Abstract
Pectinid-dominant molluscan assemblage is first reported from the upper middle Miocene Shimotezuna Formation of the Taga Group in the Takahagi district, Joban area, southern part of northeast Honshu, Japan. Stratigraphic horizon of this assemblage is correlative with the upper part of the Denticulopsis praedimorpha Zone (NPD 5B) of Yanagisawa and Akiba's (1998) diatom zonation, and is estimated to be between 12.0 Ma and 11.5 Ma in age. This assemblage is interpreted to be an indigenous shallow-water marine pebbly sand bottom community on the basis of lithology, generic composition and mode of fossil occurrence. Specific composition of the pectinids in this assemblage differs from those in the contemporaneous assemblages in the central part of northeast Honshu, by containing more relict elements from the latest early to early middle Miocene Kadonosawa Fauna and less characteristic elements of the middle middle to early late Miocene Older Shiobara-Yama Fauna. Such differences may have resulted from a remarkable latitudinal gradient of shallow marine climate in the Pacific coast of northeast Honshu due to the late middle Miocene climatic cooling.
