Late Quaternary Tephras in the Tokachi Plain and the Hidaka Range, with Special Reference to Glacial Sediments

概要

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Pleistocene tephras are significant to examine the late Quaternary glaciation in the Hidaka Range, Hokkaido, Japan. Six marker tephras are found in the glacial deposits around Mt. Tottabetsu and Mt. Esaoman-Tottabetsu. On the basis of their mineral compositions and the refractive indices of volcanic glass and orthopyroxene, these tephras are correlated with such tephras in the Tokachi Plain as Toya, Kuttara-6 (Kt-6), Rakko-3 (RP3), Kuttara-3 (Kt-3), Shikotsu-1 (Spfa-1), and Eniwa-a (En-a). Identification of these tephras makes it possible to discuss the precise chronology of glaciation of the Last Glacial stage and the occurrence of larger glaciation during the penultimate glacial stage in the Hidaka Range. Maximum expansion of valley glacier occurred around 40-42ka represented by Spfa-1. En-a indicating approximately the Last Glacial Maximum is found in the glaciofluvial deposits at many localities. Kt-6 identified in the glaciofiuvial deposits implies that the glaciation had already occurred during Oxigen Isotops Stage 5b.Nozuka pumice (NzP, in this paper) is newly identified in the Tokachi Plain; it is probably a widespread marker tephra indicating the early time of Oxygen Isotope Stage 6.