三国山地平標山に分布する第四紀末期のテフラ層 Latest Quaternary Marker Tephras in Mount Tairappyo, Central Japan
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.<BR>The followings were identified : <BR>1) the <I>Haruna-Futatsudake-Ikaho</I> as 1.44 ka, 2) the <I>Haruna-Futatsudake-Shibukawa</I> as 1.47 ka, <BR>3) the <I>Myoko-Otagirigawa</I> as 4.7 ka, 4) the <I>Asama-Tairappyosan</I> 1 as 4.9 ka, 5) the <I>Asama-Tairappyosan</I> 2 as 6.3 ka, 6 and 7) the <I>Myoko-Akakura</I> and the <I>Asama-Tairappyosan</I> 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.<BR>Among them, the <I>Myoko-Akakura</I> and the <I>Asama-Tairappyosan</I> 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 <I>tephra X</I> found from the Ozegahara bog about 60 km east of Tairappyo. Depositional conditions of these tephras imply that the eruption of <I>Myoko</I> volcano followed that of <I>Asama</I> without long interval, possibly less than few years, in the mid Holocene. A thin layer composed of the <I>Myoko-Akakura</I> and the <I>Asama-Tairappyosan</I> 3 is important as a key bed, as well as the <I>Haruna-Futatsudake-Ikaho</I>, the <I>Kikai-Akahoya</I>, and the <I>Asama-Kusatsu</I> all of which have been described as wide spread tephras. All the tephras provide Quaternary studies in this area with datum planes.