大桑層中部 (1.3-0.9Ma) に見られる4.1万年周期の氷河性海水準変動による堆積サイクル

抄録

At least ten cyclic changes in sedimentary facies and accompanying molluscan fossils can be recognized in the middle part of the Omma Formation in Central Japan. The middle part of the Omma Formation is dated as early Pleistocene (1.3-0.9Ma) which is a period that is known to have experienced major fluctuations in sea-level and climate caused by glacio-eustacy. Glacioeustacy can therefore be selected as a strong candidate for explaining the observed cyclic changes in both lithofacies and molluscan fossils. In order to test the validity of this interpretation, we compared the water-depth curve inferred from molluscan fossils in the middle part of the Omma Formation with the oxygen isotope record of DSDP Site 607. The bottom and top of the Jaramillo Subchron (the most precise datum horizons) and three nannofossil biohorizons can be used to give very good time constraint. The results show that the pattern of changes in water depth recorded by the fossil and sedimentary records exactly coincides with the oxygen isotope record at Site 607. The close match between the two records lead to the following conclusions:
1) the depositional age of the middle part of the Omma Formation corresponds to the oxygen isotope stages from 46 to 28; and
2) the observed cyclic changes in sedimentary facies and molluscan fossils can be best explained as the result of glacio-eustacy with a period of the 41, 000-years, which corresponds to the period of orbital obliquity of the earth.