Revised radiolarian zonation of the Upper Cretaceous Izumi inter-arc basin(SW Japan)

摘要

The radiolarian zonation of the Izumi Group is revised for an integrated biostratigraphy across the Campanian/Maastrichtian boundary, together with the ammonite occurrence, fission track and magnetostratigraphic calibration of the acidic tuff key beds. The significance of direct correlation in the same sections, between macro- and micro-fossils (i.e. between relatively near-shore nektonic and turbiditic planktonic-fauna) is discussed in relation to the litho- and bio-facies of the Izumi group. Late Cretaceous paleoceanographic changes inferred from the studied radiolarian fauna represent a regional marker event for the chronostratigraphic correlation of continental shelf to turbidite deep-sea sequences, which accumulated on a mid latitudinal margin facing towards the NW paleo-Pacific. The proposed Campanian/Maastrichtian boundary is at the limit between Pachydiscus awajiensis and Nostoceras hetonaiense ammonite zones. This boundary corresponds to the radiolarian last occurrence of Amphipyndax pseudoconulus (= A. enesseffi), Archaeodictyomitra sliteri, Dictyomitra formosa, D. koslovae and Rhopalosyringium magnificum at the top of the Myllocercion acineton Assemblage Zone (uppermost Campanian), and the first occurrence of Clathrocyclas gravis and Dorcadospyris sp. A, at the base of the Maastrichtian C. gravis Assemblage Zone. The new data settle partly a chronostratigraphic discrepancy between radiolarian and ammonite zonations in the Izumi Group, which was pointed out previously by Ishida et al. (2010).