No abstract

Molluscan and plant fossils from ancient mangrove swamps are known from upper Lower to lower Middle Miocene sediments in Southwest Japan. Given that the mangrove-swamp-dwelling bivalve Geloina was recently found in the Tate Sandstone Member of the Kadonosawa Formation in the Ninohe district, northeastern Japan, it was expected that mangrove pollen would also be found at this site. To investigate this possibility, samples were collected from the Tate Sandstone Member at two sites Tate and Kitadate . The pollen composition at both sites is characteristic of the NP-zone of Neogene pollen zones Yamanoi a . Mangrove pollen Kandelia sp. fossils were first found in the uppermost part of the Tate Sandstone Member at the Kitadate site, representing the northernmost record of Miocene fossil mangrove in Japan. Based on presentday mangrove assemblages and distribution, the average wintertime temperature in the Ninohe area at the time represented by the pollen fossils is estimated to have been about higher than that of today.CThe Geological Society of Japan 2010 114

CThe Geological Society of Japan 2004 765

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.