Bulletin of the Mizunami Fossil Museum

Bulletin of the Mizunami Fossil Museum No. 3

Stratigraphy and molluscan fauna of the Miocene Setouchi Series in the northern part of Okayama and Hiroshima Prefectures are studied based on newly obtained field data and molluscan specimens.

1. Relation between the Shiomachi formation and the Bihoku group

The Miocene series in the area is divided into two parts, the lower Shiomachi formation and the upper Bihoku group. The latter overlies the former disconformably. The phenomena of disconformity relation found in many outcrops shows characteristics as small relief, difference in lithofacies in both formations and a thin conglomerate bed at the base of the upper formation. This type of disconformity is common in the Miocene Setouchi Series showing a small gap between two small sedimentary cycles.

2. Molluscan fauna of the Bihoku group

Various molluscan assemblages are found in the lower member (mainly composed of sandstones and sandy shales) and the upper member (mainly composed of shales) of the Bihoku group as follows;

lower member

Geloina assemblage

Crassostrea-Batillaria aassemblage

Crassostrea-Cyclina-Geloina aassemblage

Turbo-Chlamys assemblage

Omphalius-Acesta assemblage

Dosinorbis-"Vasticardium" assemblage

Dosinorbis-"Vasticardium"-Operculina assemblage

Miogypsina-Operculina assemblage

upper member

Palliolum-Fissidentalium assemblage

Each assemblage indicates sundry paleoenvironments of deposition. Geloinas bearing assemblage is especially interesting because it represents a tropical or subtropical condition.

Some fossils are common in molluscan species of the other Miocene Setouchi Series as the Mizunami group and of the Miocene groups in Hokuriku district (Japan Sea side of central Japan) as the Kurosedani and Higashi-innai formations. The Bihoku fauna has few species in common with the Miocene faunas in San-in district (Japan Sea side of southwest Japan) which is a neibouring district. It is supposed that this fact indicates differences in sedimentary environments of both districts.

Four new species and one new subspecies are described. They are as follows;

Placopecten todaniensis n. sp.

Nipponomarcia nakamurai imobarensis n. subsp.

Monodonta minima n. sp.

Pictoneritina kibiensis n. sp.

Tugurium makiyamai n. sp.