Subdivision and age of the Silurian fish-bearing Kuanti Formation in Qujing, Yunnan Province
Subdivision and age of the Silurian fish-bearing Kuanti Formation in Qujing, Yunnan Province
复制标题
DOI：
10.19615/j.cnki.1000-3118.200513
复制
发表时间：
2020-01-01
期刊：
影响因子：
--
通讯作者：
Zhu Min
中科院分区：
文献类型：
--
作者：
Cai Jia-Chen;Zhao Wen-Jin;Zhu Min
云南曲靖地区志留系连续发育，出露化石丰富，是我国志留系地层古生物研究的理想地区之一。自2007年曲靖周边地区发现大量鱼类化石以来，宽梯组已成为世界早期脊椎动物研究者关注的焦点，并最终于2009年发现和建立了潇湘脊椎动物群。曲靖宽梯组自1914年命名以来，虽经多次生物地层学和岩石地层学的尝试，但其地层划分、对比及地质时代仍存在争议。本文根据近年来在潇湘水库周边地区进行的详细野外地质调查和一个连续剖面（上铁路-东坡剖面）的实测资料，结合前人的古生物和地层学工作，重新厘定了曲靖地区志留纪鱼类丰富的宽梯组，并进一步讨论了其地质时代。根据主要的岩性变化和古生物特征，这里废弃了此前被一些研究者从宽梯组下部分离出来的岳家山组。本文重新定义了关帝组，自下而上可划分为四段。一段（岳家山段）为黄绿色、灰绿色页岩，下部和中部夹薄层细砂岩或粉砂岩及少量含鱼类化石碎片的薄层细粒砾岩。二段（崇家湾段）为灰绿色、紫红色页岩，夹有浅灰色中薄层状或透镜状灰岩或生物碎屑灰岩，其中腕足动物化石较多。三段（彩莲段）以紫红色、灰绿色粉砂质、钙质泥岩或泥灰岩为主，夹少量紫红色、黄绿色页岩或粉砂岩，含腕足类、鱼类化石和粗壮管状遗迹化石。段底部通常发育一套厚度较小的中厚层状细砂岩，成为三段与二段分界的明显标志。四段（东坡段）为灰绿色、黄绿色泥岩、页岩夹薄层状或透镜状泥质灰岩、泥灰岩。在下部泥质灰岩中发现了丰富的潇湘脊椎动物群鱼类化石，在上部泥质灰岩中发现了珊瑚化石。主要依据鱼类化石、牙形石等古生物资料的记录，认为潇湘脊椎动物群主要含鱼地层和牙形石Ozarkodina crispa所在的宽堤组三段至四段的时代应归属于志留纪卢德洛的勒德福阶，一段至二段的时代可归属于志留纪卢德洛的戈尔斯阶。根据目前的地层资料，不排除其底部向下延伸至文洛克的可能性。
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.