Calcareous Nannoplankton Biostratigraphy of the Miyazaki Group, Southeast Kyushu, Japan

The Miyazaki Group is a typical late Cenozoic marine stratigraphic sequence in Japan and its stratigraphy and paleontology have been carried since the beginning of this century. Especially in them, SHUTO's integrated works give us indispensable basic informations on the geology of the Miyazaki Group. The Miyazaki Group consists of the Tano, Aya, Tsuma, Sadowara and Takanabe Formations in ascending order and is composed of conglomerate and sandstone in the lowest part and siltstone to mudstone in the lower to upper part. The greater part of the group yields abundant well preserved molluscan fossils and calcareous microfossils. Calcareous nannoplankton biostratigraphy was clarified on the Miyazaki Group developing in the northwestern hilly land of Miyazaki City. Based on the present study the Miyazaki Group extends from late Miocene to earliest Pleistocene in age and its age assignment agrees well with the same proposed by the planktonic foraminiferal biostratigraphy. The lower part of the Miyazaki Group is characterized by the joint occurrence of Reticulofenestra pseudoumbilica and Sphenolithus abies, and both biostratigraphically noticeable taxa extinct in the Sadowara Formation. Pseudoemiliania lacunosa appears in the lower part of the Sadowara Formation and coexists with Reticulofenestra pseudoumbilica and Sphenolithus abies. In the upper part of the Takanabe Formation planktonic foraminifer Globorotalia truncatulinoides and calcareous nannoplankton Gephyrocapsa oceanica appear in the close horizon and Discoaster brouweri extincts just under the appearing horizon of Gephyrocapsa oceanica. Therefore the Pliocene-Pleistocene boundary is expected in the neighborhood of them. Based on the occurrence mode of Gephyrocapsa oceanica, the Pleistocene in the Miyazaki Group would be limited in the lowest part of the Pleistocene