湖北兴山中寒武统覃家庙群微生物岩及其古环境意义

Microbialites in the Middle Cambrian Qinjiamiao Group in Xingshan,Hubei Province:Implication for Paleoenvironmental Reconstruction

通过野外考察及对岩石光面的观察,发现湖北兴山古洞口剖面中寒武统覃家庙群微生物岩的类型主要为波状-半球状叠层石,少量为凝块石。显微镜下观察发现叠层石及凝块石中保存有蓝绿菌类丝状微生物Girvanella,依据Girvanella丝状体在微生物岩中的赋存状态,认为覃家庙群的微生物岩是由底栖微生物群落的钙化和粘结、捕获等共同作用形成的。此外,古洞口剖面中寒武统微生物岩的宏观形态和巨型构造及其伴生的碳酸岩沉积相,指示其形成环境位于深潮下带和浅潮下带。通过对比研究,发现扬子地块表现出的中寒武世微生物岩再繁盛现象具有全球性,推测后生动物的匮乏或衰退是其主要控制因子,反映了生物礁结构由元古代的叠层石礁演变为显生宙的后生生物礁的重要转折。

Abundant microbialites, from the Middle Cambrian Qinjiamiao Group in the Gudongkou section near Xingshan County of Hubei Province, south China, were described in detail by macro- and microscopic observations. The microbialites in the Qinjiamiao Group consist of a great amount of stromatolites and minor thrombolites, and contain some distinguishable filamentous fossils of Girvanella. The relationship between Girvanella filaments and sediment particles strongly suggests that the microbialites in the Qinjiamiao Group were formed by microbial mats trapping and binding of carbonate sediment and the calcification of Girvanella filaments. The morphologic and structural patterns of the microbialites and associated sedimentary features argue that these microbialites were deposited in the deep to shallow subtidal settings. The Middle Cambrian Microbialites Resurgence Event, distinguished from the Qinjiamiao Group in Yangtze Platform and elsewhere in the world, might be controlled by virtually barren of metazoan, and could be recognized as a performance of the unique stutter-step transitional interval from Proterozoic stromatolitic- thrombolitic reefs to Phanerozoic metazoan-dominated reefs.