, may be due to the fall of sea-level which is indicated by the ten and five meters raised beach terraces developed on the sand bars. It is thought that the rise of sea-level in the Holocene history of the area began with the Ostrea Bay, and continued until the early Raeta Bay. In the late Macoma Bay, volcanic ash fell over the lagoon and its vicinity. The lateral change of the deposits suggests that the change in sea-level is an important factor in controlling the sand bar development. Interpretation of the terraces and dunes on the sand bars reveals that the sand bar development is strongly influenced by long-shore drifting, wave and wind actions. The Holocene history of the Hachiro-gata is summarized as shown in Fig. 4.
- 東北大学の論文
- 1960-05-05