Marine diatom biostratigraphy and biohorizons of the middle Miocene sequence in Ichinoseki area, Iwate Prefecture, northeastern Japan

摘要

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 sandstone, 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 micro fossil biohorizons are utilized for precise correlation between Ichinoseki and southwestern Sendai areas, although the last occurrence of nannofossil Cyclicargolithus floridanus shows slight diachroneity.