Abstract
A diatom assemblage containing Neodenticula seminae and Proboscia curvirostris as well as numerous reworked specimens, is reported from a sample from the Yuchi Formation distributed in the Tenpoku area, northwestern Hokkaido. The formation has been generally regarded to be Pliocene in age on the basis of the occurrence of the molluscan Takikawa - Honbetsu Fauna and the recognition of the diatom "Denticulopsis seminae var. fossilis - D. kamtschatica" Zone. The occurrence of N. seminae suggests that the reported "D. seminae var. fossilis" in the formation may include both N. koizumii and N. seminae, and the finding of the Pleistocene species P. curvirostris implies that the formation is Pleistocene in age at least partially. The present result further indicates that other strata correlated to the Yuchi Formation in various areas of Hokkaido also need re-examination of their geologic ages in order to evaluate the timing of the youngest Cenozoic marine sedimentation in Hokkaido more precisely.
