碳酸盐岩生屑颗粒定量研究--点计数法的理论分析与应用

Quantitative study of bioclastic grains in carbonate rocks:Theoretical analysis and application of point-counting method

碳酸盐岩中的生屑丰度数据为研究化石类群分异度、重建古生态环境提供了重要依据。点计数法易于定量统计各类生屑颗粒的丰度,比目估法、图像分析方法更适于碳酸盐岩组分的定量研究。在实际应用过程中,点计数法的最佳计点数常不易确定,往往会造成统计结果的精度不足,或统计工作耗时多、工作量大。应用点计数法和图像分析方法分别对视觉百分含量对比图和碳酸盐岩显微图像进行定量统计,分析沉积组分特征对点计数法测量误差的影响,以确定点计数法在碳酸盐岩生屑丰度统计时的最佳计点数。研究结果表明,点计数法适用于碳酸盐岩生屑颗粒丰度研究。对于大部分碳酸盐岩薄片定量分析工作,500点的计点数量基本可以将点计数法的测量误差控制在±2.5%以内。但是点计数法统计结果的精度受到薄片中生屑颗粒丰度、粒径大小、分选性甚至生屑分异度等沉积组分特征的影响。因此在实际工作中,应根据具体情况,适度调整计点数,从而在统计工作量和统计数据误差之间获得平衡。

The abundance of bioclastic grains in carbonate rocks is of great value in researches of fossil diversity and reconstruction of paleoecological environment. It can be easily valued with point-counting method, which is more suitable for carbonate rock than the other two quantitative approaches-visual estimation and image analysis. However, few studies have been attempted to determine how many points should be counted to yield adequate and precise results, which made the point-counting procedure time-consuming. By comparative analysis of the comparison charts for visual percentage estimation and microphotographs of the Ordovician carbonate rocks, this study focuses on how these attributes influence the precision of point-counting method, and discusses a moderate number of points which facilitates the method. As a result, a number of 500 points or so is appropriately enough for precise measure of the abundance of the referred component in most of carbonate thin sections with an error of ± 2.5%, and should be adjusted with abundance, grain size, sorting as well as biodiversity of bioclasts in thin sections.