Non-marine diatom fossils from the Wada Formation in Takahata Town, Higashiokitama District, Yamagata Prefecture, Japan, with reference to their depositional age

抄録

The depositional age of the Wada Formation in Kamiwada, Takahata Town, Higashiokitama District, Yamagata Prefecture, Japan, has been unknown. Correlation with other formations in the surrounding area has been difficult because the Wada Formation occurs only in the Wada Basin. To estimate the depositional age of the formation, diatom fossils in tuffaceous mudstone taken from an outcrop located along the Oguro River were identified. The fossil diatom flora was dominated by two taxa, Cyclotella iris Brun & Héribaud sensu lato (s.l.) and Mesodictyopsis miyatanus Tanaka, with rarer Fragilaria and other araphid taxa forming ribbon-like colonies. Based on age ranges of the occurrences of C. iris s.l. and Mesodictyopsis species in the Japanese Islands, the depositional age of the Wada Formation is identified as latest Miocene or early Pliocene, not much earlier than 6 Ma.