Freshwater lacustrine centric diatom fossils from the Neogene marine sediments in the Dewa Mountains, Akita Prefecture, Northeast Japan

Abstract

Neogene marine deposits in Japan sometimes contain considerable amounts of lacustrine planktonic diatom fossils transported from inland lakes as silt-sized particles through rivers. This study describes morphology and stratigraphic range of such allochthonous lacustrine centric diatoms preserved in the late Miocene and early Pliocene marine sediments of the Onnagawa, Funakawa and Tentokuji formations distributed in the Dewa Mountains, Akita Prefecture, Northeast Japan. Ten lacustrine centric diatom species belonging to seven genera, excluding Aulacoseira, were identified from these formations. Age of the first occurrence (FO) of each taxon is estimated by a sediment accumulation rate curve constructed based on marine planktonic diatom biochronology. Five significant events of lacustrine diatoms on generic level were recognized: the FO of Mesodictyon (8.3–8.5 Ma), the FO of Cyclotella (5.9−6.1 Ma), the FO of Tertiariopsis (4.5–4.9 Ma), the FO of Stephanodiscus (4.4–4.7 Ma) and the FO of Lindavia (3.5−3.9 Ma). Of these events, the FOs of Mesodictyon, Cyclotella and Lindavia are almost synchronous between the Dewa Mountain and Niigata area, suggesting a possible biostratigraphic utility of these biohorizons. Stratigraphic occurrence pattern of lacustrine diatoms preserved in marine sediments may reflect turnover of lacustrine diatoms in Japanese Islands through geologic time.