Bulletin of the Mizunami Fossil Museum

Monograph of the Mizunami Fossil Museum No. 6

The Tertiary formations developed in the Yuyawan Bay - Kottoi area, Yamaguchi Prefecture can be divided into the Tsuo Basalt, the Tsunoshima Dacite, the Hioki Group and the Yuyawan

Group and the Alkali Olivine Basalt in ascending order. Among them, the Hioki Group is subdivided into the Juraku, Kiwado, Taoyama and Hitomaru formations in ascending order, and all these formations are stratigraphically conformable.

The Hioki Group, especially the Kiwado and Taoyama formations yield rich molluscan fossil remains characterizing the so-called “Ashiya Fauna”. Based upon their generic combination in each locality, eight assemblages are recognized, namely; the Corbicula assemblage suggesting fresh water condition, the Crassostrea assemblage of tidal environment, and the Meretrix-Spisula assemblage, the Glycymeris-Dosinia assemblage and the Venericardia assemblage which indicate sandy shallow bottom environment, the Venericardia-Acila assemblage, the Siliqua assemblage and the Venericardia-Periploma assemblage which show muddy shallow environment.

These molluscan assemblages can be stratigraphically allocated into the Kiwado Formation

to the basal Taoyama Formation (in Kottoi area) and the basal Hitomaru Formation (in Yuyawan area) in ascending order; they are Venericardia-Acila A. (A: an abbreviation to assemblage) → (Crasssostrea A. in East Yuyawan area) → Glycymeris-Dosinia A. or Venericardia A. → Crassostrea A. → Glycymens-Dosinia A. → Siliqua A. → Glycymeris-Dosinia A. or Venericardia A. → Venericardia-Periploma A. →Corbicula A. (in Yuyawan area); Glycymeris-Dosinia A. → Venericardia A. → Corbicula A. → Crassostrea A. → Glycymeris-Dosinia A. → Meretrix-Spisula A. (in Kottoi area).

This kind of transformation in the molluscan assemblages from the Kiwada Formation to the

basal Taoyama Formation roughly corresponds to the change in sedimentary environment: Neritic Z. (Z: an abbreviation to zone) → (Tidal Z. in E. Yuyawan area) → Neritic Z.→ Tidal Z. → Neritic Z. (in Yuyawan area); Neritic Z. → Hypohaline and/or Freshwater → Tidal Z. → Neritic Z. (in Kottoi area).

The stratigraphic succession of the fossil molluscan assemblages reveals the sedimentary cycle of the group from shallower to deeper and then shallower again. This cyclic change in environment corresponds to that from marine transgression to regression, and can be recognized in the Hioki Group from the stratigraphic and geographic position of the marine molluscan assemblages.