关键词: 铁石口剖面, 二叠—三叠系界线, 界线粘土岩, 综合研究

Abstract: This paper is the first synthetical study on Permo-Triassic boundary(are calledPTB for short) of Tieshikou section in Xinfeng county, Jianxi Province, which is loccated in South China folded belt. It is based on the research of followingaspects: the biostratigraphy of conodonts, ammonites and bivalves; geochemical features of elements; oxygen and carbon isotope signature; sedimentary facies analysis and paleoenvironmental reconstruction; X-ray diffraction analysis of clay rocks around PTB; and grouping the ICP analysis data of rock samples collectedaround PTB by optimal division method. Through such synthetical studies, we determine the PTB of Tieshikou section at the base of No. 26 layer. The reasons arelisted as follows: 1. Faunas of Tieshikou formation (above and including No. 26 layer) include“Permiantype” and “Triassictype”forms, but faunas of Changxing formation(under No.26 layer)is “Permiantype” form. 2. Both the analysis of sedimentary facies and the geochemical features of elements indicate that No. 26 layer is the turning pointfrom wide carbonate platform environment to lagoon and shallow water seashore environment. 3. the JP26 clay rock from Tieshikou formation (No. 26 layer) is characterized mainly by both Illite and Kaolinite and a little Mixed Montmorillonite-Illite, the semiquantitative analysis shows the content of Illite, Kaolinite and Montmorillonite is 49%, 45% and 6%, respetively. In contrast to this, the main mineral of clay rocks from Changxing formation is Illite and with a little Chlo-rite, Kaolinite and Mixed Montmorillonite. Also by means of semiquantitative analysis, the content of Illite, Chlorite, Montmorillonite and Kaolinite is 80%， 8%， 7% and 5%, respectively. Thus No.26 layer is the difference limit line of clay rocks. The JP26 clay is a new type of PTB clay which is the first reported inthis paper. Besides this No. 26 layer is also the lithostratigraphic boundary. 4. The result of carbon isotope analysis indicate the values of carbon isotope changed obviously near the PTB. Strata under No. 26 layer all show positive valuesof δ¹³ C, but strata above No. 26 layer(including this layer)show negative values of δ¹³ C.The changing amplitude of δ¹⁸ O values under No.26 layer(5.4%)is larger than that of above No. 26 layer (0.64%).5. The ICP analysis of rock samples indicates that the trace elements fluctuated around the PTB at Tieshikou section. We have used the optimal division method to calculate these ICP analysis data. The No. 26 layer is the first important segement point.

Key words: Tieshikou section, PermoTriassic boundary, clay, synthetical study