美濃帯犬山地域のジュラ紀中世(Bajocian)放散虫類の群集変遷

抄録

This paper reports on a faunal change of Middle Jurassic (Bajocian) radiolarians from manganese carbonate nodules in the Inuyama area, Mino Terrane. Five radiolarian assemblages, distinguished from five horizons (IN-16, IN-10, IN-7, IN-3 and IN-1 in ascending order) of the Unuma section, are well-preserved and extremely diversified in specific composition. About 300 species of radiolarian fossils are obtained from each manganese carbonate nodule. Approximately two thirds of radiolarian species are composed of common species between two horizons. The value of Spumellaria/Nassellaria (S/N) ratio is high in the lowermost horizon (IN-16) and low in the uppermost horizon (IN-1). Although the extinction and origination rates of radiolarian species are totally constant through the section, the origination of nassellarian species in the uppermost horizon shows a high rate. On the basis of these faunal analyses, the radiolarian faunal change was constant during Bajocian time except for a little change of the S/N ratio and the origination rate of nassellarian species. It is suggested that there was not the large oceanic environmental change during Bajocian time in the western part of the Panthalassa.