华南下扬子区下奥陶统仑山组:岩石学、地层学和古地理学

On the Lower Ordovician Lunshan Formation in Lower Yangtze Region,South China:Its petrology,stratigraphy and palaeogeography

华南地区下奥陶统仑山组为奥陶系最底部的岩石地层单位之一,其以大套白云岩为主要特征,广泛分布于下扬子区。通过对安徽池州洪家剖面仑山组的系统分析,并结合下扬子区多个相关剖面的资料,进一步确定仑山组岩性为白云岩、灰质白云岩和白云质灰岩,夹有部分亮晶颗粒灰岩和亮泥晶颗粒灰岩。整体上以亮晶生屑灰岩或泥晶生屑灰岩的出现作为仑山组顶界,在下扬子区内部、北部及东南部地区以白云质灰岩或灰质白云岩出现为其底界,而在下扬子区西南部大部分地区则以白云岩化出现为其底界。整个仑山组所含生物化石种类和数量较少,自下而上可建立5个牙形石带,分别是Monocostatus sevierensis带、Acanthodus lineatus-A.costatus带、Scolopodus？pseudoplanus带、Glyptoconus quadraplicatus带和Paltodus deltifer-Tripodus proteus带,推测其时代为早奥陶世特马豆克早期。仑山组主要形成于浅潮下带至深潮下带环境中,其地层总厚度和其中的白云岩厚度受到古地理因素的控制。通过岩石学特征分析,推断仑山组白云岩主要为准同生条件下的多种白云岩化作用叠加中等埋藏条件下的白云岩化形成。

The Lunshan Formation is one of the lowest stratigraphic units of the Lower Ordovician in the Lower Yangtze Region in South China.The formation is characterized by massive dolostone in comparison with other contemporaneous lithological units,and extends widely.A detailed analysis on the Lunshan Formation at Hongjia section of Chizhou County,Anhui Province,as well as a comprehensive review of the previous studies,indicates that the Lunshan Formation consists of dolostone,lime dolostone,dolomitic limestone,and minor,intercalated grainstone and packstone.The upper boundary of the formation is marked by the occurrence of medium-to thick-bedded bioclastic grainstone and packstone in the study area.The lower boundary of the formation is marked by occurrence of dolomitic limestone or lime dolostone in inner,northern and southeastern Lower Yangtze Region and occurrence of dolostone in most parts of southern Lower Yangtze Region.Although fossils are commonly rare in the Lunshan Formation,five conodont biozones are established in an ascending order：Monocostatus sevierensis biozone,Acanthodus lineatus-A.costatus biozone,Scolopodus？ pseudoplanus biozone,Glyptoconus quadraplicatus biozone,and Paltodus deltifer-Tripodus proteus biozone,corresponding to the early Tremadocian of Early Ordovican.The Lunshan Formation was deposited in shallow subtidal to deep subtidal settings.The spatial changes in the thickness of the formation as well as those of dolostone were mainly controlled by palaeogeographic factors.The sedimentary properties of dolomite in the Lunshan Formation imply that these dolostones were mostly formed by penecontemporaneous dolomitization superimposed with moderate burial dolomitization.