On the Pliensbachian/Toarcian boundary in the Lower Jurassic Toyora Group in southwest Japan

Abstract

The Toyora Group, consisting of continental shelf sediments, is exposed in southwest Japan. It is divided into three formations, the Higashinagano, the Nishinakayama, and the Utano formations in ascending order. The Nm Member, the lower part of the Nishinakayama Formation, has yielded abundant ammonites, indicating successive ages of Early Toarcian. The ammonite zonation was established on these materials, but the Pliensbachian/Toarcian (P/T) boundary was not defined clearly in previous studies. A total of 99 ammonite specimens including 11 genera and 16 species were collected along the Sakuraguchi-dani Valley. The first occurrence of [i]Dactylioceras helianthoides[/i] is the most suitable candidate for drawing the P/T boundary in the Toyora Group, the boundary being drawn at least 35.5 m lower than the previously indicated.