房総半島南部の新生界の層位学的研究
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概要
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The Boso Peninsula is situated in the southeastern part of the Honshu Arc. In the southern terminus area of the peninsula, deformed forearc sediments are widely developed and are divided into major stratigraphic units as follows : Eocene to Lower Miocene Mineoka Group, Lower Miocene Hota Group, lower Middle Miocene Sakuma Group, Middle Miocene to Lower Pliocene Miura Group, Upper Pliocene to Lower Pleistocene Chikura Group, and Lower to Middle Pleistocene Toyofusa Group. The Mineoka Group comprises the Kamogawa Ophiolitic Complex and the Shirataki and Haccho Formations. The Hota Group encompasses upward sequence the Maejima, Takazuru, and Aokiyama Formations, and the Hashimoto Basalt. The Aokiyama Formation conformably overlies both the Maejima and Takazuru Formations. The Takazuru Formation is intruded by the Hashimoto Basalt and intercalates a conspicuous layer defined as the Ishihata Conglomerate Member. The Sakuma Group comprises the Okuzure, Okuyama, Nakaobara, Kojima, Shiroyama, and Futatsuyama Formations. The Okuzure Formation includes the Homyo Basalt Member. The Miura Group is distributed in the north, middle and south areas. In the north, this Group is divided into the Kinone, Amatsu, Senhata, Inakozawa, and Hagiu Formations ; in the middle, into the Kinone, Amatsu, and Hagiu Formations ; and in the south, into Amatsu, Hedate, and Kagamigaura Formations, in upward sequence. In the north and middle areas, the Miura Group rests unconformably upon the Hota Group. The Chikura Group is distributed in the north, north-middle, middle, and south area. In the nor h, it is composed of the Sagashi and Nekata Formations ; in the north-middle, the Mano Formation ; in the middle, the Shiramazu, Mera, Minamiasahina, Hata, and Kanamari Formations ; in the south, the Shirahama, Shiramazu, Mera, Hata, and Kanamari Formations in upward sequence. In the north, north-middle, and middle areas, the Chikura Group unconformably overlies the Amatsu Formation. The Shirahama Formation intercalates one distinct lithologic unit defined as the Nojimazaki Conglomerate Member and the Mera Formation also includes a similar unit called the Rendaiji Conglomerate Member. The Toyofusa Group comprises, in upward sequence, the Kamo and Higashinagata Formations in the north area, and the Kanamaribata and Higashinagata Formations in the south area. The Higashinagata Formation includes two distinct layers defined as the Takigawa Conglomerate and Okinoshima Sandy siltstone Members in its uppermost part. Age of these formations are established by means of planktonic microfossils as follows : The Mineoka Group ranges in age from Middle Eocene to early Early Miocene ; the Hota Group is correlated with an interwl from the radiolarian Cyrtocapsella tetrapera Zone to Calocycletta costata Zone ; the Sakuma Group from Zone N. 8 to N. 9-10 of planktonic foraminifera ; the Miura Group from the CN3-4 Zones to CN12A Subzone of calcareous nannofossils ; and the Chikura and Toyofusa Groups, respectively, from CN12A Subzone to CN14A Subzone and from CN14A Subzone to CN14B Subzone. Benthic foraminiferal data indicate that, the Hota Group was deposited at depth near the CCD or greater, the Sakuma Group in the lower part of the middle bathyal zone, the Miura Group in a depth range from the lower bathyal zone to lower part of the middle bathyal zone, the lower part of the Chikura Group in the lower part of the middle bathyal zone, the upper part of the Chikura Group in a transitional zone between the lower and upper parts of the middle bathyal zone, and the Toyofusa Group in the upper part of the middle bathyal zone. During the Early Miocene, deposition of the Mineoka Group proceeded with the accumulation of basaltic volcaniclastics and hemipelagic sediments. On the other hand, coarse sediments, including terrigenous material, prevailed during the time of deposition of the Hota Group. During the earliest Middle Miocene time, the major part of the Sakuma Group nearly filled up the grabens developed on the western side of the Mineoka Tectonic Belt. Since 16 Ma, the Miura Group had been deposited in tectonically controlled minor sedimentary basins that were developed on a forearc slope. By 3 Ma, this Group became folded strongly with a southerly vergence on a trench, slope. Since 3 Ma, the lower part of the Chikura Group became folded and formed an uplift zone at the southernmost terminus of Boso Peninsula, and the upper part of the Chikura Group and the overlying Toyofusa Group filled a slope basin called the Tateyama Basin that was developed shoreward of the uplift zone.
- 東北大学の論文
- 1992-02-28