新潟県中部下田地域の上部中新統-鮮新統の珪藻化石層序と年代層序

抄録

Marine diatom biostratigraphy is established for the upper Miocene to upper Pliocene sequence (the Shigekurayama, Taira and Minagawa Formations) in the Shitada area, Niigata Prefecture, central Japan. The Shigekurayama Formation is composed of dacite lava and pyroclastic rocks with large mudstone blocks which contain diatom assemblages correlative with the diatom zones NPD 5C to NPD 6B. The age of Shigekurayama Formation is inferred to be 8.0-8.4Ma on the basis of diatom ages of the underlying Minamiimogawa and overlying Taira Formations. The Taira Formation is divided into the lower and upper parts by a late Miocene unconformity ranging from 7.2Ma to 5.4Ma in age. The lower Taira Formation is correlated to the diatom zones NPD 6B to NPD 7A, whereas the upper Taira Formation contains diatoms correlative with the zone NPD 7Bb. The Minagawa Formation yields no age-diagnostic diatoms, but may be late Pliocene in age based on tephrochronology.