Bathonian/Callovian (Middle Jurassic) ammonite biostratigraphy of the Kaizara Formation of the Tetori Group in central Japan
Handa, Naoto; Nakada, Kentaro; Anso, Junko; Matsuoka, Atsushi
Newsletters on Stratigraphy Volume 47 Number 3 (2014), p. 283 - 297
published: Sep 1, 2014
ArtNo. ESP026004703003, Price: 29.00 €
The definition of the Global Boundary Stratotype Section and Point (GSSP) of the Bathonian/Callovian (Middle Jurassic) boundary is under debate in Europe. The sections that include the Bathonian/Callovian boundary are still limited in East Asia. In Japan, the Middle Jurassic Kaizara Formation, which is located within the Tetori Group, is exposed in Fukui Prefecture, central Japan. This formation contains abundant ammonite fossils. On the basis of ammonite fossils, the Kaizara Formation has been assigned to the late Bathonian to early Callovian age. Therefore, the Kaizara Formation is an important formation encompassing the Bathonian/Callovian boundary in East Asia. However, the fossil-bearing horizons have not been observed clearly in previous studies. In the present study, a detailed analysis of the molluscan fossil-bearing horizons of the Kaizara Formation was conducted utilizing three stratigraphic successions. On the basis of seven identified genera and nine species of ammonites, the presence of three successive assemblage zones first proposed by a previous study is confirmed: the Pseudoneuqueniceras yokoyamai, the Kepplerites japonicus, and the Oxycerites assemblage zones. Of these, the Pseudoneuqueniceras yokoyamai Assemblage Zone was assumed to correspond to the Orbis – Oppeli Zone (late Bathonian), and the Kepplerites assemblage Zone was assumed to be comparable to the early Callovian age. These results indicate that the Bathonian/Callovian boundary lies within the middle part of the Kaizara Formation.