Stratigraphy and depositional environment of the Miocene in the Tajibe area, Okayama Prefecture, Southwest Japan

Abstract

Here we define a new Miocene formation, the Tajibe Formation, which is distributed in the intermontane basin of Chugoku Mountain, Okayama Prefecture, Southwest Japan. We describe the lithology and identify the biostratigraphy based on planktonic microfossils. We make novel discoveries of several index fossils showing the planktonic foraminiferal Zone N8 of Blow (1969) and the calcareous nannofossil Zone CN3 of Okada and Bukry (1980) from the upper part of the Tajibe Formation. From this we infer that the formation was deposited between 17.0 and 14.9 Ma and is correlated in the main to the Korematsu Formation of the Bihoku Group in Hiroshima Prefecture, Southwest Japan. We also reconstruct the spatiotemporal change of the depositional environment based on the lithofacies and fossils such as mollusks, planktonic foraminifers, and ostracodes. The depositional environment of the Tajibe Formation is interpreted to have changed from that of a warm inner bay or sandy coastal environment to that of a warm shelf.