湖北兴山早奥陶世沉积环境

兴山下奥陶统由西陵峡组、南津关组、分乡组、红花园组、大湾组和牯牛潭组组成。岩石类型包括泥晶灰岩、生物碎屑灰岩、鲕粒灰岩、内碎屑灰岩、白云岩等，为一套典型的台地或沉没台地型陆表海碳酸盐沉积。颗粒灰岩在剖面中占绝大多数，其中，鲕粒灰岩、内碎屑灰岩和多数生物碎屑灰岩均为亮晶方解石胶结。鲕粒和砂屑的粒度参数和曲线分布特征说明，砂屑和鲕粒在形成过程中均经历了较强的水动力搬运或搅动。早奥陶世从西陵峡组沉积开始到牯牛潭组沉积结束，沉积环境经历了从局限台地潮坪环境到开阔台地再到浅海陆棚的演化。其中，潮坪环境包括云坪、灰坪、灰云坪等，而开阔台地环境为各种颗粒滩和生物礁。环境的演化反映了从早到晚海水逐渐加深的过程。

SEDIMENTARY ENVIRONMENTS OF THE EARLY ORDOVICIAN IN XINGSHAN, HUBEI PROVINCE

The Lower Ordovician in Xingshan consists of the Nanjinguang Formation,Fenxiang Formation, Honghuayuan Formation, Dawan Formation and Guniutan Formation.The rock types include limemud limestones, bioclast limestones, oolites, intraclast limestones and dolomites, indicating a typical set of epeiric sea carbonate platform or submerged platform deposits. Grainstones are the main part of the section, among which the oolites, intraclast limestones and most bioclast limestones are cemented by sparry calcite. The grain size distribution and parameters of oolites and intraclastic graistones show that those grains had been transported and agitated during their process of formation. From the beginning of Xilingxia Formation deposition to the end of Guniutan Formation deposition, the sedimentary environments evolved from the tidal flats of restricted platform to open platform and shelf. The tidal flats include dolomitic flat, limemud flat and dolomitic limemud flat, whereas the open platform mainly consists of grain banks and reefs.