Jour. Geol. Soc. Japan, Vol. 119, No. 4, p. 249–266, April 2013 JOI: DN/JST.JSTAGE/geosoc/2012.0048 doi: 10.5575/geosoc.2012.0048 Molluscan fauna of the“ Miocene” Namigata Formation in the Namigata area, Okayama Prefecture, southwest Japan Abstract Takashi Matsubara The molluscan fauna of the Namigata Formation, traditionally ac- cepted to be of Miocene age, are reexamined taxonomically, and the Received 27 February, 2012 geologic age of the formation and its paleogeographic implications Accepted 12 June, 2012 are discussed. The formation is subdivided into the main part and two new members (the Senjuin Shell-Sandstone and Ônishi Con- Division of Natural History, Museum of Na- glomerate members). The Namigata Formation yielded 13 species of ture and Human Activities Hyogo, 6 Yayoiga- Gastropoda, 16 species of Bivalvia and 1 species of Scaphopoda. The oka, Sanda 669-1546, Japan occurrences of Molopophorus watanabei Otuka, Acila (Truncacila) nagaoi Oyama and Mizuno, Chlamys (Nomurachlamys?) namiga- Corresponding author: T. Matsubara,
[email protected] taensis (Ozaki), and Isognomon (Hippochaeta) hataii Noda and Fu- ruichi indicate that the molluscan age should be revised to the late Late Eocene–Early Oligocene. Taking account of the latest elasmo- branch 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 em- bayment connected to the northwest to the Sea of Japan.