In Search of the the Bathonian–Callovian Boundary in Central Japan

The Kuzuryu Group (the lower part of the Tetori Group (sensu lato)), the Middle Jurassic mainly shallow marine siliciclastic sequence, is distributed in the Kuzuryu area in northern Central Japan. Three ammonoid zones: the Pseudoneuqueniceras yokoyamai(Upper Bathonian), the Kepplerites japonicus, and theOxycerites (Lower Callovian) Assemblage Zones have been established in the Kaizara Formation of the Kuzuryu Group. However, recent studies suggest that this zonation needs revision. Furthermore, since the stratigraphic range of the genus Kepplerites or the superfamily Keppleritinae is not limited to the Early Callovian, further investigations of Japanese Kepplerites species are necessary to confirm the previous hypothesis of the assignment of the earliest Callovian age to the Keppleritesjaponicus zone. Thus the horizon of the Bathonian–Callovian boundary in the Kaizara Formation remains uncertain. Recently, age-diagnostic ammonoid taxa are newly discovered from the Kuzuryu Group. Epistrenoceras from the Tochimochiyama Formation, which underlies the Kaizara Formation, suggests the correlation with the Retrocostatum Zone (Upper Bathonian) in the Sub-Mediterranean region, and Kepprelites (Gowericeras) from the uppermost part of the Kaizara Formation suggest the correlation with the middle Lower Callovian. This study reports new occurrences of diverse Hecticoceratinae ammonoids, probably correlated with the Callovian, also from the uppermost Kaizara Formation. In addition, the geographical distribution of the Yambarazaka Formation, which overlies the Kaizara Formation, is probably very limited, and its purported Middle Oxfordian age (Kranaosphinctes matsushimai Assemblage Zone) also needs to be confirmed in future. Thus, the stratigraphical range of the Kuzuryu Group probably covers the Late Bathonian to the Callovian, though the exact timing of the start of its deposition is still uncertain. Further studies of the ammonoid fauna from the upper part of the Kaizara Fomation is necessary to reveal the exact horizon of the Bathonian–Callovian boundary in this area.