| 概要 |
The Kozaki formation is exposed in narrow belts along several tectonic lines in the lower course of the Kuma river. It is limited on the lower by a fault or intrusions of granite or serpentine, and is... overlain by a basal conglomerate of the Carnian Tanoura formation with a distinct unconformity. It is divided into two subformations (fig. 2). The lower subformation (180m) consists mainly of alternating thick conglomerate with intercalated sandstone and black mudstone, and is accompanied with dark grey to black lenticular limestone at four horizons. The upper subformation (180 m) comprises thick conglomerate in the basal part and predominantly mudstone with thin siltstone and occasionally a few thin chert in the main part. The conglomerate is intraformational, showing abrupt lateral and vertical changes into sandstone or mudstone. It contains roundstones, up to 50 cm in diameter, of biotitegranite, quartz-porphyry, porphyrite, diabase, andesite, andesitic tuff, schistose hornfels, etc. There also occur abundantly in local slabs and chips of mudstone and uneven or ragged blocks of dark grey to black limestone of various sizes up to 2 m, which are probably the product of the contemporaneous erosion. Isolated roundstones of igneousv rocks are found also in mudstone, which is, in turn, black, usually massive, and commonly contains pyrite crystals of sedimentary origin. The sandstone is usually of muddy sublithic type, with a large amount of clay-matrix. The limestone, less than 7 m thick, is mostly bioclastic calcarenite and calcirudite consisting mainly of crinoid oscicles, remains of calcareous algae and foraminifers and subordinately of bryozoas, sponges, and brachiopods, with a spary matrix. Not uncommonly oolite with a spary matrix and occasionally algal pisolite (stromatolite) with a microcrystalline calcite matrix are also accompanied. Many of the limestones are more or less argillaceous or arenaceous, and in some parts slightly bituminous, and often contains sedimentary pyrite. The identified fusulinid species listed in Tables 1, 2, and 3 demonstrate that the lower subformation, the lower part of the upper subformation and the uppermost part of the formation are safely correlated with the Cancellina nipponica zone (Nn), Neoschwa gerina craticulifera zone (Nc), and Yabeina globosa (= Y. inouyei) zone (Ng), respectively, of the Akasaka Limestone (OZAWA, 1927), though the second zone was led indirectly from the datum in the equivalent unit of the tectonically separated mass nearby the type section. C. nipponica was recorded from only one other locality (Tominoho; KANUMA, 1958) outside Akasaka, so it may be inappropriate to chose it as the index name of the zone. However, the coexisting species with C. nipponica in Akasaka define a particular zone within the Japanese province on account of its unique assemblage comprising N. simplex OZAWA, Verbeekina sphaera OZAWA, Pseudodoliolina ozawai YABE & HANZAWA, Misellina claudiae (DEPRAT), Yangchienia compressa (OZAWA), Schubertella giraudi (DEPRAT), etc. In addition to them, primitive Cancellina allied to C. nipponica or C. schellwieni and a small subspecies of Pseudodoliolina pseudolepida may take the place of C. nipponica and P. ozawai, respectively. The first two species and C. nipponica are the most primitive species of each genus, as I (1957) previously discussed, and define the lowest part of the so-called Neoschwagerina zone in the Japanese and Chinese provinces. Of these N. simplex which commonly occurs is the most leading species of the zone. It is, therefore, better to denominate the zone as the N. simplex zone in place of the C. nipponica zone. Recent investigations clarified that the most of the conglomerate-bearing Permian formations in Japan can be referable to either of the lower Middle Permian N. simplex zone or the lower Upper Permian Y abeina gubleri-Lepidolina toriyamai zone which was previously described (KANMERA, 1953, 1954). It should be noted that the conglomeratebearing formations of the N. simple北zone in the Kuma Massif occurs only along the thrust zones. Being away from these restricted zone, on the north, east, and south there occur widely equivalent formations of muddy and cherty rock-facies. This suggests that the Kozaki formation might have originally deposited along the Islands of the old tectonic zones which were exposed above the sea-level during the Neoschwagerinan age and have provided the sediments with the material of igneous rocks, especially of andesite and its pyroclastic rocks.続きを見る
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Mendeley出力
