千葉県の低地と海岸における完新世の地形変化
付. 都川・古山川合流点付近沖積層の珪藻群集
1978 年 17 巻 4 号 p. 189-205
詳細
1978 年 17 巻 4 号 p. 189-205
Geomorphic change of alluvial plains and coasts in the Holocene Epoch has been significant for human beings as an environmental change in Chiba Prefecture as well as in the Kanto District. The successive changes of these landforms are described for the past 6000 years, that is the relatively stable time in sea level after the Yurakucho (Flandrian) transgression.
Some sea cliffs facing the Pacific Ocean have been continuously eroded by waves from 6000 years ago to the present; some ones facing Tokyo Bay had been eroded to the historic time; while some others near delta plains along the coast of Tokyo Bay had been formed by the historic time and the abrasion platforms in front of the cliffs have been buried by deltaic sand.
Rivers such as Yoro and Obitsu have protruded deltas into the drowned valleys formed by the Yurakucho transgression, and then to Tokyo Bay in the same 6000 years. Much smaller rivers like Miyako in Chiba City could not have extended deltas to Tokyo Bay, but drowned valleys were closed by baymouth bars, and peat has accumulated in thus built lagoons from about 3500 years ago to the historic time.
Retrogradation of sea cliffs, progradation of deltas, and the processes of baymouth bar or lagoonal deposition are generally understood, when we assume, as a first approximation, a stable sea level, uniform rate of uplift or subsidence, and continuing processes of waves and rivers under given local circumstances in that term. However, for features diverged from those made by uniform processes, such as three definite groups of sand ridges in the Kujukuri coastal plain, we need remodelling the first approximation, in this case for example, adding three small fluctuations to the stable sea level.
It is known through this study that both the first and the second approximative approaches are necessary to understand the geomorphic development in the past 6000 years and also to predict the future geomorphic change.
In the note attached, diatom assemblages found in 17 samples from a boring to Holocene deposits are described. The data obtained were useful to reconstruct the process of alluviation in the small valley of Miyako River.
