Correlation among the Tephra Layers in the Lowest Daisen Volcanic Ash Formation, Western Japan, on the Basis of Their Petrographic Properties

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We determined the detailed petrographic properties (grain composition, heavy mineral composition, morphology and refractive indexes of volcanic glass shards, and refractive indexes of orthopyroxene, hornblende and cummingtonite phenocrysts) for the following nine tephra layers of the Daisen Volcanic Ash Formation in the Chugoku District: the Daisen Okutsu Pumice (DOP), hpm1 Pumice (hpm1), hpm2 Pumice (hpm2), Daisen Beshyo Pumice (DBP), and Daisen Hiruzenbara Pumice (DHP) Beds of middle Pleistocene age, and the Shitano-hoki (Sh), Odori Volcanic Sand (Od), Ueno-hoki (Uh), and Misen Pumice (MsP) Beds immediately above the Aira-Tanzawa tephra. The tephra layers are distinguishable by ratios of heavy mineral crystals to light ones and contents of cummingtonite and brown hornblende phenocrysts, and by refractive indexes of glass shards, hornblende, and cummingtonite crystals in spite of their similarity of heavy mineral composition, morphology of glass shards, and refractive indexes of orthopyroxene phenocrysts. In these tephra layers, it is possible that DOP is correlated with hpm1, because they have very similar petrographic properties and occur at the same relative stratigraphic and chronostratigraphic horizon with fission-track ages. A 50-cm thick tephra layer found in the northern part of Hyogo Prefecture, about 100km east of Daisen Volcano, is newly correlated with a DBP, indicating wide spread at least in the northern Kinki District.