中国上泥盆统阳朔阶牙形类生物地层研究

CONODONT BIOSTRATIGRAPHY OF REGIONAL YANGSHUOAN STAGE(UPPER FAMENNIAN,DEVONIAN)IN CHINA

阳朔阶最早由殷保安于2008年提出,命名源自桂林市阳朔县,用以指代上泥盆统锡矿山阶与邵东阶之间的一个地方阶,大致相当于国际年代地层单位法门阶上部。阳朔阶底界以牙形类Palmatolepis rugosa trachytera的首次出现为定义,顶界则以有孔虫Eoendothyra regularis的首现为标志。阳朔阶典型剖面为广西桂林市铁山剖面,自下而上包括“五指山组”和融县组,形成于孤立碳酸盐岩台地边缘,以浅水相的内碎屑、鲕粒泥粒灰岩、粒泥灰岩和颗粒灰岩为特征,局部夹有少量黑色钙质泥岩。在桂林铁山剖面和邻近的小龙门剖面中共采集牙形样品149件,其中94件样品产丰富的牙形类分子,该牙形动物群以Palmatolepis、Neopolygnathus、Polygnathus、Pseudopolygnathus、Bispathodus、Icriodus、Rhodalepis、Mehlina和Branmehla属分子为特征,自下而上可识别出6个牙形类生物带,包括Pa.marginifera marginifera带、Pa.rugosa trachytera带、Pa.perlobata postera带、Pa.gracilis expansa带、Bi.costatus带和Bi.ultimus ultimus带,为环古特提斯洋地区法门阶地层对比提供了高精度的生物地层格架。基于详细的牙形类生物地层和沉积相分析,在铁山剖面Pa.rugosa trachytera带上部和Pa.gracilis expansa带顶部浅水相碳酸盐岩序列中识别出两次黑色富有机质沉积事件,对应于相对海平面快速上升和海洋贫氧,可能是法门期全球性Annulata和Dasberg事件在华南边缘海区的沉积响应。

The regional Yangshuoan Stage,named after the Yangshuo County in Guilin,Guangxi,represents the penultimate Devonian stage in China and is approximately equivalent to the upper Famennian.Its base is defined by the first occurrence datum(FOD)of the index conodont Palmatolepis rugosa trachytera and its top by the FOD of the foraminifera Eoendothyra regularis.The stratotype section of the Yangshuoan Stage is near Tieshan in Guilin City,and it comprises~236 m of shallow-water intraclastic,oolitic wackestone or grainstones with minor intercalations of black,organic-rich calcareous mudstones that accumulated on an isolated carbonate platform.A total of 149 conodont samples were collected at intervals between 0.25 m and 18 m within the“Wuzhishan”and Rongxian(=Yonghsien)formations at Tieshan and adjacent Xiaolongmen sections,among which 94 samples yielded conodonts and 55 were barren.Amongst the conodonts,reworking processes cannot be ruled out finally;however,there are no differences in the way they are obtained.Representatives of the genera Palmatolepis,Neopolygnathus,Polygnathus,Pseudopolygnathus,Bispathodus,Icriodus,Rhodalepis,Mehlina,and Branmehla have been collected from these successions and the faunas enables the establishment of six conodont biozones,including the undifferentiated and regionally expanded Pa.marginifera marginifera,Pa.rugosa trachytera,Pa.perlobata postera,and Pa.gracilis expansa zones as well as the Bi.costatus and Bi.ultimus ultimus zones in ascending order,providing a high-resolution correlation with other equivalent sections in the peri-Palaeotethys region.Based on a detailed analysis of conodont biostratigraphy,two intervals-involving sudden intercalations of black,organic-rich rocks within very shallow-water shoal carbonate sediments-were recognized in the upper part of the regionally expanded Pa.rugosa trachytera Zone and at the top of the likewise regionally expanded Pa.gracilis expansa Zone,respectively.These two major lithological changes represent sedimentary responses to sea-level rises and marine dysoxic events,probably paired with unsteady eutrophic conditions in the South China marginal sea and can be correlated with the global Annulata Events and Dasberg Crisis.