Pliocene Molluscan Fauna from Sasagami Hills in the Northeastern Part of Niisrata Plain, Central Japan

Abstract

The Pliocene molluscan fauna of the Annogawa Formation in the Sasagami hills, Niigata Prefecture, is described and analysed. 1. The Annogawa formation is mainly composed of massive sandy siltstone. At locality 1 this formation includes some conglomerate lenses in the middle horizon and molluscan fossils occur densly in them with some other invertebrate remains. It is assumed that all of them are allochthonous in occurrence, because shells are fragmental and most of bivalves are disarticulated. They can be divided into four types of assemblages based on the bathymetrical range, the bottom character and^the life form of living equivalent species as follows. Type i: Dwellers of gravelly or sandy bottom from the intertidal to the upper sublittoral zones. Type ii: Infaunal dwellers of muddy bottom from the intertidal to the lower sublittoral zones. Type iii: Infaunal dwellers of muddy bottom in the lower sublittoral zone. Type iv: Epifaunal dwellers of gravelly bottom from the intertidal to the lower sublittoral zones. They are sessile or adhering. One attempt was made whether the bottom character of each type was correspond to the sedimentary facies of the Dainichi Formation or not. This analysis leads to the following-conclusion : the assemblage is composed of a part of the "biocoenosis" of muddy bottom in the lower sublittoral zone (Types ii and iii) and transported elements from various biotopes in the intertidal and the upper sublittoral zones (Type i and a part of Type iv). 2. From the result of the analyses of the molluscan fauna, it is assumed that the Annogawa Formation was deposited in the lower sublittoral zone and that an intertidal zone near by shore was developed not so far from there in the same age. 3. This fauna characterized by the elements of open-sea muddy bottom (Nuculacea, Pandorella and so on) is older than the Plio-Pleistocene Haizume fauna (KOBAYASHI, 1983) which includes many endemic species of the Omma-Manganji fauna. 4. Occurrence and characteristic form of cylindrical trace fossils created by Pholadidae pelecypods are described in outline.