日本新第三紀の化石放散虫分帯 [in Japanese] Neogene Radiolarian Zonation in Japan [in Japanese]
Access this Article
Search this Article
The writers have been carrying out the investgation of radiolarian stratigraphy in the Japanese Neogene formations (see fig. 1). The combination and the dominant occurrences of important species are characteristic, and the radiolarian assemblages can be classified into the following high order assemblages: Mh, Mm, Ct-Lt, Ln, Form B, Tj-Ar, Tj and S assemblages. Of these assemblages, Mm, Ct-Lt, Ln and Tj are restricted in the stratigraphic occurrences (seefig. 2). Four radiolarian zones distinguished by the characteristic assemblage are proposed by the writer for the Neogene formation in Japan. They are as follows in ascending order : (1) Melittosphaera magnaporulosa zone, (2) Cyrtocapsella tetrapera zone, (3) Lychnocanum nipponicum zone and (4) Thecosphaera japonica zone. (1) Melittosphaera magnaporulosa zone: This is characterized by the Mm type assemblage, and represented by the dominant species Melittosphaera magnaporulosa, M. hokurikuensis, Spongoplegma variabilium, Eucyrtidium calvertense, Sphaerostylus yatsuoensis etc.. The top is defined by the abrupt decrease of M. magnaporulosa, and by the extinction of M. hokurikuensis, Sphaerostylus yatsuoensis, Haliomma subglobosum, Cladococcus yatsuoense, Actinomma minocenicum, Cannartus violina, Calocyclas ovata and C. cylindrica. This zone corresponds to the lower and middle part of the Nanatani stage in the Niigata Basin. (2) Cyrtocapsella tetrapera zone: This zone is defined by the Ct-Lt type assemblage represented by Cyrtocapsella tetrapera. The base is distinguished by first appearance of Hexacontium nadauraense and Lithopera renzae restricted in this interval. The top of the zone is defined by the abrupt decrease of Cyrtocapsella tetrapera and Lithatractus tochigiensis. This zone corresponds to the upper Nanatani and the lower Teradomari stages in the Niigata Basin. (3) Lychnocanium nipponicum zone: This is distinguished by the common occurrences of Lychnocanium nipponicum, Sethocyrtis japonica, Theocyrtis redondoensis, Stichocorys delmontensis, Spongoplegma variabilium and Cyrtocapsella japonica. The top of this zone is defined by first appearance of Thecosphaera japonica which is characteristic of the next zone. This zone corresponds to the upper Teradomari stage in the Niigata Basin. (4) Thecosphaera japonica zone: The base is defined by the first occurrence of Thecosphaera japonica and Spongurus inouei. Stichocorys delmontesis and Theocyrtis redondoensis extinct in the middle part of this zone. The zone corresponds to the Shiiya and the Nishiyama stages in the Niigata Basin. The stratigraphic distribution of the important species of the Neogene radiolarians is shown in fig. 3. The radiolarian zones in Japanese Neogene formations are correlated with those proposed by RIEDEL and SANFILIPPO (1970, 1971) (see fig. 5).
地質学論集 (8), 23-33, 1973