群馬県,碓氷峠南方に分布する中新統の珪藻・放散虫化石

抄録

The Usui Pass area is mainly composed of pyroclastic rocks and marine sedimentary rocks of Neogene system. The writers have carried out diatom and radiolarian analysis on marine sediments from the site A and B in this area. In this study, the diatom flora of site A is characterized by Actinocyclus ingens, Coscinodis cusendoi and Denticulopsis hustedtii. The radiolarians of site A are characterized by Cyrtocapsella japonica and Lychnocanoma nipponica magnacornuta. These facts show that the sediment of A had deposited during the upper Middle Miocene. The diatom flora of site B is characterized by Actinocyclus ellipticus var. moronensis, A. ingens, Coscinodiscus endoi, C. lewisianus, Denticulopsis lauta, D. nicobarica, D. ptmctata, Mediaria splendida, Melosira sol, Pyxilla americana, Synedra jouseana, Thalassionema hirosakiensis, T. robusta and Thalassiosira praeconvexa. Radiolarians of site B are characterized by Calocycletta costata and Eucyrtidium asanoi. These facts that the sediment of B had deposited during the lower Middle Miocene.