Abstract
The Tsubusagawa Formation is distributed in a small area of the northern part of Kyushu Island, and is composed of Pliocene fluvio-lacustrine deposits. Many important fossils for understanding the fauna of this age have been found from this formation, and are called the “Ajimu Fauna” (Takahashi, 2001). The age of the lower part of this formation, which is 3.5 Ma, was established by tephra correlations between this formation and the Miyazaki Group (Satoguchi, 2016).
The location where fossils are excavated from, in the Hirayama area, is a middle part of this formation, and is composed mainly of organic mud with thin sand beds. The depositional facies is estimated to have been around a swamp and/or pond. Two new described tephra beds, which are named the Hirayama 1 and 2 volcanic ash beds, are intercalated in this formation at this location. These beds are not correlated with any volcanic ash beds in this area.
