黔西北桐梓水坝塘下奥陶统桐梓组碳酸盐岩微相

CARBONATE MICROFACIES OF THE LOWER ORDOVICIAN TUNGTZU FORMATION AT SHUIBATANG,TONGZI,NORTHWEST GUIZHOU PROVINCE

黔西北桐梓水坝塘剖面下奥陶统特马豆克阶桐梓组沉积时位于上扬子区陆表海极浅水区,抑或部分时段存在盐度异常的云坪,与下伏寒武系第四统娄山关群和上覆下奥陶统红花园组均呈整合接触。桐梓组岩性多为灰色、深灰色灰岩,后生动物大化石和陆源碎屑岩含量显然偏低。常见内碎屑滩相堆积,偶见含砾屑滩,砾屑边缘弱磨圆,属非强水流的近源搬运颗粒类型;少许鲕粒灰岩层。微生物岩颗粒丰富并作为碳酸盐岩的主要供物来源,部分微生物群落形成的细纹层粘结岩属小型叠层石礁相单元,表明在贵阳乌当和桐梓红花园剖面的叠层石可北延超过100km到达水坝塘地区,构建了狭长的叠层石礁相带。黔西北桐梓组部分碳酸盐岩层中出现单一而广适性的钙质微生物造礁群落,与特马豆克阶全球生物礁演化发展阶段相吻合,在奥陶纪生物大辐射的初期,生物礁具有地理分布局限、低生物多样性以及钙质微生物占主导地位的特征。

The Tungtzu Formation from the Shuibatang section, Tongzi, NW Guizhou Province was formed in a shal- low epicontinental marine setting of the Upper Yangtze Platform paleogeographieally, partly with anomalous salinity condition（dolomitic flat） during the Early Ordovieian. This formation conformably overlies the Stage 4 of Cambrian Loushanguan Group and is overlain by the Hunghuayuan Formation. The lithological characteristics are mostly of gray to dark gray medium-thick-bedded limestones and obviously less in metazoanal fossils and shales. Intraelastie bank-dominated faeies are commonly occurred. Flat-pebbles and oolitic beds are occasionally yielded~ and the margin of the former are poor rounded indicating transposition by weak current. Microbial grains are richly occurred, thus microbial biota are considered to play the key role for the carbonate production. Some of them are shown as bind- stones with fine laminations. They formed stromatolitic facies. Northward extension of the ealcimicrobial units with distance more than 100 km along the Wudang, Guiyang, Honghuayuan and Shuibatang, Tongzi sections is thus dis- tinguished as a narrow stromatolitic belt. Nevertheless, simple but eurytopieity of the caleimicrobial community from the Tungtzu Formation in the northwestern Guizhou is consistence to the reef evolutionary stage of the Tremadoc iime. In the earliest episode of the Great Ordovician Biodiversity Event, most of reef cases worldwide are restricted in distribution, lower biodiversity and composed predominantly of microbial components.