山形県上山市に分布する泥部層（新第三系ないし最下部更新統）から見出された湖沼性珪藻化石と年代

抄録

Freshwater lacustrine diatom fossils are reported from samples of the Dorobu Formation around Dorobu, eastern Kaminoyama City, Yamagata Prefecture, Japan. The Dorobu Formation is composed of caldera lake deposits (about 250 m in thickness) of the Kaminoyama Caldera, consisting of tuff in the lower part, alternating beds of sandstone, mudstone and tuff in the middle part, and unstratified tuff breccia in the upper part. Freshwater diatoms occur in thinly laminated mudstone in the middle part of the formation. The authors have examined six samples and identified a total of 64 taxa belonging to centric diatoms (5 genera, 7 taxa), araphid pennate diatoms (9 genera, 13 taxa), monoraphid pennate diatoms (6 genera, 10 taxa) and biraphid pennate taxa (19 genera, 34 taxa), including unidentified taxa. The diatom assemblages of this formations are characterized by an abundance of Cyclotella comta (up to 94%), Tertiariopsis cf. costata (up to 86%) with few taxa such as Cyclotella antiqua and Pliocaenicus omarensis. Cyclotella comta from the Dorobu Formation may be the oldest record of the taxon in Japan. This paper is the first reliable report in Japan, describing the occurrence of C. antiqua, which is a rare diatom species limited in subarctic lakes of low pH and low conductivity. The abundant taxa and some accompanying ones are shown in LM and SEM photographs. The age of the Dorobu Formation has been previously considered to be late Miocene to Pliocene based on plant fossil assemblages. However, the diatom fossils and the K-Ar ages of the volcanic products unconformably covering the Dorobu Formation suggest that the age of this formation may be Pliocene to earliest Pleistocene.