北京下苇甸剖面张夏组鲕粒特征及其白云化机制

北京下苇甸剖面张夏组作为华北寒武系的标准剖面之一，鲜有人对其中的鲕粒类型及其成岩作用进行细致研究。在充分调研前人研究的基础上，通过野外观察、实测，并结合室内薄片鉴定、XRD测试等技术，对下苇甸剖面张夏组的鲕粒类型进行总结，并对鲕粒白云化机制进行分析。结果显示：(1)下苇甸张夏组鲕粒类型主要为放射鲕、同心放射鲕及单晶鲕/多晶鲕，还有少量同心鲕、复鲕及藻鲕；(2)鲕粒发生白云化，填隙物未见白云化，且放射圈层白云化程度高于同心圈层，并且放射圈层白云石颗粒围绕核心呈放射状分布，同心圈层围绕核心呈同心状分布；(3)鲕粒内部白云石颗粒多为中细晶，自形-半自形，回流渗透白云化是造成鲕粒白云化的主要成因，因此造成亮晶鲕粒石灰岩白云化程度高于灰泥鲕粒石灰岩。

Characteristics of Oolites and Their Dolomitization Mechanism of theCambrian Zhangxia Formation at Xiaweidian Outcrop in Beijing

The Xiaweidian outcrop in Beijing is a typical Cambrian stratigraphic profile in North China. Previous studies focused mainly on the depositional environment of the Zhangxia Formation, whereas the oolitic types and their diagenesis of the formation are poorly understood. In this paper, field geological and microscopic petrographic observations and XRD analysis were conducted to identify the oolitic types and elucidate the mechanism of oolitic dolomitization.It comes to a conclusion as follows: (1) Major oolitic types include radial ooids, concentric radial ooids and monocrystalline/polycrystalline ooids, and minor concentric ooids, compound ooids and algae ooids. (2) Oolites are dolomitized whereas the interstitials are not, and the degree of dolomitization in the radial layer is higher than that in the concentric layer.Dolomite particles in the radial layer are radially distributed around the core, and the concentric rings occur around the core. (3) Dolomite grains in the oolites are mostly mesocrystalline or fine grained, euhedral to subhedral, and returning infiltration of dolomitization was likely the main cause of the oolitic dolomitization, which resulted in a higher degree of dolomitization of the luminescent oolite limestone than the stucco oolite limestone.