Pleistocene calcareous nannoplankton biostratigraphy in the Choshi district, Chiba Prefecture, Japan

Abstract

Calcareous nannoplankton biostratigraphy extending from the Naarai Formation up to the Toyosato Formation in the Choshi district, Chiba Prefecture, was carried out with a scanning electron microscopic technique. In the present stratigraphic sequence, lithostratigraphy and magnetostratigraphy are well established by MATOBA (1967) and NIITSUMA (1970) respectively and interpreted micropaleontologically by KOIZUMI and KANAYA (1976).In the section studied, eleven calcareous nannoplankton events were recognized, that is, the appearance of Gephyrocapsa caribbeanica and the first occurrence of Gephyrocapsa oceanica in the Naarai Formation, the first occurrence of Gephyrocapsa protohuxleyi, the extinction of Cyclococcolithus macintyrei and the first occurrence of Helicopontosphaera hyalina in the lower part of the Obama Formation, the extinctions of Gephyrocapsa protohuxleyi and Reticulofenestra pacifica and the disappearance of Coccolithus pelagicus in the uppermost part of the Yokone Formation, the extinction of Pseudoemiliania lacunosa in the middle part of the Kurahashi Formation and the disappearance of Gephyrocapsa caribbeanica and the extinction of Reticulofenestra japonica in the lower part of the Toyosato Formation in ascending order. Any specimen of Emiliania huxleyi cannot be detected in the section.In this district the first occurrence of Gephyrocapsa oceanica is recongized prior to the appearance of Plio-Pleistocene marker species of planktonic formainifer Globorotalia truncatulinoides which first appeared at ca. 1.8m.y. ago (HAQ et al., 1977). On the other hand the extinction level of Pseudoemiliania lacunosa is estimated at ca. 0.45m.y. ago (THIERSTEIN et al., 1977).Based on the both micropaleontological frame work, normal magnetic zone in the Kasuga Formation is correlated to the Orduvai Event, normal magnetic zone in the lower part of the Yokone Formation is correlated to the Jaramillo Event and magnetic reversal in the upper part of the Yokone Formation is supposed to be the boundary of Matuyama-Brunhes magnetic Epochs.Accordingly the Kasuga, Obama, Yokone, Kurahashi and Toyosato Formations spread from uppermost Pliocene to middle Pleistocene and their age is estimated at ca. 2.0m.y. to 0.4m.y.Biostratigraphical range of Gephyrocapsa protohuxleyi extended from ca. 1.5m.y. to ca. 0.5my. ago. Extinctions of Reticulofenestra pacifica and Reticulofenestra japonica are conservatively estimated at ca. 0.5m.y. ago because of their abundant occurrences and high possibilities of secondary fossils.Present stratigraphic sequence will offer an important land-based micropaleontological control for the marine Pleistocene in mid-latitude area.