北多摩地区の地下地質

Abstract

In order to study hydrogeological problems such as land subsidence and groundwater pollution by organic chlorine compounds in Tama District, the west part of Tokyo Metropolis, Tokyo Metropolitan Government made 18 stratigraphical drillings from 1972 to 1991 which were from 402m to 1, 003m in depth. As the result of this inverstigation, the basal Pre-Tertiary rocks were found at the deep underground in the westernmost part of Musashino Terrace, Mizuho town, Tachikawa city, Musashimurayama city, Akishima city and Hachiouji city. The Quaternary sediments covering these rocks with unconformity are divided stratigraphically into two groups: the lower is the Kazusa Group, and the upper the Tokyo Group. The Kazusa Group comprises the Kitatama Formation and Higashikurume Formation, in ascending order, the Tokyo Group comprises the Toneri Formation and the Edogawa Formation.The Kitatama Formation of the Kazusa Group, the geological oldest sediments discussed here, is developed throughout the Musashino Terrace and consists chiefly of impermeable massive silty layer. On the other hand, formations overlying the Kitatama Formation consist chiefly of permeable sand and gravel layers, and so acts as confined aquifers in this area.The geologic structure of the most western part in the Musashino Terrace is rather different from the east one. In the most western part, the structure of each formation undulates between two axes elongating parallel to the NW-SE trend. It is ascribed to the two reverse faults named Tachikawa Fault and Mizuho One.