Cyclic changes in lithofacies and molluscan content in the early Pleistocene Omma Formation, Central Japan related to the 41,000-year orbital obliquity

Abstract

Eleven cyclothems based on change in sedimentary facies and molluscan fossil associations are identified in the middle part of the Omma Formation (early Pleistocene), in Kanazawa, Central Japan. The systematic change in the molluscan fossil associations within individual cycles indicates that the oceanic conditions changed in parallel with water depth fluctuations, such that an increase in water depth corresponds to a marine climatic warming. These changes in water depth and oceanic conditions exactly coincides with the oxygen isotope fluctuations reported for deep-sea sediments. The cyclothems identified here in the Omma Formation can be explained best as the result of glacio-eustasy with a period of the 41,000-years, which corresponds to the period of orbital obliquity.

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