四万十帯北帯の構造区分と白亜系砂岩組成  [in Japanese] Tectonic division and Cretaceous sandstone compositions of the Northern Belt of the Shimanto Terrane, Southwest Japan  [in Japanese]

四万十帯北帯を北から佐伯・蒲江・丹生ノ川の3亜帯に分け, 各亜帯に分布する白亜系の時代的範囲と砂岩組成及び堆積物供給源について論述した。また, 秩父帯南帯のジュラ系砂岩についても言及した。秩父帯南帯から四万十帯にかけて地層は南に向かって段階的に若くなり, 相接する地帯間では時代がオーバーラップする。蒲江亜帯では東に向かって地層が若くなる傾向がある。秩父帯南帯の砂岩は長石質で石英がかなり多く, カリ長石/長石が大きい。佐伯亜帯の砂岩も長石質であるが, 秩父帯南帯のものより石英が少なく, 岩片に富み, カリ長石/長石が小さい。蒲江亜帯は火山岩片に富む石質砂岩で, 丹生ノ川亜帯は石英や長石に富み, カリ長石/長石の大きい砂岩でそれぞれ特徴づけられる。佐伯・蒲江両亜帯の砂岩は, かなり顕著な東西方向の地域的組成変化を示す。蒲江亜帯以北の堆積物は主として西南日本内帯から, 丹生ノ川亜帯のものは南側から供給されたと考えられる。

The Northern Belt of the Shimanto Terrane is underlain by the Cretaceous Lower Shimanto Group, and is subdivisible in to the Saiki, Kamae and Nyunokawa Subbelts from north to south. The group ranges in age from (Hauterivian?) Barremian to Santonian in the Saiki Subbelt, Cenomanian to Maastrichtian (Paleocene ?) in the Kamae Subbelt and Campanian to Maastrichtian in the Nyunokawa Subbelt. The Cretaceous rocks have a tendency of eastward younging in the Kamae Subbelt. In the Chichibu Terrane, Jurassic clastic rocks occur in the Southern Belt and uppermost Jurassic to Lower Cretaceous ones in the Sanbozan Belt on the north of the Saiki Subbelt. Thus, on the whole the strata become successively younger southward in the southern Chichibu and Shimanto Terranes, though overlap in age between adjoining belts or subbelts. Modal analysis of 1,219 sandstone samples from the Jurassic and Cretaceous reveals that their compositions are distinctly different among the belts or subbelts mentioned above. The Southern Belt of the Chichibu Terrane is characterized by feldspathic sandstones rich in quartz with a high K-feldspar/feldspar ratio. Such sandstones occur together with chert arenites in the Sanbozan Belt. The sandstones of the Saiki Subbelt are also feldspathic but are easily distinguished from those of the Chichibu Terrane in having less quartz and higher contents of rock fragments and lower K-feldspar/feldspar ratio. Lithic sandstones rich in volcanic rock fragments characterize the Kamae Subbelt. The Nyunokawa Subbelt is characterized by feldspathic sandstones similar to the Jurassic ones in some respects. The rock fragments in sandstones are composed mostly of felsic to intermediate volcanic rocks through out the Jurassic and Cretaceous. The sandstones of the Saiki and Kamae Subbelts show a considerable lateral variation in composition from west to east. Sandstone petrography, paleocurrents and other available data suggest that the Jurassic to Cretaceous sediments in the Kamae Subbelt and those to the north were derivedmainly from Mesoz01c volcano-plutonic complexes in the Inner Zone of Southwest Japan, and that the Cretaceous clastic material of the Nyunokawa Subbelt was derived from a missing land mass to the south.