環境

To confirm the eastern distribution of the Kazusa Group exposed in the Tama Hill and a river floor of the Tamagawa River, we expanded the study area to below the Musashino-daichi and Tokyo low land using drill cores. Then, we studied the litho-stratigraphy of the Kazusa Group and the geological correlation of the group among those areas. Sedimentary environments of the Kazusa Group in those areas were reconstructed by sedimentary facies analysis, resulting in a new solution to the age of the tephra in contrast to previous studies. Based on characters of molluscan assemblages, paleo-depth of Pliocene basin of the areas was estimated, and the influence of warm and cold currents in these areas, as well as the stratigraphic changes were analyzed. Then, the influence of the results caused by the Glacial Sea level changes were discussed. To check their influence in land area, we also studied Palynostratigrahy of the Kazusa Group in the Tama Hill and a river floor of the Tamagawa River. And we presented updated outcrops of the Kazusa Group in those areas. We mentioned that a site on a river bed of the Tachikawa and Hino area is the best site for teaching field geology, and reported its study results for middle school and for teacher’s workshop.

1. Study of stratigraphy and environmental reconstruction of underground of Musashino-daichi and Tokyo low land, and the influence of Glacial Sea level changes: molluscan fossils in drill cores were analyzed for paleo-depth and influence of sea level changes for the Kazusa Group. As a result, it is expected that the age of the group range from ca. 1.8 Ma to 1.1 Ma. 2. Study of aquifers: we can present litho-stratigraphic correlation of the Kazusa Group between above and below-ground strata, which likely shows correlation of aquifers in those areas. 3. Reconstruction of sedimentary environments of the Kazsusa Group along the Tanagawa River system based on methods of sedimentary facies analyses. This is a good method for non-fossil occurrence strata. It is necessary to measure succession of strata, and we must work quickly because many good exposures have been destroyed and buried by river improvement.

Katsuyoshi Baba，
Keiichi Hayashi，
Kazuto Koarai，
Reiji Kukihara, Makiko Miyaguchi）