Abstract
The T 1 and T 4 volcanic ash layers in the Sarumaru Formation around Nagano City were correlated to the Ykp ash layer in the lower Hamatsuda Formation and the SK 110 ash layer in the Uonuma Group in the Niigata region respectively. The T 1 and Ykp ash layers are crystal-vitric, and hornblende and hyperthene are dominant as heavy minerals. Glass shards are fibrously vesiculated. Chemical composition of glass shards, hornblende (mg*=0.70-0.72) and hypersthene (mg*=0.60-0.65) is coincident. The T 4 and SK 110 ash layers contain biotite as dominant heavy mineral and shape and chemical composition of glass shards are coincident. Thus, the Sarumaru Formation can be correlated to the Hamatsuda (lower Nishiyama) Formation at base and to the Uonuma Group in the Niigata region, which indicates the Sarumaru Formation to be from lower Pliocene to lower Pleistocene (about 3.9-1.6 Ma) in age.
