安徽东至早奥陶世红花园组生物礁:奥陶纪生物大辐射前的微生物礁

通过对安徽东至建新剖面红花园组的生物礁、微生物岩进行研究,发现蓝绿菌类Girvanella在生物礁中大量分布,是生物礁的主要造礁成分。其他造礁生物包括瓶筐虫、石海绵、腹足类、腕足类、棘皮类等。Girvanella以散乱分布、包壳、组成内碎屑以及与海绵伴生等4种方式在本区生物礁中存在,并且在生物礁成礁过程中以生物本体矿化作用及粘结、捕获颗粒的作用形成生物礁格架;瓶筐虫及石海绵为生物礁中的居住者,因此,生物礁性质为瓶筐虫/石海绵-微生物礁,微生物起到了主要的造礁作用。此外红花园组典型含生物礁沉积序列是由底部的生屑灰岩基底、中部的海绵层(少量微生物岩)及顶部的微生物岩组成的。通过与其他地区同时代生物礁的对比发现,东至建新剖面红花园组中瓶筐虫/石海绵-微生物礁的发育及微生物的繁盛是全球性的,是由于底栖生物扰动对潮下带微生物发育影响尚未彰显造成的。

Lower Ordovician Reefs in the Honghuayuan Formation at Dongzhi,Anhui:Microbial Reefs Just Prior to the Ordovician Biodiversification

Morphological and ecological diversity of organisms increased during the global Great Biodiversification Event from the late Early Ordovician.Those changes were accompanied with a marked declines in microbial reefs in the subtidal settings.The reefs were built by benthic microbes,Calathium,and lithistid sponges in the middle Early Ordovician Honghuayuan Formation of Jianxin,Anhui Province.The reefs exhibit three main rock types:1)massive skeletal grainstones,2)lithistid sponge Calathium-microbial bindstones,and 3)microbial boundstones.Girvanella,one of representative calcimicrobes,is classified into four categories in the reefs:1)dispersed,2)encrusting,3)forming into intraclasts,and 4)infilling within sponges.These types of Girvanella play a significant frame-building role in the Calathium/lithistid sponge-microbial reefs of the Honghuayuan Formation.Such kind of microbial reefs commonly occur in the Yangtze Platform and elsewhere in the world just prior to the first acme of the Ordovician biodiversification.Metazoan competitions with microbes might be closely related to marked declines in microbial sediments decline after the deposition of the microbial reefs of the Honghuayuan Formation.