四国西部の四万十帯における白亜紀末〜古第三紀初期の砂岩組成とその意義  [in Japanese] Geochemical and petrographic features of the latest Cretaceous-early Paleogene sandstones from the Shimanto Belt, western Shikoku, Japan, and their significance  [in Japanese]

四国西部の伊の岬周辺に分布する四万十累層群は, 北側から中村層, 浮鞭層(U-1,2,3部層), 伊田層(Id-1,2部層)および伊の岬層(In-1,2,3部層)に区分され, それぞれの堆積年代は, 中期〜後期カンパニアン, (後期カンパニアン〜)マーストリヒチアン, 後期暁新世および中期始新世と推定される.砂岩の構成物や化学組成から次の結論が得られた.1)白亜紀後期の砂岩は, 西南日本内帯の珪長質火山岩類から主に供給された.2)後期暁新世の砂岩は, 三波川変成帯〜秩父累帯北帯から主に供給された可能性が高い.とくに伊田層Id-2部層下部の後背地には, NiやCrに富む苦鉄質岩類が露出していたと考えられる.3)中期始新世の砂岩は, 内帯の花崗岩類から主に供給された.4)浮鞭層や伊田層中の含礫頁岩は, 前弧海盆もしくは大陸斜面に堆積した砂岩や頁岩が三波川帯の上昇に関連して崩壊・再堆積することにより形成されたと推定される.

The Shimanto Supergroup on and around Ino Cape, western Shikoku, is divided into the Nakamura Formation, Ukibuchi Formation (U-1,2 and 3 Members), Ida Formation (Id-1 and 2 Members) and Inomisaki Formation (In-1,2 and 3 Members) from north to south. Their depositional ages are middle-late Campanian, (late Campanian-) Maastrichtian, Late Paleocene and Middle Eocene, respectively. Geochemical and petrographic studies on the sandstones led us to the following conclusions. 1) late Cretaceous sandstones (Nakamura and Ukibuchi Formations) were mainly derived from felsic volcanic rocks in the Inner Belt of Southwest Japan. 2) Late Paleocene sandstones (Ida Formation) are inferred to have been supplied from the Sanbagawa Metamorphic Belt and the Northern Chichibu Belt. In particular, provenance including ultramafic or mafic rocks such as serpentinite and greenstone is inferred for the lower part of the Id-2 Member. 3) Middle Eocene sandstones (Inomisaki Formation) were mainly derived from granitic rocks in the Inner Zone of Southwest Japan. 4) Uplift of the Sanbagawa Belt may have induced slope failures of forearc basin or slope basin deposits to form pebbly shales in the Ukibuchi and Ida Formations.