Preliminary report on the radiolarian age of the Upper Cretaceous Matoya Group (Shimanto belt) in the Toba District, Mie Prefecture, Southwest Japan

Abstract

A detailed field mapping and radiolarian dating in the Toba District, the Shima Peninsula (eastern tip of the Kii Peninsula) have revealed Coniacian–Campanian mudstones of the Upper Cretaceous Matoya Group. This group belongs to the Shimanto belt (northern subbelt) that formed along the plate boundary where the Kula plate has been subducting beneath the paleo-Asian continent. Seven out of 51 radiolarian-bearing samples from different outcrops of mudstone yield relatively well-preserved radiolarian assemblages, which are concentrated in three age-groups: (i) Early Coniacian, (ii) Early Campanian or Middle Santonian–Middle Campanian, and (iii) Middle–Late Campanian. This evidence has a potential to chronologically and stratigraphically divide the Matoya Group into several units.