application/pdf

The results of the study on the surface and subsurface geology of the alluvial and diluvial deposits distributed along the projected New Tokaido Line extending from Tokyo to Osaka are described and discussed in this work. Using the data of both shallow and deep borings, deep wells and other exacavation records for construction purposes, observations through topographic maps, aerial photos, and route geological survey works, the general features and characteristics of the alluvial deposits distributed on the surface along the Tokaido coastal regions and under the coastal plains interesting and new facts of the deposits and some depositional features could be clasified as described in this article. The alluvial plains along the Tokaido coastal regions are considered to have developed according to the following processes. During the last ice age at a time of lowered sea level, the rivers dissected their valleys downward to that level. Then, these valleys were drowned in consequence of the post glacial rise of sea level and gradually filled with the sediments supplied from the loaded rivers and coastal currents. At last, these valleys which were completely aggradated with inland sea marine deposits and fluvial deposits changed into alluvial plains, such as fans, flood plains, and deltas. The depth of the aggradated valleys near to the present coastal lines are 30～70 m under the present sea level in the alluvial plains around the inland bays and-100m± in the plains directly facing the open sea. Sometimes, terraces and wave cut benches are found in the aggradated valleys. In the filled valleys of the Tama and Tsurumi Rivers, the so-called Tachikawa Loam beds (Volcanic ash beds) are found distributed on such submerged terraces. At the foot of Mt. Ashitaka, the lower skirts of the volcanic slopes which are covered with loam (volcanic ashes) beds, are distributed under the marine alluvial deposits. The bottoms of the drowned valleys are incised into these loam beds in both areas. In the fans and flood plains, the alluvial deposits are generally formed with fluival deposits as the surface layers and under these layers marine sediments commonly exist. Further more, the layers in the valley bottom are fluvial in origin and were deposited during the regression stage before the succeeding transgression to the early stage of the post glacial transgression. The sequence of the deposits in the drowned valleys are in general as follows, Fluvial-→Marine-→Fluvial This forms one depositional cycle. In this paper, writer's point of view stands as follows. The marine transgression that formed drowned valleys originated from the glacial eustatic movements of sea level, which began by the melting of the continental glacier after the last maximum ice age. And the deposits accompanying this marine transgression in the drowned valleys are named "Alluvial deposits." The alluvial deposits are classified in connection with the post glacial sea level fluctuations, as follows. [Table. 1] In the fans and the natural levees of such rivers which are of comparatively high grade, namely the rivers of Tama, Sakawa, Fuji, Abe, and Tenryu, the river sands and gravels are remarkable in development, whereas the thickness and the scale of the marine sediments are small. On the other hand, at the sides of the small rivers with low capacity of load transport, such as the rivers of Tsurumi, Tomoe, Ota and etc., fluvial deposits cover only the surfaces. And under these surface layers, shallow water marine deposits mainly of clay, silt and fine sands with abundant molluscan shells occupy the greater parts of the drowned valleys. Moreover, large or small sand bar built by the coastal currents are distributed along the coastal lines of the Tokaido regions. The most important elements that control the depositional features of the A1luvial sediments are the correlation of the velocity between post glacial rise of sea level and aggradation in the drowned valleys. Thus, the general rules are as follows. Case (A) rising velocity of sea level > aggradation velocity-→ transgression Case (B) rising velocity of sea level ≒ aggradation velocity-→equilibrium Case (C) rising velocity of sea level < aggradation velocity-→regressron The products in case (A) are marine deposits, in case (B) littoral or deltaic deposits and in case (C) fluvial deposits. In the case of rest or fall of sea level, the depositional features are controlled by the conditions of aggradation and fluctuation velocity of the sea level. In real fluctuations of sea level, the velocity of fluctuation are changed and temporary falls or rests of sea level occur during the post glacial rise of sea level. This seems to be very complicated, but the deposits are controlled fundamentally by the correlation of the speeds between sea level fluctuation and aggradation. According to the fluctuation curves during post glacial sea level by Curray and Shepard, there is a high possibility that three depositional cycle layer are formed in the drowned valley having -90～-100m depth under the present sea level, two cycles in the drowned valley with -50～-60m depth and one cycle in that of -20～-30m depth respectively in accordance with correlation of speeds between aggradation and fluctuation of sea level. As the alluvial deposits in drowned valleys deeper than -50～-60m depth, are indistinct in their sediment arrangement the existence of three cycle layers are not clear. The layers with a tendency of two cycles are recognized in the coastal area around Tokyo Bay, near by Yaizu City and sides of Ota River. Aggradation velocity in the drowned valleys are controlled by the topographic and geological conditions as well as their environments as follows. These factors are the relief of the back grounds, discharge amounts of the rivers and characters and hardness of the rocks. From various data, the alluvial deposits under the coastal plains, can be classified into the next eight fundamental types based upon the difference of aggradation velocity at each deposional place and their resulting columnar sections. [Table. 2] Crustal movements also give important affect on the depositional features of the alluvial deposits. Continuous and periodical up and down movements of the crust affect the relative velocity of sea level flucutations. If the grounds have heaved, the rising velocity of sea level decrease relatively and if the grounds have subsided the rising velocity of sea level accelarates relatively. During the post glacial age, comparing with the speeds of the sea level fluctuations, that of the crustal movements are considered to be slower and therefore the effects of the movements on the features of the alluvial layers may be of the secondary order. During the later half of the Holocene period, it is considered that, there is a tendency of upheaval of the grounds in the region of South Kanto districts and subsidence of the grounds at the foot of Mt. Ashitaka, adjacent areas of Yaizu City and western part of Nobi plain in consequence of the depositional features of the alluvial deposits of each mentioned regions.

紀要類(bulletin)

8092994 bytes