地向斜砕屑堆積層中の一堆積型式
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概要
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Sedimentational features and microlithofacies of clastic sediments in geosynclines give us important clues to understand environments of sedimentation and tectonic characteristics in this particular tectotope. Much interest, however, seems to have been paid to the internal sedimentary features of strata and sedimentary cyclicities in minor scale, as was promoted by BOUMA (1962). The present author has turned his attention to the sedimentational pattern of unit sequences in geosynclinal clastic sediments and has recognized a coarsening-upward sedimentation in the Santonian sediments in the Yezo geosyncline, Hokkaido and in the Santonian to Campanian strata in the Shimanto geosyncline, Shikoku. The coarsening-upward sequences in the Yezo geosyncline are characterized in ascending order by (A) fossiliferous claystone member, (B) bioturbated siltstone member, and (C) poorly fossiliferous cross-stratified sandstone member, as deseribed by OKADA and MATSUMOTO (1969) (Fig. 1). Such unit sequences, ranging from 125 m to 400 m in thickness, recur in the marginal facies of the sedimentary basin and display a cyclic sedimentation called the Haboro cycle. This cyclicity becomes vague in the off-shore facies. A detailed deseription of the Haboro cycle is given by OKADA and MATSUMOTO (1969). The coarsening-upward sequence recognizd in flysch sediments of the Shimanto geosyncline consists in ascending order of (A) mudstone member characterized by mature turbidites, (B) alternation member of thin beds of cross-stratified, coarse-grained arenite and siltstone, and (c) massive arenite member (Fig. 5). No fossil was found throughout this coarsening-upward sequence, but elsewhere above this sequence Inoceramus fragments were detected. It is not obvious whether this type of sedimentation constitutes a cyclicity throughout the studied section of the Shimanto geosyncline, owing to unfavourable exposures. These sedimentation patterns are comparable to those recognized in the Upper Carboniferous of the Hercynian geosyncline in Devon, (DE RAAF et al., 1965) and North Spain (READING, 1970). Comparison with these examples suggests that the coarsening-upward sedimentation deseribed here indicates a regressive sequence due to forward advancement of deltas. Therefore, this sedimentation pattern is important to define sedimentary environments in the geosynclinal tectotope.
- 日本地質学会の論文
- 1971-07-31