三国山地平標山に分布する第四紀末期のテフラ層

Abstract

Sum of eleven tephras occur in Mount Tairappyo (36° 48' N, 138° 49' E, 1, 984 m asl) ofthe Mikuni Mountains, central Honshu Island, Japan. Although these tephras are thought to be useful for Quaternary studies in this area, they have not been described systematically. We investigated stratigraphy, lithofacies, petrographic properties, and refractive indices of volcanic glass shards and phenocrysts of these tephras in order to correlate them with the known ones. We also estimated the calendric ages of the tephras based on their stratigraphy and the radiocarbon ages in the previous references.The followings were identified : 1) the Haruna-Futatsudake-Ikaho as 1.44 ka, 2) the Haruna-Futatsudake-Shibukawa as 1.47 ka, 3) the Myoko-Otagirigawa as 4.7 ka, 4) the Asama-Tairappyosan 1 as 4.9 ka, 5) the Asama-Tairappyosan 2 as 6.3 ka, 6 and 7) the Myoko-Akakura and the Asama-Tairappyosan 3 as 6.6 ka, 8) the Asama-Tairappyosan 4 as 6.8 ka, 9) the Kikai-Akahoya as 7.2 ka, 10) the Asama-Tairappyosan 5 as 7.5 ka, and 11) the Asama-Kusatsu as 16 ka.Among them, the Myoko-Akakura and the Asama-Tairappyosan 3 are very unique. The former overlies the latter without intercalation of soil layers, and these two tephras form a thin layer. A set of the two can be correlated with the tephra X found from the Ozegahara bog about 60 km east of Tairappyo. Depositional conditions of these tephras imply that the eruption of Myoko volcano followed that of Asama without long interval, possibly less than few years, in the mid Holocene. A thin layer composed of the Myoko-Akakura and the Asama-Tairappyosan 3 is important as a key bed, as well as the Haruna-Futatsudake-Ikaho, the Kikai-Akahoya, and the Asama-Kusatsu all of which have been described as wide spread tephras. All the tephras provide Quaternary studies in this area with datum planes.