Latest Quaternary Features of Tokyo Bay and Its Environs

Abstract

The areas around Tokyo Bay offer the standard stratigraphic column of the Quaternary system in Japan. The writers have investigated the geomorphology and surface geology of the latest Quaternary in these areas. The results are summarized as follows:1) The latest Quaternary marine and fluvial deposits (theso-called Alluvial deposits) in and around Tokyo Bay were accumulated during the regression and transgression that had occurred since the beginning of the Late Wisconsin (or WurmIII) glacial age or about 35, 000 B. P..2) Subsurface data from boring cores in the coastal areas is now sufficient for presenting the distribution of the depths of the base of the latest Quaternary deposits (fig. 2) and for presenting the section of sedimentary facies of the latest Quaternary deposits around Tokyo Bay from Hommoku Cape to Futtsu Cape (fig. 3). The deposits can be divided into two members, the Upper and the Lower, based on the cycle of sedimentation.3) According to the mineral composition of the volcanic ash beds covering the terrace surfaces, the ashes on Aoyagi terrace (fig, 4, right) is correlated to the upper part of ones on Tachikawa terrace (fig4, left).4) Fig. 5 shows the detailed isopach map (numericals in meters) of the latest Quaternary deposits along the lower part of the Tama River.5) The longitudinal profiles of the terraces, the buried terraces and recent water course of the Tama River are projected in fig. 6. From the figs. 5 and 6, the gravel bed forming Haijima terrace can be correlated to the buried gravel bed of 60m below sea level at the mouth of the Tama, and the gravel bed forming Tachikawa terrace to the buried gravel bed of 20-40m below sea level. These two terraces have a steep gradient caused by the lowering of the sea level during the Late Wisconsin glacial age.6) In conclusion, the writers have established the landform evolution of the areas during the latest Quaternary epoch. The sequence of the features is as follows:1. Deposition of the gravel bed forming Tachikawa terrace and the buried gravel bed of 20-40m below sea level at the mouth of the Tama.2. Deposition of the gravel bed forming Aoyagi terrace and Egota (or Ekoda) Conifer bed.3. Deposition of Tachikawa volcanic ash beds during 1 and 2.4. Deep entrenchment of rivers……deposition of the gravel bed forming Haijima terrace……forming of submarine terrace of 100m below sea level.5. Rapid transgression……deposition of the Lower member of the latest Quaternary formation.6. Retarded transgression (Allerod)……forming of terrace now buried 20-30m in depth.7. Re-transgression……deposition of the Upper member of the latest Quaternary formation.8. Building of wave-cut terrace now buried less than 10m in depth and terrace of less than 4m in depth……forming of alluvial plain of Shitamachi.7) The above evolution reflects world-wide events. The correlation between the development of landforms of Tokyo Bay and the eustatic changes of sea level is estimated as shown in fig. 7.