Sedimentological Studies of the Quaternary System in Tokyo District with Special Reference to the Thickness Changes in Sedimentary Layers during Compaction History

Abstract

The area of Tokyo Metropolis occupies low hills, plateaus, fluvial and coastal plain which are underlain by thick Quaternary sediments. Tokyo is situated in the west central part of the Kanto Tectonic Basin which has been developed since the Late Neogene, and the greatest thickness of the Japanese Pleistocene sediments was attained in this basin. On the other hand, concentrated disasters by earthquakes and landsubsidence have been caused from thick unconsolidated sediments of the lowland area of Tokyo.
The present results were obtained from subsurface prospecting for the geologic structure and groundwater during the past about 10 years, in which the writer has been engaged. Twenty-two wells were drilled specially for the purpose of this investigation, among which the eight were located on the low plateau, the Musashino Upland, and fourteen on the fluvial and coastal plains, the Shitamachi Lowland. Mechanical and paleontological analyses were made on the drilled cores, and density logging was made at wells for porosities of the layered sediments.
This article dealt with subsurface geology, as to the problem of the basin forming movements. The main results are as follows.
(1) The Quaternary subsurface sediments are divided stratigraphically into two groups, the lower is the Kazusa Group and the upper the Tokyo Group. The Kazusa Group comprises the Kitatama and Higashikurume Formations, in ascending order. The Tokyo Group comprises the Toneri, Edogawa and Takasago Formations, in ascending order. The Kitatama Formation is mainly of massive siltstone, and the Higashikurume Formation of sandstone with siltstone intercalations. The Toneri, Edogawa and Takasago Formations consist of sand-rich alternations of silt and sand layers with gravels.
(2) A gently upheaved zone is recognized from Setagaya Ward to the northern part of Katsushika Ward. On the west of this zone, the strata generally strike E-W with very low dip to the north, and an undulated structure is found in the Kitatama area. A gentle homoclinal structure is developed on the east of the upheaved zone with general strike of NNW-SSE. The center of the sedimentary basin is considered to be in the northern part of Chiba Prefecture to the studied area.
(3) Evaluation of the initial thickness of these formations was based on the relation between porosity and burial depth. The relation was figured on silt and sand separately from the results of density logging and mechanical analyses of the present material referring to the published ones.
The initial porosity of silt from the surface to 10 m in burial depth seems to be 64 percent. The porosity decreases gradually with depth to 41 percent at 1, 000m, and 34 percent at 2, 000m. The initial porosity of sand from surface to 10m in burial depth seems to be 50 percent, then decreases with depth to 43 percent at 500m and 41 percent at 900m in burial depth.
(4) Decompaction number (D) indicates the ratio of initial thickness of a layer (Te) to thickness at a given burial depth (Tp) as follow.
D=Te/Tp
The decompaction number is considered to vary with burial depth, particle size of the sediments, and some other factors. The average decompaction number is about 1.53 in the upper part of the Kitatama Formation consisting mainly of siltstone, 1.13 in the Higashikurume Formation of sandstone, 1.13 in the Toneri Formation, 1.08 in the Edogawa Formation and 1.07 in the Takasago Formation.
(5) During deposition of additional layers, the surface of each formations had been sinking according to compaction of the lower formation and the own. At a sedimentation stage, both values of initial thickness and final one and compaction values, are generally larger in the northern part than in the southern part of the area.