On the Study of Calcareous Nannoplankton from the Anno Formation in Chiba Prefecture

Abstract

In the Boso Peninsula, the Cenozoic marine strata, especially of younger Cenozoic age, are well exposed successively, and full of fossils along the Yoro, Obitsu and Minato Rivers. Although calcareous nannoplanktons are abundant in these strata, the study on them has rarely been done except by TAKAYAMA (1967), NISHIDA (1976, 77) and MAEDA and SAWANO (1978).
This study was planned in order to find out the stratigraphic distribution of the assemblages of calcareous nannoplankton from the Anno Formation along the Inokawa River, which is the uppermost tributary of the Obitsu River in the Central Boso.
Calcareous nannoplankton floras were obtained from 32 horizons as shown in Fig. 2. A total of 16 species of calcareous nannoplankton belonging to 10 genera were identified. Among calcareous nannoplankton floras, Reticulofenestra japonica was very abundant through the lower horizon to upper one of the formation. The lower horizon was characterized by the abundance of R. pseudoumbilica, Cyclococcolithina leptopora and Cy. macintyrei, and the middle horizon by existence of R. pacifica and Coccolithus productus. On the contrary, in the upper horizon, Pseudoemiliania lacunosa and Gephyrocapsa caribbeanica increased in number, while the genus of Discoaster decreased distinctly.
It is worthy of note that R. pseudoumbilica could not be seen in the uppermost horizon of the formation. Based on the remarkable facts of the first appearance of R. pacifica, Ps. lacunosa and G. caribbeanica, and the extinction of Sphenolithus abies and R. pseudoumbilica, the calcareous nannoplankton floras of the Anno Formation were divided into 6 assemblage zones.
Judging from the characteristics of the assemblages, it is thought that the Anno Formation was deposited from the late stage of early pliocene to middle pliocene in geologic age.