抄録
The Yamatoda diatomaceous mudstone member (10∼15 m thick), which is characterized by the dominance of freshwater diatoms, unconformably overlies a two-pyroxene andesite and its agglomerates (Anamizu formation, F2, Middle Miocene), and is disconformably overlain by the marine Hamada mudstone member., The Yamatoda diatomaceous mudstone member also contains several plant fossils such as Comptoniphtyllum sp., Liquidambar sp., etc., and its geological age is refered to later Middle Miocene (F3)., By a detailed research on the diatom remains in the mudstone (Table 2), it is revealed that the marine diatom species (Actinocyclus sp., etc., ) are frequently found at the several localities in the eastern half, while the western half is characterized only by fresh-water species such as Melosira granulata, etc., Therefore, it is suggested that the Yamatoda diatomaceous mudstone member accumulated in a small lake which was connected with the sea (perhaps by a channel) on its eastern end., Latest the sea level rose gradually, and at last, marine water abruptly invaded the whole area, forming the interesting disconformable relation between the Hamada and the Yamatoda mudstone members (Fig 2).,
