A widespread volcanic ash layer of about 2.7Ma in central Japan; Correlation of the Habutaki I(Osaka Group), the MT2(Himi Group) and the Arg-2(Nishiyama Formation) ash layers.

Abstract

A late Pliocene widespread volcanic ash layer in central Japan was detected both on the Pacific Ocean side and the Sea of Japan side. The Habutaki I ash layer in the Osaka Group and the Minamidani 1 ash layer in the Tokai Group around Ise Bay were correlated to the MT 2 ash layer in the Himi Group around Toyama Bay and also the Arg-2 ash layer in the Nishiyama Formation of the Niigata region. They are waterlaid airfall ash layers of 13-150 cm in total thickness. These ash layers are placed in the Gauss chron and are assumed to be about 2.7 Ma in age. They commonly consist of fine-grained vitric ashes and dominant in bubble-wall type glass shards. The mineral association is characterized by high-quartz, orthopyroxene and hornblende, and chemical composition of glass shards is also coincident, namely SiO_2=79.5-81.5%, TiO_2=0.18-0.28%, Al_2O_3=11.5-12.4%, FeO=0.95-1.15%, MnO<0.08%, MgO=0.13-0.19%, CaO=0.70-0.88%, Na2O=1.1-2.0% and K_2O=2.7-4.0% on water-free basis. The Minamidani 1 ash was also so far correlated to the Arigaya I ash layer in the Kakegawa Group around Suruga Bay. The name of the Habutaki I-MT 2 ash layer is proposed as the general name of these widespread ash layers. The ashfall area of the Habutaki I-MT 2 ash attains 430 km×280 km in central Japan.