Biostratigraphy and correlation of the Lower to Middle Miocene in the Setouchi Geologic Province of the Kinki District, Southwest Japan

Abstract

This study focuses on the biostratigraphy of diatoms, planktonic foraminifera, and ostracods from the Miocene Ayugawa, Tsuzuki, Yamabe, and Yamagasu groups in the Setouchi Geologic Province of Kinki District, southwestern Japan. The results of this study are as follows. The Ayugawa Group in Shiga Prefecture does not contain fossils of planktonic foraminifera and index species of diatoms, so a detailed depositional age of the group could not be determined; however, the first discovery of fossil ostracods, along with descriptions of fossil mollusks from previous studies, allow correlation of the Tsuchiyama and Kurokawa formations of this group with diatom biozone NPD2B (18.2-16.9 Ma). The Okuyamada Formation of the Tsuzuki Group in Kyoto Prefecture contains a few fossil ostracods and fossil diatom index species, the latter suggesting correlation with diatom biozone NPD2B. The first discovery of fossil diatoms in the Hayama Formation of the Yamabe Group in Nara Prefecture suggests correlation with the diatom biozone NPD2A (19.9-18.2 Ma) based on index species. Fossil planktonic foraminifera and ostracods were discovered in the Igami Formation of the Yamagasu Group in Nara and Mie Prefectures, but no index fossils were found. The first fossil diatoms discovered in the Nakatarō Formation of the Yamagasu Group suggest correlation with diatom biozone NPD2A. The Yamabe and Yamagasu groups are therefore the oldest marine Miocene strata in Setouchi Geologic Province. Based on these results and prior studies, the Miocene strata in Setouchi Geologic Province were deposited during four global sea-level rise intervals from 19 to 15 Ma.