云南曲靖地区的志留系出露较好,连续发育,化石丰富,是研究我国志留系理想的地区之一。自2007年在关底组中发现大量鱼类化石(2009年命名为潇湘脊椎动物群)以来,曲靖地区的关底组便成为国际上早期脊椎动物研究者密切关注的对象与焦点。然而从1914年关底组命名以来,其划分方案及地质时代归属等至今存有较大争议。结合近年来在潇湘水库周边地区详细的野外地质考察以及一条连续剖面(上铁路-东坡剖面)的实测资料,并在前人古生物学研究与地层学划分对比工作的基础上,对曲靖地区含丰富志留纪鱼化石的关底组重新进行了厘定与划分,并对其地质时代开展了分析讨论。根据主要岩性变化及化石特征,取消后期一些学者从关底组下部划分出的岳家山组,并将其划分为4段,从下往上依次为关底组I段岳家山段、关底组II段崇家湾段、关底组III段彩莲段及关底组IV段东坡段。含潇湘脊椎动物群及丰富牙形类Ozarkodina crispa的关底组III-IV段的地层可归入到罗德洛统卢德福特阶,而含牙形类O. crispa的关底组II段上部及不含牙形类O. crispa的关底组I-II段中下部的地层目前暂归于罗德洛统高斯特阶为宜,但不排除其底部未见化石记录的关底组I段下部地层下延到温洛克世的可能。

The continuous Silurian strata are well developed and exposed with abundant fossils in Qujing Area of Yunnan Province, which makes Qujing as one of ideal areas in China for the research of Silurian stratigraphy and paleontology for a long time. The Kuanti Formation has become the focus of attention for early vertebrate researchers in the world, since tremendous amount of fossil fishes were found in the formation exposed in the surrounding areas of Qujing in 2007, which eventually led to the discovery and establishment of the Xiaoxiang Vertebrate Fauna in 2009. However, the stratigraphic subdivision, correlation and the geological age of the Kuanti Formation in Qujing still remain contentious, although many biostratigraphic and lithostratigraphic attempts have been made since the formation was named in 1914. Based on the detailed field geological investigation in recent years in the surrounding areas of Xiaoxiang Reservoir and the measured data of one continuous section (Shangtielu-Dongpo Section), together with many previous paleontological and stratigraphic works, this paper redefines the Kuanti Formation containing abundant Silurian fishes in Qujing Area and further discusses its geological age. According to the main lithological changes and paleontological characteristics, the Yuejiashan Formation which was separated by some researchers from the lower part of the Kuanti Formation is abandoned here. In the paper, the redefined Kuanti Formation can be subdivided into four members in ascending order. Member I (Yuejiashan Member) is characterised by yellow-green and gray-green shales intercalated with thin-bedded fine sandstones or siltstones and several thin-bedded fine-grained conglomerates bearing fragments of fossil fishes in its lower and middle parts. Member II (Chongjiawan Member) is represented by gray-green and purple-red shales, intercalated with light grey middle- to thin-bedded or lenticular limestones or bioclastic limestones containing many brachiopod fossils. Member III (Cailian Member) is dominated by purple-red and gray-green silty and calcareous mudstones or marls intercalated with minor purple-red or yellow-green shales or siltstones, containing brachiopods, fossil fishes and stout tubular trace fossils. A set of middle-thick-bedded fine sandstone with small thickness is usually developed in the bottom of the member, which becomes the obvious marker of the boundary between Member III and Member II. Member IV (Dongpo Member) is composed of gray-green and yellow-green mudstones and shales intercalated with thin-bedded or lenticular argillaceous limestones and marlites. Abundant fossil fishes of the Xiaoxiang Vertebrate Fauna were found in the argillaceous limestones in the lower part, and coral fossils in the upper part. Mainly based on the records of fossil fishes, conodonts and other paleontological data, the age of Member III to Member IV of the Kuanti Formation, containing the main fish-bearing strata of the Xiaoxiang Vertebrate Fauna and conodont Ozarkodina crispa, should be assigned to the Ludfordian Stage of the Ludlow, and Member I to Member II can be referred to the Gorstian Stage of the Ludlow, Silurian. Based on the current stratigraphic data, the possibility of its bottom extending down to Wenlock is not excluded.