Ryazanian (Berriasian) molluscs and biostratigraphy of the Dutch and Norwegian North Sea area (south of Viking Graben)

Abstract

Herein, Ryazanian (Berriasian) macrofossils from three well cores in the Central Graben (wells B18-02, L06-02, The Netherlands) and on the Jæren High (well 7/7-2, Norway) in the southern North Sea region are described. Macrofossils are mainly represented by buchiid bivalves (Buchia volgensis) and ammonites (Surites, Lynnia and Praetollia?). The genus Lynnia is recorded for the first time outside its topotypical area, and its systematic position and stratigraphic ranges are discussed. Additionally, the studied core sections yielded coleoid remains and a single limid bivalve. Based on the stratigraphic ranges of key ammonite genera (Lynnia, Surites and Bojarkia), the zonation of the Ryazanian stage is reconsidered. Uppermost Volgian to Ryazanian ammonite faunas are quite consistent and diverse but showing a higher degree of similarity throughout the Panboreal Superrealm as compared to those from rest of the Upper Volgian and the Middle Volgian. Buchia volgensis is the only species known from the southern North Sea and East Anglia, which is in strong contrast to the high diversity of Buchia in East Greenland and the remainder of the Boreal Realm. We hypothesise that such differences in the distribution of ammonites and bivalves in general, and the absence of buchiid species other than Buchia volgensis south of East Greenland in particular, are the result of anoxic bottom water conditions in the southern Viking Strait. The unusually wide geographic range of B. volgensis, which is known from such distant areas as Mexico and the Crimea, suggests a potential higher tolerance of this species to adverse conditions.