著者

木村 敏雄

出版者

東京大学地震研究所

雑誌

東京大学地震研究所彙報 (ISSN:00408972)

巻号頁・発行日

vol.44, no.2, pp.561-607, 1966-10

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Turbidites are developed in the Permian-Triassic Sambosan and the Jurassic-Cretaceous Shimanto group in Japan. The Turbidite sequences are well distributed in the Tamagawa district, the Kanto mountainous land, and in the Oigawa district (Fig. 1). The sequence in the latter district occupies the southern wing of a grand recumbent fold. Here, turbidite-poor shale, turbidite (the A formation), non-turbidite with turbidite (the B), turbidite (the C) and turbidite-poor shale formations are distributed. Average thickness and standard deviation of the sandstone and shale beds within each 5m column of strata were measured (Tables 1-3). Average thickness of sandstone beds varies with the horizon, especially with the major cyclic sedimentation. On the other hand the average thickness of shale beds is fairly constant in the turbidite sequences (the A and C formations). Thickness of the sandstone beds in the turbidite sequences shows log-normal distributions (Figs. 16, 17). However, that of the shale in the turbidite sequences shows distributions similar to the normal ones. The distributions are probably due to the fact that the shale deposition is proportional to time and that the time interval of turbidity currents is fair constant. This is confirmed by a fact that the number of turbidites during the deposition of a unit shale is fairly constant (Table 4). The standard deviation of thickness distribution is related to the thickest bed within each 5m column, especially for the sandstone beds. Thickness distributions of sandstone and shale beds in the turbidite-poor and the turbidite-rich sequences (the B formation) are quite different from those of the turbidite sequence (the A and the C formation) in the Oigawa (Fig. 18, Table 2) and show much wider distribution. The thickness of sandstone beds is not related to that of the overlying and the underlying shale beds in the A and C formation (Figs. 22, 23). However, turbidite beds in the turbidite-poor shale formation underlying the A formation are composed of a very thin sandstone part and rather thick silt part. In this case thickness of the overlying silt and shale appears to be related to that of the underlying sandstone part. The thickness of turbidite sandstone beds is also related to the clasticity (Fig. 24) and sedimentary structures. Sole marking is not common on the underside of the thinner beds (70cm or less). Laminated structures, convolute structures and other are common in the thinner beds (10cm or less), but not common in the thicker beds. In the turbidite together with the non-turbidite sequences there are cyclic sedimentations. Sandstone-shale diagrams (Figs. 6, 10, 11, 14, 15), total thickness (Figs. 4, 7, 13), average thickness and the thickest bed (Fig. 26) within 5m column, and accumulation of sandstone during the deposition of a unit thick shale (Figs. 8, 10, 15) show the cyclic sedimentations which are classified into major and minor ones. The major cyclic sedimentation is most well shown by the distribution of thickest bed within each 5m column (Fig. 26). The bed usually corresponds to the thickest bed within each minor cycle (Fig. 19). The major cycle may be related to transgression and regression and is not related to the chert deposition at Unazawa (Fig. 7). Minor cyclic sedimentations are further classified into "increasing" and "decreasing" types (Fig. 12). The "increasing" type appears to be generally formed during the regression, the "decreasing" type during the transgression. The minor cyclic sedimentations are well shown by the sandstone-shale diagrams as well as by the thickness distribution of sandstone beds according to the sedimentation order (Figs. 12, 20). There was a basin of the normal sedimentation with some steep cliffs at the regressional stage (Fig. 27), a part of this basin becoming the drainage of turbidity currents when the sea level was rather high. At the stage of transgression, shale deposited principally with muddy turbidites. The major cyclic sedimentation may have been formed under such a circumstance. The frequency of great earthquakes and the tectonic position in the westernmost Pacific near Japan are comparable with those of turbidite sequences in the Oigawa district, the turbidites having been probably produced by such earthquakes.二畳紀-三畳紀の三宝山層群,ジュラ紀-白亜紀の四万十層群にはTurbidites層が発達している.このTurbidite層は多摩川地域・南部大井川地域によく見られる(Fig. 1).大井川地域のこの層群は大きな横臥せしゅう曲の南翼をなしており, Turbiditeの少ない頁岩層, Turbidite層(A層), Turbiditeを伴うnon-Turbidite層(B層), Turbidite層(C層), Turbiditeの少ない頁岩層からなつている. 5mの厚さの地層の中の砂岩層の平均層厚(Table 1-3)は層準に応じて,特に堆積の大りんねに応じて変化する.一方頁岩層の平均屈厚はA, C層内では層準にかかわらずほぼ一定である.