島根県江島の更新世玄武岩と鳥取県弓ヶ浜砂州南東端粟島の中新世流紋岩のK-Ar年代

抄録・要旨

Igneous activity in the Shinji Rift System related to opening of the Japan Sea began in the Early Miocene and continued through to the Pleistocene. New K-Ar ages have been determined for a Pleistocene basalt from a borehole at Eshima bridge and a Miocene rhyolite from Awashima, southern Yumigahama Sandbar. The Pleistocene basalt yielded a whole rock K-Ar age of 0.19±0.01 Ma. A biotite age of 17.48±0.40 Ma for the Miocene rhyolite indicates that it belongs to the Kawai Formation. The Eshima borehole shows that the Eshima-Daikonjima basalts were erupted on land, and that their base is now situated 60 meters beneath present-day sea level. This suggests that sea level at 0.19±0.01 Ma was at least 60 meters lower relative to that of the present day.