東京都板橋区および大田区地下に産出する前期更新世テフラの層序と対比

Abstract

Two boring cores obtained by boring surveys conducted at Akatsuka Park of Itabashi Ward and at Haginaka Park of Ota Ward in the eastern part of the Musashino Uplands, Tokyo metropolitan area, were reexamined. Shapes, refractive indices, and chemical composition of volcanic glass shards, mineral composition, and refractive indices of heavy minerals of several tephra layers from these cores were determined. As a result, it was revealed that 6 tephras correlate with the tephras in the Kazusa Group of the Boso Peninsula, where magnetostratigraphy, biostratigraphy, and tephrostratigraphy had been well established. A-22, A-16, and A-10 tephras collected from Akatsuka Core are identified as U8 tephra (0.85-0.95 Ma) in the lower part of the Umegase Formation, O18 tephra (1.07-1.16 Ma) in the middle part of the Otadai Formation, and Kd5A tephra (1.21-1.27 Ma) in the upper part of the Kiwada Formation, respectively. H-27, H-7, and 11-2 tephras collected from Haginaka Core are identified as Kd21 (>1.45 Ma), Kd24, and Kd25 (1.60-1.65 Ma) tephras positioned in the middle part of the Kiwada Formation, respectively. Thus, these correlations indicate that the sediments (mostly of marine origin on the continental shelf) approximately-50 m and -170 m depth in the northern part of Central Tokyo correlate with the lower part of Umegase Formation to the upper part of the Kiwada Formation, and the sediments (showing oceanic and bathyal environment) -50 m t -100 m depth in the southern part of Central Tokyo correlate with the middle part of the Kiwada Formation. Comparing sedimentary facies and thickness between central Tokyo and the Boso Peninsula, it is suggested that the Kazusa Group in the Boso Peninsula is composed of semi-pelagic sediments accumulated in a subsiding forearc basin called the Kazusa Trough, and that the sediment environment in central Tokyo had changed from semi-pelagic in the Kazusa Trough to shallow marine on continental shelf around 1.3 Ma.