The beginning and the formation process of Kurotaki Unconformity base of the Kazusa Group, related to the origin of Kanto plain

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The Kurotaki Unconformity is a conspicuous geologic gap recognized at the base of Plio-Pleistocene Kazusa Group in Boso Peninsula. Various opinions such as structural movement or submarine landslide on the origin of the unconformity have been examined, but the clear answer is not provided. This study clarified the age of the strata covering the Kurotaki Unconformity at various places in Boso Peninsula through the identification and confirmation of chronological age of the wide spreded tephras intercalated in the Kazusa Group covering the unconformity. As a result, the author found that the sedimentation age of the base layer of Kazusa Group covering the Kurotaki Unconformity ranged from 3.2 to 1.9 Ma in Boso Peninsula, but there was no sedimentary gap between the Plio-Pleistocene Kazusa Group and Pliocen Miura Group in Miura Peninsula. From these facts, author regarded the Kurotaki Unconformity as the result of several local submarine landslides occurred sequentially in 2.3 - 1.9Ma.