Postorogenic Sediments

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Various kinds of postorogenic sediments are found in Japan, where three cycles of orogeny are recognized in her tectonic history, namely the Permo-Triassic Akiyoshi, the Jurasso- Cretaceous Sakawa and the Tertiary Oyashima cycles. In the late Triassic metaorogenic stage of the Akiyoshi cycle several small basins were beought in the mountain range in West Japan, whence a huge volume of coarse detritus was deposited in the Mine and other basins. Frequent changes in facies, composition and thickness as seen in the Alpine Molasse, show strong crustal movements repeated in the orogenic zone in the Mine epoch. Another characteristic orogenic sediments of the Akiyoshi folded mountains were accumulated in the Kuruma basin of the Hida plateau, where the Liassic Kuruma series after the Rhaetic Toyogatake phase of culmination attains more than 8000 m in thickness. It is related to the fragmentaion which occured in the axial zone.<BR>The Sakawa folded mountains were built up from latest Jurassic to middle Cretaceous through 3 principal phases called Oga, Oshima and Sakawa. The Cretaceous Tosa group in the periorogenic zone of West Japan contains 3 deltaic conglomerates at bases of the Ryoseki, Oshima and Miyako series, each showing the above phase of the Sakawa cycle. Through Oshima phase the hydrography and also the mountain structure were greately changed.<BR>In the axial zone of the Sakawaiden, on the other hand, the thick and coarse sediments of the Onogawa series were accumulated in Kyushu during the Gyliak and Urakawa epochs. They suggest the fragmentation along the axis of the orogenic body which developed into the Izumi subgeosyncline along the median tectonic line.<BR>The third group of the postorogenic sediments is found in the hinter basin of the Sakawa folded mountains. In West Japan, the brackish Yoshimo basin turned out land and Wakino lake was produced through the Oga phase of orogeny. Later the lake developed into the Tsushima red basin, in which the Inkstone series was deposited.<BR>The folded mountains of the third cycle were built up in the outer part during the Tertiary period. Postorogenic sediments in the Oyashima epoch are found in Hokkaido and some other places. As the oronization of the new mountain and the anoronization of the old mountains took place at the same time in epoch, the mode of sedimentation was so highly complicated, that one must be very cautious to distinguish orogenic deposits from others.
社団法人東京地学協会の論文

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徳山明
東大理学部地質学教室

Postorogenic Sediments

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