石灰質微化石群集に基づく七谷層と西黒沢層の生層序学的考察
1982 年 47 巻 6 号 p. 374-379
詳細
1982 年 47 巻 6 号 p. 374-379
Biostratigraphic studies of both calcareous nannofossil and planktonic foraminifera were made on the Nanatani Formation exposed in the Imogawa section and the Nishikurosawa Formation in the Oga Peninsula, northeast Japan.
The studies revealed the interrelation between calcareous nannofossil datum planes and planktonic foraminiferal ones shown by means of their occurences in ascending order as follows: Datum level a: Last appearance of Helicosphaera scissura (NN4/NN5 boundary),
Datum level b: Abundant top of Discoaster def landrei (CN3/CN4) and first appearance of Orbulina suturalis (N8/N9),
Datum level c: Abundant base of Reticulofenestra pseudoumbilica and first appearance of Globorotalia peripheroacuta and G. miozea (s. 1.) (N9/N10),
Datum level d: Last appearance of Sphenolithus heteromorphus (CN4/CN5a or NN5/NN6).
These fossil criteria indicate that the Nanatani Formation is CN3 to CN5a and N8 to N10. The Nishikurosawa Formation is the lower part of CN4 which corresponds to N9.
Abrupt increment of Reticulofenestra pseudoumbilica in the late Nanatani stage implies the influence of cold water in early Middle Miocene to the area of Japanese islands which had been under the subtropical climate.
