四国中央部黒瀬川帯美良布層の放散虫年代と二枚貝群集

Jurassic-Cretaceous sediments are widely distributed in the Inner and Outer zones of SW Japan. In the Outer Zone, the micro-and mega-fossil biostratigraphy of the Birafu Formation, one of the Jurassic-Cretaceous sediments, is investigated in detail. Based on the tectonic subdivision of East Shikoku, the Birafu Formation belongs to the Sakashu Belt (south Kurosegawa Belt) that is composed of the Permian accretionary complexes. On the contrary, the Nankai Group probably belongs to the Yoshigahira Belt of the post-Neocomian tectonic unit.
The Birafu Formation is lithostratigraphically subdivided into seven members. Radiolarians are abundantly found in nine horizons of the members A2, A3, B2 and C. In the type section, rich molluscan faunas are found in the Member B1. Ataxioceras kurisakense of early Kimmeridgian ammonite has been reported from the lower part of the Member A3. The type section is subdivided into three radiolarian assemblage zones (AZ), i.e. the Kilinora spiralis AZ (Oxfordian: Member A2), Loopus primitivus AZ (Tithonian: upper part of the Member A3), and the Pseudodictyomitra carpatica AZ (Berriasian - lower Valanginian: Members B2 and C) in ascending order. The Birafu Formation is at least attributed to the total time-range from Oxfordian to Berriasian.
Bivalves are found mainly in the Member B1 that is situated stratigraphically between the Loopus primitivus AZ (Tithonian) and Pseudodictyomitra carpatica AZ (Berriasian - lower Valanginian). Bivalves are subdivided into two groups. One is marine Late Jurassic to earliest Cretaceous, and the other is marine and non-marine Early Cretaceous. Early Cretaceous bivalves occur in the base of the Member B1. It suggests that the J/K boundary is located between the upper part of the Member A3 and the lowest part of the Member B1. Further suggestion is that the transition between Jurassic and Cretaceous bivalve-types took place in the Berriasian. It is important to clarify the bivalve evolution across the Jurassic - Cretaceous boundary.