北アルプス,白馬岳東方,松川北股入のモレ-ンを覆う示標テフラ層の発見とその意義

概要

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A terminal moraine of the Yoshihara glacial stage was considered as the oldest glacial advance in the Matsukawa Kitamata Valley (KOAZE et al. 1974). By trenching the top of the terminal moraine (Figs. 1 and 2), the authors discovered the Aso-4 ash fall and Epm pumice fall in the tephra layer covering the glacial till. The total thickness of the tephra layer overlying the till is 270 cm, and the horizons of the Aso-4 ash fall and the EPm pumice fall are about 245 cm and 175 cm deep from the ground surface respectively (Fig. 3). Both marker tephra were identified by the examination of their petrographic characteristics such as heavy mineral composition and reflaction index of glass, orthopyroxene and hornblende (Table 1 and Fig. 3). According to the inferred age of these marker tephra (MACHIDA <I>et al.</I> 1985, MACHIDA and ARAI 1979), the age of the Yoshihara glacial stage is, at least, before 70, 000 y. B. P.