用化学地层学研究新元古代地层划分和对比

随着传统地层学向多学科相互渗透的综合地层学方向的发展,笔者开展了湖北峡东地区新元古界震旦系和南华系的化学地层学研究。分别介绍了碳酸盐岩地层的稳定同位素和碎屑岩化学蚀变指数的研究方法和成果并依此提出新元古界划分和对比方案。震旦系碳酸盐岩中所获得的δ13C变化曲线和87Sr/86Sr比值与世界诸多剖面的变化曲线基本一致。南华系碎屑岩的化学蚀变指数(CIA)的特征是莲沱组下部CIA值为55~65(代表寒冷环境的CIA值),莲沱组上部CIA值则增至65~75(代表温湿环境),而莲沱组顶部两个样品的CIA值再次降为寒冷指数60~65;南沱组CIA值均为60~65,除了顶部(接近震旦系)的两个样品分别为69·4和70·9。上述CIA值的变化表明南华系以寒冷气侯为主,并且经历过“寒冷—温湿—寒冷”气候的演变条件下的沉积物。沉积相的多样性说明即使在“冰期”期间古构造、古地理环境对沉积物特征的影响依然存在。

Chemostratigraphic studies to explain Neoproterozoic stratigraphic division and correlation

Instead of the traditional stratigraphyic studies the authors have studied the chemostratigraphy of Neoproterozoic Sinian and Nanhua systems in the Yangtze Gorges, Hubei Province. This paper introduces the study methods and results of stable isotopes in carbonate rocks and chemical index of alteration (CIA) in clastic rocks and advances a new scheme of Neoproterozoic division and correlation. Using δ~ 13 C curve and ~ 87 Sr/~ 86 Sr ratios, obtained from carbonate rocks in the Sinian System, which are the same as those in the equivalent sections in the world. The study of chemistratigraphy in clastic rocks in the Nanhua system focused on CIA. The value of CIA varies between 55-65 in the lower part of the Liantuo Formation, it increases to 65-75 in the upper part, and two samples in the top turn to 60-65 again. However, except for two samples at the top of the Nantuo Formation near the Sinian System, which show 69.4 and 70.9 separately, other samples in the formation all have values ranging from 60 to 65 in cold weather conditions. The results suggest that the sediments developed mainly in cold climatic conditions and that there were cyclic climate changes of cold-damp-cold. The diversity of sedimentary facies explains that the character of sediments in the ice period has been influenced by palaeotectonics and paleogeography.