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

本文重点研究了北海道北部中顿别地区白口群放射虫生物地层学。据此，建立了晚白垩系的两个放射虫带——Protoxiphotractus perplexus带（早-中坎帕纪）和Chlathrocyclas gravis带（晚坎帕纪-早马斯垂纪）。前者对应于上白垩世热带太平洋和日本西南部的Amphipyndax enesseffi带，后者对应于上白垩世热带太平洋和日本西南部的Amphipyndax tylotus带。区分了博渊组和日高超组之间的差异。即在Hidaka超群中发现了amphipyndax enesseffi和A. tylotus组合，而在Hakobuchi群中则完全没有这些组合。日本西南部博渊群、Hidaka超群和上白垩系的区系差异表明了古地理和沉积物聚集地点的差异。在博渊群中发现了晚白垩世放射虫的突然减少，但白垩纪型的一些属一直持续到早古新世。

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.