上総・下総両層群の層序に関する2・3の問題

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Some basic problems on the stratigraphy of the Kazusa and the Shimosa Groups, which are the most typical Upper Pliocene to Pleistocene marine deposits in Japan, are discussed and reconsidered. (1) The horizon of Plio-Pleistocene boundary in the Kazusa Group is not fixed at present. Magneto-stratigraphical and bio-stratigraphical studies came to a conclusion that the boundary is located at the lowermost horizon of the Kiwada Formation. The fission track dating of the volcanic ashes in the Formation, nevertheless, shows fairly old ages. (2) It is the general opinion that the boundary between the Kazusa and the Shimosa Groups lies at the base of the Jizodo or the Kongochi Formation. A reliable clino-unconformity named the Tokyo Bay unconformity, however, isappropriate to this boundary. (3) The upper limit of the Shimosa Group is the top of the Joso Clay bed and theAnegasaki Formation, which are the last aqueous deposits in the paleo Tokyo Bay. (4) The Shimosa Group is subdivided into the Ichijuku, Nagahama, Sanuki, Mandano, Kasamori, Kongochi, Jizodo, Yabu, Kamiizumi, Kiyokawa, Kamiiwahashi, Kioroshi and Anegasaki Formations in ascending order, which are re-examined in the present paper. The base of the Jizodo Formation divides the Group into two parts. (5) Nine cyclothems closely relating with the glacial eustatic movements are recognized in the Shimosa Group. The curve of sea level change dedueed from the sedimentary facies and structure of the Shimosa Group is drawn. This curve shows certain in formation related to the sedimentary environments and ages.
日本地質学会の論文
1988-04-25

上総・下総両層群の層序に関する2・3の問題

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