南岭地区上三叠统-下侏罗统双壳类生物地层及三叠-侏罗系界线

Study on the Upper Triassic-lower Jurassic Bivalve Biostratigraphy and the Triassic-Jurassic Boundary in the Nanling Region, South China

南岭地区在晚三叠世-早侏罗世沉积了一套海陆交互相含煤地层,其中双壳类化石十分丰富,对其详细的生物地层学研究可以为这套地层的划分和对比、识别该区侏罗系与三叠系界线提供重要的古生物资料。本文对湖南浏阳石康、湖南宜章下坪和广东揭西灰寨三条剖面采集的海相和非海相双壳类化石进行了系统研究。结果显示研究区上三叠统-下侏罗统具有4个双壳类生物带,即:半咸水相Unionites?emeiensis顶峰带、海相Retroceramus subinconditus-Hiatella组合带、海相Ryderia guangdongensis-Parainoceramus jinjiensis组合带和半咸水相-陆相Kija ovata-Waagenoperna mytiloides组合带。对各生物带代表化石在区域上的时代延限分析表明,Unionites?emeiensis顶峰带时代延限主要为瑞替期,Retroceramus subinconditus-Hiatella组合带主要为赫塘期,Ryderia guangdongensis-Parainoceramus jinjiensis组合带主要为辛涅缪尔期,但有可能向上延伸至土阿辛期。经与海相同类生物带的对比,Kija ovata-Waagenoperna mytiloides组合带时代延限大致为赫塘期至土阿辛期。根据综合对比分析认为南岭地区半咸水相-陆相T-J界线应位于Unionites?emeiensis顶峰带与Kija ovata-Waagenoperna mytiloides组合带之间,这与四川盆地陆相T-J界线(位于Modiolus weiyuanensis-Unionites?emeiensis组合带与Pseudocardinia(=Kija)kweichouensis-Cuneopsis sichuanensis组合带之间)和新疆准噶尔盆地陆相T-J界线(位于Waagenoperna-Yananoconcha(=Kija)组合带与下伏郝家沟组之间)大致相当。

The Upper Triassic-Lower Jurassic in the Nanling region consists of continuously exposed, fossiliferous ahemation of marine and nonmarine coal-bearing strata. Bivalves, including both marine and non-marine types, are very abundant fossils in the strata. Detailed study on these bivalves will provide important data on the correlation of marine and nonmarine Upper Triassic-Lower Jurassic and erection of the Jurassic/Triassic boundary in this area. This paper studies the bivalve fossils from Shikang and Xiaping sections in Hunan province and Huizhai section in Guangdong province. Four upper Triassic-lower Jurassic bivalve zones are identified, i.e., brackish water Unionites？ emeiensis peak zone, marine Retroceramus subinconditus-Hiatella assemblage zone, marine Ryderiaguangdongensis-Parainoceramus jinjiensis assemblage zone, and brackish-fresh water Kija ovata-Waagenoperna mytiloides assemblage zone. Based on the analysis of the distribution of representative bivalves in each zone, this paper roughly assigns the Unionites？ emeiensis peak zone to Rhaetian, the Retroceramus subinconditus-Hiatella assemblage zone to Hettangian, and the Ryderia guangdongensis-Parainoeeramus jinfl＇ensis assemblage zone largely to Sinemurian, but probably to as young as Toarcian. By correlating with marine bivalve zone, the Kija ovata-Waagenoperna mytiloides assemblage zone is assigned to Hettangian to Toarcian. Consequently, the nonmarine T-J boundary is inferred to be located between the Unionites？ emeiensis peak zone and the Kija ovata-Waagenoperna mytiloides assemblage zone in Nanling region. It is roughly equivalent with the nonmarine T-J boundary in Sichuan Basin （between the Modiolus weiyuanensis-Unionites？ emeiensis assemblage zone and the Pseudocardiuia （=Kija） kweichouensis-Cuneopsis sichuanensis assemblage zone） and the nonmarine T-J boundary in Junggar Basin （between the Waagenoperna-Yananoconcha （=Kija） assemblage zone and the underlaying Haojiagou Formation）.