鲕粒粒度特征及其指示意义

沉积物的粒度资料是确定沉积环境的重要依据，一直以来多应用在碎屑岩方面，较少用于碳酸盐岩的分析。以北京西山寒武系张夏组鲕粒石灰岩为研究对象，在野外实测、室内薄片粒度统计的基础上，通过分析鲕粒粒度参数、粒度频率曲线及概率累积曲线特征，明确了鲕粒粒度在分析鲕粒灰岩沉积环境上的重要作用，并建立了下苇甸鲕粒粒度特征与鲕粒滩在空间上的耦合关系。研究结果表明:鲕粒粒径大小、分选系数、峰度及概率累积曲线组分截点可以很好地反映鲕粒沉积水动力环境；一般情况下，水动力环境较稳定的间歇高能滩沉积的灰泥鲕粒石灰岩鲕粒粒径较小，分选好，峰度尖锐，频率曲线主要为单峰态，鲕粒以悬浮搬运为主；水动力较为动荡的高能滩沉积的亮晶鲕粒石灰岩鲕粒粒径较大，分选较差，峰度多为平坦、中等，频率曲线主要为多峰态，概率累积曲线多为一跳一悬两段式；鲕粒的粒径与鲕粒圈层类型密切相关，粒径较大的鲕粒多以同心鲕、同心放射鲕为主，粒径较小的鲕粒多为放射鲕。研究结果为分析颗粒石灰岩的沉积环境提供了理论和依据。

Characteristics of Ooid Size and Its Environmental Significance: A case study from the Cambrian Zhangxia Formation at Xiaweidian outcrop,Beijing

The grain size characteristics of sediments are important evidence for indicating the sedimentary environment, though they have been mainly used in clastic rocks and rarely in the analysis carbonate rock environments. In this paper, the oolitic limestone of the Cambrian Zhangxia Formation at Xiawedian outcrop, Beijing is taken as the research object. Based on field measurement and ooid size statistic analysis of thin sections, combined with the grain size parameters, granularity distribution frequency curves, and cumulative probability curve, ooid size characteristics are discussed and their influence on the oolitic beach sedimentary environment revealed. The spatial coupling relationship between ooid size characteristics and the oolitic beach sedimentary environment is established. The results show that ooid size, sorting coefficient, kurtosis, and the intersection of the component on the cumulative probability curve can reflect the hydrodynamic environment of ooid sedimentation. In general, ooids in a stable hydrodynamic environment of an intermittent high?energy beach have a smaller size, better sorting, and sharper kurtosis, the granularity distribution frequency curves have more unimodality, and the ooids are mainly suspended. In contrast, the ooids in a turbulent hydrodynamic environment of a high?energy beach have larger size, relatively poor sorting, and relatively flat kurtosis. Their granularity distribution frequency curves are more multimodal, and the cumulative probability curves mainly consist of a “one bouncing population?one suspension population” bi?segmented pattern. The ooid size is closely related to the type of layer structure: the layers of the larger ooids are mainly concentric or concentric?radial, and the smaller ooids are mainly radial. The results provide a theoretical basis for the analysis of the granular limestone sedimentary environment .