Tephrostratigraphy of the Kazusa Group in the Boso Peninsula and the comparison of tephrozones among the Kazusa, the Osaka, and the Uonuma groups, Japan

Satoguchi, Y.

Chikyu Kagaku = Earth Science 51(2): 104-116


Accession: 020211651

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Volcanic ash layers of more than 250 beds intercalated in the Plio-Pleistocene Kazusa Group. Boso Peninsula, Japan were examined petrographically and lithologically. The ashes are classifiable into biotite-rich type (B-type), amphibole-rich type (A-type) and clinopyroxene-rich type (C-type). The ashes are vitric, crystalline, pumiceous and scoriaceous of various grain sizes. A particular type of ash layer tends to predominate at certain stratigraphic levels in the Kazusa Group, and it is able to establish tephrozones. Established tephrozones are Kz I. Kz II, Kz III, and Kz IV in ascending order. Kz I includes mainly C-type coarse ashes and a few A-type coarse ash layers. Kz II is characterized by B-type fine ash layers. Scoriaceous coarse ashes of C-type are dominant in Kz III. Kz IV contains many C-type pumiceous ash layers with intercalation of a few B-type ash layers. The Osaka Group in southwestern Japan and the Uonuma Group in Niigata Prefecture were deposited in the same period as the Kazusa Group. Previously established tephrozones in these sedimentary basins were compared with those in the Kazusa Group. The petrographic characteristics of the tephrozones among the separated sedimentary basins are quite different, even those of the same age. This indicates that the activities of volcanoes near the basins are largely responsible for the formation of tephrozones in each sedimentary basin. Petrographic study of tephrozones in basin-fill deposits is expected to provide general information of temporal-spatial changes in volcanic activity. Such information would be valuable for reconstruction of the volcanic activities of old and missing volcanoes in the Plio-Pleistocene.