地質学雑誌
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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紀伊半島西部,和泉層群最上部からの古第三系の発見
磯﨑 行雄長谷川 遼益田 晴恵堤 之恭
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2020 年 126 巻 11 号 p. 639-644

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Detrital zircon U-Pb ages were measured for nine non-fossiliferous sandstones from the uppermost Izumi Group, a “Late Cretaceous” forearc clastic sequence in the eastern Izumi Mountains of western Kii Peninsula, SW Japan. Seven out of nine sandstones yielded Paleocene grains. These results confirm that the uppermost Izumi Group was deposited in the Paleogene (Selandian-Thanetian or younger), extending the total depositional duration of the group to ca. 27 myr, almost double the previous estimate. The new age data raise the possibility that a stratigraphic interval across the Cretaceous-Paleogene extinction boundary may be preserved within the group. The occurrence of Paleogene strata further constrains the onset of the low-angle Median Tectonic Line to post-Paleocene.

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