International correlation of the Pliensbachian/Toarcian (Lower Jurassic) ammonoid biostratigraphy of the Nishinakayama Formation in the Toyora Group, southwest Japan

Author： Nakada Kentaro Matsuoka Atsushi

Publisher： E. Schweizerbart'sche Verlagsbuchhandlung

ISSN： 0078-0421

Source： Newsletters on Stratigraphy, Vol.44, Iss.2, 2011-06, pp. : 89-111

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Abstract

The Toyora Group, Jurassic continental shelf sediments exposed in southwest Japan, is divided into three formations. The lower member of the Nishinakayama Formation, the middle part of the group, is characterized by an abundance of ammonoid fossils. Ammonite zonal schemes have been established, although these frameworks were difficult to correlate with European zonations. This study aims to revise the ammonite zonation of this member and to correlate the new zonal scheme with the European zonations. Four successive ammonoid zones are proposed (in ascending order): the Canavaria japonica Zone, the Paltarpites paltus Zone, the Dactylioceras helianthoides Zone and the Harpoceras inouyei Zone. These new zones are compared to the zonations established in European provinces. The base of the P. paltus Zone corresponds to the Pliensbachian/Toarcian boundary. In the Upper Pliensbachian, the lowermost part of the C. japonica Zone analyzed in this study is correlated with the Apyrenum Subzone. In the Lower Toarcian, the base of the D. helianthoides Zone seems to be compared to a horizon within the Clevelandicum Subzone, and the base of the H. inouyei Zone is correlated with the base of the Exaratum Subzone. The uppermost part is probably compared to the Fibulatum Subzone.