仙台付近の海岸平野における微地形分類と地形発達

粒度分析法を用いて

抄録

The Sendai Coastal Plain is located in the northeastern part of Honshu Island. The studied area in this paper is the southern part of this coastal plain and is about 30km long from north to south and 8km wide from east to west. Natural levees, beach ridges and other micro-landform units are commonly recognized in this area, but in place exist some landform units with slight elevation which can not be classified into either natural levee or beach ridge according to their similar topographic forms. The present author attempted to discriminate these uncertain landform units into natural levee or beach ridge from statistical parameters which depend on the shape of grain size frequency curves.
For this purpose the author first of all clarified from the grain size distributions of sediment on typical natural levees and beach ridges, that standard deviation versus skewness and standard deviation versus mean grain size are effective in distinguishing the sediment of natural levee from that of beach ridge (Figs. 3, 4). The beach ridge sand and the natural levee sand are mainly separated by standard deviation, that is to say, beach ridge sand is more sorted than natural levee sand. And these sands are much more distinguishable by an analysis of the relation between standard deviation and other parameters of skewness and mean grain size.
As the result of land classification, with consideration of sedimentary structures of alluvium and distribution of historical remains, the geomorphological development of this coastal plain was considered as follows (Figs. 8, 9).
i) About 7, 500yrs. B. P. this area was under shallow maritime environment and the shore line was indented into the hilly area.
ii) And by the beginning of the Yayoi period (about 2, 000yrs. B. P.), the 1st (innermost) beach ridge in this area was formed along the foot of hill-land, which resulted in shallowing of sea bottom by deposition of the terrestrial sediment, according to the reduced rate of sea level rise.
iii) The 2nd beach ridge range appeared on the shallow off-shore bottom, before the middle Kohun period (about 1, 500yrs. B. P.), following to accmulation of terrestrial sediments on sea bottom. The marsh between 1st and 2nd beach ridges was gradually filled by fluvial sediment and peaty sediment, and the later period of this stage, the natural levee, to the north of Abukuma river, was formed on the marsh.
iv) The 3rd beach ridge range was formed near the present shore line in the last stage. The marsh formed between 2nd and 3rd beach ridges was gradually accumulated and modified by natural levees while the Abukuma river changed its course due to repeated floodings and cut the 2nd beach ridge, then the natural levee, to the south of the river, was formed.