2022 Volume 128 Issue 1 Pages 307-312
We present a diatom assemblage from two carbonate concretions hosting ammonite specimens from the Saku Formation (upper Turonian, lower Upper Cretaceous) in the Teshio-Nakagawa area, Hokkaido, northern Japan. Hemiaulus, Triceratium, and Costopyxis are the major taxa in the assemblage, and they are typical of upper Upper Cretaceous (Campanian-Maastrichtian) deposits. In contrast, the primitive cylindrical diatoms that are found mainly in Albian and Cenomanian deposits are largely absent, except for Gladius sp.
Previous records of mid-Cretaceous diatoms are rare, and our study documents the first well-preserved specimens from late Turonian deposits that have been reliably dated by refined molluscan biostratigraphy. We confirm that mid-Cretaceous diatoms overturned dramatically during the Cenomanian-to-Turonian interval, at the onset of the extreme paleoceanographic crisis known as the Ocean Anoxic Event 2.
