Abstract
Radiolarians were studied from the typical section of the Neogene of the Hokushin district, northern part of Nagano Prefecture. The studied section covers the Aoki, Ogawa and Shigarami Formations, which have been dated mainly by radiometric dating methods applied for volcanic rocks and ash layers. Radiolarians were found from the Arakurayama Pyroclastic Member and the Ogikubo Sandstone and Mudstone Member of the Shigarami Formation but were not from the other formations. The radiolarian assemblage from the Arakurayama Pyroclastic Member is that of Spongurus pylomaticus Zone (5.2-4.5 Ma). The Ogikubo Sandstone and Mudstone Member yielded radiolarians characteristic of the Pliocene (S. pylomaticus Zone-lower Cycladophora sakaii Zone; 5.2-2.6 Ma). These estimated ages show good agreement with the radiometric ages reported by the previous study.
