北海道釧路國阿寒地方の地質について

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The area now dealt with is situated to the north of Kushiro City in East Hokkaido. The Yubetsu and Shitakara coal-fields are included in this area and Lake Akan, a famous national park lies to the north. The annexed table shows the geologic column which is revised by the writer on the basis of his field work from what was previously prepared by Y. Sasa in this region including the Kushiro coal-field near Kushiro City. Owing to economic importance, many geological studies on the Paleogene Urahoro group, the lower part of the Tertiary sediments of this area and on intercalating most productive coal seams have been entensively carried out. On the contrally, the Onbetsu group and the overlying Neogene sediments are little known. So, in this report the stratigraphy of the younger formations is mainly discussed. The Urahoro group which unconformably overlies the Upper Cretaceous Sensho group comprises nine formations and consists mainly of sandstone, conglomerate, and shale with intercalated numerous workable coal seams at three horizons, namely Harutori, Yubetsu, and Shakubetsu formations. The Shitakara and Omagari formations which occupy the middle and uppermost parts of the Urahoro group respectively are of marine in origin and have yielded many molluscan and foraminiferal fossils which resemble closely those of the Poronai group in the Ishikari coal-field. The Onbetsu group consists mainly of siltstone and sha1e with rich pyroclastic materials in the upper part and also contains marine fauna as in the above-mentioned Shitakara formation. The unconformity below the Omagari formation recognized by Y. Sasa and others in other areas cannot be observed in this area. The two groups just mentioned are referred to Paleogene, probably Upper Eocene to Oligocene. The Neogene sediments of this area are divisible into three groups as shown in the annexed geologic column. Namely, the Akan, Fuppushinai, and Honbetsu groups in ascending order. The Akan group which forms the foundation of the Akan volcanoes and sporadically crops out in small areas along the upper course of the Akan River below the younger volcanics. It consists of green tuff with intercalated, thin bands of fossiliferous. tuffaceous sandstone. This group may be correlated with the Kawabata group in the Ishikari coal-field by its lithology and paleontological evidence. The stratigraphic relation between this, group and the underlying Onbestsu group is conjectured to be unconformable by mean of mapping, although it could not be observed in anywhere due to the lacking of the continued exposures of these formations. The. Fuppushinai group is a part which included in the Honbetsu group in previous work and may be assigned probably to Upper Miocene in age. Unconformably resting on the Onbetsu group, it consists mainly of tuff and agglomerate and intercalated thin beds of sandstone and siltstone in alternation. A few foraminiferal and molluscan fossils have been collected from the lower part of the group. The Hobetsu group consists. mainly of tuffaceous sandstone and has yielded many marine fossils which are characteristic to the Pliocene Takikawa group of the Ishikari district. It rests on the various formations already stated with a remarkable unconformity. The Pleistocene Kushiro group which occupies the eastern hilly land of the area now in question shows a intimate relation to the pyroclastic rocks from the Akan volcanoes in the pre-caldera time. The fossils from this group are very few, only Ostrea sp was collected at the cliff near Shimo-Onnenai. The Tertiary sediments form four anticlines and three sycnlines with NE-SW trends which equally pitch down northwards and these folds are in accordant to the Kurile Arc. The faults observed in this area are classified into three systems in direction namely N20°-30°E, E-W, and N20°W. Among them, the Shitakara and Onbetsu faults are most conspicuous and bound the eastern borders of the Yubetsu and Shitakara coal-fields. The folding and faulting just mentioned are referred by the writer to have occurred during the time of the post-Honbetsu and pre-Kushiro groups. The igneous rocks intercalated in the Tertiary formations are mostly flows of andesite and its agglomerate, and those of the Akan volcanoes are also of the same nature and largely extruded during the Quaternary period, although their first eruption seems to have already taken place in the Pliocene.
東北大学の論文
1953-12-17

北海道釧路國阿寒地方の地質について

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