对南华系下统莲沱组CIA值寒冷气候认识的几点商榷——以鄂西神农架、湘西北南华系莲沱组为例

本文根据华南南华系莲沱组化学蚀变指数(CIA值)的不同曲线分布特征,把莲沱组CIA值不同曲线的解释与莲沱组沉积特征的相-古气候分析法进行对比,发现二者古气候变化特征不相吻合。再用类比方法,与南沱组寒冷气候进行类比分析,发现南沱组冰期沉积物十分单调,而莲沱组沉积物非常丰富,沉积构造呈多样性。CIA值精度受采样岩性、采样精度、汇源关系等多种因素控制。可以说,有什么样的母岩成分和风化剥蚀方式,就会在盆内形成什么类型的沉积物。此外,成岩作用和区域后期改造作用也不可小视。因此,CIA值必须与传统的相-古气候分析法以及其它相关方法紧密结合,才能相互验证,互相吻合,收到携手共进的效果。

Some opinions concerning the understanding of CIA cold climate of the Liantuo Formation of Nanhua System: Exemplified by the Nanhua System from Shennongjia in western Hubei and western Hunan

In this paper, the authors analyzed the characteristics of the CIA distribution curve of Liantuo Formation in Nanhua System in South China, and compared different CIA values as well as their explanations for the paleoclimate. Compared with the lithofacies-paleoclimate analysis method, the result does not show consistency in the paleoclimate characteristics. By means of analog method and in comparison with the cold paleoclimate of the overlying Nantuo Formation, it is found that the interglacial period sediment is very monotonous in Nantuo Formation, whereas it is abundant in Liantuo Formation and has various sedimentary structures. The factors controlling the accuracy of CIA values mainly include the lithology of samples, sampling precision and the source-to-sink system. It can be said that the source rock composition and weathering denudation determined the types of sediments in the basin formation. In addition, the later diagenesis and regional transformation should not be ignored. Therefore, it is believedthat the CIA value application must be combined closely with the traditional sediment facies and paleoclimate analysis, which can prevent any missing in the explanation and obtain the best results.