Petrographic Properties of Visible Tephra Layers in SG93 and SG06 Drill Core Samples from Lake Suigetsu, Central Japan

Abstract

In this paper, we report the petrographic properties of visible tephra layers sampled from two drill cores（SG93 and SG06）from Lake Suigetsu, central Japan, because no fundamental petrographic data including the refractive indices of minerals and volcanic glass shards, the shapes of volcanic glasses, and mineral compositions of the Suigetsu tephra samples have been reported. The Suigetsu tephra layers can be broadly classified into those derived from calderas on Kyushu Island（e.g., Aso, Ata, and Aira）and those that derive from Daisen and Sambe volcanoes in the Chugoku district of southwest Japan. The tephra layers correlated with the Ulleung-Oki tephra from Ulleung Island on Japan Sea, the Sakate tephra which has a close relationship with the Sambe-Ukinuno tephra, and the scoria layer from an unknown source volcano were also found in the Suigetsu core samples. The Suigetsu tephra layer correlated with the San’in 1 tephra found in the drill core bored from Japan Sea can be also correlated with the Kuju-1 tephra which erupted from the Kuju caldera on Kyushu Island, based on the petrographic properties. The petrographic characteristics provide important constraints on the correlation and identification of tephras, which cannot be obtained by only the major element compositions of volcanic glass shards. However, the correlation of some tephra layers remains ambiguous. Therefore, additional geochemical data from volcanic glass shards including the concentrations of various trace elements are also necessary for a more rigorous identification and correlation of the Suigetsu tephra.