摘要
贵州紫云长兴期生物礁礁前礁后和礁核的顶部都有白云岩。如下证据说明这种白云岩是潮上萨勃哈环境蒸发交代成因的:①这种白云岩的氧同位素值都比其灰岩原岩明显高得多,②其微量元素Sr、Fe的含量范围与陕甘宁盆地奥陶纪马家沟组马五段蒸发交代成因的泥晶白云岩的很相似,③白云石的晶体都是泥粉晶大小的自形一半自形晶体,④出现在礁体的顶部,⑤含有藻纹层、鸟眼构造、干裂缝等典型的潮坪环境构造。该礁的出露可能是海平面下降造成的。意大利的Dolomites盆地在二叠纪末由中陆架环境变浅到鲕粒滩环境,再到大气水环境。我国下扬子盆地在二叠纪末由碳酸盐补偿面以下的深水盆地变浅到碳酸盐补偿面以上的深水盆地。这些事实都说明,二叠纪末曾经发生全球性的海平面下降。这种大规模的海平面下降可能是引起二叠纪末生物礁集群绝灭的机制的表现之一。
Dolostones occur on the top of the reef core, reef front and back reef sequences of the Upper Permian The Changhsingian reef in Ziyun County, Guizhou Province, southwestern China. Comprehensive study on them reveals that these dolostones are of the supratidal Sabkha genesis; (1) All have δ18O values higher than those of their precursor limestones, (2) All have Sr and Fe contents similar to those of known typical supratidal Sabkha evaporative finely crystalline dolostones from a well in the Ordos Basin, Shaanxi -Gansu -Ningxia provinces, China, (3) All are composed of finely crystalline euhedral-subhedral dolomites, (4) All occur on the top of the reef core, reef front and back reef, (5) Algal laminated structure, bird-eyes, mudcracks and crustose limonite occur in the reef front and back reef. These features indicate that this reef was once emerged at the terminal Permian. The emergence of the Changhsingian reefs at the terminal Permian might be caused by sea-level drop. This inference is in agreement with the sedimentary environmental changes in China and North Italy. The Cadore Basin in North Italy changed from a mid-shelf environment to a meteoric phreatic diagenetic environment at the end of the Permian. During the Permian-Triassic transition, the water depth of the Lower Yangtze Basin changed from more than 1,000 meter (below the carbonate compensation depth) to less than 1000 meter (near the carbonate compensation depth). The sea-level drop indicated by evidence from not only reefs but also non-reef deposits might be one aspect of the mechanism that caused the mass extinction of biota at the end of Permian.
出处
《地质学报》
EI
CAS
CSCD
北大核心
2003年第3期289-296,共8页
Acta Geologica Sinica
基金
国家自然科学基金(编号40172007
49102017)
中国科学院资源与环境重大项目(编号Kz95l-B1-409)
中国石油勘探开发研究院油气储层重点实验室开放基金
中国科学院矿产资源重点实验室的资助成果
