Abstract:
Widespread tephras correlate with Aso-4, Toya, Aso-3, and B-Og have been previously reported from the Shibikawa and Katanishi Formations at the Anden Coast, Oga Peninsula. Among these tephras, identification of Aso-3 is questionable considering the distribution of Aso-3 in NE Japan. B-Og, on the other hand, is reported only from Oga Peninsula, and its age is not yet defined. In order to revise the tephrochronological stratigraphy at the Anden Coast, the authors analyzed the major elements, compositions and refractive indices of volcanic glasses for these tephras and compared the results with the representative values. We also analyzed the major elements, compositions, and refractive indices of tephras in the upper Quaternary sediments of the Sea of Japan recovered from ODP Sites 794 and 797, where delta (super 18) O stratigraphy is established. Our results suggest that the tephra previously correlated to Aso-3 at the Anden Coast is different in FeO (super *) and CaO contents from Aso-3 but is indistinguishable in composition from Aso-1 (rich in FeO (super *) , CaO, Na (sub 2) O and K (sub 2) O). Tephras with similar composition were found at Sites 794 and 797, and the ages of both are estimated as about 255ka isotope substage 8b). This age is consistent with the age of Aso-1, which further supports our correlation of these tephras to Aso-1. Tephras whose compositions are similar to B-Og (rich in FeO (super *) , Na (sub 2) O and K (sub 2) O) are also found at ODP Sites 794 and 797, at 5 horizons whose ages range from 419ka to 538ka. Taking into account the distribution of these tephras and the low sea level at the time of B-Og deposition at the Anden Coast inferred from sedimentary facies analysis, we conclude that the tephra corresponding to isotope substage 12b (about 448ka) is probably correlated to B-Og. Based on these results, the age for the base of the Shibikawa Formation, which has previously been considered to be about 300ka, is estimated as older than 450ka. Thus, our results significantly revise the chronostratigraphy of the upper Quaternary at the Anden Coast, one of the type localities on the Japan Sea side of NE Japan.