Bulletin of the Mizunami Fossil Museum

Monograph of the Mizunami Fossil Museum No. 6

The Early to Middle Miocene Bihoku Group is exposed in the Takami area, Mizuho-cho, Shimane Prefecture. The Group, resting unconformably upon the basement rocks, is divided lithologically into the conglomerate, sandstone, and mudstone (members) in ascending order (Table 1 and Fig. 1). The Group shows a very gentle basin structure.

Fossil marine molluscs, belonging to the Yatsuo-Kadonosawa and Higashibessho fossil molluscan faunas, are collected from 23 localities in the sandstone and mudstone (Table 2). Representative molluscan assemblages in sandstone are Tateiwaia-Saxolucina (Loc. D), Anadara daitokudoensis-Euspira-“Vasticardium” (Loc. F), and Balanid (Loc. M), which lived in the tidal to upper sublittoral zones in a tropical to subtropical marine climate. Representative assemblage in mudstone is Limopsis (Loc. P), which lived in the lower sublittoral zone in a climate somewhat different from the one above-mentioned.

Judging from the distribution of sedimentary facies and molluscs, the sea, which accumulated the Bihoku Group in this area, must have invaded from the northern San-in region to the southeastern First Setouchi region.