JpGU-AGU Joint Meeting 2020

Presentation information

[E] Poster

B (Biogeosciences ) » B-CG Complex & General

[B-CG07] Phanerozoic biodiversity change: radiation and extinction

convener:Yukio Isozaki(Department of Earth Science and Astronomy, Multi-disciplinary Sciences - General Systems Studies, Graduate School of Arts and Sciences, The University of Tokyo), Yusuke Sawaki(The University of Tokyo)

[BCG07-P03] Taxonomy of Silurian conodonts from the Yokokurayama Group, Kurosegawa tectonic zone, distributed in Kochi Prefecture

*Akifumi Ito1, Sachiko Agematsu1, Yuichi Fukushima1 (1.University of Tsukuba)

Keywords:Silurian conodonts, limestone

Paleozoic sedimentary rocks distributed around Mt. Yokokurayama, Kochi Prefecture, are classified into the Yokokurayama Group consisting of the Gomi, Fukata, Ichiyama, Joryu, Nakahata, and Ochi formations, in ascending order. Although there have been sporadic reports and descriptions on trilobites, corals, and others macrofossils, the area is one of the best known Paleozoic fossil localities in Japan. Based on these paleontological studies, the depositional age of the Yokokurayama Group has been thought to be Silurian to Devonian. The authors are recently conducting reviews and taxonomic reexaminations of Paleozoic conodonts in Japan. In this presentation, we show a result of the study on the Yokokurayama Group.

The group is mainly made up of clastic sedimentary rocks, except for the Fukata Formation composed of carbonate rocks. The Fukata Formation, which is about 200 m in thickness, consists of light grey, massive limestones and limestone conglomerates. Kuwano (1974) reported several Ordovician and Silurian conodonts from the formation (The Ordovician specimens are thought to be a reworked assemblage). The massive limestone comprises lime mudstone an wackestone, under a microscope, and contains macrofossil fragments and very fine-sized quartz grains. Three conodont species belonging to three genera are identified from the formation: Ozarkodina sp., Panderodus unicostatus, and Wurmiella excavata. These species indicate that the Fukata Formation was deposited during middle to late Silurian.