Study of latest Cretaceous Radiolarian Assemblage and the faunal change in northern Japan

作者重点研究了北海道北部中通贝苏地区客口群的放射虫生物地层学。在此基础上，建立了晚白垩世两个放射虫分带--原氧磷束丛带(寒武纪早期-中期)和克氏旋回带(寒武纪晚期-马斯特里赫特早期)。前者对应于Amphipyndax enesseffi带，后者对应于热带太平洋和日本西南部上白垩统的Amphipyndax tylotus带。Hakobuchi组合和Hidaka Supergroup之间的狂热差异是明显的。也就是说，在Hidaka超群中发现了Amipyndax enesseffi和A.tylotus组合，而在Bakobuchi群中完全没有这些组合。日本西南部的箱口群、东中超群和上白垩统的动物群差异反映了古地理和沉积堆积地点的差异。9白垩世晚期，箱口群放射虫急剧减少，但一些白垩纪类型的属一直持续到古新世早期。

The authors investigated especially the radiolarian biostratigraphy of the Hakobuchi Group of Nakatonbetsu region in the northern Hokkaido. As a result, two radiolarian zonations of the latest Cretaceous System - Protoxiphotractus perplexus zone (early - middle Campanian) and Chlathrocyclas gravis zone (late Campanian - early Maastrichtian) were established. The former zone corresponds to Amphipyndax enesseffi zone and the latter corresponds to Amphipyndax tylotus zone of the Upper Cretaceous in the Tropical Pacific Oceans and the southwestern Japan. The fanual difference between the Hakobuchi Group and the Hidaka Supergroup was distinguished. Namely, amphipyndax enesseffi and A. tylotus assemblage were found in the Hidaka Supergroup, but these assemblages were completely absent in the Hakobuchi Group. Faunal difference among the Hakobuchi Group, Hidaka Supergroup, and the Upper Cretaceous Systems in the southwestern Japan suggests differences of paleogeography and accumulation sites of sediments9 Abrupt decline of late Cretaceous radiolarians was detected in the Hakobuchi Group, but some genera of Cretaceous type persist into the early Paleocene epoch.