RECENT DEPOSITS AND BURIED LANDFORMS IN THE TAMAGAWA LOWLAND

Abstract

The Tama River flows from Mt. Kasatori, which rises in the southern part of the Kanto Mountains, down to Tokyo Bay. The lower reach of this river forms an alluvial plain which is known as the Tamagawa Lowland. Recent deposits and their basal landf orm, i.e. buried landf orms in this lowland were evaluated by using some thousands of borehole records. Buried landf orms, which have vario-us geneses, were formed in the past about thirty thousands years.. In this paper, Recent de-posits denote the fills above the buried landf orms, which have been depositing in and after the main stage of the Wurm ice age. Many geologic sections were constructed in order to examine the borehole records. Some of them are shown in Figs. 1, 2, 4, 6, 7 and 8, and their locations are shown in Fig. 5. It is said that the difference in f acies, soil mechanic properties, contained materials and colors of deposits are useful indicators to analyze stratigraphy. On the basis of the difference in these indicators, longitudinal and transversal continuities of deposits were examined. As a result, the deposits above the basement of the Miura group (BR) were divided into Recent deposits composing buried river terraces and deltaic plains (T1 G, T2 G, TS and TL), diluvial Tokyo formation I (DU) and II (DL). The characteristics of indicators to identify each deposit are shown in Tab. 2. Based on the datum geologic sections and indicators, many other borehole records were examined to discriminate Recent deposits from the others and to draw their basal landform by contour lines. On the other hand, the surface of unconformities which appear in the geologic sections, can be regarded as the buried landforms. Accordingly, the buried landforms are able to be restored by tracing the surface of unconformities. The main results obtained in this paper are as follows: 1) Recent deposits are classified in to UA, US, UC, MS, LC and BG on the basis of lithostratigraphic units (Tab. 1). UA, US, UC and MS compose the upper part, LC and. BG compose the lower part. It is common in other coastal lowlands in Japan that Recent deposits consist of two parts. But it could not be made clear in, the Tamagawa Lowland whether the upper part was lying unconf ormably on the lower part as in the Tokyo Lowland or not. 2) The basal landform of Recent deposits was analyzed as shown in Fig. 5 by contour lines. The buried landforms which are shown in Fig. 9, can be classified into three types, i, e. buried river terraces and deltaic plains, buried valleys and buried coastal terraces. Buried river terraces and deltaic plains: Tachikawa terrace which is covered by Tachikawa loam, the uppermost part of the Kanto volcanic ashes, is buried under the alluvial plain about two kilometers upstream from the location of section line A-A' (Fig. 5). This buried Tachikawa terrace is divided into two parts-T1 and T2 (Fig. 9). If we extend T1 downstream, we can find deltaic plain Ts which are 10-20 meters below sea level. Ts is composed of sandy deposits (TS) with distribution restricted within the lower reach of T1 and on DU. It cannot be directly seen if T1 continues to TS, since T2 and the palaeo Nomi River separate them. But the longitudinal section (Fig. 6) and distribution of Ts suggest that it is appropriate to regard Ts as the deltaic area of T1. Considering that the lower part of the Tachikawa loam had fallen about 25, 000 years B. P., we might regard TS and the upper part of DU in the Tamagawa Lowland as the deposits which were formed during the Wurm interstadial around 30, 000 years B. P..