Late Miocene marine diatom biostratigraphy is established for the upper Arakawa Group distributed in eastern Tochigi Prefecture, central Japan. The upper Ogane Formation and the Tanokura Formation are assigned to the upper Thalassiosira yabei Zone (NPD 5C) of the North Pacific diatom zonation (Yanagisawa and Akiba, 1998). The overlying Irieno Formation is correlated to the Denticulopsis dimorpha Zone (NPD 5D). The age of the upper Arakawa Group is estimated to be between 10.5 Ma and 9.2 Ma by combining data from diatom, radiolarian and nannofossil biostrati graphy, and a potassium-argon dating.
Paleobathymetric change is reconstructed for the upper Miocene sequence of the upper Arakawa Group distributed in eastern Tochigi Prefecture, central Japan, based on quantitative diatom analysis combined with analyses of benthic foraminifers and molluscs. The upper Arakawa Group constitutes a single sedimentary cycle with a upward bathymetric transition from outer sublittoral through middle bathyal to outer sublittoral environment. The middle part of the sequence represents an middle bathyal environment and includes three maximum flooding surfaces. Of the three, the first maximum flooding surface is the largest slightly above the base of the Tanokura Formation, where diatom bathymetric indices show a maximum value.
Marine diatom biostratigraphy is established for the upper Miocene Kubota Formation in the Higashi-tanagura area, Fukushima Prefecture, northeastern Japan. The formation is composed of muddy fine to medium-grained sandstone in the lower part, cross-bedded coarse-grained sandstone and bioturbated fine-grained sandstone in the middle part, and cross-bedded tuffaceous coarse-grained sandstone in the upper part. The Kubota Formation intercalates a number of key tuff beds (Kt-1 to Kt7). Diatoms are contained only in muddy trace fossil Rosselia or thin layers of tuffaceous mudstone. The interval between the tuff beds Kt-1 and Kt-4C is assigned to the upper Thalassiosira yabei Zone (NPD 5C) of Yanagisawa and Akiba (1998) because of the occurrence of Denticulopsis simonsenii and D. vulgaris without D. praedimorpha nor D. dimorpha. The lowest horizon of the upper part of the formation is also correlative with the upper T. yabei Zone (NPD 5C). Direct correlation of diatom biostratigraphy to calcareous nannofossil, planktonic foraminiferal and radiolarian zonations is established in the Kubota Formation, but this is not coincident in part with Saito’s (1999) Neogene magnetobiostratigaphic time scale. The middle part of the formation represents an outer sublittoral environment and includes three relative sea level changes with three marine flooding surfaces. Of the three, the first maximum flooding surface is the largest at a horizon two meters above the key tuff Kt3, where diatom bathymetric indices show a maximum value. Relative sea level changes recognized in the Kubota Formation are correlated to those in the upper Arakawa Group distributed in Karasuyama area, Tochigi Prefecture. They were precisely synchronous between the two areas. This implies that this relative sea level changes were not local but more regional events, and may possibly be controlled by global eustasy.
Marine diatom biostratigraphy is established for the middle Miocene Shimokurosawa and Kamikurosawa Formations in Ichinoseki area, Iwate Prefecture, northeastern Japan. The Shimokurasawa Formation is composed of fine- to medium-grained sandstone in the lower part and cross-bedded very coarse- to coarse-grained sandstone in the upper part. The formation is assigned to the Denticulopsis simonsenii Subzone (NPD 4Bb) of the Denticulopsis hyalina Zone of Yanagisawa and Akiba (1998). The Kamikurosawa Formation consists mainly of tuffaceous fine-grained sand stone, and overlies the Shimokurosawa Formation with partial unconformity. The diatom flora in the Kamikurosawa Formation is correlative with the Crucidenticula nicobarica Zone (NPD 5A),Denticulopsis praedimorpha Zone (NPD 5B) and the lowest Thalassiosira yabei Zone (NPD 5C). The transient decrease of C. nicobarica, a biohorizen recognized in the lower part of the D. praedimorpha Zone (NPD 5B), might be stratigraphically useful for correlation. At least eight microfossil biohorizons are utilized for precise correlation between Ichinoseki and southwestern Sendai areas, although the last occurrence of nannofossil Cyclicargolithus floridanus shows slight diachroneity.
Marine diatom biostratigraphy is established for the Pliocene sequence in the western part of Joetsu City, Niigata Prefecture, central Japan. The Kawazume Formation is composed mainly of conglomerate and sandstone and contains diatom assemblages correlative with the diatom zones NPD 7Bb of Yanagisawa and Akiba’s (1998) North Pacific diatom zonation. The overlying Nadachi Formation consists of gray mudstone, and is correlated to the diatom zones NPD 7Bb to NPD 9. The Tanihama Formation is composed of greenish gray tuffaceous mudstone containing diatoms correlative with the zone NPD 9. The Tanihama Formation also yields age-diagnostic warm water diatoms, and is assigned to the diatom zone NPD 15 of Barron’s (1985) low-latitude diatom zonation. Diatom chronology and tephrochronology indicate that the Kawazume, Nadachi and Tanihama Formations were deposited during the latest early Pliocene to the earliest late Pliocene (3.9-3.6 Ma~3.2 Ma), the early late Pliocene (3.2-2.4 Ma) and the late late Pliocene (2.4-2.0 Ma), respectively. Diatom assemblages from the Kawazume Formation and the upper Nadachi Formation (including the lowest Tanihama Formation) are characterized by cold water diatoms without any warm water taxa, suggesting that cold water condition was prevailing during these times. On the contrary, diatom floras from the lower Nadachi Formation and the main part of the Tanihama Formation are admixtures of both warm and cold water species. The occurrences of such warm water diatoms as Thalassiosira convexa, Nitzschia fossilis, N. reinholdii and Rhizosolenia praebergonii reflect incursions of warm water masses into the Sea of Japan during times of climatic warming in the mid Pliocene and latest Pliocene time.