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In the Soya area (Fig. 1), northern part of central Hokkaido, the Cretaceous system is overlain unconformably by the Tertiary system. The time gap represented by the unconformity is great, since a par...t of K 6 (Maestrichtian plus Campanian) and the most part of Lower Tertiary are lacking in spite of the apparently parallel attitude of the strata on both sides of the unconformity. The so-called Ichannai formation, which was assigned to the uppermost Cretaceous, has proved to be nothing but the Tertiary Magaribuchi and Onisashi formations on the evidence of molluscan fossils. In the main part of the area the basal part of the Tertiary sequence is occupied by the Magaribuchi formation, which can be correlated with the Oligocene Momijiyama formation of the Ishikari coal-field on the evidence of fossils. In the northeastern part of the area the Miocene Onishibetsu formation directly overlies the Cretaceous. The Cretaceous rocks which come immediately below the plane of unconformity vary in age from place to place, ranging from upper K6P (Maestrichtian) at Magaribuchi (MG in Fig. 1) in the southwest to lower K6a (Campanian) at Tomariuchi (TU) and further down to K5r (Lower Campanian) at Higashiura (HU) in the northeast. More or less greenish sandstones of the two systems are similar in lithological constituents, being characterized by the prominence of volcanic rock fragments in the major constituents and hornblende (and sometimes also pyroxene) in the accessory heavy minerals, although there is some difference in the detailed characters (see Table 2 and Fig. 11).続きを見る
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