Bulletin of the Mizunami Fossil Museum

Monograph of the Mizunami Fossil Museum No. 6

Plio-Pleistocene Haizume Formation in the Nishiyama district of the Niigata oil-field contains abundant and well preserved marine molluscs. The molluscan fossils belong to the Omma-Manganzi fauna that dwelled in the Japan Sea borderland.

The Haizume Formation unconformably partly conformably, covers the Nishiyama Formation and consists of five rock facies, namely mudstone facies, silty sandstone facies, sand facies, calcareous sandstone facies and fine alternation facies of sand and silt. The stratigraphic

distribution is shown in Figs. 2 and 3.

Six fossil assemblages are discriminated in the Haizume Formation of the Nishiyama district according to dominant and characteristic species, namely Limoρsis tokaiensis-Glycymeris nipponicus, Limopsis tokaiensis-Nuculana yokoyamai-Robaia robai, Monia macroschisma-Chlamys cosibensis, Glycymeris yessoensis-Mizuhopecten yessoensis yokoyamae, Mizuhoρecten yessoensis yokoyamae-Glycymeris yessoensis-Acila insignis and Lucinoma annulata-Tucetilla pilsbryi assemblages. Their stratigraphic distribution is shown in Fig. 3.

The Haizume molluscan fauna is of Late Pliocene-Early Pleistocene in age. These marine molluscs lived on rock, sand and silty sand bottoms mostly in littoral and sublittoral zones, partly abyssal zone. Characteristic molluscs from the formation mainly are cold water species, but some warm water species, especially of shallower sea, are also yielded.