Bulletin of the Mizunami Fossil Museum

Monograph of the Mizunami Fossil Museum No. 6

The Pliocene Kume Formation developed in the Hitachi-Ota area, Ibaraki Prefecture, is mainly composed of sandy siltstone, yielding abundant molluscan fossils that characterized by offshore sandy to muddy bottom dwellers. Six types of molluscan assemblage (Types I- VI) in the Kume Formation are recognized by the composition, abundance, ecology of species and sedimentary facies.

Type I consists mainly of nearshore sandy bottom dwellers such as Anadara amicula elongate and Mizuhopecten ibaragiensis, preserved in the storm sheet sandstone on offshore muddy bottom; Type II is mainly of Mizuhopecten ibaragiensis associated with nearshore sandy bottom and offshore sandy to muddy dwellers transported by storm from shelf to slope; Type III is of brackish to offshore species in the slump deposits at the base of slope; Type IV is of only offshore species in the slump scar deposits of lower slope ; Type V is of autochthonous offshore species in the slope deposits on slumpings; and Type VI is of offsore species in the debris flow deposits on the lower slope.

In near shore sandy bottom dwellers of Types II and III, warm water forms including important elements of the Kakegawa Fauna such as Anadara castellata, Glycymeris nakamurai, Chlamys satoi, Venus foveolata, Placamen tiara and Suchium suchiense subsuchiense occur in association with cold water forms of the Omma-Manganzi Fauna such as Anadara amicula elongata, Chlamys nipponensis, Mercenaria stimpsoni and Callithaca adamsi. The diatom assemblage from the Kume Formation is assignable to the Denticulopsis kamtschatica Zone (NPD 7B), latest Miocene to early Pliocene in age (N17-19).