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Silurian island-shelf-type reef frameworks and their organic constitutions

志留纪岛架型礁架及其有机构成

基本信息

批准号：
14540435
负责人：
KAWAMURA Toshio
金额：
$ 1.79万
依托单位：
Miyagi University of Education
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2002
资助国家：
日本
起止时间：
2002 至 2003
项目状态：
已结题

项目摘要

Silurian limestones in the Japanese Islands, representing the Silurian reef carbonates on arc-island shelves, are investigated in the methods of stratigraphy, sedimentology and paleontology. Based on detailed works including lithostratigraphic subdivisions, sedimentary facies estimations, and fossil organic compositions, the Silurian reefs and their surroundings are reconstructed and then compared with the other coeval reefs in the world. Elucidated points are summarized as follows:1)Silurian carbonates in the South Kitakami Terrane, newly subdivided into three members, may have been formed by whole sedimentary process from the deposition of shallow carbonates on unstable island-shelf, consequent formation of organic reel-mound, through the collapsing of carbonate bank slope into the off-shore basin.2)Silurian carbonates in the Kurosegawa Terrane indicate the same lithostratigraphy as those of the South Kitaikami Terrane, presuming the similar tectonic environment and sedimentary process around the island-arc.3)Red-colored limestones of the both Silurian carbonates strongly suggest the emergent weathering of carbonates, possibly caused by hydro-eustatic changes in Late Silurian age.4)Organic constitution of reef-mound carbonates includes various metazoan fossils, which compositions are equivalent to those of coeval reefs of the world.5)Reef-mounds newly reconstructed were mainly built in the manner of encrusting or binding by red algae and calcimicrobes, not of framing by metazoans previously estimated.6)Internal frameworks of the island-shelf-type reefs must have been more rigid than those of the continental-shelf-type reefs, because larger amount of calcimicrobes may have joined and closely packed the interstices between skeletal frames.7)Records of microfacies pictures collected from the Silurian carbonates possess an archive value for the coming geological researches and scientific educations.

日本列岛志留纪灰岩是岛弧陆架志留纪生物礁碳酸盐岩的代表。通过对志留纪生物礁的岩石地层划分、沉积相分析和生物化石成分分析，重建了志留纪生物礁及其周围环境，并与世界上其他同时代生物礁进行了对比。阐述要点总结如下：1）南北上地体志留系碳酸盐岩新划分为三段，可能是经历了从不稳定的岛礁上的浅层碳酸盐岩沉积，进而形成有机质的卷丘，再到碳酸盐岩岸坡的崩塌进入近海盆地的整个沉积过程而形成的; 2）黑濑川地体志留系碳酸盐岩与南北上地体志留系碳酸盐岩具有相同的岩石地层学特征，推测岛弧周围的构造环境和沉积过程相似; 3）两个志留系碳酸盐岩的红色石灰岩强烈地反映了碳酸盐岩的突发风化作用，礁丘碳酸盐的有机组成包括多种后生动物化石，其组成与世界同时代生物礁相当;新重建的礁丘主要由红藻和钙质微生物以结壳或胶结方式建造，不是先前估计的后生动物的框架。6）岛架型礁的内部框架必须比大陆架型礁的内部框架更坚固，因为大量的钙质微生物可能已经加入并紧密地填充了骨骼框架之间的空隙。志留系碳酸盐岩中的微相图记录对今后的地质研究和科学教育具有档案价值。