Radiolarian biostratigraphy of the Miocene Kumi Formation in Oki Dogo Island, Southwest Japan

Abstract

Miocene sedimentary sequences are distributed widely in Oki Dogo Island, Shimane Prefecture, Southwest Japan. Among them, the lowermost part of the Kumi Formation recorded a transition from freshwater to marine environments associated with the opening of the Japan Sea. We analyzed radiolarian fossils collected from the Kumi Formation in its type locality. We identified Eucyrtidium inflatum, Subzone and Lychnocanoma magnacornuta Zone in the upper part of the Kumi Formation and the Melittosphaera magnaporulosa Zone in the lower part. In addition, the lowe part of theKumi Formation yielded the P-C assemblage composed of two species: Pentactinosphaera hokurikuensis and Cyrtocapsella tetrapera. This assemblage is unique to the earliest stage of the Japan Sea opening because it was identified immediately above non-marine sedimentary rocks. In contrast to Miocene in coastal area of Japan Sea, the specific assemblage is considered to be the most similar to that of Okushiri Island based on the composition of siliceous microfossils.