Molluscan fauna of the “Miocene" Namigata Formation in the Namigata area, Okayama Prefecture, southwest Japan

Abstract

The molluscan fauna of the Namigata Formation, traditionally accepted to be of Miocene age, are reexamined taxonomically, and the geologic age of the formation and its paleogeographic implications are discussed. The formation is subdivided into the main part and two new members (the Senjuin Shell-Sandstone and Ônishi Conglomerate members). The Namigata Formation yielded species of Gastropoda, species of Bivalvia and species of Scaphopoda. The occurrences of Molopophorus watanabei Otuka, Acila (Truncacila) nagaoi Oyama and Mizuno, Chlamys (Nomurachlamys?) namigataensis (Ozaki), and Isognomon (Hippochaeta) hataii Noda and Furuichi indicate that the molluscan age should be revised to the late Late Eocene-Early Oligocene. Taking account of the latest elasmobranch data and preliminary strontium isotope ratio, the age of the formation is confined to the late Late Eocene. The present and recent results show that the First Seto Inland Sea was actually composed of two sea areas that existed at different times: the Paleogene sea area is estimated to have been an open sea facing south to the Pacific Ocean, whereas that in the Miocene is thought to have been an embayment connected to the northwest to the Sea of Japan. Therefore, the concept of the so-called Miocene First Seto Inland Sea should be reassessed in the Cenozoic geohistory of the Japanese Islands.Selected molluscan species including Felaniella (Felaniella) namigataensis n. sp. are taxonomically described and/or discussed.

Stratigraphy and Paleoenvironment of Domi-1 and Sora-1 Wells, Domi Basin

“…These depths correspond to 0.52-0.65 s and 0.92-1.15 s in the seismic profiles across the Domi-1 Well. 일본 큐슈 북서부 지역에는 탄층을 함유한 에오세와 올리고세 층들이 넓게 분포하고 있고 (Karakida et al, 1992), Sakito-Oshima 지역의 대부분 퇴적물들은 에오 세 중기에서 말기에 퇴적되고, Sasebo, Karatsu, Yuyawan 지역에는 올리고세 초기 및 말기에 퇴적된 것으로 알려져 있다 (Matsubara 2002;Kurita, 2004).…”