Radiolarians, bivalves and the J/K boundary in the Birafu Formation, southern Kurosegawa Belt, Central Shikoku, SW Japan

摘要

The Oxfordian-Berriasian Birafu Formation in the southern Kurosegawa Belt (Permian accretion terrane), Central Shikoku, SW Japan, yields micro- and mega-faunas that contribute to the biostratigraphy across the Jurassic-Cretaceous boundary. The type section consists of turbiditic sandstone and mudstone with an upward-fining tendency in the lower members (A1–A3). The middle members (B1–B2) are coarse sandstone and mudstone with muddy limestone intercalations, whereas the upper member C is characterized by a fine-grained turbiditic succession. Radiolarians define the assemblage zones (AZ):Kilinora spiralis AZ (Oxfordian) in member A2,Loopus primitivus AZ (Tithonian) in the upper part of member A3, andPseudodictyomitra carpatica AZ (Berriasian—lower Valanginian) in members B2 and C. These give at least an Oxfordian—Berriasian total time-range to the Birafu Formation. The mixed marine and brackish bivalve assemblage of members B1 and B2 (lower part) comprisesGrammatodon takiensis Tamura,Pterotrigonia toyamai (Yehara) andCtenoides tosanus Kimura, the range of which is Late Jurassic to Earliest Cretaceous, andAguilerella nagatoensis (Ohta),Miltha japonica Tashiro andIsocyprina japonica Tashiro and Kozai, having an Early Cretaceous range. The concurrent range of Jurassic to Cretaceous bivalves and exclusively Cretaceous species is significant for the clarification of bivalve evolution across the Jurassic—Cretaceous boundary. The Berriasian appearance of Cretaceous marine and non-marine bivalves takes place while Late Jurassic marine bivalves still survived. We consider Member B1 as Berriasian with the J/K boundary situated at its base.