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Ishiga-diagram as a geologic time scale

作为地质时间尺度的石贺图

基本信息

批准号：
07640623
负责人：
ISHIGA Hiroaki
金额：
$ 1.28万
依托单位：
Shimane University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
1995
资助国家：
日本
起止时间：
1995 至 1996
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-07640623/
关键词：
sedimentary cyclicity geologic time scale geologic boundary southwest Japan bedded cherts Mesozoic Paleozoic 道前香緒里・石賀裕明

项目摘要

Ishiga-diagram displays sedimentary cycles of bedded cherts and is useful for evaluation of change of primary production and oceanic condition. Examination of this diagram has been carried out in the Carboniferous to Jurassic bedded cherts sequence, where detailed radiolarian and conodonts biostratigraphic age was examined previously. The sedimentary cycles consists of first order cycle of ca. 1 Ma, and this is a combination of 5 second order cycles. The second order cycle is ca. 200 kys, and form a peak of productivity.Mode of change in productivity varies in ages, and significantly rhythmic cycles appeared in Upper Triassic to Lower Jurassic. The decrease in productivity shown in the Ishiga-diagram well coincides to the geologic boundary. Thus primary production can be linked to depression or distinction of the other macro-fauna.This research was expanded to the geochemical examination of the geologic boundary, such as Permian/Triassic and geologic event e.g. Toarcian anoxic event. This is because these events are marked by appearance of organic black shales, which show depression of radiolarian productivity and paleooceanic environmental change. Although the productivity of bedded cherts in the Uppermost Permian gradually became relatively higher than the Middle and Lower Permian, cherts color change into black and includes significantly high contents of organic carbon. The Middle and Upper Permian boundary was also geochemically examined and that was characterized by anoxic basin formation in marginal basin. This might be caused by eutrophication by supposed enrichment of river-born nutrient supply by active volcanism of the continental margin.Ishiga-diagram is useful for evaluation of the oceanic condition change, and geochemistry of the sediments both pelagic and terrigenous may be fundamental for systematic examination of the earth environment.

石贺图显示了层状燧石的沉积旋回，可用于评估初级生产和海洋状况的变化。该图的检查是在石炭纪至侏罗纪层状燧石序列中进行的，之前已检查了详细的放射虫和牙形刺生物地层年龄。沉积旋回由大约一级旋回组成。 1 Ma，这是 5 个二阶周期的组合。二阶周期约为。 200 kys，形成生产力高峰。生产力变化方式随年代不同，上三叠统至下侏罗统出现明显的节律旋回。石贺图井中显示的产能下降与地质边界一致。因此，初级生产可以与其他大型动物群的衰退或区别联系起来。这项研究扩展到地质边界的地球化学检查，例如二叠纪/三叠纪和地质事件，例如。 Toarcian缺氧事件。这是因为这些事件的特点是有机黑色页岩的出现，这表明放射虫生产力的下降和古海洋环境的变化。尽管上二叠统层状硅质岩的生产力逐渐高于中、下二叠统，但硅质岩颜色变为黑色，且有机碳含量显着升高。对中、上二叠统边界也进行了地球化学检查，其特点是边缘盆地形成缺氧盆地。这可能是由于大陆边缘的活跃火山活动使河流产生的养分供应富集而引起的富营养化。石贺图可用于评估海洋条件的变化，而远洋和陆源沉积物的地球化学可能是系统检查地球环境的基础。