中国早古近纪陆相地层划分框架研究

AN INTRODUCTION TO RECENT ADVANCE IN THE STUDY OF THE CONTINENTAL EARLY PALEOGENE STAGES IN CHINA

近十年来,在广东南雄盆地、江西池江盆地、湖南衡阳盆地和内蒙古二连盆地开展的生物地层学、化学地层学和磁性地层学的研究工作,为我国陆相古近系上湖阶、浓山阶和岭茶阶的建立提供了可靠的依据,并为其与海相地层、全球界线层型的对比提供了可能。古新统上湖阶的界线层型剖面选在广东南雄盆地大塘剖面(即鹅颈岭—逆龙坑一线),根据脊椎动物化石和岩石地层学的研究将紧挨南雄群-上湖组界线的一套黄灰色厚层黏土砾岩作为其底界。最新的古地磁和同位素研究表明,上湖组底界处于C29R极性段的上部,接近白垩系-古近系界线,可以作为上湖阶的底界。古新统浓山阶的界线层型在江西池江盆地田心里—狮子口村剖面,该剖面下古新统狮子口组和中、上古新统池江组的哺乳动物群有明显变化。同时,磁性地层学研究表明,狮子口组与池江组界线上、下出现C27n极性段向C26r极性段转换,可与海相地层中的"丹尼最晚期事件"对比。这表明浓山阶的底界位于C27n/C26n转换带,相当于丹尼阶最上部,与塞兰特阶底界接近,与北美哺乳动物分期当中Torrejonian-Tiffanian的界线基本一致。始新统岭茶阶(古新统-始新统)界线层型选在我国湖南衡东盆地成家冲东北的岭茶—霞流公路旁,剖面上发现的古新统-始新统界线碳同位素负向漂移,可与国际古新统-始新统界线层型剖面——埃及Dababiya剖面(GSSP)的碳同位素负向漂移对比,该剖面的碳同位素负向飘移可以作为岭茶阶底界,年龄接近55.8±0.2Ma。

Recent studies of biostratigraphy, isotope stratigraphy and magnetic stratigraphy in the Nanxiong Basin of Guangdong, the Chijiang Basin of Jiangxi, the Hengyang Basin of Hunan, and the Erlian Basin of Nei Mongol （Inner Mongolia） provide reliable evidence for the establishment of the Shanghuan Stage, the Nongshanian Stage and the Lingchan Stage of the continental early Paleogene in China. In addi- tion, these studies provide a basis to correlate these Chinese continental stages to the marine strata and the global boundary stratotype. The Datang section （from Ejinling to Nilongkeng） in the Nanxiong Basin of Guangdong is proposed as the stratotype section of the base of the Shanghuan Stage （lower Paleocene）. Paleomagnetic results indi- cate that the base of a yellow-gray clay sandstone bed, representing the boundary between the Nanxiong Group and the Shanghu Formation, lies in upper part of the Chron C29r, fairly close to the K/T boundary. This inference is consistent with recent radiometric dating results. Together with the fossil mammals and lithostratigraphic data, it is currently acceptable to use the base of a yellow-gray clay sandstone bed as the lower limit of the Shanghuan Stage. The Tianxinli-Shizikou section in the Chijiang Basin of Jiangxi is the stratotype section of the base of the Nongshanian Stage （middle-upper Paleocene）. The section documented a distinct faunal change from the Shizikou Formation （lower Paleocene） to the Chijiang Formation （middle-upper Paleocene）. Paleomagnetic analysis reveals that the boundary between the Shizikou Formation and the Chijiang Formation, hence the Shanghuan-Nongshanian boundary, coincides with the Chron C27n-C26r transition. Therefore, the Shanghuan-Nongshanian boundary can be roughly correlated to the Danian-Selandian boundary of the Global Geologic Timescale and the Torrejonian-Tiffanian boundary of North American Land Mammal Ages. The Lincha-Xialiu Section in the Hengyang Basin of Hunan is selected as the stratotype section of the base of the Lingchan Stage. The negative carbon isotope excursion recorded from the Paleogene strata of the Hengyang Basin marks the base of the lower Eocene Lingchan Stage, which is correlative to the Paleocene-Eocene boundary of the Global Geologic Timescale.