Abstract
Ng-1 (0.30Ma) is a widespread tephra covering the Tokai, Kinki, and Shikoku areas, and the location of its source vent is assumed to be in the Hida Mountains. However, the existence of proximal deposits of this tephra has not been confirmed around the vent. At the same time, a pumice fall deposit named the Takayama Pumice Layer (Tky) has been distributed in and around the Takayama Basin, west of the Hida Mountains. Some previous studies identified Tky as one of the Omachi A Tephra Layers (APms: 0.33-0.40Ma) derived from a volcano in the Hida Mountains. However, other studies concluded that Tky was derived from the Hakusan Volcano, about 45km west of the Takayama Basin.
This study clarifies that Ng-1 is correlated to Tky, not to APms, and assumes that the source is most likely to be in the Hida Mountains. The following similarities confirm the correlation between Ng-1 and Tky: mineral assemblage, refractive indices of the volcanic glass, hornblende and orthopyroxene, and the chemical composition of titanomagnetite. Moreover, the distributions of Ng-1 and Tky are limited in the southwest area of the Hida Mountains.
The age of Ng-1 is estimated by previous works to be 0.30Ma, which means that Tky provides a significant datum plane of 0.30Ma around the Takayama Basin.
