南関東における後期更新世の示標テフラ層――特性記載とそれに関連する諸問題

Abstract

This paper presents a catalog of late Pleistocene tephras in the South Kanto, showing their source volcanoes, ages, petrographic characteristics, distribution and volumes for convenient use in stratigraphic and chronological studies. Fourty-nine markers formed after the last Interglacial (ca. 130, 000-140, 000 YBP) as a result of eruptions from the Hakone, Fuji and other volcanic centers, are documented. Here, the unit of the tephra as a rule represents a deposit of one cycle of eruption, representing a brief moment in geologic time.Among the valuable characteristics for identification of the tephras are their petrographic properties. Above all, the refractive indices of certain phenocrysts (especially of orthopyroxene and hornblende) and the volcanic glass are probably the most useful. In most cases the range of petrographic characters in the marker is small. Exceptionally, however, there exist specific tephras (KlP-9-KmP-5), the petrographic features of which differ considerably between the upper and lower units of the tephra sheet. Detailed analysis of the compositional variation of the KmP-1, one of such specific markers, erupted from the Hakone volcano approximately 100, 000 YBP, possibly indicates that the magma reservoir underwent some zonation as a result of differentiation during crystallization.From the stratigraphic relationships between the marker-tephras and adjacent marine sediments, the chronology of the marine terraces and various related problems of the late Pleistocene are discussed in detail. The pumice-fall deposit TAu-12, one of the most prominent markers (dated at 140, 000-150, 000 YBP by the fission-track method), formed during the Shimosueyoshi transgressive stage, since it lies within the Shimosueyoshi marine formation at several localities. The KlP-1 and following tephras (KlP-2 to KlP-13, dated at approximately 132, 000 to 120, 000 YBP) formed during the regressive stage immediately after the highest sea level. The Shimosueyoshi rise in sea level possibly progressed rapidly in contrast with the slow fall during the regressive stage. Such changes in sea level are inferred to resemble those of the Recent deglacial hemicycle. After the Shimosueyoshi stage, the following three marine terraces were formed: the Hikihashi terrace (ca. 100, 000 YBP), the Obaradai terrace (ca. 80, 000 YBP), and the Misaki terrace (ca. 60, 000 YBP). These are easily recognizable from the tephrostratigraphy over most areas of the South Kanto.