Abstract
The stratigraphy and molluscan fauna of the lower part of the Miyazaki Group distributed in the Aoshima area, southern Kyushu, were reexamined on the basis of the new biostratigraphic and paleontologic evidence. The Miyazaki Group in this area is lithologically divided into the Boroishi, Kaichigo, Gonoharu, Janokawauchi, Aoshima and Tano Formations in ascending order. The upper part of the Gonoharu Formation and the Janokawauchi Formation can be assigned to planktonic foraminiferal Zone N. 16 of Blow (1969), while the Aoshima and Tano Formations can be assigned to Zone N.17-N.18. Although the geologic age of the lower two formations could not be determined because of poor preservation of planktonic foraminiferal fossils, the fining-upward lithologic sequence caused by the increase of the bathymetrical depth and the occurrence of the tropical to subtropical molluscan fossils from the Kaichigo Formation suggest that these formations may have deposited in Zone N. 16 when the global marine transgression was recorded.The molluscan fauna of the upper Miocene Kaichigo Formation consists of the Megacardita taiwanensis-Donax sp. assemblage and the Amusium cf. pleuronectes-Amussiopecten sp. A assemblage. These assemblages have never been reported in the Neogene of Japan. The diagnostic species of these assemblages such as Megacardita taiwanensis, Amusium cf. pleuronectes have been known from the Miocene strata in Taiwan. On the other hand, the molluscan fauna of the uppermost Miocene to lower Pliocene Tano Formation consists of the characteristic species of the Zushi Fauna of central to southwest Japan. The molluscan fauna of the Tano Formation differs in species composition from that of the Kaichigo Formation. In fact, several genera such as Amussiopecten and Megacardita are represented by different species between these two formations. In this paper, the Kaichigo Fauna is newly proposed for a late Miocene tropical to subtropical molluscan fauna prior to the Zushi Fauna.
