Abstract
The authors carried out a geological survey of the Yaeyama mountain area located in the northwestern part of Kagoshima Prefecture and as a result of these findings 12 stratigraphic units were defined in the area. They established a stratigraphic framework for the Pliocene lacustrine sediments, such as the Koriyama Formation, which were difficult to correlate within the study area due to the sporadic distribution of those sediments. The Koriyama Formation is divided into four members: the Atsuchi Mudstone, the Toge Sandstone, the Yuya Mudstone and the Hanao Tuff Breccia in ascending order. Furthermore, the Koriyama Formation is accompanied by intrusive andesitic rocks (the Nasubida Andesites) while the Yuya Mudstone Member is interbedded with two pyroclastic flow deposits (the Kusakidan and the Miyawaki Pyroclastic Flows). K-Ar age obtained from the Miyawaki Pyroclastic Flow deposits is about 2.5 Ma. Fossil diatoms collected from the Koriyama Formation are divided into three assemblages within the Yuya Mudstone Member: the Stephanodiscus astraea assemblage in the lowest horizon, the Cyclotella michiganiana assemblage in the lower horizon and the Aulacoseira ambigua assemblage in the upper horizon. Accompanying these three subdivisions are the two previously mentioned pyroclastic flow deposits. It has been made clear that the Koriyama Formation sporadically distributed in the Yaeyama mountain area is synchronous lacustrine sediments based on correlations with the interbedded pyroclastic flow deposits and fossil diatom assemblages.
