Younger Cenozoic Foraminiferal Assemblages from the Choshi District, Chiba Prefecture
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概要
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The vertical distribution of the planktonic and benthonic Foraminifera from the younger Cenozoic deposits distributed in the Choshi district and superposed upon the Permian and Cretaceous Systems was studied. The lowest of the Cenozoic units or the Metogahana Formation was previously considered to be Miocene in age from stratigraphic position. The Na-arai Formation rests upon it with clino-unconformity, and is succeeded upwards with conformity by the Iioka and Toyosato Formations. They are covered by the "Katori" Formation with slight clino-unconformity and the latter is overlain with three units of volcanic ash deposits, each of which is unconformable with one another. The Iioka, Toyosato and "Katori" Formations yield abundant Foraminifera and by the benthonic forms are divided into seven units of Bulimina striata-Stilostomella, Bulimina aculeata-Uvigerina akitaensis, Cassidulina carinata, C. subglobosa, Bolivina pacifica, Nonionella-Elphidium, and Pseudononion-Rosalina zones in ascending order. Distinct changes in the coiling direction are seen in Globigerina pachyderma, Globorotalia menardii s. l. and Pulleniatina obliquiloculata in the Choshi section. There is a distinct short interval of sinistrally coiled P. obliquiloculata at the base of the Iioka Formation. This may correspond to the same coiling pattern of the species in the basal part of the Lower Pliocene in the Philippines (Bandy, 1963). Based upon the planktonic foraminiferal fauna, the basal part of the Na-arai Formation can be correlated to Saito's Upper Miocene Globorotalia menardii menardii/Globigerina nepenthes Zone. Although the other part of the Na-arai Formation yielded no Miocene index species of planktonic Foraminifera, the formation is inferred to be below Bandy's Miocene-Pliocene boundary. The basal part of the Iioka Formation is correlated to the basal part of the Kurotaki Formation along the Obitsu River; the upper part of the Bulimina aculeata-Uvigerina akitaensis zone in the Iioka Formation is correlated to the upper part of the Umegase Formation, and is the basal Pleistocene of Asano et at. (1958); the middle part of the Bolivina pacifica zone corresponds to the uppermost part of the Kakinokidai to Chonan Formation. The relations of the benthonic foraminiferal zones between the Choshi district and the Yoro River area in the Boso Peninsula, the sedimentary environment and the tectonic movement in the region are discussed. Two species of benthonic Foraminifera are described as new to science.
- 東北大学の論文
- 1967-03-27
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