Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XXI, No.4


Late Cretaceous Radiolarians of the Yubetsu Group, Tokoro Belt, Northeast Hokkaido

Iwata, Keiji;Tajika, Jun

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The Yubetsu Group represents a Mesozoic sequence constituting the western wing of the Tokoro Belt and occupies the eastern margin of the Central Axial Zone of Hokkaido. Due to the absence of mega-fossils, the age of this group has not been fully known until now. It is mainly made up of very thick Flysch type alternating beds of sandstone and shale, often intercalating conglomerates, pebbly sandstone and shale, acidic tuffs, tuffaceous shales, vari-colored shales, and calcareous nodules. This group shows almost a conformable sequence with an easterly dip and its total thickness is estimated to exceed 14,000 m. A radiolarian biostratigraphic study of all the eight formations of this group reveals that it is entirely Late Cretaceous (Early to Late Campanian) in age. Lithostrobus sp. A ― Protoxiphotractus perplexus assemblage zone is proposed to represent the lower half of this Group assemblage zone is further subdivided into a lower Spongostaurus (?) hokkaidoensis subzone and an upper Eusyringium (?) sp. A subzone. No zonation, however, could be established in the upper half of this group on account of very poor and irregular occurrence of radiolarians, except in some horizons of the Onari Formation. The radiolarian assemblage of the Yubetsu Group is to some extent similar to that of the Uppermost Yezo Group, the Rakichinskaya Series of East Sakhalin, and the Campanian equivalent sequence of California, but there is still some differences in them. It is also less similar to the contemporaneous assemblage of southwestern Japan (Shimanto Group, etc.) and the Tropical Pacific. Yubetsu radiolarian fauna may represent an intermediate latitude population of the northwestern Pacific during Campanian times.