Pliocene-Pleistocene sequences deposited in lacustrine and bay environments are widely distributed in the Kinki District, and many water-based volcanic ash layers are intercalated in those sediments, though active volcanoes of Quaternary age are not located. The main interest in the study of Kinki district tephras is related to tephra identification for use as chronostratigraphic horizons.
The history of research on tephras is divided into three stages. During the first stage, fundamental tephra discrimination was carried out on the basis of thickness, color, grain size, and succession in the field. In the second stage, petrographic characters (heavy mineral composition, glass shape, refractive index, magnetic characteristics, etc.) were analyzed for tephra identification. Recently, we have carried out tephra identification on the basis of the single grain method using EDX chemical analysis and refractive index data obtained by the thermal immersion method (RIMS 86), adding petrographic data.
One of the recent topics of tephra study in the Kinki District is obtaining continuous and sequential sections in Lake Biwa and Osaka Bay. In Osaka Bay, the volcanic glass stratigraphy labeled V1-V17 (in descending order) was constructed using 65 drilling core samples. Two of them are correlated with the widespread tephra, and V17 is correlated with the Azuki volcanic ash layer that is the most valuable key bed in the Osaka Group. In Lake Biwa, two drilling operations (at 200m depth and 1, 400m depth) were carried out. The uppermost part (T bed) is composed mainly of massive clay and reaches a thickness of 250m. It is thought that this continuous clay includes most of the tephra that has fallen in the Kinki District during about 400, 000 years. Thirty-eight volcanic ash layers are intercalated in the T bed. Some of them (especially in the upper part) are correlated with some of the widespread tephras. The Lake Biwa and Osaka Bay sequences are very valuable in the identification of a tephrostratigraphy for the Kinki District.