浅议上扬子区下奥陶统桐梓组

A BRIEF DISCUSSION ON THE LOWER ORDOVICIAN TONGTZU FORMATION IN THE YANGTZE REGION,SOUTH CHINA

华南上扬子区桐梓组为下奥陶统最底部的岩石地层单位之一。通过贵州习水吼滩剖面桐梓组的系统分析,并结合上扬子区多个相关剖面的资料,确定桐梓组以中厚层白云岩、白云质灰岩、页岩为主,偶夹薄层生屑灰岩。桐梓组主要形成于浅潮下带至深潮下带环境,其沉积厚度具由西向东逐渐增大的趋势,白云岩所占比例由西向东逐渐减少。通过对白云岩岩石学特征分析,初步得出桐梓组白云岩成因:在台地浅滩附近以混合水白云岩化为主,在台地内部以回流渗透作用为主。整个桐梓组自下而上可以建立5个牙形石带,分别是Cordylodus angulatus带、Glyptoconus quadraplicatus-Scolopodus?pseudoplanus带、Paltodus deltifer-Triangulodus proteus带、Triangulodus bifidus带、Serratognatus divers带。桐梓组上下界线均为穿时界线,底界由西向东向上穿时,顶界向下穿时。

The Tongtzu Formation is one of the lowest stratigraphic units of the Lower Ordovician in the upper Yangtze Region in South China. A detailed analysis on the Tongtzu Formation at the Houtan section of Xishui County and a comprehensive review of the previous studies indicate that the Tongtzu Formation consists of dolostones, dolomitic limestones, shales and thin-beded skeletal limestones. The Tongtzu For- mation was deposited in shallow subtidal to deep subtidal settings. The spatial patterns of its thickness and the proportional abundance of dolostone were mainly controlled by paleogeographic factors. Five conodont biozones are established, including in an ascending order, the A. lineatus-M, sevienisisd biozone, the G. quadraplicatus-S. ？ pseudoplanus biozone, the Paltodus delti fer-T, proteus biozone, the Triangulodus bifidus biozone, and the Serratognatus divers biozone. The diachroneity of the Tongtzu Formation has been verified： its top boundary becomes progressively older and the lower boundary becomes younger from west to east. Based on petrographic observation, most dolomite in the Tongtze Formation was formed at shallow burial depths from the mixing of meteoric water, seawater, and recharging brines.