Geologic age of the lower Josoji Formation,Shimane Peninsula,Southwest Honshu,Japan: Implications for an abrupt change to deep-water during the earlier opening stage of the Japan Sea |
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| Authors: | Ritsuo Nomura |
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| Institution: | Faculty of Education, Shimane University, Matsue, Japan |
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| Abstract: | The lower part of the Josoji Formation, Shimane Peninsula, contains clues for figuring out changes in deep-water characteristics during the opening of the Japan Sea. The foraminiferal assemblage includes early to middle Miocene biostratigraphic index taxa such as planktonic foraminiferal Globorotalia zealandica and Globorotaloides suteri. The occurrence of these two species, together with the absence of praeorbulinids, suggests that the lower part of the Josoji Formation is assigned to the top of planktonic foraminiferal Zone N7/M4 (16.39 Ma). The benthic foraminiferal assemblage, which is characterized by Cyclammina cancellata and Martinottiella communis, clearly suggests that the lower Josoji Formation was deposited at bathyal depths, and that it developed in association with the abrupt appearance of deep-sea calcareous forms. Such bathyal taxa are the main constituents of the Spirosigmoilinella compressa–Globobulimina auriculata Zone of the Josoji Formation and also of the Gyrodina–Gyroidinoides Zone at Ocean Drilling Program Site 797 in the Japan Sea. The base of these benthic foraminiferal zones can be correlated with the base of the nannofossil Sphenolithus heteromorphus Base Zone (= CNM6/CN3); thus, its estimated age is 17.65 Ma. This biostratigraphic information suggests that the lower Josoji Formation was deposited from shortly before 17.65–16.39 Ma in upper limit age. Evidence that fresh to brackish and shallow-water basins formed in the rifting interval of 20–18 Ma in the Japan Sea borderland suggests that the abrupt appearance of deep-sea calcareous foraminifera occurred about 1 my earlier in this area than in other sedimentary basins and suggests that a significant paleoceanographic change occurred in the proto-Japan Sea at 17.65 Ma. |
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| Keywords: | bathyal depth early Miocene foraminiferal biostratigraphy opening of the Japan Sea paleoceanographic change Shimane Peninsula |
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